forest plantations sustainability of · 4 forest plantations the evidence forest plantations the...

Sustainability ofForest Plantations

The evidence

A review of

evidence

concerning the

narrow-sense

sustainability of

planted forests

DFID Department forInternationalDevelopment

issu

es

Plantation Forestry 27/3/00 10:52 am

Sustainability ofForest Plantations

The evidence

A review of

evidence

concerning the

narrow-sense

sustainability of

planted forests

Report commissioned by

The Department for International Development (DFID),

94 Victoria Street, London SW1E 5JL, UK

This document is an output from a project funded by

the UK Department for International

Development (DFID) for the benefit of developing countries.

The views expressed are not necessarily those of DFID.

May 1999

by

Professor Julian Evans OBE

Eucalypts and pines planted in the highveld of Swaziland

Plantation Forestry 27/3/00 10:53 am

F o r e s t P l a n t a t i o n s t h e e v i d e n c e F o r e s t P l a n t a t i o n s t h e e v i d e n c e

Forest plantations are becoming an increasingly important resource worldwide, a trendthat is expected to continue strongly. This trend follows the pattern seen in farmingand food production, with the important distinction for forest products that it may helpalleviate pressure on the natural resource. This review examines the evidenceconcerning the narrow-sense sustainability of forest plantations, and raises thequestions: is growing trees in plantations a technology that can work in the longterm? Is plantation silviculture biologically sound, or are there inherent flaws which willeventually lead to insuperable problems for this way of growing trees?

The principal conclusions from the review are:

• Measurements of yield in successive rotations of trees suggest that there is, so far, no significant or widespread evidence that plantation forestry is unsustainablein the narrow sense. Where yield decline has been reported, poor silvicultural practices and operations appear to be largely responsible.

• Evidence in several countries suggests that current rates of tree growth, includingthose in forest plantations, exceed those of 50 or 100 years ago.

• Plantations and plantation forestry operations do have an impact on the sites where they occur. Under certain conditions nutrient export may threaten sustainability, but usually more important for maintaining site quality are care withharvesting operations, conservation of organic matter, and management of the weed environment. Plantation forestry appears to be entirely sustainable under conditions of good husbandry, but not where wasteful and damaging practices are permitted.

• Plantations are at risk from damaging pests and diseases. New threats will inevitably arise and some plantations may become more susceptible owing to climate change factors, but the history of plantation forests suggests that these risks are containable with vigilance and the underpinning of sound biological research.

• There are several interventions in plantation silviculture which point to increasing productivity in the future, providing management is holistic and good standards are maintained. Genetic improvement, in particular, offers the prospect of substantial and long-term gains over several rotations.

• Environmental changes will undoubtedly have an impact on plantation forests. Some changes may yield improvement, others damage. Most plantation species are resilient and broadly based genetically, and are unlikely to suffer seriously from the kinds of climate change scenarios currently predicted. It will be prudent to maintain genetic diversity and minimise stress to planted trees.

Summary Contents

Summary 2

Introduction 4

Global overview of plantations 4The question of sustainability – broad and narrow sense 5Narrow-sense sustainability – this position statement 5

Evidence of productivity change 8

Successive rotations 9Within-rotation yield class/site quality drift 17Special case of coppice crops 19

Site change induced by plantation forestry 22

Soil chemistry – nutrient budgets, nutrient cycling 25Soil physics – compaction, erosion, water use, harvesting impacts 29Organic matter dynamics 32Weed spectrum and intensity 33

Risks to which plantations are exposed 34

Pest and disease incidence 35Storms and fire 43

Interventions to sustain yield 44

Genetic improvement 45Role of different silvicultures 49Fertilising 50Site preparation practices 51Organic matter conservation 51Holistic management – as crop follows crop 53

Environmental impacts on yield over time 54

Pollution impacts CO2, NOX etc. 55Climate change 56

Conclusions 57

References 58

Acknowledgements 64

Plantation Forestry 27/3/00 10:53 am Page 4

4 F o r e s t P l a n t a t i o n s t h e e v i d e n c e F o r e s t P l a n t a t i o n s t h e e v i d e n c e 5

Sustainability

The question of sustainability in plantation forestry has two components. There is thegeneral, broad issue of whether using land and devoting resources to tree plantationsis sustainable in the economic, environmental or social sense. Is such developmentunsustainable because it is economically questionable, or environmentally damaging,or a threat rather than a help to people’s livelihoods and way of life? The samequestions may be increasingly asked about sustainable agriculture. Each of these, andrelated questions, are important in their own right and fundamentally depend onnational policies governing plantation development, understanding their impacts, andensuring full public participation in the process. For example, it is generally acceptedthat plantations should not be established on land obtained simply by clearing naturalforest formations, as there is plenty of land already degraded by past clearance orpoor farming practices and which is of no importance for conservation, but will growtrees well (indeed, plantations can help restore degraded land). Thus plantationsshould not conflict with natural forests but should be complementary to them. Hencethe debate pointing out the inferiority of plantations to natural forests in terms ofbiodiversity becomes largely irrelevant. Compared with most natural forest, plantationsare impoverished in wildlife; but compared with abandoned degraded land, perhapsdominated by rank grass, plantations can add diversity and lead to enrichment ofwildlife. These and other issues relate to what is labelled ‘broad-sense’ sustainability.

The second issue concerning sustainability is the question: is growing trees inplantations a technology that can work in the long term? Is plantation silviculturebiologically sound, or are there inherent flaws which will eventually lead to insuperableproblems for this way of growing trees? This is ‘narrow-sense’ sustainability and is thesubject of this paper.

Narrow-sense sustainability

The question raised is: can tree plantations be grown indefinitely for rotation afterrotation on the same site without serious risk to their well being? More specifically,can their long-term productivity be assured, or will it eventually decline over time? Aresome sites, crops or practices more at risk than others? These questions arepertinent due to the increasing reliance on plantation forests, and are alsoscientifically challenging: in previous centuries, trees and woodlands were seen asimproving rather than impoverishing the soil. Are today’s silvicultural and managementpractices more damaging because of greater intensity and the high timber yieldsachieved (typically two to four times that of natural forest)? And are geneticimprovements, refined fertiliser treatments, more sophisticated manipulation of standdensity, etc. likely to lead to crop improvement with time, or could they mask ordisguise evidence of genuine site degradation or increasing risk of damaging pestsand diseases?

Plantation forests

The present extent of planted forests worldwide probably exceeds 150 million ha. Newplanting in both tropical and temperate regions is leading to a significant net increasein forest plantation each year. It is predicted that a greater proportion of industrialwood will soon be sourced from plantations than from the exploitation of naturalforests, and that this trend towards increasing reliance on planted forests for woodproduction will continue (Apsey and Reed, 1996; Kanowski, 1997; Pandey and Ball,1998). This trend follows the pattern seen in farming and food production, ofdomestication and development of ever more intensive systems, especially in the past30 years. In the case of forest products, it is a trend that may help to alleviatepressures on the natural resource.

Planted trees, woodlands and forests are also an increasingly important resource forfuelwood, building poles, fencing materials, food and fodder supplies and otherdomestic needs, that are particularly important in many tropical and subtropicalcountries. They can be a significant means of generating income to sustain rurallivelihoods.

Plantation forest can also provide certain environmental services, one of which,creating carbon offsets, may come to dominate developments in the next decade.When trees are planted to rehabilitate degraded land, to provide shelter and shade, tohelp control soil erosion, and to provide amenities, they are used for the influencethey bring rather than the product they grow. Such services are part of many ruraldevelopment forestry initiatives.

Already several countries rely heavily on plantations, including Australia, Chile, ChinaDenmark, India, New Zealand, South Africa, Swaziland and the UK. And many othercountries are expanding tree planting, including forest-rich countries such as Brazil,Canada, Sweden and the USA. For example, Canada planted over five millionhectares between 1979 and 1995. Plantation forestry appears set to become amajor, perhaps dominant, form of forest development in the future. But as a way ofgrowing trees, is it sustainable?

Introduction


F o r e s t P l a n t a t i o n s t h e e v i d e n c e 7

Understanding sustainability also applies to non-industrial uses. Sustaining thenumerous benefits people derive from plantations should be a top priority and shouldarise out of good management. Does the perpetual gathering and removal of leaves,twigs and litter from beneath tree stands, so widespread in India and China, forexample, simply loot the site of nutrients? And what of the flow of non-timberproducts, often of more value than wood, and perhaps less directly damaging to siteswhen harvested? These questions are relevant to plantation forestry, even if it is notalways possible to provide adequate answers.

This review looks at evidence worldwide, mainly from industrial plantationforestry, to address what appear to be five critical elements in narrow-sensesustainability:

• what factual evidence is there of productivity change over time?

• what changes to a site may be induced by the practice of plantation forestry?

• to what particular risks are tree plantations exposed?

• what silvicultural interventions can be made to sustain yields?

• how will global impacts of climate change and air pollution affect plantation sustainability?

Data have been gathered from research and mensurational records in severalcountries, from personal communications, and from a review of literature covering thepast 20 years – a partial annotated bibliography of some 280 references for theperiod 1980–1998 is available separately.

Earlier reviews of this theme are provided by Evans (1976, 1990) and Whitehead(1981). This paper focuses on recent evidence and data.

Signs of three rotations in theUsutu Forest, Swaziland: olddecaying first rotation stump(indicated), second rotationstump cut six years ago andthird rotation stand of treesin background. On most sitesthe third rotation is moreproductive than eitherpreceding crop.

Gatheringfuelwood in

Southern Ethiopia.Supplies need to be

sustainable.

Small woodlot for sticks and poles to beused in house and hut construction.


F o r e s t P l a n t a t i o n s t h e e v i d e n c e 9

Background – agriculture and horticulture

The question of sustainability, at least in the narrow biological sense, has long been aconcern in agriculture, particularly for arable cropping. Several long-term experimentsexist in different countries, of which the oldest and most famous is Broadbalk field atRothamsted Experimental Station, Harpenden, UK. Since 1843, successive crops ofwheat have been grown and assessed continuously. One of the experimentaltreatments has excluded any fertiliser input or other cultural amelioration beyond veryoccasional soil fumigation and weed control. Over a long period yields from thiscontrol treatment have remained low but stable; yield levels are largely determined bynutrient availability, and there is no evidence of the land becoming inherently unfit or‘wheat-sick’ and unable to sustain continued cropping (Johnston, 1994). Thisexperiment, along with many other long-term investigations in agricultural andecological sciences, e.g. Sanbourn field at Columbia, Missouri, USA and several trialsin Australia, were reviewed in 1993 on the occasion of the 150th anniversary of JohnLawes’ original and far-sighted trial at Rothamsted (Leigh and Johnston, 1994).

For some horticultural crops and legumes the situation is less clear, with evidence thatcertain crops render the site unsuitable for a successor of the same species. Fruitssuch as citrus, apples, cherries and peaches may show this effect, due, it is thought,not to nutrient depletion but to a complex disease dynamic labelled ‘specific replantdisease’ (Savory, 1966) and probably related to the accumulation of soilborneorganisms such as Phytophthora, nematodes, etc.

Productivity change in successive forest rotations

Problems with data

For forest stands (crops), hard evidence of productivity change over successive rotations ismeagre, with few reliable data. Compared with agriculture, the long cycles in forestry makedata collection difficult. Records have rarely been maintained from one rotation to the nextor have simply been lost; funding for such long-term monitoring is often a low researchpriority; measurement conventions and even measurement units may change confoundingeasy comparisons; detection of small changes is difficult; and often the exact location ofsample plots is inadequately recorded (Evans, 1984). In addition, because few forestplantations are second rotation, and even fewer third or later rotation, even the opportunityto collect data has been limited. Unfortunately without data it is difficult to demonstratewhether plantation silviculture is robust, and so refute (or otherwise) claims that successiverotations of fast-growing trees inevitably lead to problems such as soil deterioration.

Moreover, the few attempts that have been made to compare productivity betweenrotations have often been initiated because of concern that yields are not holding up orabout stand health. Thus investigations have tended to focus on problems; the vast extentof plantations where no records are available or studies undertaken probably suggest nogreat concern and the likelihood that managers are not finding any very obvious declineproblem. The data available in the older literature are probably biased to problem areasand tend to report the exception rather than the rule. More recent studies appear lessdirectly problem-led, including the European Forestry Institute survey (Spiecker et al.,1996), and also several objective trials such as the recent CIFOR initiative ‘Sitemanagement and productivity in tropical forest plantations’, incorporating systematicestablishment of sample plots.

