Effects of precommercial thinning intensity on growth of Fagus orientalis Lipsky stands over 6 years

Deniz Güney·Fahrettin Atar·?brahim Turna·Alkan Günlü

Abstract Determining the most suitable intensities for precommercial thinning (PCT) in oriental beech (Fagus orientalis) stands,which cover substantial areas in Turkey and have a high economic and ecological value,is essential to provide higher economic return and obtain well-formed stems.The effects of various rates of PCT interventions on the growth characteristics of natural stands,located in the northern part of Turkey,were explored to determine appropriate thinning rates;24 experimental PCT plots (4 PCT rates×3 replications×2 sites) were established in young thicket-stage stands and four precommercial thinning rates such as light (1– 1.5 m spacing),moderate (1.5?2.0 m spacing) and heavy (2.5–3.0 m spacing) thinning,including a control plot (i.e.,untreated),were applied.At the end of the 2007 and 2010 growing seasons,the diameter and height of the trees were measured and growth analyzed based on the 3-year and 6-year incremental growth.PCT levels were found to have significant (P <0.05) effects on diameter,height,total basal area and total volume increment at the end of each period.The mean diameter and height increment proportionally increased with increasing PCT intensity in the third and the sixth year.In both sites,the moderate PCT provided the greatest total basal area and total volume increment.The early results indicate that the moderate PCT(1.5–2 m spacing) boosted stand development over time.

Keywords Tending operation·Stand development·Precommercial thinning·Thicket stage

Introduction

Developing an appropriate set of silvicultural interventions and applying them at the right time at the right rate are essential for effective wood production in forest ecosystems(Turna et al.2017).In this context,silvicultural treatments such as precommercial thinning (PCT) help control early stand development over time.Forest tending interventions have important roles in tree growth and stand density management by increasing wood quality and yield,shortening the rotation time,improving the stand stability and increasing biodiversity (Bachofen and Zingg 2001;Zeide 2001;Nutto et al.2005;Makinen et al.2006).

Precommercial thinning is a principal silvicultural intervention in which trees with little or no economic value are removed from dense regenerated and immature stands to improve and accelerate the growth performance of the remaining trees.In the management of forests for timber production,PCT is a common preharvest silvicultural treatment to alter stand structure,promote individual tree growth and improve growing conditions (i.e.,regulate soil,light,water and nutrients) at an early stage of stand development (Vestlund 2005;Fahlvik et al.2018).Studies on PCT are critical for developing optimal treatments to obtain the desired wood characteristics and quality in the future.The purpose of PCT is to regulate the stand development processes in accordance with management objectives,which prioritize the ecological characteristics of the site and the biological characteristics of the species by providing the necessary interventions to increase the stand quality (Gen? 2011).Reducing stand density by removing undesired individuals during early development stages (e.g.,regenerated and immature) to acquire a desired composition and spacing will also alleviate stress,decrease competition and improve the suitability of forest resources (Chase et al.2016).Therefore,as a silvicultural intervention,PCT is often applied to maintain species composition and accelerate growth of selected individuals in even-aged stands (Weiskittel et al.2009).

Precommercial thinning,like all tending interventions,should be implemented at the right time and appropriate intensity (Rice et al.2001;Simard et al.2004;Huuskonen and Hynynen 2006;Río et al.2008).PCT in young stands usually targets the removal of the smallest trees to reduce stand density and stimulate growth of the remaining larger trees.Because the results of forest management operations emerge over the long term,any lack or improper practices of PCT in early stand development can lead to economic losses that are not easily recoverable in the following years(Eler et al.2004).Thus,the rate or intensity of PCT should carefully be adjusted to provide optimal site conditions that will keep the overall growth and yield in the stand as high as possible.Therefore,accurately determining the appropriate rate of PCT is crucial (Kaymak?? et al.2000;Odaba?? et al.2007;Erkan et al.2017).

