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The tree species diversity, distribution and anthropogenic disturbances along the altitudinal gradients were evaluated in Bhutan's Jigme Singye Wangchuck National Park located in inner Himalayas. The sparse information of plant species where management claims high priority to species conservation necessitated this study. The research involved 92 forest inventory sample plots covering 3.68 ha in the park which was conducted along altitudinal gradients across four sites where each site covered 23 sampled plots. In the sampled plots the disturbance regimes were also registered and the stand structure and diversity of tree species were evaluated subject to different disturbance intensities. Result showed the total of 1276 individuals and 144 species belonging to 59 families. Altitude was significantly associated to species distributions and diversity. Stand structure and species richness was significantly different in highly disturbed sampled plots while no significant differences is detected between the no recorded human disturbances, low disturbances and moderately disturbed sampled plots. Based on our results we concluded that environment plays important role in shaping forest resources condition in the park and we suggest that moderate disturbances may not require the management strategy to exclude the human activities despite the utilization of forest resources in JSWNP. However, activities that leads to higher activities may need to incorporate sustainable long term strategy that guarantees the sustainable livelihoods of the park people and the conservation of its own sake.
Biodiversity is essential for economic and aesthetic wellbeing to humans and stability for proper functioning of the ecosystem (MEA, 2000; Singh, 2002; Sagar et al., 2003). Interest in biodiversity conservation has currently raised issues ranging from direct anthropogenic disturbances to indirect such as climate change leading to extinction of species (Ehrlich and Wilson, 1991; Markham, 1996; MCneely, 1994; Sodhi et al., 2004). Many kinds of environmental changes influence the diversity of species. Therefore, conservation of biodiversity is rather one of the noteworthy issues that require significant scientific efforts required to understand this complex phenomenon. Biodiversity conservation, particularly forest protection with quantification of forest species diversity is an important aspect that, provides resources and habitats for many other species (Cannon et al, 1998; Sagar et al, 2003; Yadav and Gupta, 2006). The growing environmental change augmented by human disturbances would occur as deleterious effects on the floristic structures that might have wider implications on ecosystem (Rai, 1985; Sheil, 1999). But to understand this phenomenon from Himalayan ecosystem regarding species diversity and human disturbances is limited. In Bhutan, although more than 50% of the country's area is under protected system, the floristic diversity and the human impacts on floristic diversity due to resources use by local people in the inner Himalayan ecosystem is lacking.
A number of factors affect the structure, distribution and composition of woody species. The factors could be climate, substrate, topography, natural or human disturbances. In relatively small scales soil chemistry (Fu et al., 2004), topography (Gould et al., 2006; Yadav and Gupta, 2006), canopy gaps and human disturbances such as livestock grazing, cutting and burning ( Lapkern et al., 2009; Sapkota et al., 2009; Thapa and Chapman, 2010) are the important factors that affects community structure, richness and diversity. At a larger scale, such as landscapes, climate and elevation (2001; Gould et al., 2006; Zhao et al., 2009) could play a significant role in determining species richness, diversity and structure.
In the subtropical dry and warm temperate forest, the available moisture plays important role to species distributions. Elevation, aspect, slope and soil chemistry are the other determinants of local species composition (Yadav and Gupta, 2006; Cielo-Filho et al., 2007). On the other hand, disturbances be it natural or human could alter the structure of the ecosystem and available resources (Lindemeyer and Franklin, 2002; Zhu and Liu, 2004 cited in Zhu et al., 2007). Authors argue that the human disturbance could be more destructive than any other natural factors, particularly, with regard to stand structure and composition of forest (ibid), however, our view is that the impacts of human disturbances on biodiversity could depend on the intensity of disturbances. Some studies from Himalayan ecosystem reported the varied disturbances impacts based on the intensity of disturbances (Sagar et al., 2003; Sapkota et al., 2009).
