Canopy gaps characteristics and structural dynamics in a natural unmanaged oriental beech (Fagus orientalis Lipsky) stand in the north of Iran

[Research]

Dynamics of canopy gap characteristics in a natural unmanaged oriental beech (Fagus orientalis Lipsky) stand in the north of Iran

M. Amiri¹*, R. Rahmani², Kh. Sagheb-Talebi², H. Habashi³

1. Dept. of Forestry for Arid Regions, Faculty of Desert Studies, Semnan University, semnan, Iran

2. Dept. of Silviculture and Forest ecology, Faculty of Forest Sciences, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran

3. Forest Research Division, Research Institute of Forests and Rangelands, Tehran, Iran

* Corresponding author’s E-mail:amiri.mojtaba@yahoo.com

ABSTRACT

Canopy gaps are one of the most important structural features of forest ecosystems, and studying of them can have useful results and implications for forest management. The aim of this study is investigation of characteristics and regeneration within canopy gaps in an intact beech stand in the Shastkalateh Experimental Forest of Hyrcanian region, north of Iran. All canopy gaps and related forest parameters were measured within a permanent plot 16ha. Considering the composition of the forest, the dominant tree species was oriental beech with 36.3% of the stem number and 56.6% of the stand volume. In total 54 canopy gaps were identified which covered about 5% of the forest area. An average of 4.32 gaps.ha^-1 existed in the permanent plot and gap sizes varied from 48.3 to 622.7m². More than three-quarters (77.7%) of canopy gaps were smaller than ≤200m². Also more than half of the gaps (53%) were formed by a single tree-fall event and beech made up 52% of gap makers and 23.4% of gap fillers. Beech had the second largest proportion on standing deadwood of gap maker in the study area, while velvet maple was the most frequent gap filler in approximately 30% of the gaps. Despite the high frequency of small gaps 100m², their proportion of the overall gap area reached only 25% that suggests the important role of intermediate and large gaps in the gap dynamics. Considering the recent occurring disturbances in the Hyrcanian Forests, the study analyzed the main characteristics of disturbance regime with the emphasis on the role of wind and longevity of trees. In general, findings of this study showed that the creating small and average gaps in intact beech forests could be based on natural disturbance regimes, And suitable conditions provides for successful regeneration of beech forests in close to nature silviculture.