PCT has been effective in terms of reducing stand density,accelerating tree growth,enhancing future understory structural development,and maintaining desired species composition (Brissette et al.1999;Pothier 2002;Pitt and Lanteigne 2008).Additionally,PCT can also increase the wind resistance (Achim et al.2005),nutrient availability(Thibodeau et al.2000) and wood uniformity (Koga et al.2002) of stands,in addition to light availability,often the most important factor affecting growth (Pothier and Margolis 1991).When applied to young stands of a variety of tree species,PCT also increases individual tree growth (Ruha and Varmola 1997;Simard et al.2004;Weiskittel et al.2009;Splawinski et al.2017).

Numerous studies on the effects of PCT and other thinning interventions have shown significant improvements in diameter,height and basal area (Ruha and Varmola 1997;S?vac?o?lu et al.2006;Bonica et al.2007;?ncül et al.2016;Guner et al.2017a;Bayar and Delig?z 2019,2020).Interestingly,other studies have shown that PCT and thinning interventions are effective only on diameter (Río et al.2008)or only on total basal area (Kaymak?? et al.2000;Diaconu et al.2015).

Fagus orientalisLipsky (Fagaceae;oriental beech),which provides high-quality timber,is one of the most economically and ecologically important native deciduous tree species in Turkey (Atik 2013;Anonymous 2013).It is native to Anatolia (Asia Minor) in the east,to the Balkans in the west,and to the Caucasus,Crimea and northern Iran.In Turkey,the species is found mainly throughout the Black Sea Region,south of the Marmara Region,in Thrace and some parts (Hatay,Seyhan,Kahramanmara?) of the Mediterranean Region (Atalay 1992;Denk et al.2002;An?in and ?zkan 2006) where it grows from sea level to 2200 m a.s.l.It covers an area of approximately 2 million ha in Turkey in pure stands and mixed with conifers and other deciduous species(Anonymous 2014).

Determining the best PCT conditions for optimizing the growth of oriental beech and obtaining well-formed stems is thus crucial for achieving the highest economical potential of the trees and has been the focus of quite a few studies onFagus sylvaticaL.(Le Goffand Ottorini 1993;Ek? et al.1995;Peter Skovsgaard et al.2006;Boncina et al.2007;?tefan?ík and Bo?e?a 2014;Diaconu et al.2015) andF.orientalis(Umut et al.1996;Tufekcioglu et al.2005;Guner et al.2017a).Related studies in Turkey on the effects of two commercial thinning interventions on growth variables of mature Oriental beech stands (Umut et al.1996;Tufekcioglu et al.2005;Yücesan et al.2015;Guner et al.2017a).However,no studies have focused specifically on the effects of PCT at various intensities on the growth,yield and basal area of young Oriental beech stands in Turkey.The lack of appropriate set of silvicultural prescriptions for young and immature beech stands is a serious def iciency for designing optimal practices for forest management and silviculture in Turkey.Thus,here we investigated the effects of various levels of PCT interventions on growth variables of young oriental beech stands over 6 years to determine best precommercial thinning practices.

Materials and methods

Study area

Plots were selected in young (thicket stage),naturally regenerating oriental beech (Fagus orientalisLipsky) stands in two areas in Sinop Province in northern Turkey:(1) Sinop-Erfelek (41°55′19″ N,34°56′51″ E) at a mean altitude of 120–140 m a.s.l.with an average slope of 0–20%;(2) Sinop-Ayanc?k plots (41°42′04″ N,34°35′22″ E) at a mean altitude of 1100–1200 m a.s.l.with an average slope of 20%–40%.The stands are homogeneous in terms of stand characteristics,and no precommercial had been applied before our study.

Experimental design and measurements

Four intensities of precommercial thinning were applied:control (C),light (L),moderate (M) and heavy (H).The PCT plots in the two geographic locations (Snop-Erfelek and Sinop Ayanc?k) were set up with three replications and four treatment levels using a randomized complete block design.