Many others argue that species diversity is often higher when the disturbance is intermediate intensity or frequency (Collins et al., 1995; Zhu et al., 2004; Zhao et al., 2009). For instance, Collins et al., (1995) concluded that disturbance of a suitable intensity increases the species richness and that agrees with the Intermediate disturbance hypothesis of Connell (1978). However, intensive and prolonged human disturbances may lead to decline in species diversity and change in species composition ultimately leading to forest degradation (Ramirez-marcial et al., 2001; Sagar et al., 2003; Sagar and Singh, 2004). Thus, the relationship between the disturbance and diversity has received more attention from natural resource managers in the recent past (Lindemeyer and Franklin, 2002; Zhu and Liu, 2004). In forest management, especially in protected areas (PAs) like Bhutan it is necessary to consider how human activities augmented with environmental factors affect tree structure and diversity. When the diversity and structure of forests are maintained despite the utilization by human, management strategy may not require the exclusion of human use of forest resources.
In Bhutan the 72.5% of the total land area is covered by forest (RGOB, 2002, 2003; Gurung, 2008). About 52.0% of the forests cover, out of the total of 72.5%, is represented by PAs and Jigme Singye National Park (JSWNP) represents roughly about 10% of the total proportion. JSWNP is the third largest national park in Bhutan centrally located and covers sub-tropical, temperate to alpine zone rising from 300 m in the south to 500 m in the north. Studies by Ohsawa (1999) and Wangda and Ohsawa (2006) in Bhutan Himalaya, concluded that the dominance structure changed from multi-dominant to mono dominant community with increase in altitude. Wangda and Ohsawa (2006) also concluded that the past disturbances lead to the uni-modal type of regeneration where as sporadic type of regeneration in case of small scale disturbances. However there is little information regarding the environment, forest relationship as well as forest people interactions from the national parks in Bhutan.
In JSWNP shifting cultivation, collection of forest products and associated livestock grazing were highly practiced by local people to supplement their subsistence based livelihoods (Siebert et al., 2008). The presence of residents in the park and their close linkages with natural resources are a source of conflicts. Past research in the JSWNP was concerned primarily with human-wildlife conflicts and attitudes of local people related to this problem (Wang and Macdonald, 2006, Wang et al 2006a, Wang et al 2006b). The local's subsistence based livelihoods is further constrained by the conservation policy that restricts forest products use (Wang et al., 2006a). In addition the conservation activities in the park are in backlash since 2006 due to the financial crisis as donor funds dry up (Kuenselonline, 2009). At this stake, how local people adjust with the growing constraints to their livelihoods is a great concern. JSWNP has been selected for this research because it is a flagship area for conservation owing to its location, geographical variations and presence of high diversity of plant species claimed to have medicinal and economic importance (RGOB, 2002). There are growing literatures regarding the conservation and factors associated with the anthropogenic disturbance from the people living in and around the parks and elsewhere. The studies regarding socio-economic status, their livelihood options, forest resources use, bio-physical conditions of forest resources and the conservation strategies are extremely important for drawing sustainable management strategy. However, there is very sparse information regarding the tree diversity, distributions and their relationships to anthropogenic disturbances and environmental variables. The lack of information regarding the impacts on forest resources by local communities through livestock grazing and collections of forest products necessitated this research.
The objectives of the present study was to: (1) document the floristic characteristics and distributions patterns in JSWNP, (2) assess the effects of environmental factors to tree species structure and composition, and (3) evaluate the different disturbance regimes and their degrees of disturbance on species composition. The detail information regarding diversity and disturbance regimes would provide the basis to determine the nature and distributions of biodiversity resources of the region being managed. The information is very much essential to set the conservation strategies in the national park like Bhutan where most of the local people are dependent on park resource. Has there been any quantitative vegetation surveys done in this park before? If not, that is a pretty good justification as well???
The study area is located in central Bhutan (Fig.1) within the coordinates of 27012'N, 0900 08' E, (West), 270 26'N, 0900 23'E (North), 270 15'N, 0900 35' E (East), 270 02' N, 0900 20' E (South)) with altitudes ranging from 200~5000 m a.s.l. It is the third largest national park of Bhutan and is spread over an area of 1723 sq km covering five Dzongkhags (districts) and eleven gewogs (sub-districts).
