© Institute of Chartered Foresters, 2009. All rights reserved. Forestry, Vol. 83, No. 1, 2010. doi:10.1093/forestry/cpp028For Permissions, please email: journals.permissions@oxfordjournals.org Advance Access publication date 3 December 2009

Introduction

In the majority of modern forest management systems, mature trees are harvested above the root collar, leav-ing stumps in the ground. Due to widespread concern about climatic change caused by anthropogenic emis-sions of greenhouse gases, increasing prices of fossil fuels and a desire to improve energy security, there is a growing interest in the exploitation of alternative sources of energy, including forest biomass ( Bjorheden, 2006 ). Commercial operations are now effectively uti-lizing the entire above-ground tree biomass to produce both timber and fuel ( Hakkila, 1989 ). A more recent development is interest in the potential for utilizing stump biomass as an additional source of renewable fuel ( Hakkila and Aarniala, 2004 ).

Historically, stumps were harvested both for fuel and for other uses including providing material for the horns of sledges, knee timber in ships and boats and ploughs ( Gayer, 1896 ). Stump wood was also used in Sweden for the production of tar between 1850 and 1950 ( Karlsson, 2007 ). Declining demand for woodfuel coupled with the industrial production of alternative materials such as steel led to reduced requirements for stump wood in central Europe towards the end of the nineteenth century.

Driven by concerns about a looming fi bre crisis (as-sociated with a shortfall forecast in small roundwood sup-plies) as well as the oil crisis (associated with increasing concerns about energy security) stump harvesting became the focus of a research programme in Scandinavia in the 1970s ( Hakkila, 2004 ; Bjorheden, 2006 ; Egnell et al. , 2007 ). The predicted fi bre crisis failed to materialize, while the cost of stump harvesting proved to be excessive ( Hakkila, 2004 ) and commercial stump harvesting for fi bre and fuel production was largely abandoned.

Since then demand for other forest fuels has continued to grow, leading to a re-evaluation of the potential of stumps as an energy source. Karjalainen et al. (2004) esti-mated that across Europe, there is the potential to source up to 9 million m 3 year � 1 of forest chips from stumps per annum, out of a total potential annual increment of 78 mil-lion m 3 year � 1 . One hectare can produce upwards of an additional 100 m 3 of woodfuel, equivalent to 100 MWh ha � 1 ( Flynn and Kumar, 2005 ), with some reports of 200 MWh ha � 1 ( Hakkila and Aarniala, 2004 ) and 250 MWh ha � 1 (Rolfsson, 2006 , cited in Eriksson and Gustavsson, 2008 : 897). The biomass available from the stump – root system offers the potential to gain an additional 20 per cent beyond that obtainable from the stem ( Richardson et al. , 2002 ).

Stump Harvesting for Bioenergy – A Review of the Environmental Impacts J. D. WALMSLEY * and D. L. GODBOLD

School of the Environment and Natural Resources, College of Natural Sciences, Bangor University, Bangor, Gwynedd LL57 2UW, UK * Corresponding author. E-mail: j.walmsley@univ.bangor.ac.uk

Summary

Stump harvesting signifi es an intensifi cation of forest management compared with conventional stem-only or above-ground biomass-only harvesting. There are many practical and perceived benefi ts of stump harvesting. These include (1) the production of woodfuel; (2) fossil fuel substitution; (3) additional revenue for forest owners; (4) improved site preparation and (5) potential reduction of Heterobasidion . However, evidence suggests that, in the absence of appropriate precautionary measures, stump harvesting will lead to many undesirable environmental impacts. These include (1) removal of soil organic matter inputs; (2) adverse impacts on forest soil carbon stores and greenhouse gas emissions; (3) increased soil erosion; (4) increased soil compaction; (5) depletion of soil nutrient stocks and changes in nutrient cycling; (6) unknown impacts on future productivity; (7) loss of valuable habitat for fungi, mosses, bryophytes and insects and (8) increase in non-forest vegetation and additional herbicide requirements. To minimize environmental impacts, best practice guidelines should be developed and communicated. Research to date has focussed on the impact of stump removal on the incidence of root diseases and has limited applicability to healthy uninfected stands. Additional research is required to understand fully the environmental impacts, particularly how stump harvesting infl uences the forest soil carbon balance and forest nutrient stocks.

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UPM Kymmene (a global forest industry group) com-menced commercial harvesting of stumps for fuel in Finland in 2001 ( Hakkila, 2004 ). Laitila (2006) estimated that stump removal took place on 6000 ha of forest in 2006, up from 2500 ha in 2005 ( Strandström, 2006 ) and 1000 ha in 2003 ( Hakkila, 2004 ). By 2010, Finland hopes to produce 5 million m 3 year � 1 of woodfuel direct from forests, 1.5 million m 3 of which is expected to come from stumps ( Hakkila, 2004 ). This will require stump removal across several tens of thousands of hectares ( Saarinen, 2006 ). Sub-sidies also exist for the removal of stumps from harvested sites in Finland, as this is thought to reduce the risk of root rot damaging the next rotation of trees ( Hakkila, 2004 ; Flynn and Kumar, 2005 ). Stump removal is also practised across parts of the pacifi c coast of the US and Canada ( Thies and Nelson, 1988 ; Thies and Westlind, 2005 ; Hope, 2007 ) and in eastern England, the primary motivation being control of insect pests and fungal diseases such as the pine weevil (e.g. Hylobius abietis (L.)) and root rot (e.g. Heterobasidion annosum (Fr.) Bref.) ( Greig, 1984 ).

The current practice of stump harvesting for fuel is de-scribed below and is based on published literature ( Hakkila, 2004 , Egnell et al. , 2007 ) and personal communications (J. Gallacher, personal communication). Stump removal should take place as soon as possible after clearfelling to ensure that brash (foliage and branch material left after harvesting of stemwood) is in good a condition to be used on machine traffi cking routes to support stump harvesting and forwarding machinery. Leaving the stumps for a year to allow the fi ne roots to die, in an attempt to reduce fi ne root removal and decrease the volume of soil removed, is not effective. On the contrary, a delay of 1 year greatly de-creases the quality of the brash mats and results in greater site disturbance from harvesting machinery. Stumps are removed from the soil using an excavator equipped with a stump removal head. The excavator operator cuts the stump into smaller pieces to break off fi ne roots before ex-tracting the stump and shaking it to remove excess soil. Stump pieces are then transported to a landing area using standard forwarders modifi ed for stump haulage by the ad-dition of extra pins or side bars to contain the load. Storage at the landing for up to a year allows rain and snowfall to wash off adhering soil and stones, which are fuel contami-nants, and has the additional benefi t of allowing the mois-ture content of the stump wood to decline substantially. Stumps are then transported and crushed into wood chips using stationary or mobile crushers.

Stumps as a fuel source are considered to have a number of benefi ts. Once dried, stump wood has a low moisture content, making it ideal for winter use when other forest fuels are moist ( Gayer, 1896 ) and as a control fuel ( Hak-kila and Aarniala, 2004 ). The low moisture content also means that stump wood has a long storage life, in con-trast to other forest fuels which may begin to decompose when stored ( Hakkila and Aarniala, 2004 ). Stump chip has a higher calorifi c value ( Eriksson and Gustavsson, 2008 ) and is more homogeneous than other sources of forest chip ( Ala-fossi et al. , 2007 ). In terms of general forest manage-ment, stump harvesting can be seen to have three distinct

benefi cial aspects – the creation of a new source of forest fuel (and therefore additional income), a management tool for tackling various pests and diseases, and, when com-bined with mounding, effi ciency gains in site preparation operations (e.g. Saarinen, 2006 ).

In forestry practice, the stump and coarse root system is often poorly defi ned. Hakkila and Parikka (2002) pro-vide relevant defi nitions for this study: (1) Stump – the unutilized above-ground biomass below the bottom of a merchantable stem, including any underground pro-jection such as the taproot − lateral roots are excluded. (2) Stump – root system – the stump below the merchant-able stem, including all roots. (3) Root – includes all side or lateral roots, but not the tap root. This defi nition is important, as the tree parts removed in the harvesting operation will infl uence the environmental impacts. Un-like forest residue harvesting/whole-tree harvesting, there has been little research into the environmental effects of stump removal. As data on the environmental impact of stump harvesting are limited, we have included relevant data from studies of above-ground biomass which remove more than the bole, such as whole-tree harvesting.

This review considers the environmental impacts of stump harvesting on soil condition, watercourses, biodi-versity, pest and disease control and future productivity.

Soil condition

Maintenance of soil productivity is the product of a number of key soil functions: (1) physical (e.g. aera-tion and porosity); (2) chemical (e.g. stocks of nutri-ents) and (3) biological properties (e.g. the ability of a soil to provide a habitat to populations of soil microbes and invertebrates). Most modern silvicultural operations and site preparation methods already cause substantial disturbance to the soil ( Schmidt et al. , 1996 ; Stiven, 1997 ; Worrell and Hampson, 1997 ; Eisenbies et al. , 2005 ) which will lead to subsequent changes to soil prop-erties and functions. This must be taken into account in trying to distinguish the additional effects of stump harvesting.

Nutrient reserves

The direct removal of nutrients with biomass is a function of tissue nutrient concentration and the volume of biomass removed. The concentration of nutrients in stump wood, bark and coarse and fi ne roots (less than 2 mm diameter) differs greatly ( Hakkila, 1989 ; Weatherall et al. , 2006 ). In stumps, analogous to stems, the amount of nutrients re-moved depends upon the relative amount of bark to stem wood. Egnell et al. (2007) suggest that stump removal is unlikely to cause any serious depletion of soil nutrient re-serves due to the relatively low concentrations of nutrients present in stump wood. Although this may be the case with mature forest crops, Hakkila (2004) and Saarinen (2006) suggest that smaller stumps (less than 15 cm diameter), which consist of a higher proportion of bark (and hence

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STUMP HARVESTING FOR BIOENERGY 19

nutrients) should not be harvested, as their removal is inef-fi cient and uneconomic due to their small root biomass. In the root systems of coniferous trees grown in the UK, on average the stump is 36 per cent (range of 22 per cent for Corsican Pine ( Pinus nigra J.F. Arnold var. maritima (Aiton) Melville) to 41 per cent for Sitka spruce ( Picea sitchensis (Bong.) Carr)) of the total tree biomass ( Levy et al. , 2004 ). There can also be considerable variation within the same species between different sites and over time within a rotation. Tobin and Nieuwenhuis (2007) re-ported the root : shoot ratio for Sitka spruce growing on six different sites to range from 22 to 47 per cent, with the root : shoot ratio generally decreasing with age. In terms of the wood yield from stump harvesting, greater quantities of below-ground biomass are obviously desirable. Produc-tivity will clearly also depend on the structure and archi-tecture of the root system. But given that the root : shoot ratio is a good indicator of soil nutrient status, with a high root : shoot ratio occurring under nutrient limitations on poor soils ( Brouwer, 1962 ), it is on these sites where stump harvesting may have the gravest consequences for soil nu-trient reserves and mineralization rates.