The spacing between individuals was determined to provide the number of trees that must remain in the area according to different intensities of thinning (Varmola and Salminen 2004;Fahlvik et al.2005;Zhang et al.2006;Weiskittel et al.2009;Bayar and Deligoz 2019).Nearly 31–39 trees were removed from the control (unthinned)sample plots,and each control plot was approximately 30–35 m2;they did not receive any precommercial thinning but were left to develop naturally.In the light PCT plots,1–1.5 m was left between each of 32–35 trees remaining in each replication;each plot was 35–40 m2.In the moderate PCT plots,1.5–2 m was left between each of 30–33 trees remaining in each replication;and each plot was 50–60 m2.In the heavy PCT plots,2.5–3 m was left between each of 30–32 trees in each replication;each plot was 90–100 m2.The sample plots were then adjusted in size to contain a similar number of trees in each type of treatment.The distances among the trees left on the plot were measured with a tape.After the PCT treatments,the number of growing individuals per hectare in control,light,moderate and heavy treatment levels were 10,300,8685,5636,3315 individuals,respectively.

All trees in each plot were“marked with a stripe”at breast height (1.30 m),and numbered.Then the diameter at breast height (DBH) of the trees was determined by measuring the circumference at the marked section.In experimental plots,individual healthy,well-formed,tall trees with smooth,unbroken and undamaged crowns were selected to grow and develop over time.In 2004,the PCT interventions were started,and DBH and height of the selected trees in the sample plots were measured.At the end of the 2007 and 2010 growing seasons (3 and 6 years of growth),the DBH and height values of the same trees were measured again.These values were analyzed.Merchantable volume and basal area were also calculated for each tree.Total stand volume and basal area were determined by converting the plot measurements to per-hectare values,and the values for all trees in the experimental plots were compiled and summarized.The stem volume(V) of each tree was calculated using the equation developed for this species by Carus (1998):

whereVis the over bark volume (m3),dis the diameter at breast height over bark (cm),andhis the tree height(m).R=0.99193,R2=0.98392,tR=605,412 (P<0.001),FH=183,210.1 (P<0.001),SE=0.22259.(R:Multiplecorrelation coefficient,R2:Coefficient of determination,t:ttest value,F:F test value,S E: Standard deviation of error.)

Statistical analysis

Variance analysis was performed on the initial values without the effects of the PCT by measuring the trees left to grow in the plots after the PCT,regardless of the homogeneity of the initial values of the treatments.The analysis indicated that the initial values for growth variables in the plots showed some differences.Therefore,growth variables for the trees were analyzed by repeated measures for analysis of covariance (ANCOVA) models to remove the effects of initial values.Specifically,the statistical significance of the differences among the PCT intensity levels was analyzed using covariance analysis including treatment effects.Treatment effects were evaluated using the following analysis of covariance model:

whereY ijis a dependent variable,μis the overall mean,aiis the mean of the response variable which is greater than the overall mean for treatmenti,βis the regression coefficient,andX ijis the covariate for the treatment level andE ijis the error in measurement for treatmentiand replicationj.

Again,the initial differences among the plots were removed by applying a continuous covariate (X) measured before the onset of the treatment,i.e.,mean diameter and height.Analysis of covariance (ANCOVA) was used to adjust the treatment means by accounting for the differences in tree sizes observed at the beginning of the study(Lindgren et al.2007).Bonferroni’s multiple comparison tests were used to locate the source of difference for each independent variable (Varmola and Salminen 2004;Bayar and Delig?z 2019).Therefore,Bonferroni test was performed (P<0.05) on the edited data.Multiple pairwise comparisons were made between the levels of PCT intensity.The significance levels for these comparisons were adjusted by the Bonferroni-adjustedt-test for multiple comparisons (?zdamar 2013):

Results

For each PCT level,diameter at breast height (DBH),height (H),total basal area (BA) and total volume (V) were measured in 2004,2007 and 2010 in Sinop-Erfelek plots and Sinop-Ayanc?k plots.Considering that the remaining individuals benefit from more water,light and nutrients due to the decrease in the number of trees per unit area after PCT interventions,it has been determined that stand growth variables generally increase with the increase in the intensities of thinning.