Fig. 1.1 Location of JSWNP in Bhutan
The park comprises a variety of habitats with vast tracts of primary forests; it is home to about 395 bird species, seven of which are listed among the globally endangered species. Park flora ranges from sub-tropical in the south to temperate and alpine in the north. According to Ohsawa (1987) Bhutan is divided into six clusters of altitudinal segments and in six clusters five vegetation zones are identified. The vegetation zones are based on the similarity in floristic composition and dominant species (Table 1.1). These forests are mostly undisturbed owing to its geographical location within the Inner Himalayas (Ohsawa, 1987).
Table 1.1: Description of vegetation boundary in Bhutan
Sub-tropical/ Warm -temperate
Source: Ohsawa, M. (ed.) Life Zone Ecology of Bhutan Himalaya, 1987.
A quantitative vegetation survey was carried out in JSWNP from June 2008 to November 2008 and 100 sample plots were surveyed randomly running along the altitudinal gradients. We used Arcview GIS version 3.3 software to generate maps and for the sample plots to be surveyed was randomly generated based on the coordinates obtained from the map. During the survey, sample plots were determined using GPS (Global Positioning System) with an interval of 100 m along the altitudinal gradients. In case of inaccessible plots such as plots located at cliffs and rock out crops sample plots were shifted from left to right or from top to bottom by 50 m. Field investigations were carried out along the altitudinal gradients at four sites; Ada-Pataley, Dovan-Jigmechhoeling, Korphu-Trong and Tangsibji-Langthel. The decision to take four transects were based on the geographical setup or landforms observed during reconnaissance survey.
For the arboreal layer, woody individuals of all tree species with diameter at breast height (dbh) larger than 10 cm, tree height using SUNTOO clinometers, canopy coverage using hypsometer were recorded within each sample plot of 20m X 20m quadrates. Vegetation crown cover was estimated using percentages scales. For the shrubs all the woody individuals with dbh smaller than 10 cm and bigger than 2.5 cm were measured. The herbaceous stratum were counted based on the species occurred. The size of the sub-plots for shrubs and herbaceous stratum were determined as 5m X 5m and 1m X 1m sapling plots nested within the sampling plots for trees. For identification of species the nomenclature followed Flora of Bhutan (Grierson and Long, 1983-2001), Flowers of the Himalaya (Polunin and Stainton, 1984, 1997; Gardner et al., 2000; Manandhar, 2002).
In each plot and sub-plot the position was determined by GPS and several environmental factors were recorded such as; elevation, slope, slope direction, Slope position, soil depth of upper horizon (A) and soil depth of lower horizon (B). The slope direction (E, S, W, N, NE, SE, SW, and NW) and slope position (ranging from 0-5 from foot to the top of mountain) were recorded as categorical variables (Zhu et al., 2007; Zhao et al., 2009).
The effects of disturbances on the plant diversity were evaluated based on the visual observations noted along the elevation gradients at the sample plots. Prior to the start of the inventory, we discussed the anthropogenic as well as natural disturbances with local forest users. All the forests in the inner Himalaya are subjected to natural or anthropogenic disturbances regimes which are of varying intensities. They are natural and anthropogenic or both. The natural disturbances include landslides, soil erosion and earthquakes. The anthropogenic disturbance categories were defined as: deforestation, grazing, lopping of tree branches for fodder and fuel wood, removal of leaf and wood litter from the forest floor. Although both types of disturbances affect ecosystem stability and retard the succession process (Todaria et al., 2010) but for the purpose of this research only anthropogenic disturbances were taken into account. The anthropogenic disturbances (Rao et al., 1990; Veetas, 1997; Sagar et al., 2003) such as "lopping for fuel wood and fodder, burning, livestock grazing, collection of various products such as forest floor biomass, fruits, fibers and medicinal plants" and shifting cultivation and disturbance intensity were registered along the elevation gradient. Based on the visual inspection of the indicators, we calculated a disturbance Impact Factor (DIF) as the total estimation (Sagar et al., 2003; Sapkota et al., 2009).
