Nasrin Nasiri?Mohammad R.Marvie Mohadjer?Vahid Etemad?Kiomars Sefidi?Leila Mohammadi?Maryam Gharehaghaji

Introduction

Different performance rates among tree species in a forest define the limits of natural regeneration systems.Understanding these differences may help in developing sustainable management practices for deciduous forests(Zhu et al.2014).An improved understanding of the effects of growth factors such as light and their interactions with various species is important for survival and natural regeneration(Lin et al.2014;Dobrowolska 2015).

Beech(Fagus orientalis)is a dominant species in the mixed forest of northern Iran(Sagheb-Talebi and Schutz 2012).This species is importantfortimber production and is one of the most important commercial species in the Hyrcanian forest(Forest Management Plan 2010).For natural regeneration in a forest exposed to different environmental conditions,different quantitative and qualitative characteristics will be needed.Many studies have focused on the effect of light on the growth and survival of beech regeneration in gaps(Lendzion and Leuschner 2008;Lin et al.2014,2015).Gaps may provide opportunities for the regeneration ofbeech(Emborg 1998)because they are a key factor determining plant species composition and the structure and dynamicsofthe beech forest(Pedersen and Howard 2004;Albanesi et al.2008;Richards and Harts 2011).They can increase the density of regenerated trees by decreasing the effects of pathogen attack(Ichihara and Yamaji 2009),by providing favorable substrate for regeneration through organic soil decomposition,which accelerates humus production(Parpan et al.2009),and by inhibiting competing vegetation(Fredricksen and Pariona 2002)thus increasing survival of regenerated trees.Canopy gaps significantly increase the number of regenerated beech trees(Naaf and Wulf2007).While lightmay be crucialforthe emergence of regenerating trees,beech treescan regeneratewheregapsare absent(Szwagrzyk et al.2001).Regenerated trees within gaps also grow more in height(Nakaige et al.1999;Ligot et al.2013)for at least 2 years after opening of the canopy(Collet and Chenost 2006).Therefore,creating gaps for the success of regeneration has been recommended;it will provide opportunities for beech regeneration and for the beech to become a dominant canopy species(Wang and Liu 2011).The growth form may also be affected in gaps(Sagheb-Talebi and Schutz 2002;Parhizkar et al.2011);Rozenbergarand Diaci(2014)found fewerforked treeswith decreasing light.

Low light intensity in a forest canopy cover does not limit germination(Madsen and Larsen 1997;Szwagrzyk et al.2001)or beech growth,at least not in the first 3(Peltier et al.1997)to 5 years(Topoliantz and Ponge 2000).Canopy cover has an important effect on density(Silva et al.2012).It can increase survival of regenerated trees and subsequently reduce interspecific competition(Lin et al.2014),the risk of frost damage(Kreyling etal.2012),grazing damage(Shakeri et al.2012),and prevent water vapor deficiency(Vilhar and Simoncˇicˇ2012).These effectsdepend on the ability ofa plant speciesto tolerate shaded understory conditions.Studieshave found thatmostregeneration undercanopy coverhad a forked growth form because of the decreasing light level under a canopy cover(Nicolinietal.2001;Sagheb-Talebiand Schutz 2002).Madsen and Larsen(1997)also showed an inverse relationship between crown density and the number of beech seedlingsand saplings.In many ofthese studies,however,the impacts of the closed canopy and gaps on beech regeneration were not clarified.In addition,the most favorable conditions for successful regeneration in forest management and for functional comparisons of closed canopies and gaps have not been determined.

