thinning in atlantic oakwoods – assessing options at … · averis, consultant bryologist),...

THINNING IN ATLANTIC OAKWOODS – ASSESSING OPTIONS AT THE STAND SCALE

Published by Highland Birchwoods. Any enquiries should be addressed to Highland Birchwoods at Littleburn, Munlochy, Ross-shire, IV8 8NN.

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This report should be cited as follows:Thompson, R (2005), Thinning in Atlantic oakwoods: assessing options at the stand scale,Highland Birchwoods, Munlochy

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assessing options at the stand scale

THINNING IN ATLANTIC OAKWOODS:ASSESSING OPTIONS AT THE STAND SCALE

by

Richard Thompson*

A report commissioned by Highland Birchwoods

* Forest Research, Roslin, Midlothian, EH25 9SY

Highland Birchwoods is a charitable company (SC 021146) limited by guarantee and registered in Scotland (no. 142892). Registered office: Redwood, 19 Culduthel Road, Inverness, IV2 4AA

• This report provides guidance to managers of mature oak coppice in Atlantic oakwoods; forthe selection of stands and identification of trees for removal / retention in thinningoperations. Information is applicable to Atlantic oakwoods where there is significant lowerplant interest.

• The Peterken Worrell report (2001) provided a model for the management of existing maturehigh forest Atlantic oakwoods on Loch Sunart. This report develops that model, identifyingopportunities at the stand scale for utilisation of oak timber and development of standstructure, in the context of maintenance or enhancement of biodiversity. Results arepresented from a survey of biodiversity and timber values, and advice is provided for theselection of stands and individual trees, and the intensity and cycle of thinning together withchoice of extraction technique.

• The surveys looked at the importance of trees for epiphytes, the potential of trees to becomeveterans, provision of shade for non-epiphytic bryophytes and timber quality.

• Results indicate that:

• Timber is unlikely to be extracted on a commercial basis. However, it has thepotential to support rural development through innovative application of locallygrown wood.

• 16% of trees had moderate to high timber quality, a further 19% of trees containedmaterial suitable for flooring, craft use or cruck frames.

• A uniform stand structure does not necessarily indicate low biodiversity values

• In the majority of stands, some compromises are needed to remove sufficient treesto realise objectives. Trees shading bryophytes of moderate importance may besuitable candidates as their removal should only reduce the suitability of conditionsfor these lower plants, rather than removing interest directly.

• Taking biological values into account, it was possible to remove 25% of standingbasal area in 7 out of 12 plots surveyed but difficult to remove more than 10% inthe remaining 5 plots.

• Atlantic oakwood lower plants indicating ancient woodland need high humidity andvarying degrees of shade. For rarer species, the balance between humidity andshade can be critical.

• Wheeled vehicles should only be used to extract timber after extensive periods ofdry weather.

THINNING IN ATLANTIC OAKWOODS

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Summary

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• Thinning can be used to increase prospects for natural regeneration, increase tree speciesdiversity, develop veterans (i.e. characterful trees with a range of niches for biodiversity /aesthetic value) and epiphytes, as well as produce timber and increase timber quality.

• Natural processes are changing the structure of Atlantic oakwoods. Changes are usuallysubtle and diversification of structure and species composition may take an extensiveperiod. Conversely, storms can have a big impact which may occur over a larger scale thandesirable in smaller woods.

• Stands suitable for thinning will have been coppiced or planted after the mid to late 19thcentury. Stands with high biodiversity values and frequent rock outcrops or steep sidedravines are unlikely to be suitable for thinning.

• Where there are signs of historic management such as wood pasture, judicious thinningaround previously open grown trees may be appropriate to maintain the health of the treeand its epiphytes.

• Thinning intensities which remove around 25% of basal area will be necessary to have anyappreciable effect on tree growth. The frequency of thinning will depend on objectives ofmanagement and response of retained trees.

• Criteria are included to select individual trees. The relative importance of lower plant speciesis specific to each woodland. Training locally employed native woodland advisors / projectofficers in lower plant identification is recommended together with employment of lowerplant specialists where appropriate.

• Use of locally available low impact equipment is recommended to extract round logs orconverted timber. Where material is extracted by a crofter or craftsman, the time requiredmay not be as big a consideration as it would be to a timber contractor.


Contents

SUMMARY ................................................................................................................................2

CONTENTS ...............................................................................................................................4

INTRODUCTION........................................................................................................................5

Scope ....................................................................................................................................6

THE SURVEY.............................................................................................................................7

Background to the experiment and surveys .........................................................................7

Survey methodology..............................................................................................................8

Survey results ........................................................................................................................9

Key observations from surveyors........................................................................................13

GENERAL RECOMMENDATIONS FOR THINNING IN ATLANTIC OAKWOODS..................17

Why thin?.............................................................................................................................17

Potential problems with thinning.........................................................................................18

The role of natural processes..............................................................................................19

The level of intervention in the context of historic management ........................................19

Selecting stands for thinning...............................................................................................20

Thinning intensity and cycle ................................................................................................21

Identifying the value of individual trees...............................................................................22

Objectives ........................................................................................................................22

Summary of criteria for tree retention / selection............................................................23

Selecting trees in practice ...............................................................................................27

Extraction and conversion...................................................................................................28

CONCLUSIONS.......................................................................................................................29

ACKNOWLEDGEMENTS ........................................................................................................30

REFERENCES.........................................................................................................................31

APPENDIX 1:

Long-rotation high forest model – extract from Peterken and Worrell (2001) ....................32


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Under the “LIFE ‘97 Restoration of Atlanticoakwoods” project, Peterken and Worrell (2001)(PW) looked at the conservation managementand utilisation of Sunart oakwoods, many ofwhich are designated as Special Areas ofConservation (SAC). They concentrated on thewoodland and catchment scale, developing astrategic approach for management to achievefavourable condition whilst supporting ruraldevelopment. This report was later developedinto a “user guide” for oakwood managers andowners (Quelch 2002). The models have beenadopted by the Sunart Oakwoods Initiative(SOI) and examples have been established fordemonstration purposes.