Coarse and structural roots make up by far the greatest proportion of the total below-ground biomass. For a sam-ple of ten 19-year-old Sitka spruce trees with a diameter at breast height range of 12 – 29 cm growing on an afforested peatland, Green et al. (2007) found that the coarse and structural roots (more than 10 mm diameter) comprised 88 per cent and the fi ne roots 12 per cent (less than 10 mm di-ameter) of the below-ground biomass. Despite comprising only a small amount of the total below-ground biomass, fi ne roots play an important role in the nutrition of the subsequent trees. Removing large quantities of roots will lead to greater direct nutrient removals from forests which, if left in situ , are proven to contribute directly and almost immediately to the nutrition of the next trees. For example, Weatherall et al. (2006) performed a pot trial experiment, where they used a stable isotope labelling technique to trace nutrient release from decomposing roots and subsequent uptake into newly planted Sitka spruce seedlings. They estimated that decomposing roots contributed (up to) 3 – 10 per cent of the N, 14 – 47 per cent of the K, 4 – 37 per cent of the Mg and 25 – 85 per cent of the Ca subsequently taken up by the new trees. Tree root architecture will also have a signifi cant impact upon the degree to which stump removal affects soil nutrient reserves. Although there is currently no supporting data, we suggest that shallow plate rooting will increase the amount of fi ne roots removed during stump removal compared with deeper rooting tree species. This will be exacerbated by the fact that shallow rooting is com-mon on poorly drained and nutrient-poor soils ( Fraser and Gardiner, 1967 ; Puhe, 1994 ).

The physical process of harvesting stumps may also in-directly cause depletion of soil nutrient reserves through changes in soil biological processes, although this is dif-fi cult to distinguish from the direct effects of nutrient removal. Removal of stumps and fi ne roots has led to changes in the nitrogen cycle ( Staaf and Olsson, 1994 ), potentially by reducing the supply of new organic matter

and associated ammonifi cation/nitrifi cation. Zabowski et al. (2008) reported that for fi ve Douglas-fi r ( Pseudotsuga menziesii (Mirb.) Franco) stands across a range of condi-tions in the states of Oregon and Washington, stump removal led to a mean decline in mineral soil N of 20 per cent over 22 – 29 years compared with areas where stumps had been left in situ . The effects of intensive harvesting can also lead to adverse changes in soil ecology (e.g. Bengtsson et al. , 1997 , 1998 ; Battigelli et al. , 2004 – see section on ‘ Flora ’ below) which could lead to further changes (reduc-tions) in forest nutrient cycling.

Site preparation frequently involves the mixing of min-eral soil with the forest fl oor and humic soil layers. In Swe-den, ~ 70 per cent of regeneration sites have been subject to mechanical site preparation methods over the past 40 years, disc trenching and patch scarifi cation/mounding being the most common methods ( Hallsby and Örlander, 2004 ). Peltola (2007) reports that in Finland, mechanical site preparation takes place on 96 per cent of harvested sites, by mounding (39 per cent of area), soil harrowing (36 per cent) or scarifi cation (21 per cent). Such approaches are thought to improve seedling nutrient supplies and raise soil temperatures during the growing period and improve the soil structure in the seedling rooting zone, when compared with planting in mineral soil patches, pits and furrows. Mechanical site preparation activities may, however, lead to leaching of soil nutrients ( Burgess et al. , 1995 ; MacKenzie et al. , 2005 ) and a potential decline in long-term site pro-ductivity ( Örlander et al. , 1990 ). For example, Schmidt et al. (1996) found that for 15 months following mechanical site preparation, the surface mineral soil (0 – 7 cm) of treated areas had either reduced or unchanged levels of total nitro-gen and carbon, available phosphorus and mineralizable nitrogen, compared with areas which had undergone har-vesting but no site preparation.

Hope (2007) also observed adverse changes in soil chem-istry following stump removal on three sandy loam sites in southern interior British Columbia. The results clearly demonstrate the deleterious effect on soil nutrient stocks of any harvesting treatment, with or without mechanical treatment, over a 10-year period ( Table 1 ). In addition, re-sults indicate that stump removal followed by scarifi cation led to signifi cant decreases in soil stocks of total carbon (by up to 53 per cent), nitrogen (by up to 60 per cent for total nitrogen and up to 70 per cent for mineralizable nitrogen), total sulphur (by up to 55 per cent) and available phospho-rus (by up to 50 per cent) in the forest fl oor layer after both 1 and 10 years ( Table 1 ), compared with other treatments and the control. Forest fl oor depth (which in this study in-cluded a Mor humic layer) was ~ 20 and 50 per cent lower following the stump removal and scarifi cation treatment after 1 and 10 years, respectively, compared with the con-trol treatment. Hope (2007) attributed the lowering of soil nutrient stocks to reduced forest fl oor depth, arising due to elevated rates of decomposition of surface organic matter following stump removal, rather than the direct removal of organic material in the form of stumps and roots, although he offered no evidence to substantiate this claim. Changes in the forest fl oor contrast notably with the mineral horizon.

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Table 1: Effects of stump removal treatment on forest fl oor and 0 – 20 cm mineral soil nutrient contents 1 and 10 years after treatment (redrawn from Hope, 2007 )

Treatment Total C (kg ha � 1 ) Total N (kg ha � 1 ) Total S (kg ha � 1 ) P * (kg ha � 1 ) N † (kg ha � 1 ) pH ‡

Forest fl oor Year 1 NT 49 130 (±8316) 1090 (±170) 110 (±21) 25.0 (±4.5) 34.5 (±8.5) 5.1 (±0.1)

O 47 040 (±7637) 1030 (±170) 98 (±18) 27.7 (±3.7) 31.2 (±7.5) 5.0 (±0.1) O+ 46 127 (±7319) 970 (±130) 92 (±14) 30.5 (±4.4) 26.5 (±5.5) 5.0 (± 0.1) S 24 660 (±4480) 490 (±90) 50 (±10) 18.8 (±3.2) 11.8 (±2.5) 5.1 (±0.1)

Year 10 NT 19 170 (±1600) 420 (±30) 44 (±4) 10 (±1.5) 15 (±2.0) 4.9 (±0.1) O 19 050 (±2210) 360 (±35) 40 (±5) 9 (±1.5) 11.5 (±1.5) 4.9 (±0.1) O+ 16 460 (±2130) 330 (±30) 33 (±3) 9 (±1.5) 10 (±1.0) 5.0 (±0.1) S 8980 (±1730) 170 (±25) 21 (±5) 5 (±1) 4.5 (±1.0) 4.9 (±0.1)

P -value Treatment Year 1 <0.01 <0.01 <0.01 0.09 <0.01 0.67

Year 10 <0.01 <0.01 <0.01 0.02 <0.01 0.88 Year 0.19 0.15 0.20 0.03 0.46 0.45 Mineral soil Year 1 NT 32 330 (±1800) 1530 (±75) 280 (±50) 110 (±25) 32 (±4) 5.0 (±0.1)

O 35 000 (±1910) 1560 (±60) 500 (±140) 125 (±25) 24 (±2) 4.9 (±0.1) O+ 35 390 (± 1,240) 1580 (± 65) 320 (± 35) 120 (± 20) 33 (± 4) 5.1 (± 0.1) S 34 980 (±1900) 1370 (±65) 270 (±50) 120 (±25) 21 (±3) 4.9 (±0.1)

Year 10 NT 29 390 (±1020) 1110 (±35) 230 (±35) 90 (±10) 30 (±2) 4.9 (±0.1) O 31 750 (±1420) 1190 (±45) 280 (±60) 85 (±10) 31 (±2) 5.0 (±0.1) O+ 34 680 (±1490) 1220 (±36) 210 (±20) 90 (±10) 36 (±3) 4.9 (±0.1) S 32 870 (±1760) 1190 (±50) 200 (±30) 90 (±10) 30 (±3) 4.9 (±0.1)

P -value Treatment Year 1 0.47 0.13 0.02 0.91 0.28 0.57

Year 10 0.05 0.26 0.39 0.82 0.28 0.57 Year <0.01 0.05 0.30 0.28 0.16 0.11

Results presented are mean values (±SE) for three sites in southern interior British Columbia. NT = no mechanical treatment; O = operational stump removal treatment, stumps left on site; O+ = stumps pulled then carefully removed off site; S = scarifi cation after stump removal. * Available P – Bray’s extractant. † Mineralizable N – anaerobic incubation. ‡ pH measured in water (forest fl oor) or 0.1 M CaCl 2 (mineral soil). [P-values are shown in italics].

The stump removal off-site treatment had the highest levels of total carbon in the mineral horizon after both 1 and 10 years (+ 9 and + 18 per cent, respectively, compared with the control). The stump harvesting treatments were all as-sociated with higher mineral horizon carbon stocks than the control. In addition, there was certainly no evidence to suggest that any of the stump harvesting treatments had reduced the nutrient reserves of the mineral horizon or led to any signifi cant changes in soil pH in either soil horizon.

Contrary to the drastic reductions in the nutrient stocks of the forest fl oor following intensive stump har-vesting reported by Hope (2007) , other researchers have found that the effects of site preparation (as opposed to stump removal per se) on soil nutrients to be generally short lived ( Schmidt et al. , 1996 – 15 months; Johnson et al. , 2002 – 15/16 years; Simard et al. , 2003 – 9 years). Örlander et al. (1996) reviewed the long-term impacts of soil scarifi cation on sandy soils planted with Scots pine ( Pinus sylvestris L.). Scarifi cation had caused close to 100 per cent disturbance of the soil surface and consistently led to declines of total C and N in soil, of up to 40 and 30 per cent, respectively, over periods of up to 70 years. Concen-

trations of mineral elements were found to be the same, or lower, following scarifi cation. On such nutrient-poor sites where P. sylvestris is cultivated, Örlander et al. (1996) con-tend that reductions in total soil nutrients do not present a problem, providing that nutrient turnover rates are ade-quate (i.e. in terms of growth, the rate at which nutrients are cycled through an ecosystem is more important than total quantities of nutrients). Clearly, the effects of site prepara-tion, of which stump removal could be considered a com-ponent, will vary with the intensity of treatment ( Weber et al. , 1985 ; Binkley, 1986 ) and accordingly infl uence changes in nutrient turnover rates. Effects may also be specifi c to individual sites and the specifi c stump removal techniques employed ( Miller et al. , 1996 ). There have been few studies to date other than Hope (2007) and Zabowski et al. (2008) which have attempted to evaluate the effect of stump harvesting upon soil nutrient reserves. Nevertheless, their results and those from other related studies suggest that where stump harvesting takes place on nutrient-poor soils or the process of removal also leads to large volumes of fi ne root removal, the effects on soil condition will be negative.

