Forest Ecology and Management 262 (2011) 1094–1099

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Forest Ecology and Management

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Canopy gaps and regeneration in old-growth Oriental beech (Fagus orientalis Lipsky)stands, northern Iran

Kiomars Sefidi a,⇑, Mohammad R. Marvie Mohadjer a, Reinhard Mosandl b, Carolyn A. Copenheaver c

a Department of Forestry, Faculty of Natural Resources, University of Tehran, Karaj, Iranb Institute of Silviculture, Teschnische Universität München, 85354 Freising, Germanyc Department of Forest Resources and Environmental Conservation, Virginia Tech, Blacksburg, VA 24060, USA

a r t i c l e i n f o

Article history:Received 11 April 2011Received in revised form 3 June 2011Accepted 4 June 2011Available online 25 June 2011

Keywords:Canopy-gap dynamicsForest regenerationFagus orientalisCaspian forestIran

0378-1127/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.foreco.2011.06.008

⇑ Corresponding author. Tel.: +98 2612223044; faxE-mail addresses: kiomarssefidi@gmail.com (K

(M.R.M. Mohadjer), mosanal@forst.tu-muenchen.de (R(C.A. Copenheaver).

a b s t r a c t

Virgin beech Fagus orientalis forests in northern Iran provide a unique opportunity to study the distur-bance regimes of forest ecosystems without human influence. The aim of this research was to describecharacteristics of natural canopy gaps and gap area fraction as an environmental influence on the successof beech seedling establishment in mature beech stands. All canopy gaps and related forest parameterswere measured within three 25 ha areas within the Gorazbon compartment of the University of Tehran’sKheyrud Experimental Forest. An average of 3 gaps/ha occurred in the forest and gap sizes ranged from 19to 1250 m2 in size. The most frequent (58%) canopy gaps were <200 m2. In total, canopy gaps covered9.3% of the forest area. Gaps <400 m2 in size were irregular in shape, but larger gaps did not differ signif-icantly in shape from a circle. Most gaps (41%) were formed by a single tree-fall event and beech made up63% of gap makers and 93% of gap fillers. Frequency and diversity of tree seedlings were not significantlycorrelated with gap size. The minimum gap size that contained at least one beech gap-filling sapling(<1.3 m tall) was 23.7 m2. The median gap size containing at least one beech gap-filling sapling was206 m2 and the maximum size was 1808 m2. The management implications from our study suggest thatthe creation of small and medium sized gaps in mixed beech forest should mimic natural disturbanceregimes and provide suitable conditions for successful beech regeneration.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

In many mature deciduous forests, canopy gap formation andthe resulting establishment of tree seedlings and saplings withinthe higher light environment of the gap provides the main sourceof regeneration in the absence of larger natural or anthropogenicdisturbances (Yahner, 2000). Canopy gaps are dynamic environ-ments with changes in size and shape triggered by branchin-growth and mortality of trees bordering the edge of the gapand establishment and growth of new saplings (Runkle, 1998).The importance of gaps to forest regeneration depends upon thesuccessional stage, with gaps in early-successional and late-successional forests providing a better opportunity for seedlingestablishment than gaps formed in mid-successional forests(Emborg, 1998).

In mature beech (Fagus) forests, canopy gaps are typicallycaused by single tree-fall events (Drößler and Von Lüpke, 2005),but average gap size can be highly variable with mean gap area

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: +98 2612249312.. Sefidi), mohadjer@ut.ac.ir. Mosandl), ccopenhe@vt.edu

ranging from 50 to 550 m2 across different forest regions (Table 1).Although gap size does not influence soil moisture availability, soilmoisture is higher in gaps compared to the surrounding forest(Gálhidy et al., 2006; Ritter and Vesterdal, 2006). However, lightintensity generally increases with gap size and a study from Hun-gary identified average light intensity was 3% higher in small gaps(gaps with a maximum diameter <1 times the tree height) and 8%higher in large gaps (gaps with a maximum diameter <0.5 timesthe tree height) compared to the understory of the surroundingclosed canopy (Miho9k et al., 2007). The higher light intensitymeasured in the large gaps favors seedling establishment andimproves height and diameter growth of beech seedlings (Colletand Chenost, 2006). If there is strong browsing pressure from wildungulates, gaps may take longer to refill with vegetation even withthe improved light and moisture availability (Kenderes et al.,2008). The time interval between gap formation is referred to asthe gap turnover time and this sometimes mirrors the maximumlifespan of the dominant tree species in the forest; thus, the mod-eled turnover time for mature beech forests is 220 ± 30 years(Drößler and Von Lüpke, 2005).

