Zafer Yu¨cesan?Sevilay O¨zc?elik?Ercan Oktan

Effects of thinning on stand structure and tree stability in an afforested oriental beech(Fagus orientalis Lipsky)stand in northeast Turkey

Zafer Yu¨cesan?Sevilay O¨zc?elik?Ercan Oktan

We studied relationships between stand structure and stand stability according to thinning intensity in an afforested orientalbeech stand.Various thinning intensities were applied in sample stands.We sampled eight plots in standsthatwere lightly thinned,eightplotsin heavily thinned standsand eightplotsin unthinned standsasa control.Height and diameter distributions of the stands were measured to assess stand structure.We quantified individualtree stability and collective stability.Heavy thinning during the first thinning operation damaged the storied structure of the stand in thicketstage and affected collective structuring ability.While most control plots had multi-storied stands,after light and heavy thinning two-storied structure became more common. Large gaps occurred in the canopy afterheavy thinning.On average,nine tree collectives were formed persampling plot in the untreated stand,seven collectivesafterthinning in 2008 and four collectives after thinning in 2009.Stable trees accounted for 17%of trees in controlplots,24%in lightly thinned plots,and 15%in heavily thinned plots.Collective stability values were 83%in control plots,82%in lightly thinned plotsand 36%in heavily thinned plots.We concludethat it is necessary to retain collective structuring capacity during thinning operationsforsustaining stand stability.

OrientalbeechThinningStand structureTree stability

Introduction

Thinning is importantto alter stand species composition for better growth in early stages of the stand.Because oriental beech is shade tolerant,canopy density is very important during the thicket stage.The common idea in managing beech forests is to grow them as densely as possible when the trees are young.However,ifthe understory saplings get insufficient light in a dense stand,their growth rate declines.Silvicultural management for the most suitable stand structure is important for timber production.

One of the most important characteristics determining stand structure is storied structure(Saatc?iog?lu 1971).Storied structure refers to the horizontallayers of tree crowns in a stand orthe ability to establish a layered canopy.Since storied structure relates directly to planning and managing stands,it is important in silvicultural operations(Avs?ar 2004).Storied structure depends on the tree species in the stand,their light-demand,and their growth rates.Biotic and abiotic factors,natural succession and silvicultural treatments also affect storied structure.Storied structure may change over time as stands age(Avs?ar 2004).Tabarietal. (2005)suggested that the density of the canopy directly affects tree survival.

Thinning exposes plantations to hazards,especially from wind(West 2006).Tree stability and stand stability are important in early stages of the stand.Stability can be increased by silvicultural treatments.Tall trees with low h[height(cm)]/day[breastheightdiameter(cm)]ratio resist wind and heavy snow(Langenegger 1979;Cremer et al. 1982;Gassebner 1986;Becquey and Riou-Nivert 1987; Lohmander and Helles 1987;Wilson 1988;Oliviera 1988; Wang 1988;Mayer and Ott 1991;Wilson and Olivier 2000;Bachofen and Zingg 2001;Hinze and Wessels 2002). Stands with higher height/diameter ratios are less resistant to damage(Valinger and Fridman 1997).Wind and snow damage to trees causes financiallosses to timberproducers. Valinger and Fridman(1997)reported thatsnow and wind damage causes economic loss due to timber loss and due to required changes in planned forest management.Stand stability is low if height/diameter is 100(Milne 1995; Wang et al.1998;Lekes and Dandul 2000).The critical stability value is a ratio of 90 for young stands depending upon the nature of snowfall(Mildner 1967;Konopka etal. 1987).Kono?pka(1999)proposed a classification forspruce stands wherein a height/diameter ratio of82 is excellent, 83–92 is good,93–101 is acceptable and C102 is unacceptable.Kramer(1988)claimed that height/diameter ratio of yellow pine cannot exceed 80 to avoid snow damage. Vicena et al.(1979)considered the optimum height/ diameter ratio to be 79 with a maximum of 83.Snow does not damage Picea abies at height/diameter ratios85 (Merkel1975).The threshold was lower than 85 for Picea orientalis(U¨c?ler et al.2001).

The goal of this study was to determine how different thinning intensities affect stand structure,individual tree stability and stand stability.