Right after the PCT,the average diameters at breast height of the remaining trees in the Sinop-Erfelek plot were 5.06 cm in the control plot,6.13 cm in the light PCT,6.53 cm in the moderate PCT and 6.86 cm in the heavy PCT,and 6 years later,the average diameters in the control,light,moderate,and heavy PCTs and plot were 5.86 cm,7.34 cm,8.83 cm,and 10.06 cm,respectively.Not surprisingly,the DBH increased proportionally with the increase in the levels of PCT intensity.In the Sinop-Ayanc?k plot,the largest DBH was measured from the moderate PCT in 2004,while the largest DBH was acquired in the heavy PCT in 2010.Similarly,in both plots,height values increased with increasing thinning intensity.For the total basal area at the end of year 6 in both trial areas,the highest values were obtained from the light and moderate PCT,and the lowest values were from the heavy PCT.

The 6-year DBH increments were 0.79 cm,1.19 cm,2.30 cm and 3.20 cm for control,light,moderate and heavy PCT treatments,respectively (Table 1).The results indicate that as the PCT intensity increases,the diameter increments increase too.Accordingly,while the 6-year total volume increment was 37.77 m3ha?1in the control plot,it was 96.57 m3ha?1in the moderate PCT,89.37 m3ha?1in the heavy PCT and 69.24 m3ha?1in the light PCT.The moderate PCT had the highest value in terms of 6-year total volume increment per hectare.Although the total volume increment in the light PCT performed plot was lower than in the heavy PCT performed plot (Table 1),the amount of total volume per hectare was higher in the light PCT plot compared to that in the heavy PCT plot (Table 2).Such differences may come from high number of trees per hectare (8,685 trees/ha)in light PCT plots compared to that (3,315 trees/ha) in the heavy PCT plots.In the control plot,however,although the number of trees per hectare was the highest (10,300 trees/ha) among all plots,the total volume per hectare was lowest since the total volume increment was very low compared to the other PCT intensities.

In the Sinop-Ayanc?k plot,6-year DBH increments were 0.78 cm in the control plot,1.04 cm in the light PCT,1.23 cm in the moderate PCT and 1.64 cm in the heavy PCT.The results showed that the total volume increment between 2004 and 2010 was 30.08 m3ha?1in the control plot,45.26 m3ha?1in the moderate PCT,38.41 m3ha?1in the heavy PCT and 39.26 m3ha?1in the light PCT.Among all the plots,the moderate PCT had the greatest 6-year total volume per hectare and the control plot had the lowest.

In the covariance analysis to test for significant differences between the growth variables with regard to the PCT intensity (Table 1),significant differences were found for the DBH,H,BA and V increments among the different PCT intensities each year in both the Sinop-Erfelek and Sinop-Ayanc?k plots.

The significance of the differences between the increments of the stand developmental parameters related to the PCT intensity levels was determined by Bonferroni test.The results indicated that the differences between all binary comparisons between the PCT intensity levels were statistically significant at 99% confidence level (P<0.001) (Table 3).