Evidence of

productivity

change

1


1 0 E v i d e n c e o f p r o d u c t i v i t y c h a n g e F o r e s t P l a n t a t i o n s t h e e v i d e n c e 1 1

Pinus radiata in Australia and New Zealand

Reports of significant yield decline in second-rotation Pinus radiata emerged in SouthAustralia in the early 1960s (Keeves, 1966), and by the end of that decade it wasclear that fall-off in productivity of about 30% affected most forests in the state. Inparts of New Zealand, notably some impoverished ridge sites in the Nelson area,there were also signs, albeit transitory, of yield decline (Whyte, 1973). These reports,particularly from South Australia, were alarming and generated a great deal ofresearch into possible causes. By 1990 it had become clear for South Australia thatharvesting and site preparation practices which failed to conserve organic matter, andan influx of weeds in the second rotation, especially massive growth of grasses, werethe main culprits. With more sensitive treatment of a site, conservation of organicmatter, and good weed control the decline problem was eliminated. With theadditional use of genetically superior stock, growth of second- and third-rotation pinewas substantially superior to the first crop, a situation which now prevails throughoutthe state (Boardman, 1988; Woods, 1990). Indeed, a substantial proportion of thesecond and third rotation has been upgraded from low site qualities (mean annualincrements, MAI 13–18 m3 ha-1 year -1) to high (25–33 m3 ha-1 year -1; Nambiar,1996). To summarise the South Australian situation in 1999, from the location of themost significant second-rotation decline problem, R. Boardman (personalcommunication) states: "Overall we have been able to restore productivity to at leastthe original levels for the older plantation sites, and exceed it on the newer ones. Wehave also raised the productivity of the ‘marginal land’ (site qualities V, VI and VII)."

In the state of Victoria, second-rotation P. radiata is equal or superior to first rotation,but practices were never as intensive as in neighbouring South Australia, and lessonscould be drawn from the South Australian experience, especially the importance oforganic matter conservation and rigorous weed control (Squire et al., 1985; Cellier etal., 1985; Turvey and Cameron, 1986a, b). The trends of yield improvements in thewestern Victoria (Gippsland) and adjacent Mount Gambier areas of South Australiaare shown on pages 14 and 20. In New South Wales, where only 5% of the largearea of P. radiata is second rotation, a survey in seven widely scattered sites on fourdifferent soil types, and involving over 40 pairs of plots, revealed no overall yielddecline but basal area and volume per hectare increases of 13 and 18%, respectively(Long, 1997). This improvement appears to diminish somewhat with the age of thecrop; it is attributed to better silvicultural practices and seed sources. In Queensland acareful study of first- and second-rotation Pinus elliottii of the same seed origin showsno evidence of yield decline, but a 17% increase in volume per hectare at 9 yearswhere organic matter was left undisturbed (Bevege and Simpson, 1980). Organicmatter conservation is being applied to minimise soil erosion when felling andreplanting Araucaria and pine plantations (Constantini et al., 1997a, b).

Review of evidence comparing yields in successive rotations

Four major studies have reported productivity in successive rotations, along with someanecdotal evidence and occasional one-off investigations. These are grouped by region.

Spruce in Saxony and other European evidence

In the 1920s, reports began to emerge suggesting that significant areas of second-and third-rotation spruce (Picea abies) in lower Saxony, Germany were growing poorlyand showed symptoms of ill health (Wiedemann, 1923). There was a fall of twoquality classes in second- and third-rotation stands, but this was only recorded over8% of the plantation area. This became a much-researched decline and wasattributed to insect defoliation, air pollution, the effects of monoculture, drought, andsimply the intensive forms of forestry practised. It is now clear that much of theproblem arose from planting spruce on sites to which it was ill suited, particularly thewater regime, as also happens with silver fir (Abies alba). It is interesting to report thattoday young stands of pure spruce in Saxony and Thuringia appear to be growingmuch more vigorously than equivalent stands 50 or 100 years ago (Wenk and Vogel,1996).

Elsewhere in Europe, reports of productivity comparing first and second rotation arelimited. In Denmark, Holmsgaard et al. (1961) indicated no great change for eitherNorway spruce or beech, although today second-rotation productivity in beech isreported as significantly better (Skovsgaard and Henriksen, 1996). In the Netherlands,growth of second-rotation forest is generally 30% faster than the first where it hasbeen assessed (van Goor, 1985). Similarly, in Sweden second-rotation Norway spruceshows superior growth (Eriksson and Johansson, 1993; Elfling and Nystrom, 1996).In France, some decline was reported from successive rotations on Pinus pinaster inthe Landes, although this was not attributed to site deterioration (Bonneau et al.,1968). In Britain, most second-rotation crops are equal to or better than the previousrotation and, in the case of restocking of Sitka spruce (Picea sitchensis), the mostwidely planted conifer, there is no requirement to re-apply phosphate fertiliser whichhad been essential for establishing the original crop (Taylor, 1990). There is noexpectation of a decrease in growth in second-rotation crops (J.C. Dutch, personalcommunication), and recent evidence points to conifer forests growing more quicklythan they used to (Cannell et al., 1998).

Restructuring ofKielder Forest in

Britain. Most secondrotation stands do not

need fertilising togrow at least as wellas the previous crop.

Owing to geneticimprovement many

are growing morevigorously.


In New Zealand, the limited occurrence of yield decline was readily overcome bycultivation and the use of planting stock rather than natural regeneration (A.G.D.Whyte, personal communication). On the great majority of sites, successive rotationsgain in productivity. In a review of the reputation of P. radiata as harming soil, Will(1992) concludes the opposite, and suggests that it is not the species grown inmonoculture that is harmful, but inappropriate management practices such as topsoiland litter repositioning, burning logger debris, and soil compaction. Pines are not soildegraders, but some management practices are.

Pines in Swaziland and South Africa

Long-term productivity research in the Usutu Forest, Swaziland by the author began in1968 as a direct consequence of the reports emanating from South Australia aboutsecond-rotation decline. For 30 years, measurements have been made over threesuccessive rotations of Pinus patula, grown for pulpwood, from a forest-wide networkof long-term productivity plots. Plots have not received favoured or research-leveltreatment, tree growth is simply recorded during each successive rotation resultingfrom normal forest management by the Usutu Pulp Company.

The most recent reports are given by Evans (1996, 1999) and Evans and Boswell(1998). Tables 1–3 (modified from Evans, 1999) present the results for second- andthird-rotation growth where comparison has been from plots on exactly the samesites. First-rotation growth data were obtained through stem analysis and from pairedplots, and are less accurate; some of these data are reported by Evans (1996).


This Usutu data set is amongst the most accurate available. Over most of the forestwhere granite-derived soils occur (Table 1), third-rotation height growth is significantlysuperior to second, and volume per hectare almost so. There had been littledifference between first and second rotations (Evans, 1978). On a small part of theforest on phosphate-poor soils derived from very slow-weathering gabbro, a declinehad occurred between first and second rotations, but this has not continued into thethird rotation where there is no significant difference between rotations (Table 2).Table 3 shows that switching species from P. patula to P. taeda led to poorer heightgrowth but superior basal area per hectare.

Realised gains in meanannual increments

(MAI) measured at age11 years for Pinus

radiata fromimprovements in

silviculture and treebreeding since the early

1970s. Sites are inGippsland, Australia.

Individual histogramswithin the major soil

groups representdifferent soil types.

(Redrawn fromNambiar (1998) from

the work of Leishout et al. (1996).

With permission Dr E K S Nambiar.

5

10

15

20

25

30

0

Uniform sands Duplex Clay loams

Pre 1971

Post 1974M

AI

(m3

ha-1

yr-1)

5

10

15

20

Age (years)

6 10 14

1st rotation

2nd rotation

3rd rotation

Mea

n he

ight

(m

)Table 1 Comparison of second and third-rotation Pinus patula on granite and gneiss-derived soils at 13–14 years of age (means of 32 plots)

Rotation Stocking Mean height Mean DBH Mean tree Volume per(S ha-1) (m) (cm) volume (m3) hectare (m3 ha-1)

Second 1381 17.4 20.2 0.217 294

Third 1267 18.3 21.0 0.233 305

Change (%) +4.9 +4.2 +3.8%

Probability P=0.002 P=0.197

Significance ** n.s.

Table 2 Comparison of second and third-rotation Pinus patula on gabbro-dominated soils at 13–14 years of age (means of 11 plots)

Rotation Stocking Mean height Mean DBH Mean tree Volume per(S ha-1) (m) (cm) volume (m3) hectare (m3 ha-1)

Second 1213 16.7 20.0 0.206 244

Third 1097 16.8 21.7 0.227 255

Change (%) +0.05 +8.3 +4.6

Probability P=0.890 P=0.480

Significance n.s. n.s.

Table 3 Comparison of third-rotation Pinus taeda with the previous crop of Pinus patula at 13–14 years of age (means of 15 plots)

Rotation and species Stocking (S ha-1) Mean height (m) Mean DBH (cm) Basal area (m2 ha-1)

Second P. patula 1373 17.4 19.4 43.8

Third P. taeda 1238 16.7 22.5 47.6

Change (%) –4.1 +16.3 +8.7

Significance * *

Age (years)

6 10 14

Mean height of three rotations of Pinus patula

on granite-derived soils in the Usutu Forest,Swaziland (average of 24 sample plots). FromEvans (1996).

Mean height of three rotations of Pinus patula

on gabbro-derived soils in the Usutu Forest,Swaziland (average of 10 sample plots). FromEvans (1996).

0

5

10

15

20

Mea

n he

ight

(m

)

0



changes, etc. Personal observation suggests that the widespread practices of whole-tree harvesting, total removal of all organic matter from a site, and intensive soilcultivation that favours bamboo and grass invasion all contribute substantially to theproblem. The question of allelopathy, and the effect on productivity of recruitingcoppice shoots for restocking, remain unresolved. Ding and Cheng (1995) concludethat the problem is that "…not Chinese fir itself, but nutrient losses and soil erosionafter burning (of felling debris and slash) were primary factors responsible for the soildeterioration and yield decline...compensation of basic elements and application of Pfertiliser should be important for maintaining soil fertility, and the most important thingwas to avoid slash burning...These (practices)...would even raise forest productivity ofChinese fir." (Words in parentheses added by the author.)

Teak in India and Java

In the 1930s evidence emerged suggesting that replanted teak (Tectona grandis) crops(second rotation) were not growing well in India and Java (Laurie and Griffith, 1942;Griffith and Gupta, 1948). Although under teak significant soil erosion is widespread,and organic matter lost as leaves are frequently burnt (see for example Bell, 1973),research into the ‘pure teak problem’, as it came to be called in India, did notgenerally confirm a second-rotation problem. However, Chacko (1995) describes sitedeterioration under teak as still occurring, with yields from plantations not coming upto expectation. There is a generally observed decline of site quality with age. Heattributes the problem to four main causes: poor supervision of plantationestablishment; over-intensive commercial taungya (intercropping) cultivation; delayedplanting; and poor after-care. Chundamannii (1998) similarly reports decline in sitequality over time, suggesting that poor management is a contributory factor. Helaments the lack of data from successive rotations, which would be the ideal way toevaluate changes in productivity – teak rotations are typically 60–80 years.

Concern about successive teak crops, soil erosion, and loss of organic carbon hasalso been expressed in Senegal (Mahuet and Dommergues, 1960). And in Indonesia,where there are about 600 000 ha of teak, mainly in Java, site deterioration isdescribed as a problem and "is caused by repeated planting of teak on the samesites" (Perum Perhutani, 1992).

The importance of the Swaziland data, apart from the long-term nature of theresearch, is that no fertiliser addition or other ameliorative treatment has been appliedto any long-term productivity plot from one rotation to the next. According to Morris(1987), third-rotation P. patula is probably genetically superior to the second rotation,being partly of orchard-quality material imported from South Africa (althoughselections were made for sawtimber parameters not short-rotation pulpwood).However, the 1980s and especially the period 1989–1992 have been particularly dry,Swaziland suffering the drought along with the rest of southern Africa (Morris, 1993a;Hulme, 1996). This was bound to have an adverse impact on yield. These data arealso of interest because plantation silviculture carried out in the Usutu Forest oversome 62 000 ha is intensive, with pine grown in monoculture, no thinning orfertilising, and on a rotation of 15–17 years which is close to the age of maximummean annual increment. Large coupes are clearfelled and all timber suitable forpulpwood extracted. Slash and debris are left scattered (i.e. organic matterconserved), and replanting done through it at the start of the next wet season. Theseplantations are managed intensively, and over three rotations there is little evidence topoint to declining yield. The limited genetic improvement of some of the third rotationcould possibly have disguised a small decline, but evidence is weak since breedinggenerally improves net primary productivity which cannot be realised if one or morenutrients is deficient. Also, it can be strongly argued that without the severe andabnormal drought, growth would have been even better than it is. Overall, theevidence suggests no serious threat to narrow-sense sustainability.

In South Africa there is no evidence of any decline in productivity over successiverotations other than localised small-scale examples arising from compacted soil.Excessive accumulation of undecomposed litter in some high-altitude stands of P.patula does give rise to concern over increasing soil acidity and nutrient immobilisation(Morris, 1993b; C.J. Schutz, personal communication). In cultivation of wattle (Acaciamearnsii) there is no evidence of yield decline with successive rotations:responsiveness to fertiliser was consistent in each rotation and mainly influenced bythe standard of weeding (Herbert, 1984).