Materials and methods

Material

The study area waslocated in Trabzon City,Du¨zko¨y County, So¨g?u¨tlu¨Basin in northeast Turkey(40850N–39450E).The study area wasa plantation thatwasplanted in 1991 to 1 yearold Fagus orientalis Lipsky saplings.The study area covered approximately 3 ha at 1,320–1,430 masl.The main aspect was west and,according to the managementplans,the site productivity was high(Table 1).Most of the trees in the sampled stands were in thicket stage and the silvicultural objective wasthinning asperthe updated managementplans. According to data obtained from the nearest meteorology station in Du¨zko¨y(850 masl)average annual temperature was 8.6C and average annual precipitation was 853.7 mm.Thornwaite(1948)developed a climate diagram that showed annual drought in July(Karagu¨l 1999). However the average elevation of the study area was 1,350 masl.So when temperature and precipitation values were interpolated no droughtwas evidentin the watershed above 850 masl(Karagu¨l 1999).

Methods

To accountfor spatialvariation over the study area we used stratified random sampling to demarcate transects in 2012. Eight sampling plots were located in parts of the stand where no thinning was done,eight plots were located in areas of light thinning and eight sampling plots were located in areas of heavy thinning.In lightly thinned areas 19%of the total basal area was removed on average in 2008 and in heavily thinned areas 40%of the total basal area was removed on average in 2009 by the local directorate of forest enterprise.Some of the dendrometric parameters in treated sample plots and in control sample plots are given in Table 2.

Each sampling plotwas 10 m wide and 20 m long.The stand profiles were determined and depicted using the simulation program(Staupendahl 2000).We measured slope gradient,aspect,stem locations of every tree,diameter at breast height(DBH),stump diameter,tree height, and height to the living crown in the sampling plots to quantify canopy density,define tree collectives in stands and depict the stand profile.Both height characteristics were measured with Vertex Forestor.All trees were classified as eitherdominanttrees(height[2/3 ofthe heightof the tallesttrees in the overstory)or suppressed tree(height 1/3–2/3 of the tallesttree heightas intermediate storey and height1/3 of the dominant tree height as understory), using the classification of IUFRO(U¨c?ler et al.2001;Genc? et al.2012).

Individual stability values per tree in every sampling plotwere calculated by the ratio of height(cm)/DBH(cm) (Langenegger 1979;Cremer et al.1982;Gassebner 1986; Becquey and Riou-Nivert 1987;Lohmander and Helles 1987;Wilson 1988;Mayer and Ott 1991;Bachofen and Zingg 2001).We used a stability threshold of 85 for h [height(cm)]/day[breast height diameter(cm)]ratios in this study.Collective stability values were calculated considering the collective structuring of the stand(Kleine 1983).In this calculation,after determining the tree collectives in the stand,we assumed thatthe non-stable trees had stable characteristics if located in a tree collective.

Results

Stand structure

Six two-storied and two multi-storied stand structures were identified in the eight control sampling plots that were planted with even-aged seedlings.After light thinning of 19%of the total basal area in 2008,sampled stands consisted ofone single-storied,five two-storied and two multistoried structures within eight sampling plots.Afterthinning of 40%of total basal area in 2009,the heavily thinned stands consisted of two single-storied,four twostoried and two multi-storied structures.

Table 1 General characteristics of the sampling plots

Table 2 Some of the dendrometric parameters in treated sample plots

Mean DBH was 6.78 cm in unthinned plots,7.46 cm in lightly thinned and 8.10 cm in heavily thinned plots.Mean stump diameters of removed trees were 7.59 cm both in lightly thinned and heavily thinned plots.Numbers of the removed trees per hectare were 1,425 in lightly thinned plots and 2,594 in heavily thinned plots.More than 50%of trees in three sampling plots in unthinned stands were of [8 cm DHB,and in five sampling plots 25–45%of trees were[8 cm DBH.In lightly thinned stands,trees of diameter[8 cm accounted for more than 50%in five sampling plots,and 15–20%in three sampling plots.Trees of diameter C8 cm accounted for more than 50%in seven sampling plots,and 45%in one sampling plot in heavily thinned stands.Sampled stands had partly entered the pole stage with DBH values ranging from 8.0 to 19.9 cm(Genc? et al.2012;Saatc?iog?lu 1971;Atay 1989).