Table 1 Covariance analysis of mean ± SE increments in growth variables by PCT intensities and years at two locations in Turkey

Table 2 Mean ± SE diameter at breast height (DBH),height(H),total basal area (BA) and total volume (V) of trees at two locations in Turkey after precommercial thinning at four intensities in 2004 (year of PCT) and after 3 and 6 years

Table 3 Bonferroni test results related to the difference in mean increments in stand development variables for various PCT intensities by years at two locations in Turkey

According to the Bonferroni test results,the greatest difference occurred between the heavy PCT and the control plot in terms of diameter and height increments.The differences between the heavy PCT and control in terms of 6-year increments for DBH andHwere 2.40 cm and 1.66 m in the Sinop-Erfelek plot,and 0.87 cm and 0.61 m in the Sinop-Ayanc?k plot.The greatest differences in total BA and V increments in the Sinop-Erfelek plot were found between the moderate PCT and the control.According to the results after 6 years of growth,trees with moderate PCT increased 9 m2ha?1more in basal area than in the control plot,while total volume increment was 58.80 m3ha?1more.In the Sinop-Ayanc?k plot,the difference in basal area increment between the light PCT and the control was the highest (0.78 m2ha?1) during the first 3 years (2004–2007).The difference in basal area increment between the moderate PCT and control was the highest (1.26 m2ha?1) in the second 3 years(2007–2010).In terms of the 6-year development of stands,the difference in basal area increment between the light PCT and control was the highest (1.60 m2ha?1) as it was during the first 3 years.As for total volume increment,the difference between the light PCT and control was the highest(4.07 m3ha?1) between 2004 and 2007,and the difference between the moderate PCT and control was the highest (11.6 m3ha?1) between 2007 and 2010.The difference in total volume increment between the moderate PCT and control was highest (15.18 m3ha?1) after 6-year of development at the end of 2010.

Discussions and conclusions

Precommercial thinning is an important silvicultural practice to manage the development of stand structure over time and support individual tree growth in the early stages of stand development (Fahlvik et al.2018).Stand development may also differ significantly based on the level of PCTintensity (Voorhis 1990;Ginn et al.1991;Cameron et al.1995;Rice et al.2001;Simard et al.2004;Huuskonen and Hynynen 2006;Río et al.2008;Y?lmaz et al.2010).This study showed that different PCT intensities had a statistically significant effect on the increments in diameter,height,total basal area and total volume.

Significant differences were found between all PCT intensities for mean DBH increment after 3 and 6 years of growth.In both plots,the lowest diameter increments were obtained in the control plot and the highest diameter increments were obtained in the heavy PCT practice.The results also indicated that the mean DBH increment enlarged proportionately as the PCT intensity increased.?ncül et al.(2016) reported that PCT had an positive effect on stand development,and heavy PCT was the most effective treatment in terms of diameter and basal area increment whenthey investigated the effects of PCT on diameter and basal area in natural Scotch pine (Pinus sylvestrisL.) stands.In a study of Taurus cedar,6-year results showed that the highest increment in diameter at breast height was achieved in plots with PCT to provide 1.5×3 m spacing (?z?elik and Eler 2009).A similar study on the effect of the PCT on the diameter,height and canopy development of natural stands of Turkish pine showed that the greatest effect on the tree growth was obtained with PCT to give 3×3 m spacing (?zdemir et al.1987).In many other similar studies,diameter growth increases unexceptionally with respect to a decrease in stem density right after the PCT application (Pettersson 1993;Ward 1995;Ruha and Varmola 1997;Graham 1998;Miller 2000;Eler et al.2004;Varmola and Salminen 2004;Weiskittel et al.2009;Gen? et al.2012 ;Bayar and Delig?z 2020).

The main reason for a positive correlation between precommercial thinning and diameter increments is that the remaining individuals benefit from more water,light and nutrients due to less competition as a result of the decrease in the number of trees per unit area (Goudiaby et al.2011).As stated in many studies,the growing stock in a unit area decreases in the year in which PCT is just applied.In the following years,however,with more favorable growth conditions,a significant increase is observed in diameter and height increments of individuals.Similarly,diameter increments and volumes increase as the nutritional-growth relationships of the remaining trees are favored by the better growing conditions (Pettersson 1993;Eler et al.2004;Gen? et al.2012).In some studies,the reduction in total basal area at the end of the intervention was compensated by the increased diameter increment in the coming years(?zdemir et al.1987;S?vac?o?lu et al.2006).However,Erkan et al.(2017) reported that the remaining trees may not fully benefit from the gaps created after very heavy PCT application.In such cases,production losses may result from uner-utilization of the site resources.Such results highlight the need for careful design of PCT intensity in silvicultural operations.