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Acidifi cation

Coniferous tree species are well known for their acidify-ing effects on forest soils, which is partly due to the lit-ter they return to the soil and partly due to the effi ciency of their crowns at intercepting acidic compounds from the atmosphere ( Miller et al. , 1991 ). This interception is an in-fl uential factor causing the acidifi cation of some forested catchments ( Adamson et al. , 1987 ). Acidifi cation of forest soils is of concern as it reduces soil fertility and leads to the mobilization of aluminium ( Grieve, 2001 ). Excessive levels of aluminium can disrupt essential plant functions including cell division in roots; root respiration; nitrogen metabo-lism; cell enzyme function and uptake of calcium, magne-sium, phosphorus, boron and water ( Tisdale et al. , 1985 ; Godbold et al. , 1988 ; Ohno et al. , 1988 ; Rowell, 1988 ).

Beyond the study of Hope (2007) , which found no evi-dence that stump harvesting had caused pH changes in ei-ther the forest fl oor or mineral soil horizon after 10 years ( Table 1 ), we have only found one experiment which has in-vestigated specifi cally the effect of stump harvesting on soil acidifi cation. Staaf and Olsson (1994) undertook a study in a Norway spruce ( Picea abies (L) Karst) forest in southwest Sweden. Soil chemistry changes were monitored following conventional harvesting, whole-tree harvesting and whole-tree plus stump harvesting. In plots where stumps had been removed, the concentrations of ammonium ions ( +

4NH ) increased markedly during the fi rst-year post-harvesting. High levels of +

4NH persisted for another year, after which nitrate ( 3NO ) concentrations remained elevated and soil water pH fell – indicative of enhanced nitrifi cation, nitrate leaching and soil acidifi cation. Acidifi cation effects asso-ciated with all treatments appeared to be greatest over the short term, with soil solution pH, +

4NH and 3NO returning to pre-treatment levels after 5 years. Other relevant studies have focussed on the removal of all above-ground biomass and provide further insight into the fi ndings of the Staaf and Olsson (1994) study. Soil acidifi cation coupled with declines in base saturation and cation exchange capacity have been observed following whole-tree harvesting com-pared with stem-only conventional harvesting, with similar results obtained across a number of different sites ( Nykvist and Rosen, 1985 ; Staaf and Olsson, 1991 ; Dahlgren and Driscoll, 1994 ; Olsson et al. , 1996 ; Rosenberg and Jacobson, 2004 ). Meanwhile, Olsson and Staaf (1995) consider the acidifying and nutrient depleting effects of residue harvest-ing represent a far greater threat to forest sustainability than the loss of soil organic matter (SOM) and suggest that such effects can be adequately addressed by returning wood ash to such sites.

Historically, many forested areas in central Europe were subject to intensive above-ground nutrient remov-als through the process of litter raking. Such activities are reported to have exacerbated soil acidifi cation and degradation, essentially leading to losses in productiv-ity ( Binkley, 1986 ; Glatzel 1991 ; Prietzel et al. , 1997 ; Hüttl and Schneider, 1998 ; Farrell et al. , 2000 ; Hunter and Schuck, 2002 ). The intensifi cation of forest harvest-ing through stump removal and associated removal of

below-ground nutrients would therefore be expected to lead to similar, if not greater, implications for soil acidi-fi cation and nutrition.

There is currently little data available to assess the ef-fect of stump removal on different soil types; however, the potential for acidifi cation will be a function of the buff-ering capacity of the soil. Greater effects can be expected on poorly buffered podzols compared with well-buffered brown earths. So despite the lack of specifi c research into how stump harvesting might infl uence soil acidifi cation, there are existing precedents to suggest that intensive stump removal will be unacceptable in those areas sensitive to acidifi cation and may exacerbate it in other areas.

Soil carbon stores

For assessment of the impact of forest management, such as stump harvesting, on soil carbon stores, it is important to consider that SOM exists as both labile and stable forms. For modelling and conceptual purposes, SOM is divided into either two pools (labile and stable) or three pools (labile, intermediate and stabile) ( Davidson and Janssens, 2006 ; Yang et al. , 2007 ). Whereas labile pools have a turn-over time of weeks or months, stabile pools have turnover times of decades or longer ( Davidson and Janssens, 2006 ). Stabile pools represent the long-term carbon sequestration pools. Decomposition of the labile pool by soil microbes is measured as heterotrophic soil respiration and is a major carbon fl ux in ecosystems ( Subke et al. , 2006 ). Disturbance can enhance the turnover of both labile and stabile organic matter leading to increase heterotrophic respiration and loss of carbon from the SOM pool ( Kuzyakov et al. , 2000 ; Jandl et al. , 2007 ; Diochon and Kellman, 2008 ; Smith, 2008 ).

That stump harvesting leads to large soil carbon emis-sions is simultaneously the subject both of most uncer-tainty, and concern, among the scientifi c community. The basis for concern is dependent on the notion that large-scale disruption of forest soils during stump removal may release CO 2 (from the decomposition of SOM) into the atmosphere and also cause elevated leaching of dissolved organic carbon (DOC). CO 2 emissions may potentially off-set any carbon savings that might be accomplished through the substitution of fossil fuels with stumpwood. In the UK, forests and woodlands contain ~ 150 million tonnes of car-bon and every year they remove ~ 4 million tonnes of car-bon from the atmosphere, equivalent to offsetting less than 3 per cent of the UK ’ s annual carbon dioxide emissions ( Forestry Commission, 2008 ). Soils contain much greater quantities (circa 4.5 billion tonnes of soil carbon in the top 1 m), representing by far the greatest carbon reservoir in the UK, around half of which is located in peats and other organic soils, located mainly in Scotland ( Bradley et al. , 2005 ). Given the size of the soil carbon store, it is clear why there are major concerns that stump harvesting may threaten the stability of this reservoir and exacerbate, rather than reduce, atmospheric carbon emissions.

In a recent review by Gough et al. (2008) , evidence was collated that common silvicultural practices such as

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forest thinning can enhance soil C pools ( Selig et al. , 2008 ). However, tillage prior to planting may increase forest soil C emissions ( Gough et al. , 2005 ) and lower the soil C pool. For stump removal, there is currently a lack of refereed, published literature on this topic. According to the fi gures provided by Hope (2007) ( Table 1 ), after 10 years at three different sites, the forest fl oor contained, on average, 19.17 t ha � 1 of carbon 10 years following no mechanical treat-ment, compared with 8.98 t ha � 1 of carbon following the most intensive stump removal technique – equivalent to a loss of over 1 t C ha � 1 year � 1 . Assuming that this differ-ence is due to the release of CO 2 into the atmosphere, this is equivalent to an additional 3.76 t CO 2 ha � 1 year � 1 , in addition to the effects of harvesting alone (19.17 – 8.98 = 10.19 t C ha � 1 difference. Molecular mass of C = 12, molecular mass of CO 2 = 44. Therefore, 10.19/12 × 44 = t CO 2 ha � 1 = 37.36/10 years = 3.76 t CO 2 ha � 1 year � 1 ).

Johnson (1992) and Johnson and Curtis (2001) under-took reviews of the literature (from studies lasting between 1 year and 83 years) concerning the impacts of various forest management activities on soil carbon. Both reviews found that forest harvesting generally did not lead to reduc-tions in soil carbon, with most studies showing no signifi -cant change (±10 per cent). Stem-only harvest was found to increase soil carbon (+18 per cent) while whole-tree harvesting led to decreases ( − 6 per cent), although this was restricted to coniferous species (Johnson and Curtis, 2001). Large net losses occurred in only a small minority of cases. However, the effects of forest harvesting combined with cultivation were found to be much more severe. Such treat-ments were found to result in large (up to 50 per cent) losses in soil carbon in nearly all cases ( Johnson, 1992 ).

A further review was carried out by Jandl et al. (2007) to evaluate the impacts of specifi c forest management strat-egies on long-term carbon sequestration in soils. Their fi ndings echoed those of Johnson (1992) and Johnson and Curtis (2002), although they also pointed out that there is a distinct lack of research regarding management impacts on stable carbon pools in mineral soils. Furthermore, unin-tended losses of soil carbon were found to be substantially reduced where efforts are made to minimize disturbance of forest soils. Their review also found that the mixed spe-cies forests can reduce high rates of SOM decomposition (and associated CO 2 emissions), thereby questioning the value of single-species forests at a time when maximizing forest carbon storage is of such importance. As already discussed, Hope (2007) found that stump removal and scarifi cation signifi cantly reduced soil carbon stores after 10 years, reportedly due to elevated rates of organic matter decomposition. In a separate study, Burgess et al. (1995) monitored the effects of various soil modifi cation techniques, including blade scarifi cation, on the soil nutri-ent and organic carbon content of an orthic humo-ferric podzol in the Great Lakes – St Lawrence forest region of Canada. Scarifi cation was found to reduce soil carbon and nutrient capital (N, P and K) by two- to threefold after 7 years in the forest fl oor layer. In this study, the forest fl oor was defi ned as all organic matter above the surface of the mineral soil. Örlander et al. (1996) also observed

declines in total soil carbon following soil scarifi cation of sandy sites after 70 years.

Johnson et al. (2002) examined the effects of harvest in-tensity: stemwood only, whole-tree harvest and complete tree harvest (all above-ground biomass plus stumps) on four mixed deciduous forest sites in the southeastern US. On one site, soil carbon levels increased over a 16-year pe-riod in all treatments, whereas there was little evidence to suggest any interaction between harvesting treatment and soil carbon levels at the other sites. However, the results show that other factors, such as understorey vegetation and competition and soil initial nutrient status, have clear impacts upon nutrient cycling and turnover rates and on some sites had more of an effect on soil carbon stores than the treatments. Further work by Johnson et al. (2007) re-vealed wide variations in soil carbon across both time and space, highlighting the practical diffi culties of accurately monitoring soil carbon levels.

