Effect of forest fragment size on tree diversity and ...

生物多样性

2010, 18 (2): 208–214

Biodiversity Science

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Effect of forest fragment size on tree diversity and population structure of humid subtropical forest of Meghalaya, India Om Prakash Tripathi1*, Khongjee Rangdajied Reynald 2 1 Department of Botany, North Eastern Hill University, Shillong-793 022, India 2 Department of Forestry, NERIST, Nirjuli-791109, India

Abstract: The present study was conducted in subtropical humid forests of Meghalaya to study the distributional pattern of species, floristic composition, community structure and tree population structure. Forest fragments of varying sizes (0.5 ha, 1 ha, 2 ha and 5 ha) were used in the study. All of the forest fragments are distributed within the same altitudinal range, and had similar rainfall and temperature regimes. Four forest fragments were sampled using random quadrats to analyze the impact of fragment size on tree diversity and population structure. Indices were used to compare the dispersion pattern of plant species, species diversity among fragments, and the heterogeneity and homogeneity of the fragments. A total of 45 tree species were recorded from all the fragments and simple correlation showed that the species richness was positively related to fragment size (n = 4, P95 cm. The density and basal area of each species were determined in above girth classes according to Mueller-Dombois & Ellenberg (1974) to study the girth class distribution. The population structure was analyzed at the community level with the help of density-diameter curve. Whitford index (abundance/frequency ratio, A/F) was used to study the dispersion pattern of plant species in different forest fragments. The dispersion pattern of the species will be regular, if the value of A/F ratio is 0.05. Whittaker β-diversity was used to compare the species diversity between the different forests with the help of βw = S/α –1, where, βw = Whittaker β-diversity, α = Species richness (mean), S = Total number of species. Shannon and Wiener (1963) index of diversity (H′) was calculated using importance value (IVI) of the species using the following formula: s

H ′ = −∑ ( ni / N ) ln(ni / N ) i =1

where, ni = IVI of the ith species, N = IVI of all the species. Simpson (1949) dominance index (Ds) was calculated by using IVI values of the species using Ds

生物多样性 Biodiversity Science

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= Σ (ni / N) 2. Sorensen’s similarity index was calculated using the formula 2c/(a+b) (where c = common species between the stands, a = species richness of stand-a, and b = species richness of stand-b). The above indices were calculated following Magurran (1988). Simple correlations for species richness among the fragments were analyzed with the help of statistical software (SYSTAT ver. 5.0).

A total of 29 families were recorded from all the fragments. Lauraceae and Fagaceae were among the most species rich families followed by Theaceae, Symplocaceae and Moraceae in all the fragments and a large number of families were represented by a single species.

Results

In all the forest fragments, majority of tree species (49–75%) showed low frequency (80% frequency except 5-ha forest fragment, i.e., 3% species were distributed in this frequency class. Castanopsis armata, C. indica, Quercus griffithii, Schima khasiana, Rhododendron arboreum, and Ilex venulosa were among the most frequently found species. Based on importance value (IVI), Castonopsis indica and C. armata were the dominant or co-dominant species in all the forest fragments and a large number of species had low IVI giving rise to log-normal curve of dominance-distribution pattern signifying high equitability and low dominance in all the forest fragments (Fig. 1). Shannon diversity index was maximum (3.15) in 5-ha fragment, while Simpson dominance index showed a reverse trend to that of diversity index (Table 1). Majority of the tree species in the forest were contagiously distributed, only 1–2 species showed regular distribution pattern. The number of species showing random distribution varied between 8 and 12 (Table 2). Greater proportion of contagiously distributed species made the forest community highly patchy/ hetrogenous in composition.

Tree species richness Altogether 45 species were recorded from all the studied forest fragments. The maximum number of species (33 species of 28 genera and 21 families) were observed in 5-ha followed by 28 species in 2-ha forest and 24 species each in 1-ha and 0.5-ha forest fragments (Table 1). Simple correlation showed that the species richness increases significantly (n = 4, P95 cm girth class accounted for only 9% to 17% in all the forest fragments (Fig. 2). Distribution of basal cover in different girth classes showed a reverse trend, that is, trees of higher girth classes, though less in number, contributed maximum basal cover in all the forest fragments followed by individuals of middle girth class (55–95 cm CBH). Although the lower girth class has contributed to the maximum stand density, its contribution towards the basal cover was only 7–11% in small fragments and about 33% in the 5-ha forest fragment. Population structure