The cause of gap creation alters the post-disturbance structureand in-filling growth pattern within a gap. Gaps created by diseasecan substantially shorten the gap turnover-time, as has been noted

Table 1Range in gap sizes for mature forests containing a high component of beech. Thespecies of beech included: Fagus crenata, Fagus grandifolia, Fagus japonica, Fagusorientalis, Fagus sylvatica, and Nothogagus pumilio. The minimum gap size was often areflection of methodology rather than a biologic minimum; therefore, we did notreport minimum gap sizes.

Source Region Maximumgap size(m2)

Mean ormediangap size (m2)

Busing (1998) USA – 50

Miho9k et al. (2007) Hungary 378 61

Fajardo and deGraaf(2004)

Chile 400 78

Kenderes et al. (2009) CzechRepublic

1288 95

Yamamoto and Nishimura(1999)

Japan 585 113

Tyrrell and Crow (1994) USA – 121Zeibig et al. (2005) Slovenia 833 137Tanaka and Nakashizuka

(1997)Japan 475 176

Mataji et al. (2008) Iran – 210Hart and

Grissino-Mayer (2009)USA 158 213

Henbo et al. (2004) Japan 3925 223Weiskettel and Hix

(2003)USA 1105 386

Delfan-Abazari et al.(2004)

Iran – 550

K. Sefidi et al. / Forest Ecology and Management 262 (2011) 1094–1099 1095

for beech forests in North America impacted by beech bark disease(Runkle, 2007). Wind-created gaps have a high volume of coarsewoody debris originating from the trees killed during the winddisturbance (Nagel et al., 2006) and because beech stems are morelikely to break than be uprooted during high winds, residual snagscreate vertical diversity and tree tops form a temporary shelteringenvironment on the gap floor (Yamamoto and Nishimura, 1999).Gaps created by small group selection cuts designed to releasesuppressed crop trees, yield higher growth rates from small-edgetrees than large-edge trees (Wiser et al., 2005). Forest managementpractices can also influence gaps and gaps in managed beechforests are generally larger (average gap area = 235 m2) comparedto average gap size (210 m2) in unmanaged beech forests (Matajiet al., 2008).

Although in many beech forests, canopy gaps are tied directly orindirectly to human activities, within the Caspian beech forests ofnorthern Iran, stand dynamics work in a system that is virtuallyfree from human manipulation (Sagheb-Talebi et al., 2005). Inthe early Cenozoic Era, European and Caspian forests were con-nected and beech-dominated forests covered most of the northerntemperate zone (Marvie-Mohadjer, 2005). During the Pleistocene,glaciations severely impacted European forests but had only mini-mal impact on Caspian Forests. Therefore, Caspian beech forestsrepresent a very stable system that date from the Pleistocene.Small-scale disturbance represents the primary form of regenera-tion and there has been minimal direct impact of humans on thedisturbance regime (Wagner et al., 2010). A study that character-ized canopy gaps in beech forest of northern Iran identified severalsubtle differences from beech forest from other regions (Delfan-Abazari et al., 2004). Most gaps were caused by the death of twotrees (rather than single trees) and mean gap size was 550 m2 (lar-ger than the mean from other beech forests, Table 1). Our studyrepresents an expansion of the understanding of canopy gaps inbeech-dominated Caspian forests. Specifically, we (1) describe can-opy gap structure in an old-growth beech forest; (2) identify andcharacterize gap makers; (3) connect gap structure with regenera-tion; and (4) assess the potential of different seedlings to functionas gap fillers.