One of the PW models, the Long-rotation HighForest Model (see appendix 1), providesguidance at the woodland scale for thediversification of mature oak woodland whichwas previously coppiced or planted. WhilstPeterken and Worrell were principallyconcerned with woodlands in SACs, this modelis also of relevance to non-designated matureplanted or coppiced oakwoods. The report alsoprovided models for the management of arange of other woodland types, includingdeveloping native woodland on clear felled

ancient woodland sites previously occupied byconifer plantations or new native woodland onpreviously unwooded sites. An appropriatemodel for this scenario is the standard rotationhigh forest model which is likely to become thefocus of hardwood timber production in thefuture as such stands mature.

The PW report recommended production of adocument to describe in detail the process ofrestructuring former coppice oakwoods forconservation benefit, and the timber outputswhich are achievable. This report aims toaddress those issues by concentrating on thestand scale, providing managers with achecklist of considerations when determininghow and where to thin and providing guidancefor the assessment of stand qualities.

There are two main aspects:

1. Presentation of results of surveys ofAtlantic oakwood stand qualities.

2. General recommendations for the selectionof stands and trees for thinning andextraction techniques.


Introduction

Figure 1: a mature oak standwhich may have been planted –an appropriate structure toimplement the Long-rotationHigh Forest Model.

This report provides guidance to managers ofAtlantic oakwood for the implementation of theLong-rotation High Forest Model. Criteria areincluded to:

1. Assist with the preparation ofmanagement plans (particularlyformulation of objectives and rationale).

2. Identify appropriate stands andfacilitate the zoning of woodlands intothe different PW models.

3. Identify trees for retention and removalto facilitate marking of selected stands.

Advice is most applicable for Atlanticoakwoods where there is significant lower plantinterest. Some information will also beapplicable to other oakwoods (e.g. generalprinciples such as selecting sites which havepreviously been intensively worked and adviceon extraction).


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Scope

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Stand management trials, co-funded by theEuropean Community LIFE programme, arebeing established in Gleann Geal Atlanticoakwood, Ardtornish Estate (Lochaber, WestHighlands of Scotland). The trials aim toassess:

1. The impact of different thinningintensities on lower plants.

2. The opportunities for production ofoak timber.

3. The response of canopy trees totreatment.

4. The light requirements of nativetree seedlings.

Prior to undertaking the thinning operations,three surveys have been completed to establishexisting values, namely: bryophytes (by BenAveris, consultant bryologist), lichens andveteran trees (by Peter Quelch, ForestryCommission Scotland’s Biodiversity Advisor)and timber quality (by Gordon Gray Stephens,Scottish Native Woods, Argyll). This reportpresents results from those surveys, andprovides an approach to synthesising theinformation obtained and discusses howcompromises may be reached.

The objective of the lichen / veteran andbryophyte surveys was foremost to ensure thatno features of a high conservation status weredirectly removed (i.e. those that occur on thetrees as epiphytes or features of the treesthemselves) and that conditions will bemaintained for high value lower plantsbenefiting from the shade and humidityprovided by the tree in question. The second

objective was to identify those criteria whichcan be assessed by non-specialists whenmarking stands for thinning.

The objective of the timber survey was to allowus to identify: a) the proportion of trees within astand which are of potential value for timberproduction; b) the proportion of those treeswhich are also of value for biodiversity, and; c)the degree to which high value trees can befavoured in a thinning operation fordevelopment. The survey also identifies arange of end uses for the selected trees (seeTable 1).

The findings of the three surveys are specific tothe woodland where they were undertaken.However, information recorded in these surveysshould provide a basis for generic guidance formanagers of other Atlantic oakwoods asconditions within this woodland are broadlyrepresentative – many stands have beeninfluenced by a period of intensivemanagement for production of raw materials(e.g. charcoal and tanbark).

In Scottish Atlantic oakwoods, the period ofindustrial use was typically between 1730 and1860 (it is unclear how old trees within thesestands are but they appear to have last beenmanaged during this period). Stand structure inthe majority of Atlantic oakwoods is thought tobe unnaturally uniform. Gleann Geal is similar toother woodlands in this area, in that there aremany signs of stand enrichment with oak, andsome whole stands appear to have beenplanted. Species composition is thought to beartificially dominated by sessile oak (Quercuspetraea) with only minor components of downybirch (Betula pubescens) and rowan (Sorbusaucuparia).


The survey

Background to the experiment and surveys

It should be noted that the followingmethodology was designed for researchpurposes and is too costly and time consumingfor normal woodland management. A morepractical approach to tree selection isdescribed later in this report.

The trial consists of twelve 0.1 hectare plots.Selection criteria for the plot locations were:

• high forest;

• comparatively uniform stands;

• >50 trees/plot, and;

• lack of management within the last 50 years.

Trees within each plot have been numbered andtheir approximate position mapped. Thesurveys took place during August andSeptember 2003.

Each surveyor was asked to classify the interestwithin these stands into three importanceclasses. The classes refer to the tree and itsassociated species / timber quality rather thanindividual species which may occur on the treeor within shade cast by it. Surveyors wereasked to examine each tree, score it and, whereappropriate, make notes on particular qualities.They were also asked to refine importanceclasses as the survey progressed and weigh upthe level of interest within this woodlandcompared to other Atlantic oakwoods in thearea.

Importance class oneTrees with the highest values (biological ortimber) in the woodland. Given the importanceof Atlantic oakwoods for lower plants, this classalso includes trees supporting or providingshade to some nationally uncommon bryophyteand lichen species.

Importance class twoTrees supporting moderate value lower plants;not particularly uncommon in the westernHighlands but rare in a European context. Forveteran trees and timber trees, of moderatecurrent value and moderate future potential.

Importance class threeTrees providing conditions for lower plantswhich are common in western oakwoods andthose of low timber quality, little potential forfuture veterans.

Limiting treesTrees limiting the development of class one ortwo timber trees were also recorded.


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Survey methodology

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Biological values

Despite the relatively uniform nature of standssurveyed, 39% of trees surveyed had class 2biodiversity values and 16% had class onebiodiversity values. Figure 2 shows the numberof counts for biodiversity values (note that onetree may have more than one count – e.g. be ofvalue for lichens and bryophytes).

The following observations were made:

• There does not appear to be a goodrelationship between those trees that aregood substrates for lichens and those thatare good for epiphytic bryophytes.

• Many of the trees that are good for lichensare also existing or potential veterans.

• There is not a clear relationship betweenstanding basal area (used as a surrogatemeasure for light levels) and biodiversityrichness.