Although gaps represent favorable light conditions for beech regeneration,abiotic and biotic factors such as grazing,drought,frost,interspecific competition in gaps threaten regeneration growth and vitality(Tabari et al.2005).To elucidate the most favorable conditions for beech regeneration,here we studied the original oriental beech forest at the Kheyrud Experimental Forest,Mazandaran,in northern Iran to compare understorey regeneration quality(vitality and growth form)and quantity(abundance and height)in canopy gaps and under closed canopies.We hypothesized that a closed canopy is not an obstacle to improving the quality or quantity of beech regeneration before reaching overstorey height.We anticipated that the presence of some closed canopy cover is necessary to reduce stresses during oriental beech regeneration.Furthermore,the canopy cover will promote nature-based forest management.To this end,regeneration in canopy gaps of various sizes and in a closed canopy in a natural beech forest was investigated,to determine(1)whether the number and height of regenerated trees differs in the gaps and closed canopies,(2)whether the vitality and growth form of the stem(architecture)differs in these two environments,and(3)thus which condition is more favorable for the regeneration of beech.

Materials and methods

Study site

This study was conducted in the Gorazbon District(36°27′–36°40′N,51°32′–51°43′E),a tract of 1000 ha in the Kheyrud Forest,Noshahr City,Mazandaran Province,Iran during the summer of 2012.This uneven-aged deciduous forest is dedicated to research and training and is under the supervision of the University of Tehran.Kheyrud Forest has been managed by single tree selection,and a few old-growth beech stands have been preserved for research.The Gorazbon District is dominated by beech(Fagus orientalis L.).Other species include Scots elm(Ulmus glabra Huds),Persian maple(Acer velutinum Boiss),European hornbeam(Carpinus betulus L.),Cappadocian maple(Acer cappadocicum Gled),and chestnut-leaved oak(Quercus castaneifolia).The climate of this region is sub-mediterranean with a mean annual precipitation of 1300 mm and mean annual temperature of 8°C.The altitude ranges from 1000 to 1300 m(Forest Management Plan 2010).Gap formation is the result of tree fall caused by windstorms or the natural degradation phase of trees.Logging and clearing is prohibited in this area,but traditional grazing occasionally takes place(Forest Management Plan 2010).

Regeneration sampling

A 50×50 m grid was established in a beech stand in Kheyrud Experiment Forest to determine sampling areas in gaps and under closed canopies.The gaps had been created by natural tree fall.The area of 30 gaps(33–671 m2)was recorded from the nearest point of intersection of each grid;30 well-established regeneration areas nearest to a grid intersection,each with the same area as the adjacent gap,were also selected(Fig.1).All saplings and seedlings were tallied in each gap and under the closed canopy.Vitality and growth form of each seedling and sapling were also recorded.

Gap area was considered to be shaped as an ellipse and thus calculated:A=πLW/4,where W is the widest and L islongest distance of the gap,respectively.Natural beech regeneration was established under a wide range of gap with different ages.Height,density,vitality,and growth form of each regenerated tree for all tree species including beech,maple,elm,and hornbeam and others within the sites(canopy gaps and canopy cover)were recorded.Regenerated trees were classified into two height categories:seedlings 0.5–1.3 m tall and saplings over 1.3 m tall with a diameter less than 5 cm at breast height(Nagel et al.2010).Mean total height for all trees was calculated.Tree vitality,based on relative health of the leaves and stem according to signs of insects,fungi,withered/faded or yellow/brown leaves,and browsing on leaves,shoots and stems(Agestam et al.2003),was categorized into three classes:high,medium,and very low.Growth form was divided into three classes based on the location of the terminal bud and architecture of the stem:excurrent,decurrent,and forked(Parhizkar et al.2011;Rozenbergar and Diaci 2014)(Fig.2).The excurrent form had one dominant leader shoot in an orthotropic direction with a vertical main axis.The fork form is less favorable for the wood industry because of its extra branches and multiple leader shoots(Rozenbergar and Diaci 2014).

Fig.1 Systematic sampling method in gaps and closed canopy cover in the study area

Fig.2 Growth forms of regenerated trees(a excurrent,b decurrent,c forked)

Statistical analyses

Regeneration variables including height and abundance were compared between gaps and closed canopy areas using Student’s independent t-tests.The same test was used to compare the abundance of regenerated trees among the different using applying the Chi square test.Statistical analyses were implemented using the SPSS 19.0 statistical package(SPSS,Chicago,IL,USA).A significance level of 5%was used.