Chinese fir in subtropical China

About six million hectares of plantations of Chinese fir (Cunninghamia lanceolata),possibly the world’s most extensively planted tree species, have been established insubtropical China. Most plantations of Chinese fir are monocultures and worked onshort rotations to produce small poles, though foliage, bark and even sometimes rootsare all usually harvested for local use. Reports of significant yield decline have a longhistory. Accounts by Li and Chen (1992) and Ding and Cheng (1995) report a drop inproductivity between first and second rotations of about 10%, and between secondand third rotations up to a further 40%. Ying and Ying (1997) quote higher figures foryield decline between first and second rotations, of 29% poorer height and 26% lessvolume. Mensurational data are difficult to obtain in order to indicate the extent ofsuch declines, but Chinese forest scientists attach much importance to the problemand are pursuing research into monoculture, allelopathy, and detailed studies of soil

Poor third-rotationChinese fir

(foreground);damaging site

preparation on thehill (background)

showing burnt debrisand exposure of soil

to erosion.

Soil erosion under teakplantations in Trinidad.



Other evidence

Other evidence is limited or even more confounded than that reported above. Forexample, Aracruz Florestal in Brazil has a long history of continually improving productivityof eucalypts owing to an imaginative and dedicated tree breeding project, so that eachyear many new clones are introduced into the planting and replanting programmes, andmany less productive ones discontinued (Campinhos and Ikemori, 1988). The same istrue of the eucalypt plantations at Pointe Noire, Congo (P. Vigneron, personalcommunication). In these situations it is difficult to judge from yield measurementswhether genetic improvement is disguising a possible site degradation problem causedby growing and harvesting plantations intensively. It is patently clear that greatlyincreased productivities are being achieved in practice; the sites so far appear capable ofsupporting productivities up to 60 or 70 m3 ha-1 year -1.

At Jari in the Amazon basin of Brazil, silvicultural practices have evolved withsuccessive rotations since the first plantings between 1968 and 1982. A review ofgrowth data from the early 1970s to the present suggests that productivity isincreasing over successive rotations due to silvicultural inputs and geneticimprovement (McNabb and Wadouski, in press).

In Venezuela, despite originally severe and damaging forest clearance practices, second-rotation Pinus caribaea shows substantially better early growth than the first rotationplanted into natural savannah (Longart and Gonzalez, 1993).

Within-rotation yield class/site quality drift

This recently observed phenomenon has two aspects – change from predicted toactual yield over time, and correlation of site quality (yield class) with date of plantingrather than only with site fertility.

Inaccuracy in predicted yield

For long-rotation (>20 years) crops it is a common practice to estimate yield potentialfrom an interim assessment of growth rate early in life, and then to allocate a stand to asite quality class or yield class. This is a good way of planning final yield, althoughimprecise for detailed outturn from individual stands. A change from predicted to finalyield can readily occur where a crop has suffered check or other damage in theestablishment phase that delays its development and site occupancy, and thus distortsearly estimates of site potential based on growth:age relationships. Similarly, fertiliserapplication which corrects a specific deficiency may also have this impact. However,there is some evidence for very long-rotation (>40 years) crops in temperate countriesthat initial prediction of yield or quality class in general will underestimate final outturn,i.e. the crops grow better than expected in later life. Either the models based onrelationships derived from data of 40 or more years ago were wrong; or they are nowinappropriate to present conditions; or growing conditions are ‘improving’ in the sense offavouring tree growth. Across Europe this appears to be the case (Spiecker et al., 1996;Cannell et al., 1998), and is attributed to rises in atmospheric CO2 and nitrogen inrainfall, better planting stock, and cessation of harmful practices such as litter raking.

Southern pines and other studies in the USA

Plantations of slash (P. elliottii) and loblolly (P. taeda) pines are extensive in thesouthern states. Significant plantings began in the mid-1930s as natural stands werelogged out (Schultz, 1997), and with rotations usually 30 years or more, somerestocking (second rotation) first occurred in the 1970s. In general it appears thatgrowth of the second crop is variable. Burger (1996) reported improved second-rotation growth of 4 and 6 m in height at 23 years, depending on site preparation. Amajor investigation in North Carolina found third-rotation P. taeda to be of a muchhigher site index than second-rotation, especially where good weed control wascarried out (NCSFNC, 1995). Haywood (1994) and Tiarks and Haywood (1996)report poorer growth. The latter authors cite 7% poorer height growth and 24% lessvolume in the second rotation (though this is not suggested as representative on allsites). Changes between rotation are attributed to differences in site preparation,although this was not consistent; and in particular to competition from understoreyshrubs and weeds. Type and amount of competing vegetation is the main influence(B. Shiver, personal communication). Where weeds are well controlled andappropriate site preparation used, such as a bedding plough, growth is often superior(e.g. Yin et al., 1998). Genetically improved stock and use of fertilisers are expected tobring further increases.

Other studies in North America are limited, as the main concern has been shiftingfrom natural to managed forest or to the first plantation crop, and it is too early toevaluate successive plantations. According to S.P. Gessel (personal communication), ifthere is to be any yield decline problem in the Pacific northwest it will relate toharvesting damage, compaction or soil loss through erosion on heavy textured orshallow soils, and to loss of nitrogen as the nitrogen supply is frequently limiting. "Ifthese two potentially detrimental changes are avoided in harvesting and regenerationin Northwest forests, there seems to be no evidence of an overall decline in growthand yield of successive plantations." Certainly Froelich and McNabb (1983) concludedthat in the Pacific northwest, reducing the area of compacted soil may be the mosteffective way of maintaining soil fertility. Fear has also been expressed over burning;however (in a story analogous to the Australian experience described above), althoughit may be important to avoid burning in inland areas low in organic matter (Jurgensenet al., 1990), an important comparison of burned and unburned plots at 44 locationsto the west of the Cascade mountain range 40 years after burning showed noreduction in volume production (Miller and Bigley, 1990). Further north, on Vancouverisland, Prescott and Weetman (1994) studied poor growth in planted and naturallyregenerated Sitka spruce (Picea sitchensis) and Douglas fir (Pseudotsuga menziesii), andconcluded that on sites inherently low in nitrogen and phosphorus, mycorrhizal fungiof the competing salal were severely reducing the amount of nitrogen available to thetrees, a problem that disappears once the salal is shaded out.

A coordinated series of experiments throughout the USA is currently assessing long-term impacts of management practices on site productivity, but it is too early forresults (Powers et al., 1994).


However, as noted earlier, the opposite is occurring with teak. High initial site qualitiesdo not yield the expected outturn, and estimates are revised downward as the cropsget older. Plantation teak does suffer soil erosion in established stands, developmentof understoreys is rare, and burning of debris, especially the large, dry leaves, iscommonplace. Like litter raking, these practices may contribute to the phenomenon.

Relation of quality (yield) class with time of planting

Closely related to the phenomenon of changing yield potential as a crop grows, is theobservation that date of planting is often significantly and positively related toproductivity, i.e. more recent crops are more productive than older ones regardless ofinherent site fertility. This shift is measurable and can be dramatic; see for exampleLeishout’s report from Australia cited by Nambiar (1998) and reproduced below.Attempts in Britain to model productivity on the basis of site factors have often beenforced to include planting date as a variable. Thus maximum mean annual growth ofSitka spruce increased with planting date in successive decades by 1 m3 ha-1 year -1 inone study (Worrell and Malcolm, 1990) and by 1.2 m3 in another (Macmillan, 1991).The equivalent values according to Tyler et al. (1996) for Douglas fir, Japanese larch(Larix kaempferi) and Scots pine (Pinus sylvestris) being 1.3, 1.6 and 0.5 m3 ha-1 year -1

in each succeeding decade. This phenomenon seems common (J. Methley, personalcommunication) and is commented upon by Cannell et al. (1998). It suggests thatsome process is happening that favours present growing conditions for trees overthose in the past, such as the impact of genetic and silvicultural improvements (andagain cessation of harmful ones), and possibly the ‘signature’ of atmospheric changesmentioned above.

The impact of these two related observations is that present forecasts of crop yields arelikely to be underestimates; yields appear to be increasing. The main exception is teak.


The special case of coppice

Plantations of some species, such as many eucalypts (Eucalyptus spp.), poplars(Populus spp.) and chestnut (Castanea spp.) are often managed by coppicing to yieldthe second, third and even fourth or more crops before (if ever) replanting. Coppicegrowth is sometimes exceptional. Almost 50 years ago, Troup (1952) cited data fromcoppice stands of Eucalyptus globulus in the Nilgiri Hills of India showing an MAI of 40 m3 ha-1 year -1 over a 25-year rotation. Initial growth is often rapid, and coppicecrops show an earlier culmination of maximum mean annual increment comparedwith planted crops, but the practice itself does not alter site potential – see forexample the study by Knockaert (1985) comparing yields of various coppicing regimesin Morocco. Coppice does not raise site potential, but only accelerates site capturethrough more rapid stocking. Conversely, some stumps usually die at each coppicing,and stocking per hectare declines over time unless gaps are made good. There issome evidence to show that eucalypt coppice exhibits earlier maturation of above-ground growth owing to the increasingly old root system on which it develops, aphenomenon also believed to be true of Chinese fir (and a rare example of a readilycoppicing conifer). This may cause premature cessation of height growth.

Overall, evidence suggests that often the first coppice crop (sometimes referred to asthe second rotation in the literature) is more productive than the original seedlingplanting, but subsequently yields diminish with each successive cut; see for exampleKaumi’s (1983) data for Kenya. In India, where four coppice rotations of E. globulusare grown after the initial seedling crop, a fall in yield is expected of 9% in the thirdrotation and 20% in the fourth (Jacobs, 1981). For short-rotation poplar coppice,Strong (1989) found this relationship held good initially, but later growth of the‘second rotation’ declined. Other writers report the ‘second rotation’ to be moreproductive (e.g. Bowersox et al., 1988). However, this pattern of improvement in thefirst coppice crop followed by decline is not always found, and sometimes the coppicecrop is poorer than the initial seedling crop.

In temperate coppice crops, few records exist of yield data over time. Coppice stemsof oak which are grown on (recruited) to high forest (stored oak) initially will outgrowmaiden oak, but later in the rotation will often grow more slowly (Groos, 1953). Thecenturies-old practice of coppicing oak (Quercus spp.) in Britain on a 20–25-year cyclefor tanbark and firewood is claimed to have led to phosphate impoverishment anddeclining yields of these and other forms of coppice in the middle ages (Rackham,1967). Attempts today to stimulate the growth of long-established coppices byfertilising have not succeeded (Evans, 1986), and there are no reliable mensurationalrecords to demonstrate growth decline or improvement. The situation is the same forchestnut (Castanea sativa), where at least eight coppice rotations have been harvestedwithout obvious decline problems, although precise yield measurements have notbeen taken. It is a common practice with coppice today to infill gaps caused by stumpdeath, either by planting new trees or by layering (Evans, 1992b).

0

5

10

15

20

1950 1955 1960 1965 1970 1975 1980 1985

Long-term trends inP. radiata MAI across

many sites, soil typesand planting years in

southern Australiaspanning 34 years.

MAI was measured atage 11 years, the lastset of measurements

being in 1995(planted in 1984).

Redrawn fromNambiar (1998) from

the work of Leishoutet al. (1996).

With permission Dr E K S Nambiar

Planting year

MA

I (m

3ha

-1yr

-1)



Coppice silviculture confuses the picture of productivity change because successivecrops are obtained either from coppice itself or by replanting. Few data are availableto suggest that the practice is markedly helpful or harmful to site conditions.

Conclusions

Two main conclusions can be drawn from this review of yield assessmentsmade over long periods and often over more than one rotation.

• Measurements of yield in successive rotations of trees suggest that, so far, there is no significant or widespread evidence that plantation forestry is unsustainable in the narrow sense. Where yield decline has been reported, poor silvicultural practices and operations appear to be largely responsible.

• Evidence in several countries suggests that current rates of tree growth, including in forest plantations, exceed those of 50 or 100 years ago.

Mixed coppice in southern England, cut every sevenor eight years, a practice carried on for centuries.

Oak woodlands in south-westEngland (Exmoor) whichwere worked regularly for

hundreds of years as coppicefor fuel and tan bark until

the 1920s.


F o r e s t P l a n t a t i o n s t h e e v i d e n c e 2 3

There are two important questions concerning plantation silviculture. Firstly, do thecommonly applied silvicultural practices, such as use of exotic species, monocultures,clear-felling systems, etc., cause or contribute to site change? Secondly, are suchchanges observed to be more or less favourable to the next crop? That is, how doesgrowing one crop influence the potential of its successor?