• Bryophytes on the ground appear to bemore influenced by the presence of suitableniches such as boulders and rock outcropstogether with localised shade rather thangeneral stand conditions.

• Lichens were frequently associated withtrees on crags where management may nothave been so intensive in the past. However,they were also encountered within uniformstands if sufficient light levels existed, sopredicting where good lichen trees werelikely to be was not straight forward andeach tree had to be checked. This was alsothe case for epiphytic bryophytes.


Survey results

Figure 2: Count of biodiversity values.

Timber values

• A mean of 16% of trees per 0.1ha plot werein importance classes 1 or 2 for timberquality.

• The number of trees per plot with timberquality in class 1 or 2 was highly variable(maximum number of trees 21, minimum 5with a standard deviation of 4.55).

• There was no relationship with timberquality and biodiversity values.

• The survey identified that many trees not inclass 1 or 2 contain timber which could beused for short lengths for flooring or similaruse, knees for boat repair, crucks for greenoak buildings or features for craft use.Overall, 19% of trees out with classes 1 and2 contained utilisable material.


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Figure 3: Count of trees in timber quality classes 1 and 2

(NB data were not collected for plots 8 and 11).

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Synthesis

Figure 4 shows the effect of combining theinterests within each plot and the influence ofone value on another. Trees which are shadingbryophytes of moderate importance are likely tooffer the best opportunities for compromise as,if felled and extracted, they would not bedirectly removing interest or altering the micro-climate of bryophytes with higher conservationstatus. The following assesses the effect ofremoving some trees in this category:

• If all biodiversity values are protected, 23%of trees with timber quality in class 1 and 2could be felled. This would rise to 38% ifthose trees which also provide shade forclass 2 bryophytes are felled.

• A large number of trees were considered tolimit the growth of timber class 1 and 2trees. Figure 4 shows that many of these“limiting trees” also have biodiversity values(lbv) and opportunities to improve the futuretimber quality are limited (particularly inplots 3 to 7). Again, there are moreopportunities to favour the development oftimber trees if some trees shading class 2bryophytes are removed.

• The development of some class 1 or 2veteran and lichen trees was also limited bytrees shading class 2 bryophytes.

In some cases, to realise objectives of thinning,some trees with moderate value epiphytes mayneed to be felled. Criteria to identify whereremoval of a tree will minimise impact areprovided later in this report.

In plots with medium biodiversity values,finding sufficient trees to remove 25% ofstanding basal area was possible if some treesshading bryophytes of moderate importancewere identified for removal. Two plots hadsufficiently low biological values to makeremoval of 25-30% of standing basal areastraightforward. We found that it would bedifficult to remove more than 10% of standingbasal area in plots with high biological values.


Explination of Figure 4.

• “low interest” indicates trees within plots which were assigned less than importance class 2 forall criteria.

• “limit only” are trees which are limiting the development of trees in timber quality classes 1 or 2.

• “timber only” are trees with timber quality classes 1 or 2 but no other interest.

• “ibv timber” are trees with biodiversity values and timber quality in class 1 or 2.

• “ibv limit ” are trees with biodiversity values in class 1 or 2 which are also limiting the developmentof timber quality trees – class 1 or 2.

• “ibv only” are trees with biodiversity values in classes 1 or 2 and low timber quality, no potentialto improve timber quality.


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Figure 4: Synthesis of biodiversity values and timber quality

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Timber quality – Gordon Gray Stephens

• The economics of timber productionsuggest that the small quantities of largelylow grade timber being produced at GleannGeal would be unlikely to be extracted on acommercial basis. However, supply oflocally produced material may be put togood use by the local community and helpto stimulate rural development.

• The activities of woodland groups andinitiatives such as Argyll GreenWoodworkers, Scottish Native Woods andHighland Birchwoods have demonstratedthat there is the potential to add value to lowgrade hardwoods in the Highlands.

• Experience indicates that wheeled vehiclesshould only be used in western oakwoodswhen there has been a prolonged dryperiod. Use of vehicles during orimmediately after wet periods can causeconsiderable difficulties.

There is a good fit between the timing of thistrial and the development of a woodland groupin Morvern. To foster wood skills and craftsamong the local community, use of some of thematerial from the thinning trials is suggested forone of the following projects:

1. A cruck-framed building constructed as acommunity facility

2. A project centred around the reconstructionof a west coast birlinn could be achieved ona number of levels from construction of anindividual boat to the development of abirlinn centre.

3. Distribution of small quantities of processedoak to local craftspeople and woodworkerswith a reward for the best innovative use ofthe material.

[In recent years, the Sunart Oakwoods Initiative(SOI) has encouraged woodland-relatedcommunity activity and rural developmentaround Loch Sunart, initially focusing on thenorth side of the loch. In 2003, the Initiative wasextended into Morvern. The MorvernWoodlands Project is funding native woodlandrestoration and access improvements, anddeveloping community involvement in thewoodlands of Morvern, through training,employment and events, delivered under theumbrella of the SOI.]


Key observations from surveyors

The following are key findings from the surveyors’ reports. Some of the comments are quoted directlywhilst others are summaries of more detailed information.

• The two most frequently encounteredbryophytes of higher conservation status inthis survey (Adelanthus decipiens andPlagiochila atlantica) are in Scotlandstrongly associated with native deciduouswoodland with a long-recorded history ofcanopy cover (‘Ancient’ woodland). Theyare most common on SE-facing slopes andappear to demand a delicate balance ofhigh atmospheric humidity and moderateamounts of light and warmth. This balanceis likely to be upset by felling, especially asthis would immediately let in much light onsoutherly aspects.

• The shade cast by trees to the S-SW ofPlagiochila atlantica appears to beimportant in shading the liverwort from thestrongest sun in the middle of the day andthe afternoon, when conditions can bedriest after morning dew has evaporated.

• Bryophytes of moderate importance requireshaded, humid woodland. They are notparticularly uncommon in the westernHighlands but are important species in aEuropean contex.


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Table 1: Timber grading.

End use Characteristics Comments

Planking / beam Minimum 2.5m straight stem Sought after timber: most western without major visible defects. oakwoods contain a very small number &Minimum dbh of 45cms. Gleann Geal is no exception.

Small beam Minimum 3m straight stem without Small beams are a valuable structuralmajor visible defects. timber, fit for most domestic buildings. Minimum dbh of 30cms.