The effects of the PCT on tree heights may well be considered essential or insignificant depending on factors such as the levels of PCT intensity,timing of the application,site and biological features of the species.Our study revealed that the different PCT intensities led to significant differences in height increments in the different years of measurement.The highest mean increment in height was obtained after the heavy PCT (2.5–3 m plots).Furthermore,the mean height increment had increased in the third year and the sixth year after the PCT.Similarly,Guner et al.(2017a) reported that,tree heights and diameters increased by 41% and 72%after heavy thinning,30% and 52% after moderate thinning,and 25% and 35% in the control in the 8th growing season after the thinning compared to the pre-intervention period in oriental beech stands.Weiskittel et al.(2009) reported a significant increase in height growth,crown ratio and crown width ofPicea rubensandAbies balsamea25 years after precommercial thinning,compared to the control stands.InPinus slyvestrisstands,mean tree heights in PCT-applied plots were also higher than in the control plots (S?vac?o?lu et al.2006).After various levels of PCT of Scotch pine stands,mean tree heights varied between 1.2 and 8.3 m(Fahlvik et al.2005).Contrary to those results,Kaymak?? et al.(2000) reported that the PCT did not effectively increase height growth in a delayed PCT study of Anatolian black pine stands,whereas basal area in the PCT-applied plots was lower than in the control plots without PCT after 5 years.Similarly,Lindgren and Sullivan (2013) found that different levels of PCT intensity inPinus contortavar.latifoliastands did not affect height increment at the end of 15 years.In a similar study,height increment decreased Scotch pine stands that were thinned to very wide intervals,and tree growth was diverted to the branches due to the decrease in light competition (Ruha and Varmola 1997).Studies on other tree species revealed that the intensity of the PCT does not significantly affect incremental growth in height (Pinkard and Neilsen 2003;Eler et al.2004;Simard et al.2004;Varmola and Salminen 2004;Rytter and Stener 2005;Gauthier 2008;Rytter 2013).

The change in basal area and volume after tending interventions is an important indicator of stand development over time.In a study of the effects of selective thinning on the growth of European beech (Fagus sylvaticaL.) stands by Boncina et al.(2007),despite the low stand density after PCT,annual basal area increments of the thinned stands were approximately 20% higher than in control stands in both periods after thinning (1980–1991 and 1991–2002).In a study on the effects of different thinning intensities on aboveground biomass of young oriental beech stands,biomass values ranged from the highest after heavy thinning to the lowest after moderate thinning or none (control).Although thinning interventions decrease the number of individuals within plots,the rate of aboveground biomass of the remaining trees eventually makes up for the difference in number of trees (Guner et al.2017b).In young stands of oriental beech,the increment in stand basal area was about 10%,18% and 27% in the unthinned,lightly thinned,and heavily thinned stands three years after PCT,respectively(Tufekcioglu et al.2005).Intensive thinning (2.13 m,or 2212 stems/ha) ofPinus banksianaLamb stands increased tree diameter by >20% and merchantable stem volume per tree by >75% compared to the control (Zhang et al.2006).Ruha and Varmola (1997) reported that precommercial thinning of young Scotch pine stands significantly increased the increments in diameter and volume growth during the second 10-year period.Similar to such studies,our results showed that the total volume was much higher than in the control plot,although the number of trees per hectare was very low,depending on the 6-year values for the three different intensities of PCT in the Sinop-Erfelek plot.The total basal area in the light and moderate PCT intensity was higher than in the control plot at the end of 6 years,but the heavy intensity yielded lower basal area than in the control plot.After 6 years,the total volume was higher in the light and moderate PCT applications compared to the control in the Sinop-Ayanc?k plot.Although the number of trees per hectare (3315) in the area with the heavy PCT was one third lower than in the control (10,300 trees/ha),the total volume in each plot were very similar.Total basal area in the light and moderate PCT was higher than in the control plot in Sinop-Ayanc?k,as in Sinop-Erfelek,but total basal area after the heavy PCT was lower than in the control plot.