INTRODUCTION

Forests dominated by oriental beech (Fagus orientalis Lipsky) as a major species are one of the most widespread and abundant vegetation types in the Hyrcanian Forest (Resaneh et al. 2011; Sagheb-Talebi & Schütz, 2002; Sagheb-Talebi et al. 2004). In the other hand, intact oriental beech stands provide a unique opportunity to study the disturbance regimes of forest ecosystems without human influence [4]. In many forest ecosystems, major disturbances are pivotal in controlling the structure of stands within the context set by environmental factors and the present species (Whit 1979; Pickett et al. 1989; Wimberly & Spies, 2000). Disturbance is a permanent feature of forest ecosystems, determining species' composition, structure and process (Atiwil, 1994; Ulanova, 2000; McCarthy, 2001; Kucbel et al. 2010) and it plays a crucial role in the forest ecosystem dynamics (Kucbel et al. 2010). Disturbance events range from the small scale disturbance of single tree-falls or crown-breaks to the large-scale disturbance caused by e.g. catastrophic windstorms. One of the most important disturbances in forest ecosystems is opening areas called “canopy gap”. Canopy gaps play an important role in the regeneration of forest by providing habitat for regenerating seedlings and saplings in the forest floor (Hubbell & Foster, 1996). In many forests throughout the world, tree- fall gaps, opening areas in the canopy caused by the death of one or more trees (Whitmore, 1989), are the dominant form of disturbance. These gaps not only help maintain the characteristic uneven-aged nature of late successional forests, but also they influence nutrient cycling, preserve soil and plant species diversity, and change the microtopography in many forests. Windthrow gaps have been documented in a variety of ecosystems on every continent. Forest gaps have also been termed nutrient “hot spots” because they tend to be areas with high rates of decomposition and mineralization, leading to increased levels of nutrients (Colines & Pickett, 1988; Poulson & Platt, 1966; Denslow et al. 1998; Ritter, 2005; Scharenbroch & Bockheim, 2007). Thus, gaps play an important role in the overall biogeochemistry of forest systems. Over the last 30 years, numerous studies have been conducted describing gap dynamics (Denslow, 1987; Kuuluvainen et al. 1998; Yamamoto, 1992; Yamamoto, 2000; McCarthy, 2001; Delfan-Abazari et al. 2004; Sagheb-Talebi et al. 2005; Sefidi et al. 2011).  These studies focus on gap impacts on forest structure, nutrient cycling, microclimate, regeneration, restoration and forest management. Although there is extensive literature on gap dynamics, it is often hard for managers to interpret results due to inconsistent methods. In shaded understorey of forests, many suppressed seedlings and saplings wait for many years for the appearance of a canopy gap (Hubbell & Foster, 1996; Forget, 1997; Ashton, 1998; Van Der Meer et al. 1998; Abe et al. 1995). Therefore, forest regeneration processes frequently depend on the natural disturbance regimes (Whitmore, 1989). In fact, canopy disturbances have a major influence in forest communities’ structure and organization, since gaps formed by disturbances will influence the germination, growth and survival of tree seedlings. The response of trees to disturbance depends on the type of species regeneration and on gap features including (dimension, age, morphology, mode and frequency of formation) (Abe et al. 1995; Van Der Meer et al. 1998; Yamamoto & Nishimura, 1999; Imai et al. 2006). Importance of gap dynamics in forest ecosystems was discovered in the early 20th century, and intensive investigation of gap dynamic features are dates back to the late 1970s (McCarthy, 2001). Gap formation changes the amount of total incident light reaching the ground level and influences nutrient and moisture availability (Denslow & Spies, 1990; McCarthy, 2001; Ritter, 2005; Mihok et al. 2005; Galhidy et al. 2005), providing potential establishment sites for regeneration. As the gap dynamic is recognized as a crucial process in forest stand development, it has received much attention during the last several decades (Denslow, 1980; Runkle, 1982; Runkle & Yetter, 1987; White & Pickett, 1985; Brokaw & Scheiner, 1989; Spies & Franklin, 1989; Attiwil, 1994; Seymour et al. 2002). Gap dynamics have been described in tropical (Van Der Meer & Bongres, 1998; Carvalho et al. 2000; Imai et al. 2006), temperate (Abe et al. 1995; Yamamoto, 1995; Coates, 2000; Choi et al. 2001; Ott & Juday, 2002; Gagon et al, 2004; Amanzadeh et al. 2004; Delfan-Abazari et al. 2004; Sagheb-Talebi et al. 2005; Arietta & Suarez. 2005; Naaf & Walf, 2007; Shabani et al. 2011; Sefidi et al. 2011) and boreal forests (Kneeshaw & Bergeron, 1998; Drobyshev, 2001; McCarthy, 2001, Table 1), and being considered a process capable of shaping the structure of plant communities, it expands environmental heterogeneity, in space and time, develops a diversity of establishment and growth chances for tree species. Differences in species shade tolerance levels may determine the ability of immature individuals to grow and survive in gaps of different sizes (Carvalho et al. 2000; Meyer et al. 2000; Schumann et al. 2003; Imai et al. 2006). Gap size is an important characteristic in natural forests and it can strongly influence vegetation growth and nutrient cycling (Zhang & Zak, 1995; Gary et al. 2002; Muscolo et al. 2007). In many studies gaps ranging from 10 to about 2500m2 have been reported (Table 1). Small gaps are originally defined to describe small openings in the forest and are created by the death of branches or one or more trees (Watt, 1947). Large gaps (>1000m²), created through fires, tornadoes, downdrafts, or hurricanes, have characteristics sufficiently different from small gaps which makes it difficult to compare them (Schliemann & Bockheim, 2011) or are created by domino effect by falling of big trees over other trees. In particular, very large openings have reduced shading from surrounding trees and consequently have higher solar radiation and soil temperature than small openings (Zhang & Zak, 1995; Gary et al. 2002; Muscolo et al. 2007) In this study, we have examined stand structural and gap characteristics of tree species in old-growth oriental beech stands of the Hyrcanian forests, north of Iran. Specific objectives were i) to examine stand structure , ii) to examine trends in gap characteristics (gap area, gap size, gap density, gapmaker traits, and number of gap makers), iii) conditions and state regeneration within canopy gaps in the permanent plot in size of 16ha. 