This is a much-researched topic, especially concerning impact on soils, and only themain themes are summarised here. Two recent books have presented the science:Dyck et al. (1994) Impacts of forest harvesting on long-term site productivity; and Nambiarand Brown (1997) Management of soil, nutrients and water in tropical plantation forests –which also includes many temperate data. Also, it is important to be cautious: treerotations are long, even in the tropics, compared with most research projects.

Assessing changes in soil

That soil changes may be caused by imposed forestry practices is usually difficult toestablish conclusively both in fact and in scale. An absence of sound baseline data iscommon, and it is questionable whether the reported change is actually induced bythe plantation silviculture. Binns (1973) pointed out many years ago that"many...studies of soil development under managed forest or plantation are renderedunproductive by the lack of a known datum, while comparison with another land useoften ignores changes inherent in that land use". For example, if conclusions aboutthe effects of tree species on soil development are drawn from comparing conditionsunder each species, is one comparing the forester’s instinct in species choice for thesite rather than the effect of the species itself?

The second question is whether the observed changes represent degradation orimprovement. There are remarkably few examples of changes supposedly induced bygrowing trees that lead to less favourable conditions for that species – with theexception of the specific replant disease problem for a few horticultural crops (Savory,1966). Equally, the irreversibility of changes has rarely been demonstrated, apart fromobvious physical losses such as erosion of topsoil. A gradual trend, perhaps observedover several decades, can be quickly reversed as stand conditions change, a pointmade long ago by Page (1968). Attiwill (1979) and Miller et al. (1979) pointed outthat the years preceding canopy closure are characterised by a major shift of nutrientsfrom soil to tree biomass, but that subsequent to this (Miller, 1995) efficient internalre-use of nutrients means that there can be a quite rapid recharge of soil-exchangeable nutrients, e.g. Lundgren (1978). Measures of available soil nutrientsbetween stages in a crop, or between stands of different species, provide meresnapshots in a very dynamic situation and can lead to erroneous conclusions. AsNambiar (1996) points out, "the most striking impacts on soils and hence productivityof successive crops occur in response to harvesting operations, site preparation, andearly silviculture from planting to canopy closure".

Site change

induced by

plantation

forestry

2


2 4 S i t e c h a n g e i n d u c e d b y p l a n t a t i o n f o r e s t r y F o r e s t P l a n t a t i o n s t h e e v i d e n c e 2 5

midsummer (when assessments are normally made) these differences had entirelydisappeared. Moreover, winter pH levels differed by more than one pH unit fromsummer levels (pH 3.0–3.7 compared with 4.9–5.0, respectively). Too little is knownabout causes of such variation; soil moisture and temperature are probably involved,but approximate matching of assessment dates appears to be as important asmatching sites if valid conclusions are to be drawn.

The above points are laboured to underline the danger of drawing conclusions fromlimited investigations covering only a few years of a rotation. Short-term studies canyield grossly misleading scenarios, none more so than when extrapolating over wholerotations and successive rotations of forest plantations. This is also the reason whyemphasis is placed in this paper on actual mensurational evidence assessing cropyield from one rotation to the next.

Soil chemical status

Three critical aspects may be affected by plantation forestry practice:

• change in nutrient ‘reservoir’ both in absolute terms and rate of change – nutrient budget, accretion and depletion, nutrient cycling;

• loss of nutrients from a site in drainage and to the atmosphere;

• change in ‘availability’ of nutrients induced by other changes such as pH, soil aeration and fixation.

The most obvious impact is the removal of nutrients, from the soil into trees as theygrow, and then from the site as trees are harvested. Less direct is the change thatmay take place in the chemistry of the soil surface as the litter layer and organicmatter are dominated by one species and the chemical composition and decaycharacteristics of the leaves, twigs, branches and other debris become more uniform.In addition, site preparation practices such as ploughing, soil drainage and fertilisingwill lead to changes in soil physical parameters, which in turn may show an impact onnutrient availability.

Soil as a mineral store

Soils vary enormously in their role as a nutrient reservoir. Thinking has been muchconditioned by two features of temperate regions: (i) arable farming treats soils as amedium in which to grow crops where nutrient supply is largely maintained by annualfertiliser inputs; and (ii) in most cases the soil store of plant nutrients far exceeds thatin the above-ground biomass. Furthermore, most analytical techniques aim to providea measure of available nutrients and do not predict the rate at which this reserve(analogous to a current account) can be recharged by weathering of soil minerals(analogous to a savings account), a weathering process that can be accelerated bythe action of tree roots and their mycorrhizal fungi. In forestry, where fertiliser inputs

What to measure

Relationships between soil nutrient content and tree growth can rarely be generalised,and species-specific and even site-specific relationships undoubtedly exist and havean impact on productivity. Fundamental relationships between soil parameters andtree growth remain difficult to elucidate and difficult to interpret, not least because theextractants and analytical methods of modern soil science were developed tocorrelate with agricultural production, and seldom prove to be of value when dealingwith perennial and deep-rooted tree species.

There are two approaches to assessing change:

• observational – to compare sites in carefully matched pairs, or to observe changes over time on the same site (chronosequences);

• deductive – by modelling ecosystem dynamics such as the nutrient budget, followed by testing theory with field experimentation.

In the past, most reports of site change in plantation forestry derived from matchedplots. Increasingly long-term observational experiments are being specifically designedto investigate change, usually following both approaches, e.g. CIFOR’s tropics-widestudy (Tiarks et al., 1998); Powers et al.’s (1994) study in the USA; and studiesmonitoring gross environmental change such as the Europe-wide extensive andintensive forest monitoring plots (level I and level II). Modelling is also widely used,but suffers in precision at site level because of assumptions made. For example,Bruijnzeel (1990), cited by Folster and Khanna, (1997) states that "Nutrient budgetscan be considered only as good approximations of ‘real world’ happenings because ofinadequacies of methodology; assumptions and extrapolations from ‘false-time’ seriesare usually required to supplement field data".

The observational approach suffers a bias in that investigations are often carried outspecifically because there is a problem which has already revealed itself in poor treegrowth or health. Only occasionally is it used to predict potential problems onuntested sites. It also suffers from soils being notoriously variable, a difficultyexacerbated on many forest sites by the kind of ground often used for plantationforests. Single-plot or small-sample comparisons can be wildly inaccurate. Forexample, Charley (1981) found that in the soils of one 12x24 m plot of Eucalyptuspilularis, pH ranged from 4.4 to 6.7, potassium from 0.17 to 0.55 meq %, andcalcium from 1.05 to 12.31 meq %. Overcoming such variability must rely onintensive sampling within a plot and the matching of many, not just a few, pairs ofplots. Soils show such high micro-variability that there will still be much statisticalnoise which will obscure changes of less than about 20% of initial values.

A second, little recognised source of variability is that measured values of many soilparameters can change radically during one year. For example, when the effects ofdifferent tree species on soil development were compared at Gisburn, Forest ofBowland, UK, the time of sampling on soil chemistry was also investigated (M.A.Anderson, personal communication). It was found that in winter there were highlysignificant differences between plots for most major plant nutrients and pH, while in



nutrition will be a problem (e.g. Rennie, 1955; Binns, 1962; Johnson and Todd,1990); yet trees continue to grow on soil where conventional soil analysis suggeststhere is virtually no calcium.

The impact of nutrient losses will depend on total nutrient reservoir, replenishmentfrom aerosol inputs, weathering of subsoil, and management practices aiding orhindering incorporation of the nutrient-rich litter. A crucial issue is what parts of thetree are actually removed – debarked log, log, whole tree including branches, etc. –owing to the highly unequal concentration of nutrients in plant tissue. A crucialmeasure of the impact is the stability ratio: anything greater than 0.3 raises seriousstability questions in the longer term, and above 0.5 in the immediate future.

In most temperate plantations the slower rates of growth, long rotations and relativelyhigher soil reserves make nutrient removal impacts less significant, except perhaps forwhole-tree harvesting technologies, see for example the review by Dutch (1994). Insome cases, as was always emphasised in older textbooks, trees augment soilsurface nutrient supplies.

Understanding these dynamics helps identify at what points on the continuum ofplantation growth throughout the world of sites, species and productivities the ratiobecomes critical for long-term stability. There appear to be few examples so far of suchlimits being reached, and there is probably only a small proportion of sites where it everwill be. And it is worth remembering that nutrient removal in forestry cropping systemsis typically only one-fifth to one-tenth that in arable farming; see Miller (1995). Theimportant issue is to undertake assays and gather mensurational data as the check.

are limited and trees perennial and generally deep-rooting, research has come tofocus less exclusively on soil reserves and more on where the dynamics of nutrientsupply is mediated – i.e. largely at the soil surface as the debris of leaves, bark,branches and twigs is decayed and re-incorporated, and nutrients recycled. Indeed,forests are highly efficient recyclers of nutrients, and almost ‘leak free’ if undisturbed.In addition, in many parts of the tropics, where recycling can be at its most efficient, ithas been necessary to recognise that the nutrients in mineral soil no longer representthe dominant proportion of the ecosystem. This explains the all-too-commonobservation that clearance of rich, natural forest often fails to reveal the fertile,productive soils that temperate observers would expect. Mineral soil, in manytemperate as well as tropical regions, often plays only a small part in nutrientexchange, and it is the surface organic, root-bearing zone, especially the annualturnover of fine roots, which is important in concentrating energy flow from decayingand dead organic matter back into living organic matter. Thus the integrity of this layerand how it is handled in plantation silviculture is critical to sustainability.

Nutrient removal

Nutrient removal in plantation forestry occurs when any product is gathered orharvested, such as leaves, fruits, litter, logs or whole trees. Many studies have beenmade; Goncalves et al. (1997) alone list 12 tropical examples. Of critical importanceto plantation sustainability is the proportion of nutrients lost in relation to the wholestore. Indeed, this ratio of nutrient export : nutrient store is advocated as a keymeasure of long-term ecosystem stability, which raises the question of what is thestore and how it can be measured. Two examples are discussed. Lundgren (1978), inan exhaustive study of two tropical plantation conifers in Tanzania, concluded thatPinus patula led to annual removals of 40 kg ha-1 nitrogen, 4 kg ha-1 phosphorus, 23kg ha-1 potassium, 25 kg ha-1 calcium and 6 kg ha-1 magnesium. For Cupressuslusitanica, the other conifer, comparable figures were generally lower, largely becausethe stands had a smaller biomass (i.e. were growing more slowly). These rates ofremoval are about one-third of those of maize (Sanchez, 1976), and in the Tanzaniastudy represented less than 10% of soil store, a stability ratio of <0.1. In contrast,Folster and Khanna (1997) report data for Eucalyptus urophylla X E. grandis hybridstands with three very different site histories in terms of the previous plantationcrop(s) at Jari in north-east Amazonia. Table 4 reproduces their results. Theremarkable aspect of this table is that in treatments (b) and (c) especially, potassiumand calcium supply in exchangeable form is extremely critical. To quote: "Twelve of thestands were in the second to fourth rotation, indicating that most of the previouslygrown Gmelina, Pinus or Eucalyptus had already extracted their share of base cationsfrom the soil and left it greatly impoverished". The stability ratio is >1 and suggests itis unstable and unsustainable. While the authors go on to state that uncertaintiesremain about the minimum amount of nutrients required by a stand, they do reportthat 87% of calcium, 48% of potassium and 68% of magnesium uptake by theeucalypts is contained in the bark, suggesting that bark removal on site may offer apartial solution. However, caution is needed. Others have predicted from a comparisonof removals in harvested biomass with available quantities in soil that calcium

Table 4 Amount of nutrients in harvested stem wood and bark of 4.5 yr old Eucalyptus

urograndis, and left in 100 cm depth of soil, for three treatments at Jari, NW Amazon. Treatmentsare: a) a first-rotation stand; b) a second-rotation stand following first-rotation of 12 yr of Pinus

caribaea; and c) a fourth-rotation stand following three rotations (altogether 14 yr) of Gmelina

arborea. Direct comparisons between treatments are hampered by lack of data on initial conditions and the possibility of differences between the soil reserves available to the different treatments.From Folster and Khanna (1997).

Treatment Weight Total Total Exchangeable Exchangeable Exchangeableand material (t ha-1) N (kg ha-1) P (kg ha-1) K (kg ha-1) Ca (kg ha-1) Mg (kg ha-1)

(a) Soil 12924 1350 150 1365 253

Wood 109 245 12 154 581 50

(b) Soil 3548 1268 45 435 117

Wood 91 204 9 128 452 42

(c) Soil 11686 3606 301 13 161

Wood 88 197 10 124 254 40



Soil physical condition

The development of plantations on a site may affect the soil’s physical condition inthree major ways:

• site preparation and establishment deliberately modify the microsite by improvingsoil physical conditions such as drainage, aeration, and compaction;

• tree growth itself modifies local hydrology by rainfall interception, moisture uptakeand possibly improved permeability of root channels, and may affect soil erosion rates through suppression of ground vegetation, build-up of litter layer etc.;

• harvesting practices frequently lead to localised soil erosion, soil compaction, anddamage to organic matter.