2. Methods

2.1. Study area

The study was conducted within the University of Tehran’sKheyrud Experimental Forest in northern Iran. The Kheyrud Exper-imental forest is 8,000 ha in size and ranges in latitude from36�270N to 36�400N and in longitude from 51�320E to 51�430E.The climate is sub-Mediterranean with mean annual temperatureof 9 �C and total annual precipitation of 1380 mm (Marvie-Mohad-jer, 1976). The forest occupies plateaus and moderate slopes withdeep soil above limestone bedrock. Most stands have an uneven-aged structure where new seedling establishment occurs withincanopy gaps (Marvie-Mohadjer, 1976). Mature forests are domi-nated by beech (Fagus orientalis has an average volume of187 m3 ha�1, which represents 74% of total stand volume), Carpi-nus betulus (21 m3 ha�1, 8%), and Acer velutinum (18 m3 ha�1, 7%).Other minor tree species include: Acer cappadocicum, Tilia platy-phyllos, Ulmus minor, Ulmus glabra, Cerasus avium, Taxus baccata,and Sorbus torminalis (Marvie-Mohadjer, 2005). Structurally, theseforests have high volumes of coarse woody debris with mature for-ests averaging 51 m3 ha�1 (Sefidi and Marvie-Mohadjer, 2010). Thepresence of coarse woody debris often indicates a present or pastcanopy gap and therefore seedling density is positively correlatedwith volume of coarse woody debris (Sefidi et al., 2008).

2.2. Field sampling

All sampling occurred during the summer of 2009 in naturally-regenerated, unmanaged beech stands. To quantitate the gap char-acteristics, three sample regions (25 ha each) were established.Sample regions were located in areas where they were not influ-enced by forest edge and were fairly homogenous with respect toslope and aspect. Within each sample region, all canopy gaps weremeasured. A gap was defined as an opening in the stand with anarea exceeding 15 m2 that appeared to have been formed by thedeath of one or more trees. When trees within the gap reachedthe base of canopy of the surrounding forest, a gap was consideredclosed and not sampled. The actual gap was identified as the areadirectly under the canopy opening and length (longest distancewithin the gap) and width (perpendicular to the length) were mea-sured for each gap. The gap maker, typically a dead tree in theupper canopy layer, was identified by species and the number ofgap makers was recorded for each gap. Regeneration was sampledin all gaps with seedlings (0.5–1.3 m tall) and saplings (>1.3 m talland <5 cm diameter at breast height) counted, identified by spe-cies, and measured for height and diameter at the root collar. Inall gaps ‘‘definitive gapfillers’’ (Lertzman, 1992) were identifiedbased on which individuals were most likely to occupy the gapaccording to their location within the gap, height, and potentialgrowth rate. This was possible to assess because very few of ourgaps (18%) were newly created – where the gap makers still hadintact bark and solid wood – and therefore differences in growthrates of saplings had already demonstrated vertical stratification.

2.3. Data analysis

Analysis of the canopy gaps was based on the combined data setfrom the three 25 ha study sites. Most gaps were elliptically shapedand therefore, we followed Runkle’s (1982) suggestion to calculategap area with the formula for area of an ellipse. Thus A = pLW/4,where A is area of an ellipse, L is length, and W is width. Gaps wereclassed by into small gaps (<200 m2), medium gaps (200–500 m2),and large gaps (>500 m2). Simple linear regression was used to testfor a relationship between regeneration density and gap size and

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shape. Seedling and sapling diversity within gaps was calculatedusing the Shannon Index such that H = �R Pi ln Pi, where H is theShannon Index of Biodiversity and Pi is the proportion of total indi-viduals represented by the ith species. Spearman’s rank correlationcoefficient was used to test the relationship between gap size andregeneration density. Tests were performed for all regenerationand as separate tests for individual, dominant species. Analysis ofvariance (ANOVA) was used to test for differences in regenerationcharacteristics across different gap size classes. All tests were con-ducted at a significance level of a = 0.05.

0100 250 450 650 850 1050 1150 1250

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ig. 1. Distribution of canopy-gap sizes in an old-growth beech forest in Kheyrudxperimental Forest in northern Iran.