MATERIALS AND METHODS

This study is carried out in the 79 ha natural unlogged oriental beech (Fagus orientalis) stand in compartment 32, district 1, located at Shast Kalateh Forest in the eastern Caspian region, North of Iran on 36° 43′ 27″ N, 54° 24′ 57″ E. A general view and location of the study area is given in Fig. 1. Elevation of the study area varies from 820 to 960m a.s.l., with an average monthly temperature of 15.4°C, with maximum and minimum temperature in July (28.7°C) and in February (8.71°C), respectively. Mean annual precipitation is 650mm. According to the De Martonne and Emberger classifications, the climate of the study area is cold and wet, having a temperate summer with short dry season. Stand total height is about 30 m and the canopy cover varies between 60 and 100 percent (Habashi et al. 2007). 

Soil texture is loam to clay-loam with a pH of 5.5 and soil is classified as a forest brown soil. Mean stand density and standing volume are 235ha^-1 and 463m3ha-1, respectively (FRW, 1995 & 2008). The compartment consists of a natural, mixed, uneven-aged deciduous old-growth forest dominated by shade-tolerant Oriental beech with minor components of other broad-leaved species including hornbeam (Carpinus betulus L.), velvet and cappadocian maple (Acer velutinum Boiss. and Acer cappadocicum), Caucasian alder (Alnus subcordata), ironwood (Parrotia persica), date plum (Diospyrus lotus) and elm (Ulmus glabra Huds.). The compartment experienced very limited human intervention and disturbance and had no silvicultural activity in the last 50 years since forest management plans started in Iran. Therefore, this stand could be regarded as an example of an intact and unmanaged natural forest.

FIELD MEASUREMENTS AND DATA ANALYSIS

The total surface of compartment 32 is 79ha and we selected 16ha (400×400m) as a permanent research plot in summer 2011. The location of the plot was selected to satisfy the following conditions: i) the permanent research plot should be placed in the natural oriental beech stand without human influence, and ii) the permanent plot should be homogenous regarding the slope and aspect. Then, the permanent plot was divided into 64 subplots (50×50m), where all individuals of natural regeneration and understory trees of DBH ≥ 7.5cm were counted within subplots (Delfan-Abazari et al. 2004; Sagheb-Talebi et al, 2005; Habashi et al. 2007). Moreover, on the entire permanent research plot, all living trees with diameter at breast height (DBH) ≥ 7.5cm, standing deadwood (snags) and fallen deadwood (logs) were measured (Lertzman, 1992; Habashi et al. 2007). The threshold values for snags was the DBH ≥7.5cm and the height > 1.30 m and for logs the small-end diameter ≥ 7.5 cm and length more than 1.30 m. For the living stems and snags the tree species, DBH and only for snags also the height was measured. Tree species, DBH, small- and large-end diameter, total length and the exact location on the sample plot were identified for the logs. On the 16ha permanent research plot, we recorded type of canopy openings—canopy gaps (Runkle, 1982). Canopy gap was the area directly under the canopy opening with the border defined by the crown projections of trees surrounding this canopy opening. When trees within the gap reached the base of canopy of the surrounding forest, a gap was considered closed and not sampled.