Craft use/flooring Small straight lengths & pippy trunks. Previous work by Highland Birchwoodsincluded acquisition of a mobile doubleslabber mill. This allowed 4 foot sections tobe converted into flooring. Craft workers can find a good use fortimber with “defects” .

Use in green oak buildings Sound timbers with swept or “bent” In addition to the beams and small beamsstems or major branches can be used certain “bent”. This is a small, but growing as cruck blades, collars and braces. market with good potential to add value.

Wooden Boat repair 90 degree bends from major Knees for wooden boats are a major marketbranches and v forks. in America, and one which has not been

exploited (to the surveyor’s knowledge) inScotland.

Bryophytes – Ben Averis

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Table 2: Importance classes for bryophytes in Gleann Geal.

(NB the status of all species below is likely to be higher in woods further east and south of the study area)

Status Species

High Adelanthus decipiens, Aphanolejeunea microscopica, Harpalejeunea molleri,

Plagiochila atlantica and P. exigua.

Moderate Bazzania trilobata, Hylocomium umbratum, Lejeunea patens, Mylia taylorii, Plagiochila

killarniensis, Plagiochila punctata, Plagiochila spinulosa, Ptilium cristacastrensis,

Saccogyna viticulosa, Scapania gracilis and Sphagnum quinquefarium.

Hymenophyllum wilsonii (Wilson’s Filmy fern) is included as an ‘honorary’ bryophyte.

• Any guidance on recognition of lichens toindicate habitat richness for thinning, shouldbe restricted to the larger, more easilyrecognisable species. Rare crustose andscriptose lichens are difficult to identify onoak and are more typically found on smoothbarked species within oakwoods (e.g.rowan) or in ash and hazel woods.

• Different tree species often supportcompletely different lichen flora; additionallythese flora change as the trees mature.Lichens do not grow at random on any treesbut in distinct communities which varyaccording mainly to bark acidity. The mostacidic trees are pine, birch and alder. Bycontrast ash, sallow and hazel are muchless acidic and have a richer flora of lichenscontaining more ancient woodland indicatorspecies. Young oak are acidic but becomeless so as they mature.

• The term veteranicity was adopted to showthe potential for the tree to become aveteran oak of the future. Class 1 wasprobably large already with an open growncrown and a well lit trunk (which is also likelyto increase its lichen substrate quality).Other features of veteranicity were: existingor incipient rot holes; forks; burrs; buttressswellings; and large coppice stoolformation. Hence, sometimes a small groupof stems was chosen to become a bigcoppice stool group of stems for the future.These were left unthinned especially if theywere fairly open grown, with each havinggood crowns.

Lichens and veterans – Peter Quelch

The colonisation and succession of lichens isakin to a gradual colonisation and successionof higher plant species over time on a bare soilsite, which we are more familiar with. The youngtree is colonised by certain common crustosespecies (e.g. Lecanora chlarotera) when it isyoung. These species are fast colonisers andtherefore of no use as ancient woodlandindicators. With continuing succession, moreinteresting species may colonise (e.g.Thelotrema lepadinum – a fairly easilyrecognised crustose species with characteristic“volcanoes”. This species is only found inancient woodlands and was fairly abundant inthe Gleann Geal plots). This is followed by a

successional stage when big foliose lichens,particularly Parmelias, dominate - the GleannGeal plots are more or less at that stage. Whenthe trees mature enough for Frullania tamarisci(a purple leafy liverwort) to become established,larger foliose lichens move in on top of thatliverwort and then lungworts and Stictadominate. Smaller foliose lichens like thePannarias come in at that stage but never seemto dominate, though Degelia can reach largesizes but mainly on ash. Rare lichens like thePseudocyphellarias seem to be at the top ofthat pyramid of succession. [This species wasencountered in the woodland surveyed butoutwith the plots].


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There are several reasons why an owner /manager may wish to thin stands within Atlanticoakwoods:

• To prepare a stand for natural regenerationand increase the abundance and diversity ofvascular plants. Dense stands which havebeen subject to intensive grazing may be ina poor condition for acorns to germinateand advanced regeneration to establish.Wavy hair-grass (Deschampsia flexuosa) isoften dominant, with little accumulation ofleaf litter; acorns are more vulnerable topredation and, in dry years, to desiccation.A light thinning combined with reduction ofgrazing animals, leads to increased lightlevels, and the development of small herbsand dwarf shrubs together with deeper litterlayers.

In addition, crowns of dominant oak shouldbe more capable of bearing seed if givenroom to expand. These conditions shouldallow for more successful germination andestablishment of oak advancedregeneration. Recruitment of thatregeneration into the canopy would ofcourse require development of largercanopy gaps subsequently.

• Species other than oak were often removedin intensively worked coppices. Therefore,today, there is only a scarce representation

of species such as birch, rowan, holly andash, which would naturally occur morefrequently. Where tree species other thanoak occur, thinning may be targeted to givethese trees more growing space and todevelop crowns for future seed production.

• All stands have a variety of tree sizes andforms. It may be desirable to develop anumber of stems in each stand to providecandidates for future veteran trees (see thelater sections on selection criteria to identifycandidates).

• Stands with very low light levels (i.e. wherestanding basal area is >38-40m2/ha) mayhave a sparse representation of old growthlichens or other epiphytes such as polypodyferns (Polypodium species). Where this isthe case and important bryophytes will notsuffer from increases in illumination, lightthinning may be appropriate to increaselichen and fern abundance, providedsufficient humidity can be retained.

• The traditional reason for thinning is todevelop timber quality and provide earlysupplies of timber. Demand for locallygrown hardwood timber is increasing and itsutilisation is seen as a potential vehicle forrural development in more remotecommunities.


General recommendations for thinning in Atlantic oakwoods

Why thin?

• Loss of shade and humidity:In general, most lichen species preferhumid, dapple shade conditions. Standswith a large number of stems per hectareappear to be too dark for an abundantlichen flora to develop. Analysis of data fromthe experiment at Gleann Geal is required toprovide clearer guidance. Some lichenspecies (e.g. Dimerella lutea and Stictas) areunderstood to prefer more shadedconditions (Averis and Coppins, 1998).

Bryophyte species generally prefer moreshaded conditions to lichens but similarlevels of humidity. As stated by Averis in thesurvey for this report, some of the rarerbryophytes prefer very humid andmoderately lit conditions found undermedium density tree canopies on southfacing slopes. It is thought that only a smallreduction in the tree canopy could causehumidity levels to drop too low forparticularly rare species to persist.