Fig. 1 Dominance-distribution of species in different forest fragments of humid subtropical forest of Meghalaya Table 2 Distribution pattern (based on Whitford index) of trees in different forest fragments of humid subtropical forest of Meghalaya. Values in the parentheses are the percentage of the total number of species in given forest fragments. Distribution pattern

Density of tree seedling: The number of tree seedlings increases with increase in fragment size, with maximum number of 16 species in 5-ha, followed by 14 species in 2-ha forest, 13 species in 1-ha forest fragment, and 11 species in 0.5-ha forest fragment (Table 1). Mean seedling density (plants/100 m2) was 800, 900, 915 and 1,025 in 0.5-ha, 1-ha, 2-ha and 5-ha forest fragments, respectively. Seedlings of Castonopsis indica, Myrica esculenta, Symplocus javanica and Eurya japonica in 0.5-ha and 1-ha forest, C. indica,

Forest fragments 0.5-ha

1-ha

2-ha

5-ha

Regular

0 (0%)

0 (0%)

2 (7%)

1 (3%)

Random

12 (50%)

8 (33%)

11 (39%)

12 (36%)

Contagious

12 (50%)

16 (66%)

15 (53%)

20 (60%)

Total

24 (100%)

24 (100%)

28 (100%)

33 (100%)

Table 3 Sorensen’s similarity index (%) and Whittaker’s β-diversity index (dissimilarity) in different forest fragments of humid subtropical forest of Meghalaya Forest fragments (ha) 0.5 and 1

Sorensen similarity index (%) 58.3

Whittaker β-diversity index 0.42

1 and 2

53.8

2 and 5

62.3

0.50 0.48

5 and 0.5

52.6

0.51

1 and 5

49.1

0.54

2 and 0.5

65.4

0.38

Basal cover of trees were found to be maximum in 2-ha (39.8 m2 /ha) followed by 5-ha, 1-ha and 0.5-ha (Table 1). Distribution of stand density in different girth classes revealed that trees of lower girth (15–55 cm) accounted for 52–86% in all the fragments except 0.5-ha fragment size where middle girth class has

Fig. 2 Density-diameter (A) and basal area (B) distribution under different girth classes in different forest fragments of humid subtropical forest

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M. esculenta, Schima khasiana and S. javanica in 2-ha forest, and C. indica, Sterculia villosa, M. esculenta and Vaccinum sprengelii in 5-ha forest fragments were among the most dominant species. Density of tree sapling: The number of tree saplings increase with increase in fragment size with maximum number of 15 species in the 5-ha and minimum of 9 species in 1-ha forest fragment. The mean sapling density also followed a similar trend to that of species richness (Table 1). The saplings of Castonopsis indica, Linderia pulcherrima, Myrica esculenta and Ficus gasperriniana in 0.5-ha and 1-ha forest, C. indica, Schima khasiana, Exbucklandia populnea and Neolitsea cassia in 2-ha forest, and C. armata, C. indica, L. pulcherrima and M. esculenta in 5-ha forest fragments were among the most common saplings. Densities of adult trees were presented in Table 1. The overall population density of seedlings, saplings and adult trees formed a pyramidal structure in all the forest fragments. Preponderance of tree seedlings, followed by a population density of saplings and steep decline in density of adult trees, indicated that the period between saplings to adult stage was the critical stage in the life cycle of the tree population, as the maximum mortality occurred during this period.

Discussion The state of Meghalaya is undergoing rapid transformation due to urbanization, commissioning of hydroelectric projects, mining and extraction of forest products, besides age-old practices of shifting agriculture. All these have led to the fragmentation of large tracts of natural forests into small patches. Forest fragmentation modifies species composition due to microclimate changes, and decrease in genetic heterozygosity, and favours colonization by invasive species from the surrounding vegetation (Tilman et al., 1994; Laurance et al., 2002). Fragmentation results in two types of forests ecosystem, interior forests that are virtually identical to the original, and the edge forests that is different from the original in respect of species composition and microclimatic conditions. Fragmentation reduces the size and quality of the habitats, with loss of corridors and continuous impact of edges which has serious ecological and environmental implications, since it often causes depletion of biodiversity and elimination of resources or their rearrangement into a new configuration (Matlack, 1994; Mishra et al., 2003; Jha et al., 2005; Sarma et al., 2008). The sacred grove at Laitryngkew is multilayered subtropical humid forest community composed of large, medium and small trees distributed in three distinct strata. The sub-canopy layer composed of me-