Canopy gap was defined as an opening in the canopy with the size at least 40 m 2 that was formed by the mortality of one or more trees from the upper tree layer (with DBH ≤ 20 cm), and where the standing or lying remnants of the gap maker were detectable. For each canopy gap, two parameters were measured—the length (L) as the longest distance within the gap and the width (W) as the largest distance perpendicular to the length. Gap form was considered as ellipse and the area of canopy gap was calculated using the formula for an ellipse: A=πLW⁄4, Gapmaker was considered to be a tree from the upper tree layer whose death caused an opening in the canopy. On the other hand, Gap makers were characterized with the following descriptors: type (snapped, root-thrown, dead standing, and leaning), species, and diameter at breast height.

The number of gap makers for each gap was recorded, and the tree species and the DBH were identified for each gapmarker as well.

Regeneration (gap filler) was sampled within all gaps with seedling (et al. 2004).

In all gaps “definitive gap fillers” were measured (Sefidi et al. 2011).  Additionally, we selected the gap filler most likely to replace each gap maker. These definitive gap fillers were chosen based upon their height, location, and a visual assessment of their overall condition (Lertzman, 1992). In most cases, we could easily designate the definitive gap fillers because they were considerably taller than other gap fillers, and they often showed a height growth release in response to gap formation that was easily seen in the field, especially for beech and hornbeam. Most definitive gap fillers in this study were pole-sized trees with DBH 7.5-12.5 cm and 8-15 m tall.

Table 1. Comparison of forest stand and canopy gaps characteristics in differences studies

Fig.1 Location of the study site in the Hyrcanian Forest

RESULTS

Stand structure and composition

The mean density and volume of the studied permanent research plot were 287 Nha^-1 and 472 m 3ha^-1, respectively. Dominant tree species was beech with a proportion of 40% in stem number and which was only counted for 55.8 % of standing volume. The proportion of stem number per ha for hornbeam and ironwood was 21.2% and 26.9%, respectively. While the volume proportion of these two species was 24.1% and 10.4%, respectively. Other species were counted to 11.5% of stem number and 9.7% of stand volume (Table 2). Although the other species present in the stand reached a relatively high number (11.6%), they accounted for only 9.5 % of volume.  Of total deadwood, standing deadwood accounted 32.6% and fallen deadwood consisted of 67.4 % within the canopy gaps (table2). Total amount of deadwood was 27.75 Nha^-1 and 45.4 m 3 ha^-1, which was counted to 8.7% of total density and 9.1% of total volume of living trees. The deadwood proportion of beech, hornbeam, ironwood and other species logs was 31.4, 32, 21 and 15.6%, respectively. The volume proportion of these three species was counted to 62.5, 25.51, 8.62 and 4%, respectively (Table 2). 

GAP CHARACTERISTICS

Overall, 54 canopy gaps were identified on the permanent research plot with a total area of 7966.5 m² (Table3) that covers 4.98% of the total area. Minimum, maximum and mean gap size

was 48.3, 622 and 147.5 m². Half of the gaps were smaller than 100 m², covering 2051.1 m², which counted for 25.7% of the total gap area. The other half of the gaps were larger than 150 m²; among them only one gap was larger than 600 m². Gaps showed different directions, mostly north, south-west and north-west directions counting for 33.3% north, 25.9% south-west and 16.7% north-west of the whole gaps. The highest mean gap size (ca.140 m²) was found on east and west directions (Table3).  Density of gap makers was 7.6 Nha^-1, and mean, minimum and maximum DBH of them were 63.16 cm, 21.7 cm and 115 cm, respectively. Of total deadwood, standing deadwood counted to 32.6% and fallen deadwood consisted of 67.4% within the canopy gaps (Table3). Range of gaps varied within gap size, that 27 numbers (50%) were in the gap class 100 m² and other species 50% were in the other gap classes. Only one gap (1.85 %) of total gaps recorded in the gap class was greater than 600 m². The largest gap area belonged to gap classes lower than 100 m² with 2051 m² and the smallest included gap class above 600 m² with (622 m²). The most frequent were the canopy gaps with the size from 48.5 to 150 m², i.e. gap classes 2 and 150 m² (68.5%). Whereas more than 70% of these categories were consisted by the gaps 2.  About seventy-eight percent of the gaps was less than 200 m² in area and only 7% of the gaps exceeded 500 m² (Table 4) gaps larger than 300 m² composing 14.7 % of total gaps frequency, but they formed 35.1% of the total area covered by gaps. The mean frequency direction of tree falls and gaps were in accordance with the prevailing winds, e.g. North (0°) and southwest (240°) (overall 58% of total gaps) in the surveyed area (Table 5). Fig. 2 shows the distribution of the total gap area according to the size categories of canopy gaps. While the small gaps (2) made up nearly 50 % of gap maker, their proportion of the total gap area reached only 25.7%. On the other hand, one largest recorded gap made up to 7.8% of the overall gap area. The most frequency of the gaps size 2 comprised about two-thirds of gap area, whereas the largest gap made up an over proportion amount of 9.6%.