• Loss of ecological continuity:Rare woodland lichens and bryophytesrequire continued woodland cover toprovide the conditions describe above. Anylarge scale disturbance which removeswhole patches of old growth species (i.e.ancient woodland indicators) either directlythrough felling and extraction of groups ofhost trees or indirectly through loss ofappropriate micro-climate conditions, willbe very slow to recolonise due to poordispersal abilities of species concernedand, for epiphytic species, the need forsuccession.

• Windthrow:As discussed below, windthrow is the mostlikely natural disturbance mechanism forAtlantic oakwoods. However, this is unlikelyto be seen as a benefit from thinning unlessit occurs sporadically.

• Loss of “wilderness” value:Due to the long period withoutmanagement, these woodlands areconsidered by many members of the publicto be ‘natural’ in their species compositionand structure. Obvious signs of recentmanagement are likely to reduce theaesthetic appeal of these woodlands in theshort-term although development of largecharacterful trees and installation of multi-purpose paths (for public access andextraction) are likely to result in net publicbenefits in the longer term.

• Extraction:A lot of thought needs to be given toextraction routes, timing and choice ofequipment to ensure that soil disturbancedoes not become a problem (see thesection on extraction and conversionbelow).


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Potential problems with thinning

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The role of natural processes

Figure 5: This stand has been thinned withinthe last thirty years. It now shows much greaterpotential for timber production than elsewherewithin the wood. A stand thinned for a varietyof objectives would exhibit greater diversity interms of tree size and form.

Stands which have arisen from intensivemanagement will eventually begin to diversifythrough natural processes. For example, self-thinning is occurring in many of the plots inGleann Geal: coppice stools are self singlingand suppressed maiden trees are dying. Overthe long term, this may lead to promotion of themore dominant stems. However, self thinningtends to lead to a uniform distribution ofdominant trees and suppression of lightdemanding species. Occasionally, naturalprocesses can cause larger scale disturbanceover much shorter time spans. Storms can havea major impact, snapping and blowing downcanopy trees to provide high volumes of largediameter deadwood and increased light levelsfor lichens, vascular plants and naturalregeneration. However, examples of theseevents are rare and relying on such processesto diversify stands in small woodlands is a riskystrategy.

The level of intervention in the context of historic management

Intensive oak coppicing regimes during the18th and 19th centuries were of small extentand duration compared to woodlands furthereast (Smout, 1997) and therefore species couldsurvive and move back into adjacent areasmore successfully. Lower plants probablysurvived more intensively worked areas withrefugia in gullies, on rock outcrops and onveteran trees on adjacent boundaries or in

adjoining wood pasture. They have achievedtheir current condition due to a long period freefrom disturbance combined with suitableclimatic conditions. Therefore, despite someevidence that patches may have been workedintensively for a short period, returning to anyform of patch clearance in ancient woodlandslarger than that needed to allow regenerationwould be inappropriate.

Stands most likely to be suitable for thinningare those which have been coppiced intensivelyor those planted and subsequentlyunmanaged. These may be large relativelyuniform areas or, more typically on brokenground in the west, smaller components withina range of different stand types (see Figure 6).

In most cases, a number of objectives will existfor each stand but the balance between themwill need to be prioritised. This will beinfluenced by characteristics such as; standorigin, the density of trees, topography withinthe stand and position in the woodland (e.g.lower top height of trees at higher elevation).For example, improvement of a stand for futuretimber production is likely to be targetedtowards stands with heavy stocking and limitedbiodiversity values; and creating gaps forregeneration is likely to be most appropriate instands of lower basal area and limited lowerplant interest. (Two plots matching thisdescription occur in Gleann Geal - the limitedlower plant interest is thought to be due tolower humidity) .

The following describes broad habitatindicators where anything more than verylocalised thinning is unlikely to be appropriate:

Although intensive management had largelystopped on the west coast of Scotland by1860s – 1880s, sections of some moreaccessible stands may have beenopportunistically worked for firewood, fenceposts etc. Where this took place longer than 50years ago, lichen communities have oftenresponded to increases in light and anysubsequent thinning may be difficult to achievewithout directly removing trees with a highabundance of important epiphytes.

Some topographic features within Atlanticoakwoods are likely to contain flora moresensitive to disturbance and are therefore bestavoided in thinning operations unless specialistadvice indicates that species would benefit.Rock outcrops and steep stream side banks aretypical examples. These create conditionssuitable for lower plants due to shelter andhumidity and often to the lack of access forlarge-scale disturbance in the past. Groundwater seepage from rocky slopes often causessoil base enrichment which provides conditionsfor small components of upland mixedashwood.

Areas of former wood pasture often containlarge open grown trees showing evidence ofpollarding. Such areas can also contain remainsof dwellings and shelter for livestock, togetherwith sunken trackways and field boundaries.Former wood pastures are likely to be rich inbiodiversity and contain important heritagefeatures, therefore thinning is unlikely to beappropriate. However, gaps may have beenenriched with planting or naturally regeneratedinto a thicket. Some judicious thinning aroundveteran trees may be appropriate here toimprove light levels for lichens and the health ofveteran tree crowns. In such cases specialistadvice should be sought.

If the stand(s) has been surveyed by alichenologist, their species list can becompared with indices of ecological continuity(Coppins and Coppins, 2002) to determine thelevel of disturbance that the site hasexperienced. Thinning will be inappropriatewhere the relevant index indicates an importantsite.


20

Selecting stands for thinning

21

Removal of 10% of standing basal area is onlylikely to produce subtle changes (e.g. enoughto increase light levels for lichens in heavilystocked stands). For most objectives, removalof 20-25% of standing basal area is likely to berequired to make any significant difference tothe growing condition of trees retained, be it forfuture timber production or development ofpotential veteran trees.

A thinning cycle of anything less than 10 yearsis likely to be too short given the age of treesand their potential to respond. Cycles in excessof 20 years may prove to be appropriate. Theword cycle is perhaps misleading, indicatingthat a similar volume may be removed insubsequent operations. In the case of Atlanticoakwoods, future operations are likely to beclearly targeted to improving the condition ofidentified timber and veteran trees, removing aproportion of remaining timber trees or creatinggaps for recruitment of regeneration.