All these influences will have an impact on the sustainability of plantation forestry. For a general review of forest operations and sustainable management of soil, seeDyck et al. (1994) and Worrell and Hampson (1997).

Site preparation and planting

Preparing ground for planting often involves several operations that modify siteconditions to a greater or lesser extent. Two important ones, cultivation and drainage,affect soil physical conditions for many years and sometimes for more than onerotation. Obviously such work seeks to improve growing conditions for trees and is notintended to impair sustainability or productivity. Longer term benefits include reductionin bulk density, increased infiltration capacity and aeration, improvement in moisturestorage, and enhanced mineralisation rates of accumulated organic matter (Ross andMalcolm, 1982). Physical disruption of indurated layers and deep cultivation such astining are actually designed to reverse undesirable soil profile development. The risk ofincreased erosion from cultivation needs to be minimised.

Impact of tree growth

Water use by trees is a much-researched subject leading to the very general conclusionthat, compared with grassland and many agricultural crops, trees exhibit higher levels ofevapotranspiration. On some sites this has been harnessed to dry the site out andsignificantly lower the water table. There are several instances of eucalypts plantedspecifically for this purpose.

It is difficult to quantify this effect on the growth of later rotations. If the plantation lostmore moisture than was received by the site in precipitation, no soil moisture rechargewill occur and reserves will be depleted. In the US mid-west, many plantationsestablished in the 1890s and 1900s thrived for a time, but eventually died onceinherent moisture reserves were used up and precipitation was inadequate to sustaingrowth (Kramer and Kozlowski, 1979). In South Africa Herbert (1984) concluded from astudy of three rotations that productivity of each rotation of Acacia mearnsii in Natal waslargely determined by summer rainfall amounts and possibly by soil moisture reserves,especially on marginal sites. In general, foresters do not seek to rely on soil moisturereserves, nor to any large extent on augmenting moisture through irrigation in order togrow tree plantations, but match species with the prevailing climatic conditions.

Litter and residues

The influence of litter on soil chemical status may be important, as leaves of differentspecies do not decay and release their nutrients at the same rate. In some tropicalpine plantations, litter accumulates and breakdown is slow. In southern Africa,substantial accumulations may develop under P. patula on certain sites (see forexample Morris, 1993b), while this is unusual beneath the more lightly canopied P.elliottii. Under broadleaved stands accumulation of litter is less common though notunknown, e.g. under beech and some oak stands on acid soils in Europe. Even underteak and Gmelina, which usually suppress all other vegetation, the large leaves readilydecay. Similarly, under the light crowns of eucalypts and ash (Fraxinus spp.), thenitrogen-rich foliage of leguminous trees such as Acacia, Leucaena and Prosopis spp.,and non-legume nitrogen-fixers such as alders and casuarinas, litter build-up is rareowing to rapid decay as numerous organisms utilise the rich organic matter.

Litter accumulation indicates at least partial nutrient immobilisation, and uniform litteritself may modify soil development because of a single food type for decay organisms,a humus developed from one litter type, and uniform chemical composition (ions andpH) of the water that percolates through to the soil.

Of greater importance than the above long-term impacts is how the litter and organicmatter layers are handled, especially during harvesting operations, and this isreviewed below.

Measured changes in soil chemistry

The above processes plainly indicate that plantation forestry practices could influencesoil chemical status, but what has been observed? Most studies have eithercompared conditions between plantation sites and those before plantations wereestablished, or examined trends as a plantation develops. Few have examinedchanges over successive rotations of the same species, and few consistent trendsemerge. (Interestingly and surprisingly, there are even fewer direct comparisonsbetween plantation silviculture and agricultural land-use practices.)

In tropical studies, decreases in carbon, nitrogen and macronutrients underplantations compared with natural forest or pre-existing conditions have beenreported, for example by Robinson (1967); Cornforth (1970); Morris (1984); andFolster and Khanna (1997). Increases have been reported by Iyambo (1973); Chijioke(1980); and Choubey et al. (1987). Kadeba and Adyaui (1982) and Adejuwon andEkanade (1988) report few significant changes. Not surprisingly, nitrogenaccumulation is widely found under nitrogen-fixing species.

There have been a large number of studies in temperate plantations, with mostattention focused on pH change, litter type, podzolisation, etc. Much recentinvestigation has been triggered by concern over acid rain, though distinguishing thisimpact on soil acidity from direct tree effects is difficult. On the whole, tree impactsare relatively small compared with the soil nutrient store.


3 0 S i t e c h a n g e i n d u c e d b y p l a n t a t i o n f o r e s t r y

In wetter, cooler climates such as western Europe, the common observation is that asite re-wets when a stand is clearfelled. This brings to an end a period of drier soilconditions which may assist soil aeration, at least for a time until the next crop isestablished and the canopy closed. This phenomenon is especially obvious on surfacewater gleys. The higher levels of evapotranspiration of trees in climates where moisturedeficit is rare may contribute to improving conditions for subsequent growth, and mayexplain in part the generally better growth of second-rotation stands in the UK.

Indirect impact of vegetation suppression

Species such as teak and Gmelina in the tropics, and many conifers in tropical andtemperate conditions, may suppress all ground vegetation when grown intensively.Where this leads to exposed soil, perhaps because litter is burnt or gathered, it oftengreatly worsens surface soil erosion. Under teak, Bell (1973) reported rates of soilerosion 2.5 to nine times higher than under natural forest – unfortunately he did notinclude any comparison with agriculture. It is clear that the protective function of treecover derives more from the layer of organic matter that accumulates on the soilsurface than from interception by the canopy. In India, raindrop erosion was ninetimes higher under Shorea robusta plantations where litter had been lost throughburning (Ghosh, 1978). Soil erosion beneath Paraserianthes falcataria plantations wasrecorded as 0.8 t ha-1 year -1 where litter and undergrowth were kept intact, but anastonishing 79.8 t ha-1 year -1 where it had been removed (Ambar, 1986). Similarly,Wiersum (1983), reporting on soil erosion under Acacia auriculiformis plantations,found that the presence of litter and undergrowth virtually eliminated erosion whetheror not there was an intact tree canopy, but that where local people removed litter,erosion occurred. In Jamaica, Richardson (1982) found that the dense needle matunder pine plantations was even better than natural forest for minimising soil erosion.

Harvesting damage

Extracting trees from a site causes damage, although the severity and form varies.Damage may arise from soil compaction, scouring of soil surface and erosion,blocking of ditches and other drainage channels, and oil spillage. The method ofextraction is the most important factor affecting damage, with draft systems usingmules, oxen etc. being least harmful and skidding with tracked vehicles generally mostdamaging. Using skidding to extract timber from a clearfelling can damage up to 35%of the ground surface (Hatchwell et al., 1970). Lacey (1993) compared differentharvesting systems on a large variety of sites, and found up to 30% of the groundsuffering serious soil disturbance with ground skidding systems. Weather conditionsand the type of soil also affect the severity of damage, and areas of compacted soilare common following harvesting, especially when extraction operations areconducted in wet conditions on heavy, high-clay-content soil.

Aerial photograph (infalse colour) of second

rotation Pinus patula

in the Usutu Forest,Swaziland, showingregular depression in

growth coinciding withthe tractor extractionroutes used to harvest

the first crop (photo: S E G Brook

– from Evans, 1992a).



Any activity in a forest that disturbs the role played by litter in the ecosystem can havelarge effects. Examples are litter accumulation where carbon:nitrogen ratios exceed200:1 and normal decay and breakdown cease; over-zealous burning of harvestingdebris where large quantities of nitrogen are volatilised and the litter layer itselfdestroyed once per rotation; and perhaps most serious of all, regular and frequentlitter raking or gathering (see page 52). It is interesting that the cost of managingdebris and site preparation when restocking plantations is high, and represents a largeproportion of the establishment costs. As Nambiar (1996) points out, one shoddyoperation can leave behind lasting problems.

It is clear from the first part of this review that the few examples of yield declinerecorded mostly relate to damaging practices regarding litter and organic matter.

Weed spectrum and intensity

Establishment of plantations greatly affects ground vegetation. Many of the operationsare designed directly or indirectly to reduce weed competition. The competitive effectsof weeds are well researched, e.g. Davies (1987) for British conditions, and thewidespread problems of eucalypts in dense grassland are well known (Evans, 1992a).The objective of weed control is to ensure that the planted tree has sufficient accessto site resources for adequate growth. Once canopy closure has occurred, weedsuppression is usually achieved for the rest of the rotation. A critical next phase ismanaging the weed problem through the harvesting and restocking process in re-establishing the crop.

Often the weed spectrum changes. Owing to weed suppression or elimination, theexposure of mineral soil in harvesting, and the accumulation of organic matter,conditions for weed species will change. Birds and animals may introduce or spreadnew weed species, and grass seed can be blown into plantations and accumulateover several years, to flourish when the canopy is removed and light is provided.Roads and rides in plantations can become sources of weed seeds. Weedmanagement is a holistic operation. The ideal is to harness the tree crop in controllingthe weeds and then to maintain this. As with a failure to handle organic mattercarefully, where yield declines have been reported, the significance of weeds has alsobeen poorly recognised, and restocked sites in second or third rotations havestruggled in unfamiliar weedy conditions.

Conclusion

Plantations and plantation forestry operations do have an impact on the siteswhere they occur. Under certain conditions nutrient export may threatensustainability, but care with harvesting operations, conservation of organicmatter, and management of the weed environment are usually more importantfor maintaining site quality. Plantation forestry appears to be entirelysustainable under conditions of good husbandry, but not where wasteful anddamaging practices are permitted.

There are many reports of impaired growth of planted trees on extraction routes andwhere soil has been compacted and suffered erosion, e.g. with Cordia alliodora inSurinam (Weert and Lenselink, 1973), and Gmelina arborea in the Amazon and Pinuspatula in Swaziland (Evans, 1992a). In addition, mortality is often higher. A usefulsummary of these impacts is given by Nambiar (1996).

Minimising harvesting damage is achieved by avoidance of log extraction in wetconditions when soil strength is weak, use of brash mats to provide some flotation,and by switching from mechanical skidding systems to gentler extraction methods onslopes and fragile, erodible soils. Planning extraction routes is also crucial. However, itneeds to be borne in mind that harvesting in plantations, particular in temperatecountries, occurs at long intervals and is not an annual event – it is worth doingcarefully.

Organic matter dynamics

Much of the above discussion has focused in one way or another on the litter andorganic matter layer at the soil surface. What happens to it appears to be cruciallyimportant to the question of sustainability, for three reasons:

• the surface litter layer contributes hugely to preventing soil erosion;

• litter and organic matter represent a significant nutrient store, albeit usually in a dynamic state of litter addition and litter/organic matter re-incorporation;

• the litter:organic matter:mineral soil interface is the seat of nutrient cycling and microbial activity, and usually has the greatest concentration of fine roots found anywhere in the soil profile.

Mule extraction of pinein the Usutu Forest,

Swaziland used onsteep slopes and fragilesoils liable to erosion or

compaction.



Pest and disease incidence in monocultures

One of the most serious dangers to a plantation is the possibility of destruction owingto a massive build-up of a pest or disease. Whether monoculture itself is moresusceptible to devastation from these causes has been a subject of much dispute,particularly as almost everywhere agriculture is characterised by decreased diversity(Way, 1977). In natural ecosystems catastrophic disruption in single-species stands iscommon (Ford, 1982), but it need not be as frequent in plantations providedattention is paid to the ecological principles of stability (Bruenig, 1986) andappropriate interventions made.

The broadly accepted ecological principle of stability dates back to the 1950s andstates that the stability of a community and its constituent species is positively relatedto its diversity – the more diverse, the more stable and the less prone to destruction.Following this reasoning, foresters have rightly stressed that substitution of naturalforest by even-aged monoculture plantations may remove many of the naturalconstraints on local tree pests and pathogens, and thus increase the risk of attack.Evidence in support of this is well documented: see for example Gibson and Jones(1977), although these authors point out that much of the increased susceptibilityhas arisen from conditions in plantations rather than from the fact that only one treespecies is present.

The relative susceptibility of monocultures to organic damage is ecologically complex.For example, applying the idea that diversity is beneficial by cultivating mixed cropsmay not offer much protection, as only small amounts of the right kind of diversity areneeded to maintain stability (Way, 1966); see e.g. Nair et al.’s (1986) study of insectpests on plantation species growing in their natural forest environment. Also, theinfluence of diversity on stability of (say) insect populations will depend on whatpopulation level is deemed desirable or tolerable. Often, stable equilibrium levels aretoo damaging and artificially low populations sought. Pest control to maintain such lowpopulation levels means adopting measures very different from those required toachieve stability (Speight and Wainhouse, 1989), as every spray in every farmers’ fieldtestifies. These authors also point out that artificially created diversity, i.e. mixedcrops, does not necessarily improve ecological stability and is certainly inferior tonaturally occurring diversity in this respect. Complexity of organisation and structure isjust as important (Bruenig, 1986).