Results

The results showed that 70%(21 gaps)of all identified canopy gaps in the study area were less than 200 m2in size,and only 3%(1 gap)were larger than 500 m2in size;8 gaps were between 200–500 m2(Fig.3).

In all,60 sampling plots in gaps and closed canopy were recorded.The frequency of beech saplings was signif icantly higher in the closed canopy area than in the gaps(P=0.003,d.f.=58,t=3.09)(Table 1).The results showed no significant difference between seedling density in gaps and in closed canopy areas(P=0.80,d.f.=58,t=0.25)(Table 1),although the gaps had more seedlings(Table 2).

Regenerated beech trees had significantly more excurrent and decurrent growth forms in the closed canopy areas than in gaps(excurrent:P=0.008,d.f.=58,t=2.756;decurrent:P=0.031,d.f.=58,t=2.231),and there were fewer forked trees in the closed canopy areas than in the gaps(Fig.4b),although the differences were not significant (forked: P=0.939, d.f.=58, t=0.077)(Table 1).Although there were fewer excurrent seedlings in the closed canopy area than in gaps,there were more excurrent saplings in the closed canopy areas than in the gaps(Table 3).

Despite the more favorable light conditions in the gaps(Agestam et al.2003),the mean height of regenerated beech trees was significantly higher in the closed canopy areas than in the gaps;other species,however,did not differ significantly in height in the two areas(beech:P=0.02,d.f.=58,t=2.3;other species:P=0.4,d.f.=58,t=0.68)(Fig.5).

There was significantly more beech trees with high vitality under closed canopy (P=0.000,d.f.=58,t=5.285)compared with gaps(Fig.4a),whereas no significant difference was seen between medium vitality and very low vitality for beech in gaps or closed canopy areas(medium:P=0.506,d.f.=58,t=0.669;very low:P=0.122,d.f.=58,t=1.587)(Table 4).Fewer low vitality beech trees,however,were found in the closed canopy areas than in gaps.

Table 2 Number of regenerated individuals of beech and other species in gaps and closed canopy

Discussion

As numerous studies have demonstrated(Zeibig et al.2005;Kucbel et al.2009;Sefidi et al.2011;Mohammadi et al.2014),most canopy gaps in beech stands were small(＜200 m2)due to single tree-fall events in the natural stands(Wagner et al.2010).Increasing the size of gaps will lead to an increased amount of light received by the understorey(Naaf and Wulf 2007).However,the smaller gaps seemed to provide better growth conditions for beech regeneration compared with large gaps because growth conditions in smaller gaps are more similar to those under a canopy,where the abundance of regenerated trees was higher.

This higher abundance under closed canopy has also been demonstrated in studies by Agestam et al.(2003)and Petritan et al.(2007),although Nagel et al.(2010)found no significant difference in density between gaps and canopies.Madsen and Larsen(1997)illustrated that very dense canopy cover leads to reduced regeneration.Moreover,Modry′et al.(2004)indicated that a decline in regeneration is associated with increased canopy openings.Such an outcome seems to be caused primarily by more browsing and grazing in gaps compared with closed canopy areas(Gharehaghaji et al.2012).Grazing by cattle will decrease the abundance of regeneration by increasing the mortality rate(Holland et al.2013).However,grazing was suggested to be a positive factor for increasing the abundance of regenerated trees by diminishing the competition when herbivores are in low density(Naaf and Wulf 2007).Second,a higher density of herbs and shrubs(Pourmajidian et al.2010)and more plant diversity in gaps leads to increased intraspecific competition for space and environmental resources such as light and water,although in some studies this factor did not affect the density of regeneration in gaps(Caquet et al.2010).Third,concentrations of soil water are higher because less sunlight reaches the forest floor(Ritter et al.2005),leading to less drought and more humidity under closed canopy.Hence,providing more appropriate conditions for the survival of this shade-tolerant species and fewer frost events under the closed canopy will lead to less mortality.A study on the effects of light variation on regeneration showed that in gaps,shade-tolerant species grew at locations that received the least light,while all shade-intolerant species grew in gap locations where more light was received(Parhizkar et al.2011).In conclusion,beech regeneration tends to occur in low levels of light,which benefits this shade-tolerant species.