Thinning intensity and cycle

Figure 6: a stand with uniform structure which indicates that thinning may be suitable

(photo: Peter Quelch).

The purpose of thinning is the firstconsideration. The following describes howdiffering objectives affect the choice of trees tomark for felling:

Regeneration

If the aim is to create gaps for regeneration,then removal of some bigger crowneddominant trees may be appropriate. Otherwise,it will be necessary to take a number ofneighbouring trees (co-dominants and sub-dominants) to create sufficient light andfreedom from other canopy interactions (e.g.canopy throughfall of defoliating invertebrates –Harding, 2002). Given the level of biologicalinterest in the Gleann Geal plots, somereduction in suitable micro-climate conditionsfor ground-dwelling bryophytes of moderateimportance and, possibly, removal of aproportion of trees with moderate valueepiphytes, is likely to be necessary to achievesufficiently low canopy cover for recruitment ofoak seedlings. (See sections on selection forcriteria to select appropriate trees).

Timber production

The aim of a thinning operation for timberproduction should not be to purely harveststems of the best form, but to leave aproportion of these (for future production and toensure that a full range of genotypes aremaintained) and take some poorer formed treeswhich are limiting the development of the futuretimber resource. It will rarely be necessary toremove suppressed trees as these will behaving little effect on the growth and form oftrees retained for development (the exceptionwill be where they are causing an obstruction to

felling of more dominant stems). The initialstocking density and range of size classes willinfluence whether a crown thinning orintermediate thinning is undertaken (crownthinning concentrates on removing co-dominant trees in the canopy and intermediatetakes some co-dominant and some sub-dominant trees).

Improving conditions for veteran trees andpoorly represented tree species

It is usually obvious which trees are acting asdirect competitors. Veterans and minor treespecies are often sub-dominant andsuppressed. Consideration should be given tothe potential for these trees to respond to theremoval of competitors and likely life span ofthe trees concerned, particularly where the treeto be removed may have other biologicalvalues. Where veteran trees are dominant,removal of competitors will generally bepositive but care should be taken with existingveterans which have adapted to a closed highforest structure, not to create too large a gaparound the crown otherwise their condition maydeteriorate (Read, 2000).

Lichens

In general, increases in light in previouslyunmanaged ex-coppice or plantation standsare likely to be beneficial for epiphytic lichens,as long as a suitable level of humidity can bemaintained. However, the degree to which thisis the case varies between species. A fieldguide for lichen identification is planned and itis recommended that information on lightrequirements is included for each of the speciesdescribed.


22

Identifying the value of individual trees

Objectives

23

Creation of deadwood

Where objectives other than timber productionare involved, substantial amounts of deadwoodmay be created by thinning. This is still likely tobe the case where timber production is a maingoal unless extraction is very straightforwardand low value material such as firewood can beextracted economically. In such circumstances,positive steps will need to be taken to retainsufficient proportions of felled timber fordeadwood (Peterken and Worrell (2001)indicate that around 40m3/ha is appropriate).

Summary of criteria for tree retention /selection

Veteran trees(NB the criteria below are not mutually inclusive)

• open grown

• well lit around the trunk (or the potential to create sufficient light)

• good epiphytes (ferns and bracket fungi as well as lichens and bryophytes)

• large stem diameter

• existing or incipient rot holes

• fused stems (see figure 7)

• forks

• burrs

• buttress swellings

• large coppice stool formation

• large diameter dead branches

• trunk hollowing

• water pockets

• fallen trees which are growing up againfrom the main stem or root (known asphoenix trees)


Figure 7: Fused coppice stem –candidate for a future veteran tree (photo: Peter Quelch).

Lichens

In order to identify whether the lichencommunity on a particular tree should beretained, the interest within each woodlandshould be calibrated. Some species areobviously of high importance (e.g. Lobariaamplisima) or low importance (e.g. Parmeliasaxatilis). However, it may be difficult to assessthe value of a number of other species and thefollowing criteria should be used to weigh uptheir status in the woodland concerned:

• National and international status: Reference can be made to Woods andCoppins (2003) to determine theconservation status of different species. Themajority of species which constitute class 1trees in this study, are classified as speciesfor which Britain has internationalresponsibility. Particular rarities such as thePseudocyphellarias have additional statusas Near Threatened, and Nationally Scarce.

• Abundance: If the lichen species is well represented inthe stand and the tree in question only hasa very low abundance, this will reduce itsrelative importance.

• Scarcity within the woodland: If a species is generally considered to be oflow to moderate importance but it onlyoccurs on a very small number of treeswithin the woodland, retention of the treesconcerned should be considered tomaintain the full suite of species.

• The community of species:Whilst individual species may be relativelycommon, if a community of lichens (e.g. theParmelietum laevigatae) is particularly welldeveloped on a tree within a stand with anotherwise poor representation, the tree maybe considered an important example to beretained.

Weighing up these criteria will require training,experience and advice from specialists (seelater).


24

Figure 8: Large patch of the lichen Lobaria virensmaking this tree an obvious candidate for retention

(photo: Peter Quelch).

25

Bryophytes

A similar approach to the one described abovefor lichens can be adopted for bryophytes. Inaddition, for non-epiphytic bryophytes, it ispossible to identify a range of habitat types withwhich they are often associated:

• Rock outcrops and boulders:On steep surfaces, small oceanic liverwortscan survive because larger, competitivebryophytes are unable to get a “foothold”(Averis and Coppins (1998) and see Figs 9and 10).

• Steep-sided ravines:The atmosphere is very humid and a gooddiversity of rock habitats are present (Averisand Coppins 1998). Trees on the edge ofsuch ravines influence the level of humidity– a belt of unthinned trees should beretained.


Figure 9: A typical boulderimportant for Atlantic

bryophytes.

Figure 10: Close up of asection of the boulder

showing PlagiochilaAtlantica (centre) a class 1bryophyte in Gleann Geal.

Timber trees

Criteria are summarised in table 3.


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Importance class Features

1 Dominant, balanced & good crown, straight main stem with timber volume, minimal

deadwood.

1-2 Dominant tree with good crown, straight main stem with some small branches

allowable, small quantities of deadwood allowed.

2 Co-dominant tree with two of the following: moderate crown, straight stem, small

quantities of deadwood.

2-3 Sub-dominant tree with two of the following: moderate crown, straight stem,

moderate amounts of deadwood.