While the ecology, and any possible scientifically based approaches to minimising riskto monocultures, are less than straightforward, it is prudent to spell out whyplantations are perceived to be in danger.

• Plantations consisting of one or two species offer an enormous food source and an ideal habitat to any pest or pathogen species adapted to them. Food supply isa basic ecological determinant of population size, and a multiplicity of suitable sites for breeding or infection strongly favours rapid population build-up.

Risks to

which

plantations

are exposed

3


3 6 R i s k s t o w h i c h p l a n t a t i o n s a r e e x p o s e d F o r e s t P l a n t a t i o n s t h e e v i d e n c e 3 7

Dothistroma needle blight in East Africa. Initial success of Pinus radiata plantations inthe East African highlands was curtailed by Dothistroma damage and halted furtherplantings of this species in the 1950s, despite its superior growth rate to thealternatives Pinus patula and Cupressus lusitanica. It is now better appreciated thatdamage is worse where P. radiata is grown in warm, wet climates and is less seriouswhere warm and dry or cool and wet conditions prevail (Zobel et al., 1987). In suitableclimates, P. radiata is one of the world’s most successful and productive tree speciesin forest plantations. Currently efforts are under way in Kenya to select for resistanceand resume planting of this valuable pine.

Diplodia in southern Africa. Diplodia (Sphaeropsis sapinea) can cause death of pines ifsevere infection occurs following exposure of damaged tissue, as for example followinga major hailstorm. In April 1973 one 20-minute storm killed hundred of hectares ofPinus patula in northern Swaziland (Evans, 1992a). Despite frequent hailstorms insouth-east Africa, the Diplodia threat is not considered serious enough to prevent useof the species.

Poplar canker and poplar rust. In Europe, poplar canker (a bacterial infection ofXanthomonas populi) and leaf rusts (Melampsora spp.) restrict commercial use ofpoplars to a few relatively resistant species – this is enforceable by law in Britain, forexample, where only approved clones may be grown. Breeding programmes of veryhigh-yielding hybrids require regular introduction of new genetic material to maintainsufficient resistance for clonal plantations. These diseases restrict the choice ofspecies, and require investment to avoid problems in clonal plantations.

Phytophthora dieback. In Western Australia, large areas of natural Jarrah forest(Eucalyptus marginata) suffered dieback in the 1950s and 1960s, raising the fear thatmuch native forest was at risk. These fears have not been realised and it is now clearthat favourable site conditions, presence in soil and susceptible host species are allrequired for the disease to express itself (Florence, 1997). Phytophthoras also causeeconomic damage to many cultivated species such as oaks (Quercus spp.) in Spainand Portugal and alders (Alnus spp.) in Britain.

Dutch elm disease. In Europe and parts of North America, mature elms have beenalmost wiped out owing to Dutch elm disease (Ophiostoma ulmi and O. novi-ulmi)since the 1960s. The high and widespread mortality results from a particularly virulentform of the disease; an efficient vector – the scolytid beetles; and elm populationsoften exhibiting narrow genetic variability. Elm is not an important plantation species,but the disease is illustrative of the potential threat. Research has greatly advancedunderstanding, but has not so far yielded a field-scale control method, althoughgenetic engineering is a promising avenue.

• The uniformity of species and closeness of trees, including branch contact above ground and root lesions in the soil, allow for rapid colonisation and spread of infection from tree to tree. Canker diseases that are often splash-dispersed or mist-carried, and insects with small effective spread, are favoured by close proximity of hosts.

• A narrow genetic base in plantations may be seen not only in a single species, but also in limited or no genetic variation (e.g. clones) in the growing stock, thus reducing the inherent variability in susceptibility to attack.

• The forest plantation grows on one site for many years. This may allow a pest or disease to build up over a long period, a situation very different from most farm crops which are harvested and replaced at short intervals, offering the opportunity to destroy infection and/or to grow a different or more resistant crop next time. The forest plantation cannot be changed quickly in the face of a devastating outbreak.

• Many plantations contain exotic species which have been introduced to the site from elsewhere and are not native. This often favours their rapid growth owing to freedom from insect pests and pathogens that occur in their native habitat. It undoubtedly contributes to the great success of eucalypts throughout the tropicaland Mediterranean regions (Pryor, 1976) where they are free from numerous leaf-eating insects that occur in the Australian environment. The converse is that many natural agencies controlling pests and diseases are also missing, and destruction can be swift and uncontrolled. It has been argued that exotic plantations experience a period of relative freedom from organic damage, perhaps for the first one or two rotations, but this cannot last indefinitely. Zobel et al’s (1987) analysis of the threat to exotics concludes that there is no evidence that stands are more at risk, other than clonal plantations, and that problems often arise because foresters have planted species that are ill suited to a site.

Examples of devastating outbreaks

There are several examples of seriously debilitating pest and disease problems thathave either prevented use of an otherwise economically desirable species, or greatlyimpaired growth, or required considerable investment in control to allow satisfactoryperformance in a plantation. The list below is illustrative, not exhaustive.

Diseases

Fordlandia. One of the causes of the failure of the rubber plantations (Heveabrasiliensis) developed in the 1930s in the Amazon was the tree-to-tree spread of adisease (Microcyclus ulei). Traditional tapping in Brazil gathers rubber latex fromindividual or tiny groups of trees often spaced hundreds of metres apart in the forestand not in close proximity. Failure also arose from establishing many of the earlyplantations on stony, inhospitable soils ill-suited to Hevea spp.



Insect pests

Leucaena psyllid. In the 1980s outbreaks of the severely defoliating insect Heteropsyllacubana began devastating plantations of Leucaena leucocephala, especially whereHawaiian hybrids were used. In a few years, 1985–1988, the psyllid spreadthroughout tropical Asia and by 1992 was established in Africa. It has rendered thisonce widely planted nitrogen-fixing fuel and fodder tree far less valuable and greatlyreduced its use.

Mahogany borer. Attempts to grow mahogany (Swietenia spp.) in plantations, andindeed almost any tree species in the Meliaceae family (Cedrela, Khaya, Melia, Toona,etc.), have often been thwarted by damage from the mahogany shoot borer, Hypsipylasp., which causes severe stem deformation. The problem can be overcome bygrowing mahogany in heavy shade, but it has severely curtailed plantationestablishment of these exceptionally valuable tropical hardwoods.

Pinus radiata at highaltitude in Ecuador and

suffering from Dothistroma

disease. Highland tropical sitesare ill-suited to this species. It

is a remarkably successfulplantation species in many

countries including Australia,Chile, New Zealand and

South Africa.

Shoot borer damageto African

mahogany (Khaya

nyasica) byMussidia

nigrivenella

(Pyralidae). Newworld mahoganies

are similarlyattacked by

Hypsipyla spp.(Pyralidae).

Mahogany (Swietenia macrophylla)

grown successfully in line planting inPuerto Rico where overhead shade

helped prevent shoot borer damage.



There remain two serious concerns.

• Environmental change – changing climate and increasing pollutants of CO2 and nitrogen compounds will add stress to established plantations, while increasing nitrogen inputs may increase insect pest and disease problems (Lonsdale and Gibbs, 1996).

• New pests and diseases will emerge: from new hybrids or mutations; from new introductions consequent upon the ever-increasing flow of material around the world, e.g. Cryphonectria canker in eucalypts in South Africa and new phytophthoras in Britain; or from the adaptation of native pests to introduced trees.

Risks associated with plantation forestry practices

Many pest and disease problems in plantations arise from the nature and intensity offorest operations, and not directly from growing one tree species in a uniform way(monoculture). Outside the nursery stage there are four broad aspects of silviculturewhich can be critical.

Harvesting and other residues

Large amounts of wood residue from felling debris and the presence of stumps arefavourable for colonisation by insect pests and as sources of infection. There aremany examples. In the UK, build-up on restocking sites of the weevil Hylobius abietis,which breeds in the freshly cut stumps, causes high mortality of untreated seedlings,and honey fungus (Armillaria spp.) infection of trees planted on former hardwoodsites. On alkaline soils, fomes (Heterobasidion annosum) can cause widespreadmortality among young replants, especially of pine. In Swaziland, young Pinus patulaseedlings suffer from weevil (Hylastes angustatus) infestation if not protected and, whenburning of felling debris was practised, the ascomycete fungus Rhizina undulatacaused major losses in young second-rotation replants.

There are many other examples, but modification of silviculture or application ofspecific protection measures can generally contain such problems, which mayincrease costs but do not seriously threaten narrow-sense sustainability.

Aphids on East African pines and cypress. In the late 1960s, the pine woolly aphid (Pineusspp.) damaged many Pinus patula plantations in East Africa. More recently and farmore damaging has been widespread defoliation and death of cypress trees (Cupressuslusitanica) by the cypress aphid (Cinara cupressi) in the industrial plantations of Kenyaand Tanzania (Ciesla, 1991). It also damaged indigenous Widdringtonia spp. Theproblem appeared to be triggered by drought, was severely damaging from 1992 toabout 1995, but has receded in its prevalence and severity.

Pine beauty moth on lodgepole pine in Scotland. In the 1970s, a moth native to GreatBritain (Bupalus piniarius) killed whole plantations of young pole-stage Pinus contortagrowing on deep peats in northern Scotland. Control is possible through aerialspraying, but use of this tree species has been discontinued owing to this pestproblem in addition to the species’ inferiority in growth and many wood qualities tothose of Sitka spruce.

Sirex mortality in Southern hemisphere pine plantations. The European wood wasp (Sirexnoctilio) was first found damaging and killing Pinus radiata in New Zealand in the1900s. It spread and became established in Australia in the 1960s where it causestree mortality, especially in unthinned P. radiata plantations where trees are understress. It also appeared recently infesting Pinus taeda plantations in Argentina, Uruguayand Brazil, while in New Zealand it has greatly diminished as a problem.

Dendroctonus micans in Britain. This borer was introduced to Britain in 1973, wasfirst identified in 1982, and has spread through many Norway spruce (Picea abies)plantations in Wales and the west midlands of England, and more recently in smallpockets in Kent. Spread is slow (5–10 km year -1) but is economically serious, and hasled to enforceable restriction of log movement in parts of the country and, perhapsperversely, maintenance of protected zones still free of the pest so that import controlof logs from Europe can be maintained. Biological control through release of thespecific predator Rhizophagus grandis has achieved substantial success (Fielding et al.,1991).

These examples (for others see e.g. Ciesla and Donaubauer, 1994) illustrate the scaleand potential threat from pests and diseases to plantation forestry. They haveprevented the planting of some species and greatly impaired the productivity ofothers, but overall have not so far caused such widespread damage to so manyspecies as to seriously question plantation silviculture as a practice. What they doillustrate is that vigilance and research are required, that new threats can readily arise,and that knowledge of pest and disease ecology is crucial to understanding how bestto avoid, contain and manage problems.



Site and species selection

Extensive planting of one species, whether indigenous or exotic, inevitably results insome areas where trees are ill-suited to the site and suffer stress. This may occureither where large blocks of one species are planted or, as has commonly happened,exotics are extensively planted before the response of the species has been fullymonitored over a whole rotation, e.g. Acacia mangium in Malaysia and Indonesia, andthe discovery of widespread heart rot. Trees under stress, or trees not well suited to asite, are more prone to organic damage. This is now believed to be a major reason forthe poor growth and susceptibility to attack by nun moth (Lymantria monacha) ofsecond- and third-rotation Norway spruce plantations in Saxony, reported long ago byWeidemann (1923).

Thinning and pruning damage

Thinning operations can damage remaining trees and provide infection courts fordiseases and, in the case of Fomes infection as stumps are colonised and throughroot lesions, may lead to death of adjacent trees. Delayed thinning, ragged pruningand poor hygiene can also increase the risk to remaining trees; several examples arecited by Evans (1992a), but none of these seriously threatens plantation sustainability– rather, these risks need to be managed by applying good husbandry.

Storms and fire

Plantation uniformity may render stands more at risk from hurricane and stormdamage, if only because trees may be planted in locations which increase theirsusceptibility. In the UK, upland plantations on moist soils are exposed to this threat,which is contained by modifying silviculture by using short rotation lengths and byrefraining from thinning. Nevertheless, sub-optimal productivity is the result, and thesites’ yield potential is not fully realised. Minimising hurricane damage in the tropicscan be helped by planting wind-firm species such as Cordia alliodora or choosing Pinuscaribaea var. bahamensis over Pinus oocarpa.

Most forest fires in plantations are caused by arson, and only a few are due tolightning or encroachment of fires from neighbouring land. While there are examplesof frequent fires finally preventing plantation development (the author is aware of twosituations, in the UK and South Africa), where this has occurred it is more to do withrelations with the local community than any inherent shortcomings of forestplantations.