3. Results

Within the 75-ha area measured in this study, a total of 231gaps were identified. Thus, the old-growth beech forest had anaverage of 3 gaps ha�1 and canopy gaps covered 9.3% of the totalforested area (Table 2). Canopy gaps ranged in size from a mini-mum of 19.6 m2 to a maximum of 2246 m2. The median canopygap size was 178 m2. Fifty-nine percent of the gaps were less than200 m2 in area and only 3% of the gaps exceeded 1000 m2 (Fig. 1).Gaps larger than 500 m2 composed only 14% of the total gap fre-quency, but comprised 41% of the overall area covered by gaps.The canopy gap size distribution followed a negative exponentialcurve with an equation of Y = 34.688e�0.037x. The perimeter-to-arearatio for gaps ranged from 0.07 to 0.8 and the mean perimeter-to-area ratio was 0.29 (Fig. 2). Gap shape of gaps <400 m2 was irreg-ular and significantly different in shape compared to a circle(F = 10.31, P = 0.00). In contrast, gaps >400 m2 were not signifi-cantly different in shape from a circle (F = 0.644, P = 0.507).

A total of 214 gap makers were identified within the 231 mea-sured gaps. F. orientalis was the most common gap maker (63%)with Acer sp. (21%) and Carpinus (12%) also important gap makers(Table 1). Alnus and Ulmus were infrequent gap makers. The mostcommon cause of canopy gaps was a single tree-fall event(Fig. 3). Gaps caused by three or more trees were relatively rare(<5%). The maximum number of fallen trees within a single gapopening was four. Most gap makers were highly decomposed andonly 14% of the gap makers were recently killed trees with the barkstill intact on the stem.

A total of 16,385 seedlings and saplings were measured in the231 gaps. Acer velutinum (39%) and F. orientalis (37%) were the mostcommon species. Carpinus betulus (18%) and Ulmus sp. (5%) werealso important contributors to the regeneration pool. Seventy-two percent of the regeneration was less than 30 cm in height.The average seedling density was 3988 seedlings ha�1 and themaximum seedling density within a single gap was 10,402 seed-lings ha�1. The Shannon index of diversity, calculated from theregeneration present in each gap, varied from 0 to 1.49. The highestdiversity of regeneration was found in gaps larger than 37 m2.However, there was no significant correlation between canopy

Table 2Characteristics of canopy gaps within mature F. orientalisforests of northern Iran.

Item Value

Minimum gap size 20 m2

Maximum gap size 2246 m2

Median gap size 178 m2

Canopy gap area (as % of total forest) 9.3%

Most common gap makersF. orientalis 63%Acer spp. 21%

Most common gap fillersF. orientalis 93%Carpinus betulus 5%

FE

gap size and the number of beech seedlings (r2 = 0.069, P = 0.986),the number of maple seedlings (r2 = 0.069, P = 0.897), or the num-ber of seedlings of all remaining species (r2 = 0.099, P = 0.134,Fig. 4). Beech and maple seedlings were present in all gaps, regard-less of size (Fig. 5). The linear regression model confirmed that gapsize was not a significant predictor of seedling density (P = 0.716),but circularity was a significant, negative predictor (P = 0.013);thus, as seedling density decreased, circularity increased.

We identified 354 definitive gap fillers with F. orientalis (93%)the leading species to fill a gap (Table 1). The minimum, median,and maximum gap area that contained at least one F. orientalisgap filler was 24, 206, and 1808 m2, respectively. Carpinus betuluswas definitive gap filler in 5% of the gaps. Although Acer spp. wasan important gap maker, it appeared as a gap filler in <1% of thegaps.

4. Discussion

4.1. Gap creation

Previous research in Iranian beech forests identified most gapsformed as a result of a double tree-fall event (Delfan-Abazariet al., 2004); however, at our study site, we found a substantialmajority of gaps were formed by single tree-fall events (Fig. 3).Weiskettel and Hix (2003), who identified a high frequency of gapscreated by multiple trees in mixed beech forests in the UnitedStates, suggested that the number of trees involved in gap creationreflect poorly-drained soils and older forests with more diseasedtrees rather than a species–specific characteristic. Although oursite represents a mature beech forest, the trees are growing onwell-drained, deep soils which may reduce the likelihood of a sin-gle tree-fall event cascading into multiple trees.