Table 3: Importance classes for timber quality.

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Selecting trees in practice

Unless surveys are undertaken by lower plantspecialists and all interest is clearly identified onthe ground, each potential candidate tree forremoval will need to be checked for epiphyticinterest, the qualities of the tree itself and thevalue of shade cast by it.

Many oakwood managers have a good workingknowledge of lower plants and identification ofthe more obvious species may be relativelystraight-forward. Where this knowledge is notavailable and specialists are not taken on tocarry out a full survey, it is suggested that sometraining is undertaken. To avoid the need for alarge number of people to be trained and toallow consolidation of training, it isrecommended that individuals likely to beinvolved in the marking of a number of woods(e.g. woodland initiative project officers) aretrained. A two-day course on Atlantic oakwoodbryophytes and another on the Lobarion lichencommunity and associated species shouldprovide oakwood managers with sufficientknowledge to identify the more conspicuousspecies.

It is worth approaching specialists even if aprofessional survey is not planned as manywoodlands have been surveyed in the past andthere may be existing knowledge about thelocation of rare species or reassurance thatinterest is very limited. At the least, some inputfrom specialists is recommended to indicatethe relative value of different species for thewoodland in question. Scottish NaturalHeritage or Forestry Commission offices shouldbe able to provide details of lower plantsurveyors.

Marking trees for thinning is a complex processof decision making, even in a conifer plantation.An initial period of familiarisation is normallyrequired when beginning to mark each newstand. As discussed previously, thecharacteristics of the stand will influence thebalance between different objectives forthinning. This will in turn affect what type of treeis selected for removal (e.g. dominants mayneed to be removed to create sufficient gaps forregeneration). The approach suggested is totemporarily mark those trees which shouldideally be removed, then assess them forbiodiversity values. It is often possible to selectan alternative tree and assess the relativeinterest between the two. The intensity ofthinning will again be tied to objectives andcharacteristics of the stand. It will be helpful toknow the standing basal area and set a targetfor removal to assess whether the correctintensity has been reached (Hamilton andChristie, 1971).

A major consideration when selecting a tree forremoval is the ease with which it can be felled.Damage to crowns of adjacent trees isinevitable when felling dominants and co-dominants but if a tree is likely to get hung upand a winch isn’t available during fellingoperations, this should not be selected onsafety grounds.


This should be a primary consideration wheretimber production is a key objective. Wheretracks c.2m wide exist, equipment such as allterrain cycles (quads) with specialistattachments or mini-forwarders may beappropriate. However, away from tracks,wheeled vehicles can cause a lot ofdisturbance in wet conditions and are notsuitable where there are sections of side slopeor frequent boulders. Horses may beappropriate for extraction across minor sideslopes but, again, they are likely to causedisturbance in wet conditions. Where volumesare very low, a range of more labour intensiveequipment may be appropriate, particularlywhere the operator is a crofter or craft workerfor whom timber extraction only represents asmall proportion of his or her working time andthe capital outlay on more sophisticatedequipment is not possible. Examples of thistype of equipment would include chainsawwinches with skid cones and chainsaw mills forconverting logs into boards, posts and beamsat stump. Extraction of low volumes overuneven terrain may be possible using simpleequipment such as the iron horse.

Routes should be continually assessed duringextraction and operations ceased if the weatherdeteriorates. Where possible, a number ofroutes should be used for more flexible optionssuch as the horse or iron horse to reduce theconcentration of ground disturbance. If logscan be extracted prior to conversion, a mobilebandmill will be appropriate where sufficientvolumes of suitable material can be collected.In more remote Atlantic oakwoods, transportand accommodation costs will need to beconsidered if hiring one of these saws fromanother area.

Unless static winching onto a hard trackwaycan be used, it is not realistic to undertakeextraction without some impact on the ground.Limited soil disturbance (e.g. cutting shallowchannels through the humus layer and the first2 or 3 cm of the A horizon) should heal within 2to 3 growing seasons. Where possible,consideration should be given to constructingnarrow all terrain cycle tracks which can beused for future extraction, recreation routes andextraction of deer carcasses. The route andspecification of these would need to be agreedwith Forestry Commission woodland officers(and Scottish Natural Heritage if the woodlandis designated).


28

Extraction and conversion

29

It is possible to put the Long-rotation HighForest Model into practice with careful selectionof stands and individual trees together with aconsidered and flexible approach to timberextraction. The following should ensure thatstructural diversity increases at the woodlandscale in line with the indicators of favourablestatus outlined in the model: protecting andenhancing directly observed biodiversity values;protection of habitats indicating richerbiodiversity; and selection of stands accordingto their access for timber extraction.

Creating gaps for regeneration may not be anissue, i.e. an open canopy may exist alreadydue to trees dying and past grazing levelslimiting regeneration. Alternatively, there maybe opportunities to expand the existingwoodland, encouraging regeneration at thewoods margins. In both cases, somemanagement will often be required to achievethis (e.g. bracken and herbivore control). Therewill be other cases where it is desirable todiversify the age structure and creation ofcanopy gaps is necessary to achieve this in theshort to medium-term (e.g. in an isolatedwoodland or a series of woods dominated byone age class, with a closed canopy and noopportunities for expansion onto adjacentground). In Atlantic woods with lower plantinterest, some thought will be needed todetermine where such gaps are created. Ideally,stands with a low basal area and little lowerplant interest should be selected. Where suchconditions don’t occur, compromises will needto be made and relative values of treesassessed as discussed in the selectionsections. In all cases, grazing pressure shouldbe sufficiently low to allow advancedregeneration to exist under a canopy beforegaps are created.

Volumes of high quality timber were low in thewoods surveyed but innovative use of smalldimension stock by local craftsmen or nichemarkets for cruck frames or boat repairsincreases the potential for Atlantic oakwoods topromote rural development.

If all but the most common lower plants areprotected from any impacts of thinning, itbecomes difficult to thin at a sufficiently highintensity to make any difference to the growingconditions of retained trees. Moderate valuebryophytes on the ground (i.e. those that arerelatively frequently encountered in westernoakwoods but are uncommon nationally) offerthe opportunity for compromise as felling treesthat shade them will not directly remove interestand such species are typically less exactingthan rarer species, and hence are more capableof withstanding changes to their micro-climate.Criteria within this report should help to identifywhich trees with moderate value epiphytes canbe removed without significantly reducingbiodiversity values at the stand scale.