Table 1 Comparison of(±SE)different beech regeneration variables between canopy gaps and closed canopy cover

Fig.4 Percentage of beech trees in the(a)vitality and(b)growth forms in canopy gaps and closed canopies

Fig.5 Mean height of beech and other species regenerating in gaps and closed canopy

Not only were there more regenerated beeches under the closed canopy,but they were also taller than in gaps.A similar study showed that in areas with the lowest relative light,regenerated trees were taller than in areas with the greatest relative light(Parhizkar et al.2011).One explanation for this result may be that trees under closed canopy have a greater opportunity to reach higher height classes because they are not browsed or grazed,which can be a serious threat to germination and height growth in the forest.This factor is rarely an issue under a closed canopy.Another factor affecting height growth is intraspecific competition,which is more pronounced in gaps than under closed canopy.Madsen and Larsen(1997)demonstrated that water content is another deciding factor on height growth.Hence,increasing soil water content leads to increasing regeneration height,which results in better regeneration in closed canopy areas than in gaps.

Table 3 Number of regenerated trees by growth form in gaps and closed canopies

Table 4 Comparison of(±SE)for variables for regenerated beech trees between canopy gaps and closed canopy cover

Regeneration vitality and growth form differed signif icantly between gaps and closed canopy sites.Madsen(1995)and Agestam et al.(2003)also found significant differences in growth form in two experiments with contrasting light levels.In our study,trees under the closed canopy had greater vitality(high vitality)and acceptable growth form(from a forest manager point of view).The effect of light on the health and form of regeneration has been a focus of several studies(Sagheb-Talebi and Schutz 2012;Javanmiri et al.2012).Parhizkar et al.(2011)found no difference in regeneration form in different light intensities,whereas Rozenbergar and Diaci(2014)found fewer fork forms when light levels were lower,in accordance with the current results.Different factors can affect the quality of regeneration;Lendzion and Leuschner(2008)showed water vapor pressure deficit to be one of the most important factors determining vitality.Water vapor pressure deficit is more severe in gaps where light has greater intensity and plausibly contributes to the differences in growth form and vitality between the sites.Browsing is also an important factor affecting form and vitality;permanent browsing can decrease the quality of seedlings(Javanmiri et al.2012)and can shift the regeneration of beech to small bonsai trees over time(Kamler et al.2010).Therefore,if browsing is managed,better form and vitality in regeneration can be expected.

Conclusions

The results of the current study highlight the role of canopy cover in the growth and vitality of beech regeneration;we can infer that the timing of creating gaps is critical for species management and conservation.Although beech regeneration is favored by the conditions under closed canopy cover as opposed to gaps,light in gaps is eventually necessary for trees to achieve overstorey height and more growth.Consequently,gaps in near-natural forests should be created only after the regeneration tree has grown adequately under a protective canopy,which may be needed to reduce potential stresses.Conversely,gaps in the selection cutting system,one of the main harvesting methods in Iran,do not start beech regeneration,but they will later promote further regeneration.Hence,to achieve a close-to-natural approach,the time of gap creation must be balanced with adequate initial regenerative growth under closed canopy.Measuring the percentage of canopy cover and light intensity and emergence and establishment of regenerated trees is recommended for future studies.Finally,further studies on regeneration in different light levels will improve our understanding of the equilibrium between early regenerative growth and later increases in light intensity to achieve overstorey growth within an appropriate time.

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