Control sample plots were characterized by dense structure.Of trees in the layers in unthinned two storied sampling plots 25%were in the understory and 75%were in the overstory.In multi-storied sampling plots 12%of trees were in the understory,76%were in the intermediatelayer and the remaining 12%were in the overstory. Lightly thinned stands were dense in some areas and had canopy gaps in other areas.In two-storied sampling plots 20%of trees were in the understory and 80%in the overstory.In multi-storied sampling plots 20%of trees were in the understory,70%in the intermediate layer and 10%in the overstory.In heavily thinned stands 65%of trees were in the understory,and 35%were in the overstory.On average 15%of trees were located in the understory,65%in the intermediate layerand 20%in the overstory in multi-storied plots.

Canopy density was about 90–100%in control plots, and trees did notgrow rapidly due to high planting density. Canopy density was about 70–80%after light thinning in 2008 and remaining trees grew to reduce the number of canopy gaps within 4 years after thinning(2008–2012).In stands heavily thinned in 2009,canopy density was around 60–70%in 2012.

Fig.1 Stand profile of sample plot 3(not thinned)

Fig.2 Stand profile of sample plot 13(heavy thinning)

Fig.3 Stand profile of sample plot 23(lightthinning)

On average 9(6–11)collective structures were formed per sampling plot(Fig.1),7(5–9)collective structures per sampling plot after thinning in 2008(Fig.2),and after thinning in 2009,four collective structures(3–6)appeared per sampling plot(Fig.3).There were 10 trees(3–15)onaverage in control sample stands,seven trees(3–9)in lightly thinned stands,and five trees(2–7)in heavily thinned stands.The collective structures formed in control plots covered about 15 m2(5–25),those in lightly thinned stands covered 12 m2(5–15),and those in heavily thinned stands covered around 7 m2(2–12).Collective stability of heavily thinned stands was lower than in lightly thinned stands.

Stand stability

Table 3 lists stands by treatment according to h/day ratios. Trees of h/day ratio 85 and lower were considered stable. The percentage of stable trees was 16.7%in controlplots, 23.6%in lightly thinned plots,and 14.5%in heavily thinned plots.The stands of greatest stability were those lightly thinned in 2008.Stability declined depending on thinning intensity in 2009.

Collective stability values are listed in Table 2.Collective stability averaged 83.4%in controlsampling plots, 81.7%in lightly thinned plots,and 36.5%in heavily thinned plots.Collective stability was lowest in stands heavily thinned in 2009.

Discussion

Treatmentintensity and stand structure

Most of the control plots had multi-storied structure.Most of the thinned plots had two-storied structure.This shows thatbeech(a shade-tolerantspecies)tended to be stratified in accordance with its biology despite thinning treatments. It is critical,particularly for seedlings,to maintain storied structure in beech stands,which tend to form a layered structure in spite oftreatment.There was little difference in storied structure between unthinned and lightly thinned plots.Large gaps increased in stands after heavy thinning and the gaps did notclose within 3 years of thinning.Large gaps in young stands can be dangerous especially in terms of wind and snow damage.Mitchel(2013)emphasized that there is a strong relation between windthrow patches,site heterogeneity and stand condition.

When compared to heavily thinned plots,the trees removed during light thinning in 2008 were of the same mean diameter classes.Though removed trees had similar diameter characteristics in lightly and heavily thinned plots,numbers of trees removed were nearly double in heavily thinned plots.In heavily thinned plots treated in 2009[50%of trees in seven sampling plots were of8 cm and greater DBH while 45%of trees in one plot were of this size class.Although this result showed that heavy thinning promoted growth better than light thinning,after light thinning in 2008,the remaining trees closed the canopy gaps within 4 years(2008–2012)and retained the collective composition of the stand as well.Storied structure and collective composition were not damaged in lightly thinned stands,and tree growth was slowest in unthinned plots.

Heavy thinning(40%tree removal)damaged canopies and storied structures,especially because it was the first thinning treatment.Light thinning(19%tree removal) allowed for retained canopy and storied structure but growth did not achieve expectations.Additional years of growth might be required for more reliable assessment of the success of thinning.