Where oakwood managers lack knowledge oflower plant identification, short courses arerecommended for lichens and bryophytes ofAtlantic oakwoods. Specialists should beapproached for any existing knowledge of thewoodland concerned and commissioned to doa survey where particular rarities are suspected.

Thinning has the potential to develop awoodland towards favourable condition,particularly for recruitment of regeneration,lichens, veteran trees and increased volumes ofdeadwood. If carried out in the wrong place,too intensively or without regard to thebiodiversity values, it also has the potential tobreak long periods of ecological continuity and


Conclusions

create an unfavourable micro-climate for somelower plants.

Choice of extraction equipment, routes andtiming are primary considerations to avoidunacceptable levels of ground disturbance.

More information is needed to predict theimpact of thinning in mature Atlantic oakwoodson lower plants. The experiment established at

Gleann Geal should allow us to assess theresponse of a range of species to very light andmoderate thinning intensities. It isrecommended that future guidance on lichenidentification includes details of lightrequirements so that managers can predict theimpact of thinning. A field guide to theidentification of Atlantic oakwood liverworts isalso recommended.


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Acknowledgements

We are grateful to Ardtornish Estate for hosting the experiment and surveys. Peter Quelch, GordonGray Stephens and Ben Averis are thanked for undertaking the surveys and providing subsequentinformation and illustrations. Dave Tracy is thanked for establishing the experiment and assisting withthe surveys. Thanks to Joe Hope and Jamie McIntyre for their comments on the report and to TimClifford, Stephen Corcoran, Phil Baarda and Life Nature for assistance with funding. Colin Saunders,Derry Neil, Jamie McIntyre, Kirsty Barr and Jake Willis are thanked for their advice on timberextraction. Feedback from Brian Eardley and Ian Collier is also gratefully acknowledged.

Conclusions cont.

31

Averis, A.B.G. and Coppins, A.M. (1998) Bryophytes, Lichens and Woodland Management in theWestern Highlands. Scottish Natural Heritage order number L000337

Coppins, A.M. and Coppins, B.J. (2002) Indices of Ecological Continuity for Woodland EpiphyticLichen Habitats in the British Isles. British Lichen Society.

Hamilton, G.J. and Christie, J.M. (1971) Forest Management Tables. Forestry Commission Booklet 34, HMSO, London

Harding, D.H. (2002) Where have all the caterpillars gone? Quarterly Journal of Forestry Vol. 96, No.3

Peterken, G.F. and Worrell, R. (2001) Conservation management of the Sunart Oakwoods SAC andthe potential for supporting rural development Report for Caledonian Partnership for the Life '97project

Quelch, P.R. (2002) The Sunart Oakwoods: A Guide to their Sustainable Management. Report to EU Life Nature

Read, H. (2000) Veteran Trees – A Guide to Good Management. English Nature, Peterborough.

Smout, T.C. (1997) Highland landuse before 1800: Misconceptions, Evidence and Realities. In Smout, T.C. (ed.), Scottish Woodland History: 5-23. Scottish Cultural Press.

Woods, R.G. and Coppins, B.J. (2003) A Conservation Evaluation of British Lichens. British Lichen Society, London


References

This model would apply to woods where an existingmature structure is to be maintained, but from whicha limited harvest of timber would be extracted. Thebasis would be rotations of 150-200 years,combined with small-scale felling and restocking bynatural regeneration, planting, or both. Felling at thisrate would allow 0.5-0.67% of the canopy to berenewed each year, but felling and restocking wouldin practice take place in episodes of, say, 10.0-13.4% restocking at intervals of 20 years. It wouldprobably be practicable to use some of the groundin such woods as pasture. Favourable status wouldbe maintained if:

• Mature and maturing stand structures to bemaintained over at least 70% of the area.‘Maturing’ in this instance would be 50 yearsgrowth or more, the age at which an oakplantation would be expected to develop anunderwood, or the age at which a semi-naturaloak-birch stand would start to appear matureand diverse. The criterion implies that up to 30%of an existing mature oak wood can be felled andrestocked in the next 50 years.

• Rotation of canopy oaks to be at least 150 years.This allows much longer rotations, but ensuresthat the stand remains generally mature.

• Fell and restock patches to be a range of sizes upto 2 ha. This mimics natural disturbance patternsand allows openings large enough to allow oak togrow into the canopy without permitting largeclear-fell coupes. Most gaps would be no morethan the spread of 2-3 mature trees.

• Thinning no more than 30% crown thin in anyone operation. This leaves a mature stand thatwould rapidly fill most canopy gaps by crownexpansion.

• At least 50% of the site to remain unthinnedduring the last 10 years. This avoids disturbing atleast half the site until the rest of the wood hasrecovered from the last operation.

• Canopy trees retained indefinitely to be at least 5per ha. These provide a minimum density of largeold trees.

• Deadwood volume to be at least 40 m3/ha. This isslightly above the values generally obtained inmanaged semi-natural stands.

• Shrub layer covers at least 10% of area in standsover 50 years old. It can hardly be higher if it is toapply to strongly acid oak-birch woods.

• Regeneration predominantly natural by seedlingregeneration or vegetative regrowth. In the eventof regeneration failure, planting confined to oakat low density on less than 50% of availableground.

• Grazing and browsing to be reduced to allowregeneration for at least one period of 15 years inevery 100 years over 50-75% of site. This allowsa pulse of regeneration over a majority in gapsand as advanced regeneration in shade withinthe lifespan of canopy trees.

• Glades and other open space habitats(temporary and permanent) to occupy at least10% of site. Gaps created by windthrow orsilvicultural operations would count as openspace habitats.

This model would maintain existing mature stands,and should be particularly suitable for mature oakwoods. It perpetuates mature stands, but alsodiversifies the structure and helps to generateyounger age-classes. It also enables timber to beutilised. The disadvantage is that stand structuresand ground conditions that have been undisturbedby felling within recent decades would be disturbed,and that some habitat trees for epiphytes would beremoved. Care would therefore be required toensure that extracted trees are not only utilisable,but do not currently support rare epiphytes.


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Appendix 1: Long-rotation High Forest Model (Extract from Peterken and Worrell (2001) p.32).

thinning in atlantic oakwoods – assessing options at … · averis, consultant bryologist),...

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