Conclusion

Plantations are at risk from damaging pests and diseases. New threats willinevitably arise and some plantations may become more susceptible owing toclimate-change factors, but the history of plantation forestry suggests thatthese risks may be contained with vigilance and the underpinning of soundbiological research.

Burning debris after clear-felling in Swaziland led to lossof nitrogen from the site anddeath of replanted pine seedlingsfrom Rhizina fungal infection.This undesirable practice wasdiscontinued in 1973.

Application of urea andother treatments to freshly

cut conifer stumps inBritain successfullyprevents build up of

Fomes (Heterobasidion

annosum) disease in thetree stand and in the crop

which follows.



The steady transition from exploitation and management of natural forest to increasingdependence on plantation forests is following the path of agriculture. Many of the samebiological means to enhance yield are available. They are outlined here only briefly, beingmore properly the substance of separate texts, e.g. Evans (1992a); Savill et al. (1997);Wright (1976). They are summarised for their potential to increase productivity, toalleviate threats, and to suggest what is best practice.

An aside here – owing to the disposition of the world’s forest resources, the presentintensification of forestry in the form of plantations brings one advantage thatdistinguishes it from what has taken place in agriculture. Plantations contribute to thedeliberate protection of natural forest formations by diverting pressure away from them intheir wood-producing role. Few examples of agricultural intensification have enjoyed thismotivation – a desire to conserve. This is not yet a dominant motive for forestplantations, but it is an increasing one, particularly where agroforestry and ruraldevelopment forestry are concerned. Intensification in plantation forestry continues thisprocess.

Genetic improvement

The forester only has one opportunity per rotation to change the crop grown. Changesin species, seed origin, use of new clones, use of genetically improved seed and,possibly in the future, genetically modified trees all offer the prospect of better yieldsin later rotations.

Species change

There are surprisingly few examples of wholesale species change from one rotation tothe next, which suggests that in most cases foresters have been good silviculturists,drawing on trials and long experience with a species before commencing large-scaleplantations. Some examples follow.

• Changing from native Pinus sylvestris to Pinus nigra var. maritima for the second rotation in Thetford Forest, UK has led to a yield increase from about 10 to 13 m3

ha-1 year -1 in average mean annual increment.

• Replacing Pinus pinaster in the Landes region of France with Pinus nigra has raised yield, and improved stem form and quality and hence outturn of millable timber.

• In Swaziland, on sites where for the third rotation Pinus patula was replaced by Pinus taeda, there has been little yield advantage but it may be economically attractive as a cheaper tree to grow (Evans, 1999) see table 3, page 13.

Interventions

to sustain

yield

4



• New poplar clones of the inter-American hybrids (Populus trichocarpa x Populus deltoides) such as Beaupre and Boelare are generally greatly superior to previously approved clones of the euramerican hybrids (P. deltoides x Populus nigra) such as Serotina and Robusta which were hitherto widely planted in Europe (Tabbush and Beaton, 1998). More recently, other hybrids are being introduced with even more remarkable vigour and with increased rust and canker resistance.

• In Queensland, Australia, use of pine hybrids of Pinus elliottii x Pinus caribaea var. hondurensis are used operationally in place of the pure species on sites with impeded drainage, leading to both increased vigour and better stem quality.

In general, where an exotic is replaced by a native species in the second rotation forreasons of conservation or public preference, productivity may diminish. Convertingconifer plantations to native broadleaves in Britain usually has this effect.

Better seed origins, provenances and land races

The impact of all these genetic improvements will affect yield and outturn both directlyand indirectly, through better survival and greater suitability to the site which may leadto increased vigour and perhaps greater pest and disease resistance. Countlessstudies affirm the benefits of careful investment in this phase of tree improvement.

It is also important to cover the range of site conditions where a species is believed tohave potential owing to the phenomenon of site–genotype interaction. The best seedorigin in one location may not be the best in another. This refinement inunderstanding offers further yield improvement.

Clonal plantations

Some of the world’s most productive tree plantations use clonal material, including botheucalypts and poplars. It is clear that both the potential productivity and the uniformity ofproduct make this form of silviculture attractive to industry, and it is likely to expandmarkedly in the future. Although clonal forestry has a narrow genetic base, carefulmanagement of clone numbers and the way they are interplanted can minimise pestand disease problems. Roberds and Bishir (1997) suggest that use of 30–40 unrelatedclones will provide security against catastrophic failure in most circumstances.

Fast-growing poplar clones bredfor vigour and disease resistancein Belgium. The trees are seven

years old

Climbing avaluable, selectedpine tree to collect

seed as part ofgenetic tree

improvement.


4 8 I n t e r v e n t i o n s t o s u s t a i n y i e l d F o r e s t P l a n t a t i o n s t h e e v i d e n c e 4 9

Responding to environmental change

Genetic improvements or changes offer a means of responding to predicted climatechange and, indeed, of seeing it as an opportunity. An example would be a speciesswitch to cope with changing rainfall patterns such as increased drought in parts ofthe tropics. In the UK, in the Sitka spruce breeding programme, the southern breedingpopulation (from Washington state, USA) is expected to play a major role owing tolonger growing seasons and lower incidence of frost to which it is better adapted.

For genetics to fulfil this role an active breeding programme is necessary, and it mustbe based on a wide genetic diversity. Part of the programme should not only beselection and multiplication but also deliberate maintenance of a broad genetic base.This may require new collections of material from across the whole of a species’natural range.

Role of different silvicultures

Silvicultural knowledge continues to increase through research and field trials oftenfocused by greater understanding of tree and stand physiology. While large yieldimprovements appear unlikely, several incremental gains can be expected from greaterrefinement in plantation establishment and management. Important examples includethe following.

• Manipulation of stocking levels to achieve greater output of total fibre or a particularproduct such as high-quality sawlogs. The object will be fuller site occupancy, less mortality, and greater control of individual tree growth.

• Matching rotation length to optimise yield – the rotation of maximum mean annual increment – offers worthwhile yield gain in many cases. Of course, financial constraints may prevail and indicate alternative rotations.

• In some localities, such as the British Isles, prolonging the life of a stand of trees subject to windthrow will increase yield over time, as almost all such threatened stands are felled or windblown before maturity. Research to predict damaging stormimpacts, and silvicultural research which increases crop stability and stem strength, assist increases in yield.

• Use of mixed crops may help tree stability and may possibly lower pest and disease threats, but is unlikely to offer a yield gain over growing the most productivetree the site can support (FAO, 1992).

• Moves to silvicultural systems that maintain forest cover at all times – continuous cover forestry practices – such as shelterwood and selection systems are likely to be neutral to slightly negative in production terms, while yielding gains in tree quality, aesthetics and probably biodiversity value.

• Crop rotation, as practised in farming, appears unlikely as a feature in plantation forestry although there are examples of tree plantations benefiting from a previous crop of nitrogen-fixing trees such as Acacia mearnsii. The expectation must be that when replanting industry will require a similar, not widely differing, species.

Tree breeding

Through an array of selection, crossing and propagating techniques, traits can be favouredthat may improve vigour, stem and wood quality, pest and disease resistance, and otherparameters such as frost tolerance. There are many notable examples of successful treeimprovement strategies, most of which are only beginning to bear fruit owing to long treerotations and the slow process of tree breeding, particularly in orchard establishment andpromotion of flowering, and in field testing of selections and progenies. Nevertheless,genetic tree improvement, still in its infancy, offers by far the greatest assurance ofsustained and improved yields from plantations in the medium and long term. It iscommonly believed that improvements in the order of 20–50% are relatively easy toachieve (Franklin, 1989). From plus-tree selection alone, Cornelius (1994) reportedgenetic gain values of 15% in height and 35% in volume based on 24 published reports.

The following examples list improvements in vigour that have been reported or arepredicted.

Sitka spruce in the UK. Predicted gains for first-generation orchard-quality Picea sitchensisshow 15% greater volume compared with the best but unimproved provenance, and riseto 20% for family mixtures produced by controlled pollination (Lee, 1990, 1994). Actualgains must await the outcome of genetic gain trials.

Eucalypts in Brazil. Selection, breeding and use of clonal techniques with eucalypts atAracruz Florestal in Brazil have more than doubled and sometimes tripled meanproductivity of the plantations between the 1970s and the 1990s. Mean increment ofpulp has increased from 5.9 to 10.9 t ha-1 year -1 (Campinhos, 1995).

Radiata pine in South Australia. Current second and third rotation plantings of Pinus radiata inSouth Australia show an increase in yield over the preceding crops, largely due to geneticimprovement (see figures on pp. 12 and 18). Johnson (1992) reports similarimprovements of between 15 and 40% in volume from breeding programmes with P.radiata in New South Wales.

Subtropical pines in Zimbabwe. In Zimbabwe a sustained 30-year programme of treeimprovement in subtropical pines has led to first-generation selections showing 15–20%yield increases, and to second-generation selections with 30–35% improvement. As aresult all recent plantings, of whatever rotation, show much higher productivitiescompared with the original plantations. Interspecific hybrids offer further promising gains.A related programme involving eucalypts has realised gains of up to 70%.

Genetically modified trees

There are no widely planted examples at the present time where genetic engineeringhas modified trees. The expectation is that these techniques will be used to developdisease resistance, modified wood properties and cold or drought tolerance, asopposed to direct increases in vigour. Examples already in progress include modifiedlignin content of eucalypts, and insertion of disease-resistant genes in elms.

Subject to their widespread public acceptance these powerful genetic tools willbecome increasingly cheap and accessible to forestry use, and offer an important aidto intensification of production.


5 0 I n t e r v e n t i o n s t o s u s t a i n y i e l d F o r e s t P l a n t a t i o n s t h e e v i d e n c e 5 1

Fertilising

Regular application of mineral fertiliser is not presently a feature of plantation forestry,though it is used in plantation agriculture such as rubber and oil palm production.Most forest use of fertiliser has been to correct known deficiencies, e.g. inmicronutrients such as boron in much of the tropics and zinc in Australia, and inmacronutrients such as phosphorus on impoverished sites in many parts of the worldand nitrogen in some locations such as the Pacific Northwest of America. In mostinstances, fertiliser addition has been required only once in a rotation to obtainsatisfactory establishment and growth. Sometimes re-application is not required for thenext crop, the one application introducing sufficient of the nutrient into the ecosystem,e.g. phosphate use in Britain (Taylor, 1990).

Forest use of mineral fertiliser is thus quite different from farming, where crop yieldsclosely match the level of fertiliser input. Such an approach to forest plantations remainslargely experimental, with the one exception of intensively grown eucalypts – seeexamples given by Evans (1992a). Also, spectacular yields have been achieved on somesites by frequent or even annual fertiliser addition as part of an intensive managementpackage including full weed control, optimal spacing, etc. Examples include the BritishForest Research experiment Wareham 156; the trials by Torsten Ingestad in Sweden;and the biology of forest growth experiment carried out by CSIRO in Australia. Thesetrials are experimental and serve more to elucidate principles of stand physiology,applied nutrition and maximum growth potential rather than to offer practical operationalprescriptions. This arises because wood is a low-value product, and seeking greateryields through such approaches is unlikely to be economically worthwhile. The scale ofimprovement, while sometimes dramatic, is dependent on other aspects of the site (e.g.weed control or use of irrigation) to be properly realised. For most tree plantations, onceactual deficiencies are overcome, nutrition is rarely the limiting factor.

Monitoring of nutrient levels in foliar analysis or fertiliser trials will have a role, but willprobably only be as an aid to good overall silviculture, not as a diagnostic tool for growthpromotion. Fertilising is likely to be the principal means of compensating for nutrientlosses on those sites where plantation forestry practice does cause net nutrient export tothe detriment of plant growth.

Site preparation and establishment practices

Ground preparation to establish the first plantation crop will normally have introducedsufficient site modification for good tree growth. Cultivation inter alia loosens soil, improvesrooting, encourages drainage, limits initial weed growth, improves water percolation, mayreduce frost risk and, perhaps importantly for the long-term health of the forest, bringsrelatively unweathered soil minerals nearer to the surface and into the main feeding zone oftree roots. Substantial new investment in site manipulation is unlikely in second andsubsequent crops owing to cost of handling stumps and the implied failure first time round.Exceptions are alleviation of soil compaction after harvesting, and measures to reduceinfections and pest problems. For example, in Britain some de-stumping and windrowing ofdebris on some alkaline sites helps avoid Fomes infection.

Weed control strategies may change from one rotation to the next owing to a differingweed spectrum and whether weeds are more or less competitive with planted trees. Theissue is crucial to sustainability since all the main examples of yield decline problems reflectworsening weed environments, especially worsening competition from monocotyledonssuch as grasses and bamboos.

Changes between rotations in treatment of felling debris and organic matter may occur,such as cessation of burning, use of windrowing, or removal from site in whole-treeharvesting. Where the consequences and impacts of such changes are informed byresearch, e.g. modelling the greater nutrient removal in whole-tree harvesting, then aconscious evaluation can be made. What is clear, however, is that the felling, harvestingand re-establishment phases are crucial to sustainable practice and need to be viewed asa whole, with the aim of seeking to minimise impacts from compaction along extractionroutes, from loss of organic matter and from soil erosion and nutrient loss, etc.