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dium and small trees and shrub had highest species richness. High species richness of sub-canopy is attributed to the presence of individuals of canopy species, which were either young or growth were arrested due to shade cast by overhead canopy as well as other under-growing species. Species richness of the grove was also due to geographical location of the northeastern region at the confluence of Indo-Malayan and Indo-Chinese biogeographical region and favourable climatic condition. Species composition and stability depend on site condition and biotic stress. The present study site was covered by a continuous forest patch of 15-ha about two decades ago, which has been broken into small fragments separated by 5–50 m grass-covered corridors due to human disturbances. The intensity of disturbance which was assessed by disturbance index reveals that though all fragments were mildly disturbed, the smaller patches were relatively more disturbed than the larger ones. As part of the same forest (sacred grove), fragments were similar to each other in species composition. However, different community parameters such as density, basal cover and importance value of species revealed changes in these parameters, which seems to be the result of forest fragmentation and associated micro-environmental changes. Tree species richness and diversity showed an increasing tendency with increase in fragment size and decrease in disturbance degree in the fragments. Though the dominance of species was similar in all the fragments, the dominance of a few species showed a progressive decrease with increase in patch size. On the contrary, some species showed a distinct increase in their dominance with increase in fragment size. Despite general similarity, forest fragments differed markedly in species richness including tree component, leading to low similarity index and high β-diversity between the stands. This could be related to the level of disturbance to which they were exposed. The log-normal dominance-distribution curves, which signify equitability and stability of the community, signifies abundance of species having intermediate dominance values in the community (Magurran, 1988) and indicates maturity and complexity of natural community. Contagious/clumped distribution pattern and low frequency of most species in stands was responsible for making the community highly heterogeneous and patchy. This type of distribution of species is often related to inefficient mode of seed dispersal (Richards, 1996). Random distribution of species in the community was a sign of frequent disturbance (Armesto et al., 1986). Comparatively higher density of tree species in 5-ha forest fragment than other forest fragments was primarily due to presence of large number of young

第2期

Tripathi OP & Reynald KR: Effect of forest fragment size on tree diversity in India

individuals. The species richness, tree density and basal cover obtained in the present study is comparable with the results of Campbell et al. (1992) from Brazilian Amazon, Pascal & Pelissier (1996) from Uppangala, Kadavul & Parthasarathy (1996) from Western Ghats, and Upadhaya et al. (2003), Tripathi (2002), Mishra et al. (2004) and Tripathi et al. (2004) from different forests stands of Meghalaya. The tree population density and basal cover in >5 ha fragment was comparatively higher than the other fragments due to presence of large number of young as well as old individuals in the community. The sapling population in small fragments was low in spite of large seedling populations which could be due to mortality caused by cattle’s or unfavourable microclimatic conditions. The poor seedling and sapling density of some species in large fragments was attributed to the low light intensity on the forest floor due to dense overhead canopy (Barik et al., 1992; Tripathi, 2002). Recruitment failure of native species due to shading has been emphasized by a number of workers (Davies, 2001; Levine et al., 2003; Lichstein et al., 2004). Reduced abundance of shade-tolerant understory seedlings in forest fragments could result from both limited seedling recruitment and increased mortality of established seedlings (Bruna, 2002). Seedlings have a limited tolerance range of light, temperature and humidity which were strongly altered in the forest fragments (Gehlhausen et al., 2000). However, the effects of fragmentation on tropical plants were not uniformly detrimental; some taxa showed enhanced reproduction and growth in the fragments. The subtropical humid forests of Meghalaya represented by sacred groves are still preserved by the tribes. These groves are mildly/highly disturbed due to use by the local village community for their timber, fuel wood and forage. In spite of these, tree regeneration was not severely affected as was evident from presence of large number of young trees in the stand. All these activities were responsible for decrease in tree diversity in the smaller fragments. Large fragments which were less disturbed harboured such species which were not present in small fragments. Based on the results the following protective measures have been recommended: (i) the loss of tree species from the forest fragments is not inevitable if they are managed well; (ii) to conserve tree diversity, it requires bringing newly fragmented forest patches to its natural condition as quickly as possible; (iii) creating a stable, protective buffer of edge species around newly fragmented forest patches, to protect the diverse core species from altered micro-climate; and (iv) controlling grazing and other anthropogenic activities in the forests to ensure further degradation.

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