By modelling tree growth, litter production and decom-position in Swedish forest soils, Ågren and Hyvonen (2003) found that residue harvesting reduced the soil carbon store by 59 Tg (1 Tg = 10 12 g or 1 million tonnes) over 150 years, equivalent to 0.4 Tg year � 1 . This is equivalent to the loss of 3.5 per cent of Sweden’s total forest soil carbon store, which is 1700 Tg. Their model also predicts that in-creases in temperatures will increase carbon losses from the soil to 0.9 Tg year � 1 . In the face of efforts to reduce an-thropogenic carbon emissions, this is clearly an undesir-able outcome. Although this model did not include the effects of stump removal on soil carbon stores, we would anticipate the practice to further exacerbate these fi ndings for residue harvesting, with further decreases in soil carbon stores.

Given the fi ndings of a number of studies and reviews considered above, there is strong evidence to suggest that mechanical site preparation techniques (which may be considered comparable to stump harvesting) may pose a substantial threat to the conservation of forest soil carbon stores. The risk of environmental damage, however, is very much dependent upon the carbon status of the soil, partic-ularly the amount of SOM, and thus will vary signifi cantly between site types.

Forest ecosystem carbon balances

Two recent life cycle assessment (LCA) type studies have been published which attempt to evaluate the net carbon emissions of various forest utilization scenarios. Eriksson et al. (2007) analysed net carbon emissions of various sce-narios involving P. abies . These scenarios consisted of (1) forest management strategies (traditional, intensive man-agement and intensive + fertilizer application); (2) residue management regimes (no removal, removal and removal with stumps); (3) product uses (construction material and biofuel) and (4) reference fossil fuels (coal and gas). The study found the greatest reduction in net carbon emis-sions to occur when the forest was fertilized, residues and stumps were harvested, wood was used as a construction

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material and the reference fossil fuel was coal. This result was mainly as this approach increased the amount of forest products available for substitution of non-forest products and fuels. The lowest reduction in net carbon emissions happens when traditional forest management was used, residues remained on site and harvested biomass was used as fuel to replace natural gas. That LCA model concluded that the greatest reduction in net carbon emissions corre-sponds with the stump harvesting treatment is perhaps sur-prising given the argument presented above – but refl ects a limitation of the method used by Eriksson et al. (2007) . The model assumed that decomposition of soil organic car-bon occurred at the same rate for all forest management regimes studied and is a refl ection of the severe lack of em-pirical research in this area rather than an omission by the authors. At the same time, the result of the Eriksson et al. (2007) study highlights the potential for stump harvesting to assist in the mitigation of climate change, particularly if soil carbon losses can be avoided.

In a separate exercise, Eriksson and Gustavsson (2008) conducted a similar LCA study looking at the net energy balance and carbon emissions related to the harvesting of stumps and small roundwood. They compared the energy inputs and carbon emissions from stump harvesting (ex-cavating with machinery, forwarding, hauling and chip-ping) against energy outputs and carbon savings through displacement of fossil fuels. The primary energy input : en-ergy output ratio for stumps was found to be 0.024, or 2.4 per cent, broadly comparable with forest residue bundles and small roundwood, confi rming that the energy inputs required to produce forest biomass fuel are very small when compared with the energy that can be gained from it. According to their results, production (e.g. harvesting, forwarding and processing) of stumpwood fuel gave rise to greater carbon emissions than other forest fuels (such as small roundwood and forest residue bundles) (in terms of kg C MWh � 1 ). However, this was more than offset by its relatively high energy content (3.8 MWh per tonne, com-pared with 2.3 MWh per tonne for forest residues) and ac-cordingly led to greater carbon reductions in terms of kg C avoided per hectare than other forest fuels (compared with coal). There are of course losses when converting any fuel into energy, so a fi gure of 2.4 per cent is never achievable in reality. However, other studies have shown that if forest biomass is used as fuel in combined heat and power plants, the energy input : output ratio can still remain high, at just under 14 per cent ( Elsayed et al. , 2003 ).

Two recent reports on the environmental impacts of stump harvesting ( Berglund and Åström, 2007 ; Egnell et al. , 2007 ) explicitly state that they do not attempt to ad-dress the issue of soil carbon emissions from stump har-vesting, simultaneously highlighting the current lack of empirical research into this subject. Indeed, Eriksson et al. (2007) also recognize this lack of understanding as a major limitation of their study, while Eriksson and Gustavsson (2008) are transparent in their ‘ system boundary ’ descrip-tion that their study does not attempt to include any poten-tial carbon emissions from soil that may arise from stump harvesting.

Soil physical structure

There is a general consensus in the literature that stump har-vesting may lead to further disruption of the physical struc-ture of the soil both due to increased traffi cking and excessive disturbance to the soil during stump removal. Certainly, this could become an even greater risk to site sustainabil-ity if stump removal is not coordinated with other harvest-ing activities such as the collection of residues. Hope (2007) suggests that severe soil disturbance associated with certain silvicultural treatments may in fact lead to losses in pro-ductivity that outweigh the initial improvements in growth which might be gained through improved conditions for restocking or reductions in the prevalence of root disease.

As discussed above, stump and root architecture will have a substantial infl uence on the nature and severity of soil disturbance caused by stump removal. Stump and root architecture vary widely between species and are signifi -cantly infl uenced by individual site factors such as slope, soil type, soil moisture regime, drainage, wind exposure, underlying geology and planting density ( Fraser and Gardiner, 1967 ; Nicoll and Ray, 1996 ; Levy et al. , 2004 ). Slopes, for example, cause trees to concentrate root de-velopment upslope ( Nicoll et al. , 2006 ), while increasing planting density tends to encourage trees to develop deeper roots and smaller root plates (e.g. Yanai et al. , 2006 ).

The work of Fraser and Gardiner (1967) focussed on rooting and stability of Sitka spruce given widespread prob-lems of windthrow in many areas of Wales, Northern Eng-land and Scotland. There were clear differences between the stump and root structures of Sitka spruce across sites. Trees growing on deep, well-drained brown earths were deep rooting, with relatively small root plates and a fairly com-pact root ball. This was in stark contrast to trees growing on peats, peaty podzols, peaty gleys and other wet soils. Root-ing on such sites was frequently found to be limited to the upper few inches of soil, with extensive lateral roots, large root plates and little evidence of a distinct root ball. The ex-tent of soil disruption due to the physical process of stump removal will accordingly vary between such soil types. The issue of stump and root architecture is critically important in the context of stump harvesting. Hakkila (1976, cited in Hakkila, 2004 ) provides conceptual diagrams showing the distribution of biomass in stumps and roots of Scots pine and Norway spruce ( Figure 1 ), highlighting the im-portance of species in determining stump and root archi-tecture. Both Norway and Sitka spruce form plate root systems with sinker roots ( Ray and Nichol, 1998 ; Puhe, 2003 ). As discussed above, on soils with both chemical and mechanical restriction such as subsoil acidity, a high water table and soil compaction, the development of sinker roots is restricted, resulting in the development of fl at plate root systems ( Ray and Nichol, 1998 ; Puhe, 2003 ). In contrast most pines typically form a tap root system on many soil types ( Tobin et al. , 2007 ), and this rooting pattern is less modifi ed by soil conditions than plate root systems. This has implications for stump removal, i.e. in general the re-moval of plate root systems will leave wide, shallow holes whereas removal of tap root systems will leave narrower but deeper holes.

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In simple terms, if adequate guidelines and controls are not in place and adhered to, stump removal on wet sites will likely lead to widespread (up to 100 per cent) distur-bance of the soil surface, leaving shallow depressions in the soil. In addition, wet sites are more sensitive to the im-pacts of machine traffi cking, potentially leading to a con-fl ict in harvesting objectives and site sustainability. Stump removal on more fertile, better draining sites may lead to the disturbance of much deeper soil layers but potentially associated with a reduction in the amount of surface soil disturbance (depending on planting density). These fac-tors should be taken into account when considering stump harvesting, as they will have implications for restocking, drainage and future productivity.

Further investigation into the links between stump and root architecture and the aforementioned factors is clearly required to develop further understanding in this area. As a potential consequence of soil disturbance and machine traffi cking related to stump removal, soil compaction and shear is an issue of particular concern. Polomski and Kuhn (2001) consider the damage caused by soil compaction (among other anthropogenic impacts) to have a far greater negative effect on tree health and root stability than acidi-fi cation or air pollution. As with other aspects of stump harvesting, there has only been a small amount of research which has specifi cally examined the impact of stump re-moval on the physical structure of soil. The only studies we have found reporting the effects of stump removal on soil structure are from North America ( Table 2 ). In general stump removal has led to increases in soil bulk density, but in general, these effects do not appear to persist and are not as severe as the impacts of machine traffi cking. Hope (2007) argues that stump harvesting, although appearing to lead to excessive disruption of soils, if carried out care-fully, will only lead to the disruption of surface soil layers.

Intensive forest harvesting methods, such as the removal of residues following harvesting or the removal of stumps, will increase the risk of soil erosion. Erosion problems re-lated to forestry have been substantially reduced in many countries in recent decades, such as the UK ( Carling et al. , 2001 ), the US ( Aust and Blinn, 2004 ; McBroom et al. , 2008 ) and Finland ( Rusanen et al. , 2004 ). This is the re-sult of much research and the implementation of effective guidelines ( Carling et al. , 2001 ). However, harvesting op-erations can still lead to highly localized, severe erosion events, occurring, for example, when a heavy rainfall event follows shortly after harvesting. A key approach to reduc-ing soil damage and erosion on harvesting sites in the UK is to utilize brash on traffi cking paths for harvesters and forwarders, particularly in wet conditions and on soft soils ( Moffat et al. , 2006 ). A potential confl ict between residue harvesting, stump extraction and minimizing ground dam-age may arise particularly on soft, wet soils. There is a strong case under such circumstances to consider retaining all residues for use as brash mats for machine traffi cking and leaving stumps in the ground.

A study by Strandström (2006) gives a useful insight into the physical effects of stump removal on forest sites. Strandström (2006) surveyed soil disruption on 10 sites of varying nutrient status and soil type, on which stump removal had taken place, in Finland. He reported using vi-sual estimates that on average 78 per cent of the soil area was disturbed, with a lower limit of 67 per cent and a max-imum disturbance level of 94 per cent. He also reported that these fi gures contrast strongly with those reported for conventional soil preparation techniques, where soil area disturbed is commonly 20 – 30 per cent (up to a maximum of 50 per cent).