Organic matter conservation

It is clear from many investigations that treatment of organic matter both over therotation and during felling and replanting is as critical to sustainability as coping withthe weed environment. While avoidance of whole-tree harvesting is probably desirableon nutrition grounds, it is now evident that both prevention of systematic litter rakingor gathering during the rotation, and not significantly harming or destroyingaccumulated organic matter at harvesting, are essential. Many authors attribute poortree growth in the past and evidence of increasing yields in many plantations today tothe cessation of litter-raking practices (Baule and Fricke, 1970; Will, 1984; Spieckeret al., 1996).


Holistic management

If all the above silvicultural features are brought together, a rising trend in productivitycan be expected. But if any one is neglected it is likely that the whole will sufferdisproportionately. For example, operations should not exclusively minimise harvestingcosts, but those of harvesting, re-establishment and initial weeding as a holisticactivity, and without impairing tree vigour. Evidence of a rising trend reflecting theinterplay of these gains is seen on page 20 for southern Australia, and an example ofwhat is predicted for the Usutu Forest in Swaziland, based on research results andfield trials for each component prepared by A.R. Morris, is shown below.

Holistic management also embraces active monitoring of pest and disease levels, andresearching pest and disease biology and impacts will aid appropriate responses suchas altering practices, e.g. delayed replanting to allow weevil numbers to fall. Carefulre-use of extraction routes to minimise compaction and erosion is a further example.

Conclusion

There are several interventions in plantation silviculture which point toincreasing productivity in the future, providing management is holistic andgood standards are maintained. Genetic improvement, in particular, offers theprospect of substantial and long-term gains over several rotations.


1995 2000 2005 2010 2015 2020 2025

Usutu Forest estimated yield gains 1992-2028

Sustainable yield (million t yr -1)1.4

1.3

1.2

1.1

1

0.9

0.8

0.7

New area (1970>)Optimum stocking

Fertilizer useAlternative species

P. patula breedingCurrent base is

taken as second-rotation crop yields

Year of harvest

Litter raking under a pineplantation in southern China, a

practice likely to causeunsustainable nutrient loss.Cessation of this practice in

Europe during this century isone reason why present rates of

tree growth are much betterthan they were in the past.



Brief comment is appropriate on the impacts of global climate change on thesustainability of plantation forests. There are direct measurable changes in thecomposition of the atmosphere such as a rise in CO2 and NOX gases and stratosphericozone depletion, and the indirect consequences are loosely referred to as ‘globalwarming’.

Pollution impacts

Many data show that carbon dioxide and oxides of nitrogen, along with other gasessuch as methane and chlorofluorocarbons (CFCs), are increasing in concentration inthe atmosphere.

Carbon dioxide

Atmospheric concentration of CO2 has risen by about 30% since the pre-industrial era,and as it is the key gas in photosynthesis, changes in plant growth can be expected.Most evidence suggests that this rise is having a positive effect on growth referred toas the ‘CO2 fertilisation effect’. It is unknown how persistent it will be, and whether theobservation of reduced stomatal density will occur in all plants leading to slightlyimproved water use efficiency and hence somewhat greater drought tolerance.

Evidence of a fertilisation effect is suggested by the observation of improved treegrowth in recent decades throughout Europe (Spiecker et al., 1996; Cannell et al.,1998) and may explain part of the missing sinks in the carbon cycle. Plantationforests seem likely to benefit from this phenomenon with possibly increasedproductivity as well as playing a potentially important role in carbon sequestration.

Oxides of nitrogen

These pollutants may also have a positive effect, particularly in regions of nitrogendeficiency e.g. the Pacific Northwest of America. It is possible that the rising trend ofwheat yields in the past 30 years in the untreated control plots on the famousBroadbalk Field experiment at Rothamsted is a response to higher inputs of nitrogenin rainfall (see page 9). Certainly in many industrial regions nitrogen deposition inrainfall has risen from 5–10 to 20–50 kg ha-1 year -1. Such increases are likely toenhance tree growth, at least marginally, on some sites and not to diminish it.

However, there could well be a negative impact, particularly in increasing foliarnitrogen concentrations making leaves and needles more palatable to browsers andinsect defoliators. There is little experimental evidence available, but such a changemay increase the risk to plantations from biotic damage. Paradoxically, one of the fewstudies relating atmospheric inputs of nitrogen to insect activity showed that elevatednitrogen inputs to stands of Sitka spruce did not increase susceptibility to attack bythe aphid Elatobium abietinum, but instead accelerated the rate at which treesrecovered after aphid attack (Thomas and Miller, 1994).

Environmental

impacts on

yield over

time

5


5 6 E n v i r o n m e n t a l i m p a c t s o n y i e l d o v e r t i m e F o r e s t P l a n t a t i o n s t h e e v i d e n c e 5 7

Other pollutant gases

Atmospheric sulphur dioxide is decreasing in many western industrialised countries, aslong-established reduction strategies have begun to take effect.

Increasing ground-level ozone episodes as sunlight interacts with NOx emissions onhot, dry days (photochemical smog) are potentially damaging, causing leaching offoliar potassium and other symptoms of chlorosis.

Other pollutants may have indirect effects, such as thinning of the stratospheric ozonelayer and increased ultra-violet radiation. Little is known about direct effects on trees,but plantations are unlikely to be especially affected.

Climate change

Global climate change models predict several changes relating to incidence of drought,mean temperatures and frost occurrence, frequency of storm events, severity of El Niñoepisodes, and so on. These can all affect forest plantations owing to long rotations andan inability to respond quickly to change. However some effects may be positive, suchas rising temperatures and less frequent frosts – and this represents an opportunity inplantation development – but others may be harmful, such as more severe droughtand damaging storms. Even rising temperatures in some regions may trigger increasedaggressiveness of some diseases or encourage establishment of certain hithertoexcluded insect pests. Susceptibility of forest plantations will depend in part on theirgenetic make-up and on silvicultural steps taken to reduce stress. It appears prudent:

• to maintain greater genetic diversity, either in individual stands or in breeding strategies by adding new propagating material;

• to manage stands to avoid stress through silvicultural techniques such as timely thinning, sanitation and removal of diseased and defective trees;

• to minimise harvesting damage and impacts on soil such as compaction or destruction of organic matter.

In suggesting a prudent and cautious approach, it must be recognised that most treespecies widely used in plantations grow well in a range of climatic environments asfound in their natural distribution, e.g. Eucalyptus camaldulensis, or are found to beremarkably adaptable to different climates, e.g. Pinus radiata. It seems highly unlikelythat predicted climatic change will render an individual species unsuitable, althoughuse of other origins and provenances may become desirable.

Conclusion

Environmental changes will undoubtedly have an impact on plantation forestry.Some changes may yield improvement, others damage. Most plantationspecies are resilient and broadly based genetically, and are unlikely to sufferseriously from the kinds of climate change scenarios currently predicted. It willbe prudent to maintain genetic diversity and minimise stress to planted trees.

• Measurements of yield in successive rotations of trees suggest that, so far, there is no significant or widespread evidence that plantation forestry is unsustainable in the narrow sense. Where yield decline has been reported, poor silvicultural practices and operations appear to be largely responsible.

• Evidence in several countries suggests that current rates of tree growth, including in forest plantations, exceed those of 50 or 100 years ago.

• Plantations and plantation forestry operations do affect the sites on which they occur. Under certain conditions nutrient export may threaten sustainability, but care with harvesting operations, conservation of organic matter, and management of the weed environment are usually more important for maintaining site quality. Plantation forestry appears tobe entirely sustainable under conditions of good husbandry, but not wherewasteful and damaging practices are permitted.

• Plantations are at risk from damaging pests and diseases. New threats will inevitably arise and some plantations may become more susceptible owing to climate-change factors, but the history of plantation forests suggests that these risks are containable with vigilance and the underpinning of sound biological research.

• There are several interventions in plantation silviculture which point to increasing productivity in the future, providing management is holistic andgood standards are maintained. Genetic improvement, in particular, offersthe prospect of substantial and long-term gains over several rotations.

• Environmental changes will undoubtedly have an impact on plantation forests. Some changes may yield improvement, others damage. Most plantation species are resilient and broadly based genetically, and are unlikely to suffer seriously from the kinds of climate change scenarios currently predicted. It will be prudent to maintain genetic diversity and minimise stress to planted trees.

Conclusions



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6 4 F o r e s t P l a n t a t i o n s t h e e v i d e n c e

This position statement was commissioned by the Department forInternational Development as a contribution to the work of the Inter-governmental Forum on Forests, in particular its session meeting in May1999.

The writer was assisted by preparation of an annotated bibliography ofabout 280 references by Mr A. Hellier. This is available as a separatedocument or floppy disc. Much of the material for the bibliography wascompiled using the facilities of the Oxford Forestry Institute and to alesser extent the Rural Resources Assessment Group of ImperialCollege, London University.

At an early stage in the review the writer visited the Food and AgricultureOrganisation (FAO) in Rome and is grateful to Mr Hosni El-Lakany (ADG– forestry) and his staff, in particular Jim Ball, for facilitating the visit.Also, several of the illustrations and some of the data reported arisesfrom the writer’s research in the forest plantations of the Usutu PulpCompany, Swaziland.

The writer is very grateful to many foresters, researchers and scientistsfrom around the world who contributed observations, data or additionalinformation, or suggested new contacts to make the review as completeas possible, including (but not exclusively) the following: R. Boardman,C. Brown, C. Cossalter, J. Culbert, R. Deleporte, G. Dolman, J.C. Dutch,H.F. Evans, P.H. Freer-Smith, S. Hirai, J. Hudson, L. Jones, S. Keeling,M. Loyche Wilkie, J.S. Maini, A. Moffat, A.R. Morris, E.K.S. Nambiar, C.Perley, B. Shiver, J. Simpson, J.P. Skovsgaard, J. Tustin, P. Vigneron, L.Whitmore and A. D. G. Whyte.

Figures on page 12 (top) and page 18 are reproduced from Nambiar (1998)from the work of Leishout et al. (1996). With permission Dr E K S Nambiar(CSIRO).

Figures on page 12 (bottom) are modified from Evans (1996).

Figure on page 53 reproduced with permission Dr A R Morris (SAPPI).

Except where stated all figures are the author's copyright.

Acknowledgements

The Department for International Development (DFID) is the Britishgovernment department responsible for promoting development andthe reduction of poverty. The government elected in May 1997increased its commitment to development by strengthening thedepartment and increasing its budget.

The policy of the government was set out in the White Paper onInternational Development, published in November 1997. Thecentral focus of the policy is a commitment to the internationallyagreed target to halve the proportion of people living in extremepoverty by 2015, together with the associated targets including basichealth care provision and universal access to primary education bythe same date.

DFID seeks to work in partnership with governments which arecommitted to the international targets, and also seeks to workwith business, civil society and the research community toencourage progress which will help reduce poverty. We also workwith multilateral institutions including the World Bank, UNagencies and the European Commission. The bulk of ourassistance is concentrated on the poorest countries in Asia andSub-Saharan Africa.

Tables 1 to 3 (page 13) are simplified extracts from tables in Evans (1999).

Table 4 (page 27) is reproduced from Folster and Khanna (1997) in Nambiarand Brown (1997). With permission Dr E K S Nambiar (CSIRO).

The writer is also very grateful to both Professor H.G. Miller and Mr P.J.Wood who carefully reviewed a draft of this report and made manyuseful suggestions. Any mistakes, errors or wrong interpretations, andconclusions drawn or opinions offered are entirely those of the author.

About the author

Professor Julian Evans OBE was formerly Chief Research Officer (S) inthe British Forestry Commission. He is presently Professor of TropicalForestry in the T.H. Huxley School of Environment, Earth Sciences andEngineering of Imperial College, and also holds an honorary chair offorestry at the University of North Wales, Bangor. He is chair of theForestry Research Programme Advisory Committee for the Departmentfor International Development (UK) and vice-chairman of theCommonwealth Forestry Association.

Julian Evans has researched the question of narrow-sense sustainabilityof plantations for over 30 years, during which time he has amongstother things maintained a network of long-term productivity plots in theUsutu Forest, Swaziland, recording growth through three successiverotations. He has visited a great many tropical and temperate countriesin connection with forest plantations. He is author of seven books andabout 100 scientific and technical papers on forestry.

Department for International Development

We are also contributing to poverty elimination in middle incomecountries, and helping the transition countries in Central and EasternEurope to enable the widest number of people to benefit from theprocess of change.

As well as its headquarters in London and East Kilbride, DFID hasoffices in New Delhi, Bangkok, Nairobi, Harare, Pretoria, Dhaka, Suvaand Bridgetown. In other parts of the world, DFID works through staffbased in British Embassies and High Commissions.

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