GAP MAKER CHARACTERISTICS

A total of 95 gap makers were identified within the 54 studied gaps, making an average of 7.6 Nha^-1.

More than third-quarter of the gaps were made by one or two gap makers, 50% by one and 31.5% by two gap maker; there was only one gap that was made by 7 gap makers (Fig. 2). The area of gaps made by one gap maker varied between 48 m² and 252 m², while that of made by 7 gap makers was 622 m² (Table 5). Beech was the most common gap maker (52%) followed by hornbeam (26.6%), ironwood (16%). Velvet maple, Caucasian alder and date plum were other gap makers with total proportion of 5.4 %. The minimum, maximum and mean DBH of the gap makers was 21.7, 63.2 and 115 cm, respectively.

The mean DBH of beech was the highest (70.6 cm) among the gap makers, whereas that of date plum (21 cm) was the lowest (Table 6).

The most common cause of canopy gaps was a single tree-fall and two tree-fall events with (52 and 34%). While gaps caused by three and more trees were very little (14%). The maximum number of fallen trees within a single gap opening was 7.  Fig. 7 showed that beech had the highest percent of gap maker among species (52%) and Caucasian maple had the lowest (2.4%). While this species had the highest frequency gap-filler among species (29%).  As the tress creating gaps were identified using woody debris analysis, a two-trees gap might be identified in which the first tree died 40 years ago and the second tree one year ago. Regarding the land area of gap size classes, a distinctive disproportion between the gap frequency and gap area was observed.

GAP REGENERATION

A total of 37167.4 seedling and sapling per hectare were measured in the 54 gaps studied (Table7). Velvet maple (28.8%) and beech (24.1%) had the maximum frequency among regeneration. Ironwood (15.4%), hornbeam (14.8%) and date plum (11.5%) were the next important contributors to the regeneration pool. Approximately, 84% of the total regeneration, namely the seedlings was shorter than 1.30 m in height class (Table 7).

The proportion of saplings with DBH less than 2.5cm was 11%, saplings with DBH 2.5-7.5 cm were 4% and saplings with DBH 7.5-12.5 cm were 1%. The frequency of regeneration of different species in different gap sizes is given in Fig. 4. With increasing gap size, the number of beech regeneration decreases, while that of hornbeam and velvet maple increases.

Fig. 2. Proportion of gaps made by gap makers.

Fig. 3. Frequency of gap makers and gap filler species in the 54 gap studied.

Table 2. Main characteristics of the living trees and deadwood on the studied permanent research plot

Table 3. Characteristics of the studied gaps in the permanent research plot

Table 4. Characteristics of gaps in relationship with geographical direction

Table5. Characteristics of the studied gaps in relation to number of gap makers

* Numbers within parenthesis are minimum and maximum gap size

Table6. Characteristics of the studied gaps based on gap maker species

Table7. Number and frequency of seedlings and saplings (Nha-1) by species in the canopy gaps on the permanent research plot

Fig 4. Proportion of regeneration of different species in different gap sizes