On the other hand,total basal area and volume decreased with increasing PCT intensity onEucalyptus camaldulensis,E.pellitaandE.urophylla(Bernardo et al.1998),onBetula papyrifera(Simard et al.2004) and onPinus nigrasubsp.pallasiana(Bayar and Deligoz 2020),and small effects on the total volume were reported onFagus sylvatica(Ek? et al.1995) and onEucalyptus nitens(Pinkard and Neilsen 2003).Although various levels of PCT applied to Taurus cedar stands had positive effects on the growth of diameter at breast height (Y?lmaz et al.2010),the total basal area per hectare in the moderate and heavy PCT-applied areas decreased with increasing intensity of PCT,and the total basal area was lower compared to the control plot for both PCT levels at the end of 10 years.Bayar and Delig?z(2019) investigated the effects of different PCT intensity on growth variables of Anatolian black pine stand and found that total basal area of the control plots was greater than in the PCT-applied plots,even though the basal area increment increased with increasing PCT intensity.In another PCT applied study conducted in Turkish pine (Pinus nigrasubsp.pallasiana) stands,diameter growth increased as the number of individuals decreased,and diameter growth was highest in stand with 3×3 m spacing,but the basal area per hectare was lower due to the lower number of stems per hectare (Odaba?? 1981;?zdemir et al.1987).Diaconu et al.(2015) showed that basal area increment significantly increased with increasing thinning intensity at the tree level.

Overall,some studies indicated that there is no increase in total basal area or volume after PCT compared to the control,while others claim that there are significant positive impacts.Our results revealed that both total volume increment and total volume increased after all levels of PCT intensity in the Sinop-Erfelek and Sinop-Ayanc?k plots,except after the heavy PCT in the Sinop-Ayanc?k plot,despite the decrease in stand density after after 6 years.Similarly,total basal area increment and total basal area were higher after all levels of PCT intensity in the Sinop-Ayanc?k plot,except for the heavy PCT,than that in the control plot.In Sinop-Erfelek plot,the total basal area increment and total basal area was far better after all levels of PCT intensity,except for the heavy PCT,than in the control plot.

In conclusion,the moderate PCT with 1.5–2 m spacing between the trees provided the best results in terms of both DBH,H,BA andVincrements and total BA and total V per hectare in both plots.After the moderate PCT,the 6-year volume increment was quite good;an increase of 58.80 m3ha?1volume in the Sinop-Erfelek plot and an increase of 15.18 m3ha?1in the Sinop-Ayanc?k plot were both the highest compared to the control.The total volume obtained in the same areas was higher in both plots,although the number of trees per hectare was lower in the moderate PCT than in the control and the light PCT.Furthermore,it is quite possible to increase branch diameters by growing wider crowns in oriental beech,which tends to spread its crown after the heavy PCT with a 2.5?3 m spacing.In such case,however,the supply of quality stem wood that is smooth,well-formed and knot-free should be taken into account.Additionally,the high number of trees removed in the heavy PCT creates more gaps,exposing the stand to more wind and snow.However,overlooking PCT interventions in young stands may result in snow damage,especially at higher elevations.When trees to keep during the advanced stage of the PCT,the individuals should be vigorous with strong growth capacity and without wounds and permanent defects,have a well-formed symmetrical crown,good natural pruning,thin branches and a smooth,cylindrical stem,and capacity for a good response to the PCT.

Declarations

Conflict of interestThe authors declared that there is no conflict of interest.