In a separate study evaluating the effect of machine traffi cking, soil bulk density was measured following

Figure 1. Distribution of biomass in stumps and roots of Scots pine and Norway spruce (Hakkila, 1976, adapted from Hakkila, 2004 : 25).

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STUMP HARVESTING FOR BIOENERGY 25

Tab

le 2

: Sum

mar

y of

stu

dies

whi

ch h

ave

inve

stig

ated

the

impa

ct o

f st

ump

rem

oval

on

soil

stru

ctur

e

Stud

y au

thor

sSo

il ty

pe a

nd s

ite

desc

ript

ion

Tre

atm

ent

Impa

ct o

n so

il st

ruct

ure

Oth

er c

omm

ents

Smit

h an

d W

ass

(199

4)

Cal

care

ous

loam

y so

il,

D

ougl

as-fi

r a

nd L

odge

pole

pine

, int

erio

r B

riti

sh

C

olum

bia,

Can

ada

Stum

p up

root

ing

Bul

k de

nsit

y in

dep

osit

s cr

eate

d by

stum

ps s

imila

r to

und

istu

rbed

soi

l,

wit

h 50

% in

crea

se in

‘ pen

etra

bilit

y ’

at

dep

th o

f 15

– 20

cm

Tra

ffi c

king

rou

tes

23%

incr

ease

in b

ulk

dens

ity

of u

pper

20 c

m o

f so

il an

d 68

% r

educ

tion

in

pe

netr

abili

ty (

not

spec

ifi ed

)

Thi

es e

t al

. (19

94)

Silt

y cl

ay lo

am, m

ixed

tre

e

spec

ies,

Cas

cade

Ran

ge,

O

rego

n, U

SA

Stum

p re

mov

alIn

crea

sed

soil

bulk

den

sity

by

7%

af

ter

near

ly 1

0 ye

ars

Incr

ease

in s

oil b

ulk

dens

ity

fo

und

to b

e si

gnifi

cant

, but

unlik

ely

to s

erio

usly

aff

ect

si

te q

ualit

y W

ass

and

Smit

h (1

994)

So

il ty

pe u

nkno

wn,

Dou

glas

-fi r

and

Lod

gepo

le p

ine,

inte

rior

Bri

tish

Col

umbi

a, C

anad

a

Stum

p up

root

ing

Les

s se

vere

soi

l com

pact

ion

than

caus

ed b

y sk

id r

oad

cons

truc

tion

Eff

ects

of

both

sit

e pr

epar

atio

n

trea

tmen

ts le

ft 3

4% o

f

the

surf

ace

area

of

the

site

unsu

itab

le f

or p

lant

ing

Skid

roa

d co

nstr

ucti

onE

ffec

tive

ly li

mit

ed s

eedl

ing

root

ing

de

pth

to u

pper

10

cm o

f m

iner

al

so

il ho

rizo

n as

a r

esul

t of

sev

ere

co

mpa

ctio

n W

ass

and

Smit

h (1

997)

G

rave

lly

sand

y lo

am,

D

ougl

as-fi

r, s

outh

ern

V

anco

uver

Isl

and,

Can

ada

Stum

p re

mov

alN

o si

gnifi

cant

incr

ease

in s

oil d

ensi

ty,

ea

se o

f so

il pe

netr

abili

ty in

crea

sed

fo

llow

ing

dist

urba

nce

of s

tum

p

rem

oval

Site

con

side

red

to h

ave

low

sens

itiv

ity

to c

ompa

ctio

n

Page

-Dum

roes

e et

al (

1998

) A

sh-c

ap s

oil,

Dou

glas

-fi r

and

Wes

tern

whi

te p

ine,

nor

ther

n

Idah

o, U

SA

Soil

com

pact

ion

Incr

ease

d so

il bu

lk d

ensi

ty b

y

15 – 2

0% t

o a

dept

h of

30

cmN

on-r

eplic

ated

exp

erim

ent.

Bot

h tr

eatm

ents

led

to d

eclin

es

in

num

bers

and

mor

phol

ogic

al

ty

pes

of e

ctom

ycor

rhiz

ae a

nd

no

n-ec

tom

ycor

rhiz

al s

hort

root

s of

bot

h tr

ee s

peci

es a

fter

1 ye

ar

Stum

p re

mov

alD

ecre

ased

bul

k de

nsit

y of

0 – 3

0 cm

hori

zon,

incr

ease

d bu

lk d

ensi

ty o

f

30 – 4

5 cm

hor

izon

, to

sam

e le

vel

as

wit

h so

il co

mpa

ctio

n tr

eatm

ent

Hop

e (2

007)

Sa

ndy

loam

, Dou

glas

-fi r

,

sout

hern

inte

rior

of

Bri

tish

Col

umbi

a, C

anad

a

Stum

p re

mov

alIn

itia

l inc

reas

e in

soi

l bul

k de

nsit

y

afte

r 1

year

, ret

urni

ng t

o pr

e-

trea

tmen

t le

vels

aft

er 1

0 ye

ars

Bul

k de

nsit

y de

clin

ed a

fter

10

ye

ars

due

to a

bund

ance

of

fi ne

ro

ots

loos

enin

g an

d ae

ratin

g so

il Z

abow

ski e

t al

(20

08)

And

isol

s, I

ncep

tiso

ls a

nd

U

ltis

ols,

Dou

glas

fi r,

Was

hing

ton

and

Ore

gon,

USA

Stum

p re

mov

alA

cros

s fi v

e si

tes,

the

re w

as a

non

-

sign

ifi ca

nt 3

% in

crea

se in

soi

l bul

k

dens

ity

to 2

0 cm

fol

low

ing

stum

p

rem

oval

aft

er 2

0 ye

ars

Incr

ease

in b

ulk

dens

ity

was

sign

ifi ca

nt a

t tw

o si

tes,

perh

aps

due

to t

heir

fi ne

r

soil

text

ure

and

heav

y ra

infa

ll

whi

ch o

ccur

red

shor

tly

befo

re

tr

eatm

ents

wer

e ap

plie

d

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FORESTRY26

clearfelling on fi ne-textured soils located near the Pacifi c coast of Washington state, USA ( Miller et al. , 1996 ). After 8 years, the soil bulk density in the 0 – 8 cm depth of pri-mary extraction routes was 41 – 52 per cent higher than non-extraction route areas. After a further 8 years, bulk density was still 20 per cent higher in the 0 – 30 cm depth of primary routes, highlighting that the initial traffi cking had resulted in longer term changes to the physical structure of the soil. A similar study was conducted by Wert and Thomas (1981) . They compared the soil of skid roads, transition zones (3 m either side of skid road) and undis-turbed areas. After 32 years, they found that the deep soil horizon (20 – 30 cm) was still heavily compacted, whereas the 0 – 15 cm horizon had recovered. Other studies have also confi rmed that compaction of the soil beneath heavily traffi cked areas can persist for many decades (e.g. Froehlich et al. , 1986 ) and we can surmise that the effect of stump removal and associated additional heavy machine traffi ck-ing will have long-term impacts upon the physical structure of forest soils.

Off-site environmental impacts on streams, rivers and lakes

Forest harvesting activities can have profound impacts on water catchments. Water courses typically experience in-creases in levels of suspended solids, DOC and aluminium and may as a result become increasingly acidifi ed ( Neal et al. , 1998 ; Croke and Hairsine, 2006; McHale et al. , 2007 ). Large quantities of nitrogen and other macronutrients such as calcium and potassium may also be released into water courses ( Likens et al. , 1970 ; Colinvaux, 1986 ; Reynolds et al. , 1995 ; Stevens et al. , 1995 ; Carling et al. , 2001 ). In order to prevent or reduce such impacts, there exist a num-ber of guidelines and protocols for managed forests which should be adhered to during harvesting activities. In the UK, for example, these are summarized in Nisbet (2007) . Loss of nitrogen from harvested sites can lead to particu-lar problems of eutrophication ( Lundborg, 1998 ), caus-ing major ecosystem damage to streams, rivers and lakes. Where whole-tree harvesting is carried out, this problem is generally reduced, as shown in the study by Stevens et al. (1995) , who observed a nitrate pulse in water courses following conventional, stem-only harvesting, but no pulse from whole-tree harvested sites. It is for this reason that residue harvesting can be considered an ecosystem benefi t in areas where atmospheric N deposition is high (Swedish National Board of Forestry, 2002; Moffat, 2003 ). There have been few studies examining whether the harvesting of stumps might lead to impacts different from those arising from other forest harvesting activities on the water courses of forested catchments.

Olsson (1995) investigated the short-term effects of residue harvesting and stump removal on soil water chem-istry. The study site was a Norway spruce stand located in south west Sweden. Residue harvesting led initially to lower concentrations of +

4NH , 3NO and K � in soil water, but after 5 years, differences in nutrient levels in drain-

age water could not be detected between treatments and were comparable to those normally found to occur from such sites. A hydrological modelling approach has also been employed in order to predict how stump removal might infl uence N cycling on clearfell sites and the ef-fect this may have on surrounding water bodies ( Laurén et al. , 2007 ). The model employed found that the recovery of forest residues and stumps did not decrease N exports to water courses, as a result of a massive decline in microbial immobilization of N following the removal of all woody debris from the site. The results highlight the complexity of nutrient cycling in forest ecosystems, whereby major sinks for N after clearfelling are immobilization by soil mi-crobes, ground vegetation and soil sorption ( Fahey et al. , 1991a ; Laurén et al. , 2005 ). The removal of stumps may further disrupt the ability of forest sinks of N to function and thereby potentially offsetting the various perceived benefi ts.

Forest biodiversity and habitats

Modern forest management aims to provide multiple func-tions from the resource – for example timber and fuel production, amenity and recreation and maintenance of biodiversity ( Ferris-Kaan, 1995 ). In unmanaged forests, natural disturbance in the form of wind, fi re, water, insect outbreaks and fungal infections maintain a continual sup-ply of deadwood. These various disturbances are heavily controlled in managed forests, greatly reducing the amount of deadwood in comparison to natural forests ( Fridman and Walheim, 2000 ) and stump removal represents a fur-ther reduction in the supply of deadwood substrate. Dead-wood can be considered the foundation of the food web in forest ecosystems and its provision essential to maintaining habitats for several groups of organisms (e.g. Bader et al. , 1995 ; Humphrey et al. , 2002 ). Understanding the implica-tions of stump removal upon biodiversity is vital to ensure that this practice does not compromise the functioning of forest ecosystems.