The size of gaps is obviously closely linked to the number oftrees involved in its creation. At our study area, we found a mediangap size of 178 m2. Research in forests in Slovenia showed that Acerrequires large gaps to successfully reach the overstory, but Fagusslyvatica was able to reach the canopy in all gap sizes and was par-ticularly successful in capturing gaps that were <400 m2 (Nagelet al., 2010). This same pattern may exist in the Iranian forestsand would explain the presence of Fagus and Acer seedlings in allgaps, but the dominance of F. orientalis as a gap filler (Fig. 5).Although seedlings and saplings in the adjacent closed-canopyforest were not measured in this study, a previous study alsolocated within the Kheyrud Experimental Forest found 5350–6400 seedlings and saplings ha�1 under closed canopy (Sefidiet al., 2011); this seedling density surpasses the seedling and sap-ling densities measured within the gaps of this study. Perhaps

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Fig. 2. Area and perimeter relationships for canopy gaps in old-growth beech stands from northern Iran. The dashed line indicates the area-perimeter ratio for a circle and theblack circles represent measurements from actual canopy gaps.

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vertical stratification and the increasing crown width of the indi-viduals filling the gap causes seedling mortality and a reductionin the density of seedlings (Parhizkar et al., 2011). The growingconditions for the young seedlings and sapling within a gap arealso depend upon gap size. Seedlings and saplings growing in thesmaller gaps found in Iranian beech forests will experience lowercover from competing herbaceous cover (Gálhidy et al., 2006)and faster litter decomposition rates (Sariyildiz, 2008).

4.2. Patterns of gap closure

Dendroecological research from beech forests demonstrated thepotential of individual beech saplings to remain in the understoryfor as long as 100 years before being released to fill an overstorycanopy gap (Pollmann, 2003). Beech’s ability to persist for longtime periods in low-light levels may explain why a majority ofthe gap fillers identified in this study (93%) were beech. It is possi-ble that these gap fillers came from saplings already established inthe understory when the gap was formed rather than from seedsthat germinated after gap formation. If the gap creator was a beechwith an existing living root system, beech stump sprouts or suckersare also successful at filling gaps (Ohkubo et al., 1996) and in nat-urally-regenerated F. orientalis stands, stump sprouts account for

9% of the successfully established advance regeneration (Erlas,2001). Gaps in beech forests primarily serve to ‘‘reorganize ad-vance regeneration’’ and beech is highly successful at capturinggaps before its competitors (Nagel et al., 2010).

The importance of gap shape to patterns of regeneration de-pends upon the uniformity and complexity of the surrounding can-opy. Within the relatively open, single-strata canopy of longleafpine stands, the presence of irregularly-shaped canopy gaps doesnot substantially alter the resource availability within the gapscompared to the surrounding understory (Gagnon et al., 2004);however, within the closed-canopy, multiple strata structure ofbeech forests, the irregularity of the small gaps creates the oppor-tunity for a greater edge effect because more saplings along theperimeter of the gap compete for the increased light and spaceassociated with the gap’s creation (Fig. 2, Mountford et al., 2006).The irregularity of the shape of the small gaps from this study,may explain the lack of a significant correlation between size ofgap and density of regeneration (Fig. 4) and because the shape ofthe gap had a significant relationship to regeneration density, thismay have been connected to the small gaps having more irregularshapes, while the large gaps were circular.

Canopy gap fraction is a reflection of stand composition, age,and management. In the mature Fagus oriental-dominated hard-wood stands of northern Iran, we found that canopy gaps covered9.3% of the total forested area. This fraction is slightly lower thanthe 15% identified in mature Acer-Liriodendron-Fagus stands fromthe Cumberland Plateau in the eastern United States (Hart andGrissino-Mayer, 2009) and slightly higher than the 4.7% identifiedfor mature Picea-Fagus forests in the Appalachian Mountains theeastern United States (Rentch et al., 2010). Although canopy gapfraction generally decreases with stand age (Hart and Grissino-Mayer, 2009), management can also influence gap fraction. Ayoung, managed Fagus sylvatica forest in northern France had only1.5% gap fraction (Planchais and Pontailler, 1999) and Acer-Betula-Fagus forests from Quebec, Canada that had been selection cut had25% gap fraction compared to the uncut control plots with 10%(Beaudet and Messier, 2002).