O¨zc?elik(2000)reported thatmostforestareas in Turkey were not subject to regular tending and that un-tended stands were characterized by slow growth.However,creating gaps with early thinning has a positive effect on the stand(Somerville 1980;Cremeretal.1982).Stand tending should be carried out adequately and at the proper timethroughout the growth stages beginning from seedling to achieve productivity that is appropriate to habitat conditions.In this regard,thinning is essentialto directthe future of a stand.Tree growth decreases incrementally as trees age.Slodicak and Novak(2006)reported a dramatic difference in diameter distributions of P.abies trees cut in thinning operations compared to control plots.Growth in their stands decelerated,especially 3–4 years after thinning.Particularly tall seedlings experience steady growth for a longer time(Vaartaja 1951;O¨rlander and Karlsson 2000).Primicia et al.(2013)concluded that thinning of Pinus sylvestris had effects on growth 3 years after thinning:canopy coverage in control plots increased while growth decelerated.Breda et al.(1994)reported that after thinning ofa 43 year Quercus petraea stand the trees in the overstory rather than in the understory exhibited faster growth.

Table 3 Percentage of stable trees by individual and collective stability

Treatmentintensity and stand stability

Individual stability values were 17%in control areas, 24%in lightly thinned areas and 15%in heavily thinned areas.Collective stability was 83%in controlplots,82% in lightly thinned plots and 36%in heavily thinned plots. We conclude that collective stability was higher than for single trees and cooperation between trees created a positive impact in sampling plots.Yu¨cesan(2006)and Scho¨-nenberger(2001)reported that the individuals in tree collectives(tree masses)had some superior characteristics when compared to single trees.Kleine(1983)proposed that stepped groups with high life force exhibited stability for a long time while other groups without youthful growth capability were usually not stable.

Trees maintained biomass and stand structure did not decompose as a resultofthe removalof19%ofbasalarea from plots in 2008.Afterremovalofan average of40%of basal area in 2009,depending on treatment intensity and treatment manner,stand structure partially decomposed, collective compositions were interrupted and the stability of remaining trees declined.In terms of stand profiles,too many gaps were created within the stand and remaining trees did not grow at expected rates.Post-thinning beech stands did not close canopy gaps even though the species tends to crown spreading.Heavy thinning had a strong and negative impacton stands when itwas the firstthinning.As Saatc?iog?lu(1971)suggested,the principle that‘‘density will remain dense’’must be obeyed and no canopy gaps should be created by thinning.Yu¨cesan(2006)reported that trees are not randomly distributed within stands but rather they constitute structures to increase stability by forming tree collectives or clusters.Even though broadleaved species in open-air conditions grow in heightfaster than coniferous species,they need more time to establish stable stands because of slower diameter growth(Yu¨cesan 2006).

Individual stability values were lower than expected in plots thinned at 19%.However,stability in these lightly thinned plots was higher than individual stability values in heavily thinned plots.Moreover,in view of stand stability in controlareas,the interaction between trees contributes to stand stability and counters low individual stability.Predicted growth rates in unthinned control plots were not achieved,probably because thinning was not performed.It could be anticipated that trees in unthinned plots grew in height rather than diameter in dense parts of the stand; diameter growth was prominent especially among trees at the edges of gaps and these enhanced stand stability. Varmola et al.(1998)reported that,depending upon the gaps in stand after thinning,tree diameters increase and stand stability values increase.Where seedlings grow densely,diameters are low and stems are weak.At such sites,intense thinning should not be undertaken but should be carried out gradually to maximize the vigor of individual trees.However,after heavy thinning in 2009, growth in diameterand heightled to stand stability thatwas lower than for stands lightly thinned in 2008.

Conclusion

We conclude thatremoving 40%of basalarea from stands during the first thinning was excessive yet light thinning was inadequate to stimulate anticipated levels of diameter growth.Although oriental beech forms a wide crown, persistent gaps in the canopy 3 years after heavy thinning suggestthattoo much ofthe stand basalarea was removed. Collective composition was degraded and both storied structure and stability declined.Removal of 19%of basal area by light thinning did not stimulate adequate DBH growth.Moderate thinning should be tested in future.

AcknowledgmentsThis study was carried foran MSc thesis Project at Karadeniz Technical University at the Institute of Science and Technology.The MSc thesis was supported by Karadeniz Technical University Research Fund,Project number 2010.113.001.11.

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12 January 2014/Accepted:23 March 2014/Published online:16 January 2015

Project funding:This work was supported by Karadeniz Technical University Research Fund,Project number 2010.113.001.11.

The online version is available at http://www.springerlink.com

Corresponding editor:Zhu Hong

Faculty of Forestry,Karadeniz Technical University, 61080 Trabzon,Turkey

e-mail:yucesan@ktu.edu.tr

S.O¨zc?elik

General Directorate of Combating to Desertification and Erosion Control,Ankara,Turkey