Stem-only harvesting leaves large quantities of dead-wood, in the form of residues, stumps and roots which, following fi nal felling, may make up to 80 per cent of the deadwood volume ( Caruso et al. , 2008 ). The removal of residues as well as stem wood has been found to remove up to 70 per cent of the material which would otherwise be left on site ( Rudolphi and Gustafsson, 2005 ) and accordingly, following residue removal, stumps may comprise up to 80 per cent of the remaining deadwood ( Egnell et al. , 2007 ). Stumps therefore represent a signifi cant component of the deadwood habitat of harvested sites, particularly where res-idue harvesting has taken place. Stump harvesting will lead to further depletion of the deadwood habitat in managed forests and has led to concerns over potential impacts on forest biodiversity ( Caruso et al. , 2008 ). In the UK, ~ 25 per cent of forest-dwelling species of animals, plants and fungi (equivalent to ~ 1000 species) are reliant on dead-wood habitats (including those provided by conifer planta-tions) ( Elton, 1966 ). Further losses of such habitat through

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stump removal may therefore pose grave consequences for a substantial number of forest species.

Several studies have highlighted the importance of re-taining snags, standing deadwood and high stumps at fi nal harvest for saproxylic beetles (e.g. Jonsell et al. , 2004 , 2005 ; Hedgren, 2007 ), and as such there are guidelines regarding how much of this material should remain. For example, United Kingdom Woodland Assurance Stan-dard (UKWAS) suggest that a minimum of 20 m 3 ha � 1 (or 5 – 10 per cent of the average stand volume) of deadwood remains after fi nal felling across the woodland area as a whole ( UKWAS, 2006 ). This guidance is drawn up with-out considering stump removal – so the UKWAS guide-line retention volume may be considered additional to the deadwood that would normally remain in the form of the above-ground component of stumps. Stumps have been identifi ed as providing a very long-lasting deadwood habi-tat for (1) a wide range of specialist toadstools and bracket fungi (2) mosses and many specialized lichen species; (3) saproxylic beetles and other invertebrates and (4) species supported by the above ( Ferris-Kaan et al. , 1993 ; Hedgren, 2007 ; Carusuo et al. , 2008). Larger pieces of deadwood (i.e. stumps) take longer to decompose than smaller pieces and thereby provide habitat for a greater length of time.

As a result of intensive management of much of the for-est estate in the UK over the past century, there is a lack of large pieces of deadwood, stumps forming a major compo-nent of the extant resource. A shortage of future sources of new deadwood supplies has also been identifi ed ( Smith et al. , 1997 ; The Woodland Trust, 2005 ). Given that as deadwood fraction size increases, conservation and habitat value also increase (due to their slow rate of decay), therefore stump removal presents a signifi cant threat to the preservation and enhancement of biodiversity in managed forests in the UK. As such, there is general agreement in the literature that for-est management requires planning at the landscape level to ensure that the full diversity of forest habitats is maintained, from newly felled and regenerating, pole stage, canopy clo-sure to old-growth and unmanaged forest in order to main-tain biodiversity and ecosystem integrity ( Niemelä, 1997 ; Niemelä et al. , 2007 ; Berglund and Åström, 2007 ).

Invertebrates

There are a wide number of studies that have reported the impacts of residue harvesting on ground-living invertebrates, but, to date, the effects of stump harvesting have not been reported. Studies have demonstrated that retention of man-made high stumps can benefi t insects as well as fungi ( Lindhe and Lindelöw 2004 ; Jonsell et al. , 2004 , 2005 ). So it would seem logical to predict that those invertebrates which feed directly on deadwood, whether in high or low stumps, such as saproxylic beetles and wood-living invertebrates, will be negatively affected by stump removal. In particular, the loss of habitat heterogeneity is likely to favour lower, rather than greater, diversity and numbers of invertebrates due to a reduction in ecological niches and substrates ( Ecke et al. , 2002 ). The impact on soil dwelling invertebrates is less

straightforward to predict but also likely to be negative. For example, Berch et al. (2007) found that mounding (but no stump removal) signifi cantly reduced species diversity of oribatid mites on the forest fl oor of clearfell areas compared with untreated areas in high-elevation clearfell sites in south-ern British Columbia.

There are other studies which have reported substantial negative effects of residue harvesting and associated effects on forest dwelling invertebrates. Battigelli et al. (2004) observed the effects of whole-tree harvest combined with forest fl oor removal and heavy compaction of subboreal spruce forest soils (deep, medium-textured luvisols, pod-zols and brunisols) on soil mesofauna in central interior British Columbia, Canada. Although this represents an extreme management intervention, similar conditions could arise on sites where stump removal is conducted. Soil mesofauna densities were found to decline by 93 per cent, relative to uncut forest. Similar results were recorded by Bengtsson et al. (1997) , who simply compared the ef-fects of residue retention or removal. The removal of resi-dues led to signifi cant decreases in abundance of spiders, predatory insects, Collembola (springtails) and gamasid mites (mesostigmata). However, no signifi cant effects on enchytraeid worms, or diplopods (such as millipedes), was detected. Bengtsson et al. (1997) concluded that the en-tire soil fauna community and structure of the food web had been altered by the removal of residues, potentially leading to long-term declines in the abundance of many soil animal groups. On both a Scots pine and Norway spruce site, residue removal led to a decrease in gamasid mites, spiders and staphylinid and cantharid beetles, com-pared with control plots where residues remained on site ( Bengtsson et al. , 1998 ). The authors expressed particular concern regarding the loss of species crucial for nutrient cycling and the maintenance of site productivity, in par-ticular declines in fungivores and predatory arthropods. The effect of organic matter removal on the soil food web was forecast to be of the order of decades ( Bengtsson et al. , 1997 , 1998 ). Although the studies of Battigelli et al. (2004) and Bengtsson et al. (1997 , 1998 ) did not specifi cally look at stump removal per se, they give an indication of the changes that occur to forest sites if resi-dues are removed – effects which are likely to be further accentuated if stump harvesting is carried out.

A strong case is presented for the retention of stumps by Hedgren (2007) , who observed early arriving bark- and wood-boring beetles and associated insect enemies in freshly cut high and low stumps of Norway spruce. Most species occurred in both the high and low above-ground components of fresh spruce stumps. The stumps were also shown to provide a microhabitat for generalist predator beetles and parasites, including predators of the damaging forest pest, the eight-toothed spruce bark beetle ( Ips typographus (L)). Berglund and Åström (2007) consider that the effects of residue harvesting on forest invertebrates to generally be moderate, when considered in relation to the effects of the act of clearfelling. However, it would appear from the evidence presented here that although clearfelling alone may lead to signifi cant impacts on forest

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invertebrates, the removal of stumps as well as residues will substantially exacerbate the effects of clearfelling.

Flora

The removal of forest residues followed by stump harvest will lead to much more exposed and disturbed clearfell sites. Such conditions will favour pioneering species of fl ora which are also tolerant of exposure and wide variations in soil moisture levels ( Olsson and Staaf, 1995 ; Åström et al. , 2005 ). Strandström (2006) observed that stump removal led to a vast increase in the level of soil surface disturbance, which he associated with a 30 – 50 per cent increase in the number of broadleaf tree saplings. Kaye et al. (2008) ex-amined the effect of stump removal on understorey veg-etation after 24 – 28 years on fi ve study sites in the Pacifi c Northwest of North America. Stump removal was found to have long-term effects on plant communities and increase the risk of colonization by introduced species – at all sites, community composition was signifi cantly affected ( P < 0.05), with graminoids, forbs and introduced species in-creasing in diversity at all study sites where stump removal had taken place.

The removal of all the above-ground biomass in an exper-iment comparing whole-tree harvesting with conventional clearfelling in Beddgelert, North Wales, revealed that after 5 years, there was twice as much vegetation on sites which had been whole-tree harvested (all above-ground bio-mass) ( Fahey et al. , 1991b ). This is because the presence of residues reduces the area of soil available for seeding and also acts as a physical barrier to the growth of vegetation. When removed, growth is much more rapid and vigorous. Such growth, particularly of undesirable fi eld vegetation, presents serious management challenges in the re-establish-ment of the next rotation and if left unmanaged will almost certainly lead to reductions in future timber productivity. Addressing this challenge is likely to require additional management interventions.

Residue harvesting has also been associated with reduc-tions in the prevalence of nitrophilous vegetation such as rosebay willowherb ( Epilobium angustifolium (L)) and some grasses (e.g. Deschampsia fl exuosa (L)) ( Olsson and Staaf, 1995 ; Bråkenhielm and Liu, 1998 ) and stump harvesting could lead to further declines in these species. Scherer et al. (2000) found that intensive residue removal led to increases in the abundance of weedy, non-forest species, which although undesirable in terms of forest management, are likely to restrict nitrogen losses and as-sociated off-site environmental impacts (e.g. Fahey et al. , 1991a ). Whether stump removal leads to drastic changes in the composition, abundance and growth of various species of forest (and non-forest) fl ora is a key question regard-ing future site productivity and management. In particular, research is required to determine whether stump removal will increase the prevalence of invasive non-forest fi eld veg-etation or those which pose particular problems to forest managers in the UK, such as heather ( Calluna vulgaris (L)), rhododendron ( Rhododendron ponticum (L)), bramble ( Rubus sp) and bracken ( Pteridium sp). There is a clear

need to ensure that large-scale stump removal will not lead to a large increase in the necessity for chemical herbicide applications in managed forests.

Mosses, liverworts, fungi and lichens

Åström et al. (2005) found that drought-intolerant and wood-inhabiting species were particularly badly affected by residue harvesting after 5 – 10 years, both in terms of species richness and numbers of unique species. Liverworts suffered the greatest declines, with signifi cantly less spe-cies found in plots from which residues had been removed compared with those where stem-only harvesting was car-ried out. Humphrey et al. (2002) reported that plantation forests and semi-natural stands are very similar in terms of their bryophyte (moss) species richness across the UK. Fur-ther analysis revealed moss species richness to be positively correlated with the amount of large, well-decayed coarse woody debris (more than 20 cm diameter) as well as the amount of stumps. Stumps were also found to be particu-larly important in recent clearfell stands for the lichen gen-era Calicium and Cladonia , typical of the epiphytic lichens used as indicators of biodiversity and continuity in forests in both the UK and Scandinavia ( Thor, 1998 ; Jonsson and Jonsell, 1999 ).