4.3. Comparisons with other Iranian beech forests

One characteristic of canopy gaps found at the Kheyrud Exper-imental Forest that differed from other forests in northern Iran was

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Fig. 4. Scatterplots of the number of (A) Fagus orientalis seedlings, (B) Acervelutinum seedlings, and (C) minor species seedlings across different canopy gapsizes. For all figures seedling density is reported as number of seedlings/m2.

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ig. 5. Shifts in proportions of seedlings, by species, across the different canopy gapzes.

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the number of trees involved in the gap creation. We found thatmost gaps were formed from single tree-fall events and identifiedno gaps that formed with more than four gap makers. In contrastSagheb-Talebi et al. (2005), who worked in the Mazandaran forestsof northern Iran, found gaps that were formed from up to six tree-fall events and Delfan-Abazari et al. (2004), who worked in theKelardasht forests, also of northern Iran, found most gaps were cre-ated during double-tree fall events. Not surprisingly, this differ-ence in the number of gap makers resulted in a correspondingdifferenced in gap sizes among the three forested regions, withthe number of gap makers directly related to the size of the gaps.Single tree-fall events at Kheyrud resulted in most gaps being<200 m2 and double-tree fall events at Kelardasht resulted in anaverage gap size of 550 m2 (Delfan-Abazari et al., 2004). Theuniformity of relatively small gaps found at the Kheyrud Experi-mental Forest may explain why we found no significant relation-ship between canopy-gap size and regeneration density, while

Fsi

other researchers who measured a wider range of gap sizes found asignificant, positive relationship between gap size and regenera-tion density (Sagheb-Talebi and Schutz, 2002). Our median gap size(178 m2) was a much closer match to the average gap size identi-fied by Mataji et al. (2008) in F. orientalis stands near Noshahr innorthern Iran than the average gap size reported by Delfan-Abazariet al. (2004) of 550 m2.

4.4. Implications for management in beech forests

In situations where forest managers are attempting to increasethe establishment of F. orientalis in managed stands through thecreation of gaps, the results of this study would indicate there isno relationship between canopy gap size and density of beechseedlings (Fig. 4A). However, it is important to differentiate be-tween seedling germination (characterized by the seedling densitythat we measured) and seedling establishment. Germination ofbeechnuts requires a chilled period of dormancy and one of thegreatest obstacles to germination is insect damage (Yilmaz andDirik, 2008). After germination, beech seedlings are able to survivelow light conditions for long periods of time; however, they can beout-competed when understory vegetation is particularly denseand seedlings are commonly browsed or killed by small mammalsand large herbivores (Wagner et al., 2010). Thus, our results indi-cate no significant difference in seedling germination in responseto gap size; however, establishment of F. orientalis appears to varyby gap size. In a gap-planting experiment in northern Iran (Tabariet al., 2005), small gaps (50 m2) had higher survival rates forplanted seedlings compared to larger gaps (600 m2). Thus, Sag-heb-Talebi and Schutz (2002) recommend using group selectioncuts where 2–4 overstory trees are removed to produce the nextgeneration of F. orientalis trees. In contrast, to these recommenda-tions Parhizkar et al. (2011) suggest that gaps of 200–500 m2 willbe most likely to produce F. orientalis with the straight boles idealfor timber production because smaller gaps produce forked sap-lings which will result in poorly-formed mature trees.

5. Conclusions

Canopy gaps are the dominant source for regeneration in ma-ture F. orientalis forests in northern Iran. Most gaps form duringsingle-tree fall events and the deep soils in this region may preventthe cascading effect of surrounding trees following the initial treefall. Thus, most gaps tend to be small (median gap size was178 m2) and small gaps were irregular in shape. Across the range

K. Sefidi et al. / Forest Ecology and Management 262 (2011) 1094–1099 1099

in gap sizes that we measured (19.6–2246 m2) there was norelationship between gap size and seedling density for either ofthe dominant species in this stand (F. orientalis and A. velutinum).F. orientalis was clearly superior at capturing canopy gaps com-pared to other species and was identified as the definitive gap fillerin 93% of the gaps.

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