Rudolphi (2007) observed the colonization by lichens and bryophytes of Norway spruce stumps of different age categories. Very little colonization was found to occur during the fi rst 4 years after harvesting, whereas stumps older than 18 years were found to be highly de-cayed and frequently had 100 per cent cover of ground-living bryophytes. Lichens, on the other hand, tended to colonize stumps more rapidly. Such results highlight the importance of stumps as a long-term habitat for these or-ganisms. Caruso and Thor (2005, cited in Berglund and Åström, 2007 ) contend that stumps are a relatively rich substrate for lichen fl ora, meaning that stump removal will lead to the impoverishment of valuable habitat for those lichens which favour stumps. More recent research by Caruso et al. (2008) has revealed the value of Norway spruce stumps as a habitat for lichen species. Their study compared the forest residues with stumps as substrates for lichens. Stumps were found to host greater numbers of li-chen species, as well as a higher number of unique species; species which were also identifi ed in the literature as na-tionally rare or uncommon. Allmér (2005) evaluated the effects of residue harvesting on wood-living fungi in boreal coniferous forests. Results suggested that residue removal is unlikely to have a negative infl uence on species diversity, as no wood-living fungi were found that specialized on this fraction of wood. Drawing on these results, Allmér (2005) postulated that, because stumps generally host a wide va-riety of wood-dwelling fungi, their removal is likely to be more severe than the effects of residue harvesting in such ecosystems.

Large-scale stump removal is likely to have serious negative implications for those species of moss, liverwort, fungi and lichen which depend on them and in this regard

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the effects of stump removal will be much greater than the effects of residue removal. Clearly, where stump removal is undertaken, it is essential that a minimum quantity of stumps be retained to guarantee at least a minimum provi-sion of valuable stump habitat.

Biotic and abiotic risks

Regardless of the potential market for harvesting stumps for fuel, there is a considerable European-wide effort to treat stumps against root rot. Thor (2002) conducted a survey for the Swedish Forest Agency, SkogForsk, and found that stump treatment activities took place on a total of 210 000 ha per annum – equivalent to ~ 75 per cent of the total forest area of Wales. Poland, Britain and Sweden account for the largest stump-treated areas, with Norway spruce and Sitka spruce the most treated species, except in Poland where treatment is targeted almost exclusively to Scots pine. Given concerns about increased incidences of H. annosum infestations, Finland, France, Poland and Sweden all forecast increases in the level of stump treatment ( Thor, 2002 ). A recent review by Vasaitis et al. (2008) found that when stump removal is undertaken to remove root rot-infested stumps, in most cases this leads to a reduction in root rot in the next generation and lends support to the case for stump removal in such instances.

Insects

Of major concern is the threat posed by the large pine weevil ( Hylobius abietis ) of the genus Hylastes ( King and Scott, 1975 ). Hylobius abietis kill conifer seedlings by gnawing through the bark, severing the phloem and even-tually ring-girdling the stems. The weevils emerge from the stumps and associated root – stump systems remaining on sites after clearfelling and then damage young seedlings. The main risk to seedlings is in autumn, when weevils emerge from stumps, or in spring, when weevils re-emerge from the overwintering sites ( Wainhouse and Brough, 2007 ). Pine weevils have also been reported to attack twigs and bark in the crowns of mature spruce trees surround-ing clearfell areas. The weevils attack young seedlings and saplings during the fi rst 3 – 5 years after clearfelling ( Petersson, 2004 ). Storage of fresh logging residues and fresh cut stumps can attract pine weevils, which may further increase mortality of seedlings planted near to storage areas. There is concern that the uprooting of stumps may encourage infestation by pine weevil, further increasing seedling mortality ( Metla, 2008 ). Other insect pests of potential concern include the great European spruce bark beetle ( Dendroctonus micans (Kugelann)) ( Fielding and Evans, 1997 ), the common pine shoot beetle ( Tomicus piniperda (L.)) and the larger European spruce bark beetle ( I. typographus (L.)) ( Croome and Cameron, 2006 ).

Wallertz et al. (2006) demonstrated that, for a Scots pine site in southern Sweden where two silvicultural treatments had been conducted (shelterwood and clearfell), a substan-

tial quantity of root bark remained in the soil humus layer (on average, a surface area of root bark equivalent to 3741 m 2 ha � 1 , over 96 per cent of which were the roots of Scots pine). Results showed that (1) pine weevils showed no pref-erence for roots of varying nutritional value; (2) no dif-ferences in bark consumption were observed between the two silvicultural treatments; (3) pine weevils almost never consumed roots of broadleaf or other fi eld vegetation and (4) conifer roots remaining in the humus layer are a major food resource which pine weevils can utilize for a consider-able period in both shelterwood and clearfell sites. These results illustrate that stump removal presents little hope for completely preventing the incidence of pine weevil infesta-tion, given that following stump removal substantial quan-tities of roots will remain on harvested sites. A reduction in the amount of breeding substrate available to pine weevils may offer a means by which the scale of infestations can be minimized, although further research is required to deter-mine whether this assertion is correct.

The results of Wallertz et al. (2006) contrast with the fi ndings reported by Petersson (2004) . Petersson (2004) evaluated different techniques to reduce pine weevil dam-age and found that scarifi cation, combined with shelter-wood and feeding barriers (chemical or physical), reduced the mortality rate of Norway spruce seedlings to less than 10 per cent. Soil disturbance leading to the mixing of humus and mineral soil horizons coupled with a reduction in the amount of shelter on harvested sites caused by scari-fi cation were considered important in reducing pine weevil damage. Pine weevils require some form of shelter close to seedlings to be able to subsequently infest and damage them – denying them, this shelter can be effective in reduc-ing their capacity to damage newly planted seedlings.

Recent work by Moore (2004) and Moore et al. (2004) has highlighted the importance of the date of felling in the determination of the likely emergence of adult pine weevils from stumps, and the subsequent damage likely to be sustained by seedlings. Management approaches have been proposed, such as the use of nematode as a biological control agent or leaving sites fallow until adult Hylobius abietis have developed from eggs. However, this does not allow us to speculate on the potential effi cacy of stump harvesting at reducing the incidence of pine weevil damage. Although stump removal will reduce both pine weevil breeding habitat and shelter, it is unclear whether remain-ing stumps will still enable viable populations to survive and infl ict measurable damage to seedlings. Additional re-search is required to determine this. Further research is also clearly required to determine the signifi cance of the stump itself as compared with the fi ne roots, some of which will remain, in the life cycle of H. abietis .

Fungi

A number of fungal infections, such as H. annosum , can cause substantial damage to forests and in many cases stump harvesting has been used as a control method (Gibbs et al. , 2002). Heterobasidion annosum causes root and butt

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rot disease which affect conifers worldwide ( Pratt, 1999 ) and is one of the most destructive conifer diseases in the northern hemisphere, causing estimated economic losses of around € 800 million per annum ( Asiegbu et al. , 2005 ). Primary infection is from spores, which infect wounds such as cut stems, and after establishment in the tree spreads by root contact. Whereas in Fennoscandia, spores are mainly formed during the growing season ( Thor et al. , 2006 ); they are present throughout the year in the UK ( Pratt, 1999 ).

There is convincing evidence that for Scots pine mon-ocultures on sandy soils, as well as in mixtures of Norway spruce on any soil, stump treatment could be a profi table means of reducing disease damage caused by H. annosum ( Rönnberg et al. , 2006 ). Given the size of the economic losses occurring due to H. annosum , there is a clear case for evaluating methods that may reduce future incidence of the disease. Stump removal is essentially an intensive site preparation method, while chemicals such as urea and bo-rates as well as biological control agents are currently used as means for reducing the spread of such diseases ( Asiegbu et al. , 2005 ).

Stump treatment methods generally lead to signifi cant reductions in the area of stump colonized by natural air-borne infection of H. annosum ( Nicolotti and Gonthier, 2005 ), with reports of up to 90 per cent reduction in the area colonized ( Thor and Stenlid, 2005 ). Stump removal has also been proposed as a means of reducing the risk of H. annosum , by reducing the amount of inoculum left on restock sites (e.g. Paananen and Kalliola, 2003 ). In a study by Omdal et al. (2001) , different stump harvesting machines were employed to remove the root systems of Ponderosa pine ( Pinus ponderosa Dougl. ex. Laws.) (infected with Armillaria ostoyae (Romagn.) Herink) and white fi r ( Abies concolor (Gord. & Glend.) Lindl. ex Hil-debr.) (infected with A. ostoyae , H. annosum or both). There were clear differences between machines in terms of the amount of root biomass they left in the ground. How-ever, in general, following the treatments, residual biomass was not thought to represent a signifi cant source of root disease, as nearly all (90 per cent) broken roots remain-ing were smaller than 5 cm in diameter and predicted to decompose very rapidly. Similar fi ndings were reported by Bloomberg and Reynolds (1988) who also compared dif-ferent stump harvesting machines, all of which recovered in excess of 90 per cent of the root volume. They concluded that the quantity of roots remaining would be insuffi cient to transmit infection to the next rotation.

Gibbs et al. (2002) have summarized research into the behaviour and control of H. annosum spanning over 50 years in Thetford Forest, East Anglia, UK. The stumps of both Corsican pine ( P. nigra J.F. Arnold ssp. laricio (Poir.) Maire) and Scots pine have been found to be equally prone to colonization by airborne spores of H. annosum . Some evidence suggests that following thinning, Corsican pine is less likely to develop the disease than Scots pine. On har-vested sites where soil pH >6.0, mortality losses of up to 30 per cent can occur during the initial 10 years, whereas where soil pH <6.0, losses are generally lower but can still average 20 per cent. Most importantly for this review, by

comparing the effi cacy of different control methods they concluded that stump removal is the only means by which adequate disease control can be accomplished.

A long-term fi eld experiment was set up in 1968 in Brit-ish Columbia in order to evaluate the effectiveness of stump removal for controlling Phellinus weirii (Murr.) Gilb., which causes laminated root rot (LRR) disease ( Morrison et al. , 1988 ). In a stand of mature Douglas-fi r where 60 – 70 per cent of stems had suffered infection or mortality from P. weirii , two harvesting treatments were compared. One treatment was whole-tree harvesting and root raking to ensure removal of most roots greater than 2 cm diameter from the upper 60 cm of soil. The other treatment was stem-only, conventional harvesting. Various tree species were planted in both harvested treatments and their mor-tality observed over 20 years. Both height and diameter growth of Douglas-fi r and lodgepole pine ( Pinus contorta Dougl. ex Loud.) were found to be greater in the treated plots, with reductions in mortality due to both fungal dis-eases, in agreement with the conclusions of Gibbs et al. (2002) .

One of the most comprehensive studies into the effi cacy of stump removal at reducing the risk of LRR disease from P. weirrii is reported by Thies and Westlind (2005) . They set up independent studies across fi ve P. weirrii stands of Douglas-fi r across Washington and Oregon (the same sites used in the study by Kaye et al. , 2008 , reported in the section ‘ Mosses, Liverworts, Fungi and Lichens ’ above). Stump removal with a bulldozer was compared with con-ventional harvesting, as well as combination with ammo-nium nitrate fertilizer. They found that stump removal was associated with a signifi cant reduction in seedling mortality caused by LRR, whereas ammonium nitrate treatments did not lead to changes in mortality rates. By following the development of 7827 trees across 239 plots for up to 27 growing seasons following treatments, mean mortality due to LRR was 2.4 per cent on stumped areas compared with 9.1 per cent on non-stumped areas. In ad-dition, where LRR was observed, there was a 25 per cent reduction in volume growth, compared with areas where it was not found.

Despite generally promising results ( Vasaitis et al. , 2008 ), there is still some uncertainty regarding whether any infected roots remaining after stump removal are capable of infecting the next generation of trees ( Metla, 2008 ). Stump removal as a means of inoculum removal is reportedly far more intensive than conventional stump harvesting, leading to the removal of not only stumps but also much of the root system ( Morrison et al. , 1988 ; Omdal et al. , 2001 ). At the same time, there is evidence to suggest that even the most thorough, intensive stump removal attempts still leave signifi cant quantities of roots in the soil ( Wallertz et al. , 2006 ), leading us to the view that stump removal cannot be justifi ed solely for achieving pest and disease control, particularly in healthy, disease-free stands.

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sign

ifi ca

ntly

hig

her

on

tr

eate

d pl

ots

( P <

0.0

5)

Con

clud

ed t

hat

mec

hani

cal

sc

arifi

caito

n im

prov

es lo

ng-

te

rm p

rodu

ctiv

ity

of s

uch

lo

w-p

rodu

ctiv

ity

site

s

Was

s an

d Sm

ith

(199

7)

Dou

glas

-fi r

, sou

ther

n

Van

couv

er I

slan

d,

C

anad

a

Gra

velly

san

dy

lo

amSt

ump

rem

oval

+ve

Tre

e gr

owth

rat

es

si

gnifi

cant

ly in

crea

sed

fo

llow

ing

stum

p re

mov

al

af

ter

10 y

ears

. See

dlin

gs

gr

owin

g on

dep

osit

s (+

12%

in h

eigh

t an

d 18

% in

diam

eter

) co

mpa

red

wit

h

undi

stur

bed

soil.

Site

con

side

red

to h

ave

low

sens

itiv

ity

to c

ompa

ctio

n

and

thus

pot

enti

ally

bene

fi ted

fro

m d

istu

rban

ce

ca

used

by

stum

p re

mov

al

Page

-Dum

roes

e

et a

l (19

98)

Dou

glas

-fi r

and

Wes

tern

whi

te

pi

ne, n

orth

ern

Id

aho,

USA

Ash

-cap

soi

lSt

ump

rem

oval

− ve

Smal

ler

root

col

lar

di

amet

ers

on s

tum

p

rem

oval

are

as o

f

Wes

tern

whi

te p

ine

3

year

s af

ter

outp

lant

ing

Red

uced

gro

wth

att

ribu

ted

to

sev

ere

soil

dist

urba

nce,

redu

ctio

ns in

por

e sp

ace

an

d m

ycor

rhiz

al a

ctiv

ity

Was

s an

d Se

nyk

(1

999)

So

uth

cent

ral B

riti

sh

C

olum

bia,

Can

ada

Gra

velly

san

dy

lo

ams

Stum

p re

mov

al a

nd

sc

arifi

cati

on − v

eL

odge

pole

pin

e, D

ougl

as-

fi r

and

wes

tern

larc

h

grow

th w

as g

reat

er

on

und

istu

rbed

soi

l

than

stu

mp-

harv

este

d

area

s

Dou

glas

-fi r

gro

wth

app

eare

d

to b

e m

ore

nota

bly

affe

cted

than

the

oth

er t

wo

spec

ies

Thi

es a

nd

W

estl

ind

(200

5)

FIve

inde

pend

ent

D

ougl

as-fi

r s

tand

s

acro

ss O

rego

n an

d

Was

hing

ton,

Nor

th

W

est

USA

Ran

ge o

f lo

amy

so

ils: s

ilty

loam

s,

sa

ndy

loam

s an

d

clay

loam

s

Stum

p re

mov

al+v

e or

nc

Sign

ifi ca

ntly

impr

oved

prod

ucti

vity

( P

< 0

.05)

at t

wo

of fi

ve s

ites

at

pr

ethi

nnin

g st

age,

no

di

ffer

ence

s at

oth

er s

ites

Impr

ovem

ents

in p

rodu

ctiv

ity

cl

osel

y re

late

d to

red

uctio

ns

in

the

inci

denc

e of

LR

R

ca

used

by

Phe

llinu

s w

eiri

i

Hop

e (2

007)

D

ougl

as-fi

r s

tand

,

sout

hern

inte

rior

of B

riti

sh

C

olum

bia,

Can

ada

Sand

y lo

amSt

ump

rem

oval

+ve

or n

cL

odge

pole

pin

e gr

owth

incr

ease

d, b

ut n

o

cons

iste

nt e

ffec

ts o

bser

ved

on

Dou

glas

-fi r

Dou

glas

-fi r

app

ears

to

be

m

ore

affe

cted

by

soil

di

stur

banc

e th

an p

ine

sp

ecie

s, w

hich

oft

en s

eem

to b

enefi

t

+ve

= po

siti

ve c

hang

e in

pro

duct

ivit

y; -

ve =

neg

ativ

e ch

ange

in p

rodu

ctiv

ity;

nc

= no

cha

nge.

Tab

le 3

: Con

tinu

ed

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STUMP HARVESTING FOR BIOENERGY 33

Future site productivity

Egnell et al. (2007) consider that results from fi eld experi-ments suggest no negative effect of stump harvesting on growth of the next tree crop. Furthermore, Egnell et al. (2007) contend that, in the short term, stump harvesting is unlikely to reduce stand productivity but acknowledge that long-term empirical evidence is lacking. Such claims contrast with effects found in residue harvesting and the recommendations which encourage the return of wood ash to forests in order to maintain nutrient levels and sustain future productivity ( Swedish National Board of Forestry, 2002 ). Where stump removal is undertaken in order to remove root rot, a comprehensive review undertaken by Vasaitis et al. (2008) discovered that in most cases, it leads to improved seedling establishment and gains in overall stand productivity.

A summary of relevant studies investigating the effects of stump harvesting as well as other effects which are as-sociated with stump harvesting is presented in Table 3 . Five studies report an increase or no change in productiv-ity, fi ve report a decrease and two report no change. Produc-tivity on sandy loams and podzols tended to benefi t from stump harvesting, whereas productivity generally suffered on more organic soils/ash-cap soils.

While the views of Egnell et al. (2007) are general, there is evidence from some of the studies presented in Table 3 to suggest wide variations in the response of different tree spe-cies to stump harvesting, primarily due to soil disturbance ( Hope, 2007 ). There are clearly differences between sites and between species with regard to the effect that stump removal has on productivity. There is evidence to indi-cate that Douglas-fi r, for example, may be more sensitive to soil disturbance associated with stump removal than other species, such as lodgepole pine and western larch ( Larix occidentalis Nutt.) ( Wass and Senyk, 1999 ; Hope, 2007 ). Perhaps this is unsurprising given that the latter are both pioneer species. We may therefore expect that pioneer tree species endemic to the UK, such as silver birch ( Betula pendula Roth.), mountain ash ( Sorbus aucuparia L.) and grey willow ( Salix cinerea L.) will become established on stump-harvested sites and may initially grow well. How-ever, there is no evidence reported in the literature to date about how stump removal might infl uence the productiv-ity of other species such as Picea sitchensis , currently the most commercially important species in the context of UK forestry ( Samuel et al. , 2007 ). Long-term studies are clearly required to address this.

Discussion and conclusions

The majority of research concerning environmental aspects of stump harvesting has focussed on situations where the primary motivation for removal has been pest and disease control. There have been very few studies to date which have evaluated the implications of stump harvesting for bioen-ergy, using disease- and pest-free stands and this represents a substantial gap in our understanding. Stump harvesting

represents an increase in the intensity of forest manage-ment activities and is highly likely to intensify the negative environmental effects of existing forestry practices, such as mechanical site preparation. But there remains a degree of uncertainty. There is an urgent need to undertake empirical fi eld studies in order to reduce this uncertainty, particularly with regard to the effect that stump harvesting may have on soil C emissions.

There is strong evidence to suggest that stump removal will have a signifi cant negative impact upon forest biodi-versity, as a result of the removal of deadwood – a highly valuable habitat which forms the foundation of the food web in forests. As such, 100 per cent removal of stumps should be avoided and efforts to retain undisturbed areas should be encouraged. Stump harvesting is also likely to favour invasive, pioneering vegetation and leads to a shift in the plant species composition on sites where it is carried out, potentially leading to additional herbicide requirements.

Set against these uncertainties and concerns, there are clear advantages of stump harvesting. These include the provision of local energy supplies, reduced dependency on fossil fuels, additional revenue for forest owners and, when combined with mounding, can lead to effi ciency gains in site preparation for restocking. LCA is a tool that should be further utilized in order to gain a clearer, more objective picture of the costs and benefi ts of forest manage-ment practices such as stump harvesting. Such assessments need to carefully consider what benchmark reference sys-tems to use for robust and meaningful comparisons to be made, both for methods of forest production and energy generation, and utilize accurate, case-specifi c reliable data sources. At the level of individual forest stands, efforts to develop and refi ne risk assessment protocols which allow forest managers to systematically identify sites where risks of adverse impacts are lowest are required.

Further fi eld scale research is also vital in order that the full implications of stump harvesting are to be understood and to ensure that the desire to source local biomass is fully compatible with other efforts to maintain the functioning of forests ecosystems and the vital services that they pro-vide us.

Acknowledgements

The authors wish to acknowledge John Gallacher and UPM Tilhill for funding an initial review on which this manuscript is largely based. We are also extremely grateful for the constructive criti-cism and comments received from both the external referees and the editor .

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