The disturbance and disturbance intensity were registered along the elevation gradient. The disturbance indicators were recorded in relative terms as undisturbed (UD); low disturbed (LD); moderately disturbed (MD) and highly disturbed (HD) (values ranging from 0 to 3). The disturbance index was described as the ratio of the particular indicator to total of all the indicators in sampled plots (Zhao et al, 2009).
The Importance Value Index (IVI) was calculated which is defined as the sum of the relative density, relative frequency and relative dominance following Curtis and Mcintosh, 1951; cited in Kent and Coker, 1995. We calculated Basal Area (BA) from DBH (Diameter at Breast Height) of the species and summed up the BA. The percent proportion of each species were calculated as relative Basal Area percent (RBA %). We used calculated RBA to determine species Dominance measure (Ohsawa, 1984). The site level analysis was also assessed to determine the dominant and co-dominant species of each site were performed on the basis of relative basal cover. This method followed the Sagar et al., (2003) with "the species having highest relative basal cover was defined as dominant and that having the second highest relative basal cover was defined as co-dominant species".
The Shannon diversity index (H') was calculated for each plot using the following equation (Magurran, 1998).
where PiÂ is the proportion abundance of ith species of each species in the sample and S equals the total number of species in the sampling plot.
The species distribution and their relationship to environmental variables in JSWNP were analyzed using Canonical Correspondence Analysis (CCA) ordination and generated diagrams using CANODRAW 4.5. CCA measures the relationship between the environmental and species data, which is a form of direct gradient analysis, and is more appropriate for studies that focuses on examining the relationships between floristic compositions constrained by environmental variables (Leps and Smilauer, 2003).
The Mann-Whitney U test used to compare the each possible independent forest pair to identify the differences in the mean dbh of tree species between the disturbed plots and undisturbed plots (Siegel, 1956 cited in Ramirez-Marcial, 2001; Sapkota et al., 2009).
1400 individuals of trees with dbh >10 cm and 533 individuals of shrubs and saplings >2.5 cm of dbh. Total species 152 recorded from 4.335 ha sampled area.
Dominance (Cm2 ha-1)
The IVI of the vegetation was analyzed according to the boundaries following Ohsawa, 1987.
Sub-tropical/ Warm -temperate
Tree Species Composition in JSWNP
Results from our field survey in four sites in a total of 92 forest inventory plots yielded a total of 1276 stems, 144 species and 59 families (Table 1.2). The highest density of stems is found in Ada-Pataley site and the least in Korphu - Trong site. The highest diversity is recorded from Dovan - Jigmechhoeling site although there is not much variation in the diversity of species across sites.
On the basis of relative basal cover, the four sites differed in the combination of dominant and co-dominant species. Ada-Pataley site exhibited Pinus roxburghii as the dominant and Albizzia lebbeck as the co-dominant species. The important Value Index (IVI) also showed Pinus roxburghii as the most important species, while at Dovan - Jigmechhoeling site Castanopsis hystrix is the dominant and most important species and Ostodes paniculatus showed as co-dominant species. In Korphu - Trong site Lithocarpus elegans is the dominant where as Quercus lanata is the co-dominant species. The IVI analysis showed Litsea monopetala and Castanopsis tribuloides as the most important species at Korphu - Trong site. In Tangsibji - Langthel site Pinus wallichii exhibited dominant and most important species and Quercus lamellosa exhibited co-dominant species.
Overall analysis showed, Pinus wallichii, Pinus roxburghii and Castanopsis hystrix as the three most dominant and most important species in JSWNP. Thus, the site wise analysis indicated the different community associations. For e.g; Ada-Pataley site exhibited Pinus - Albizzia community; Dovan - Jigmechhoeling site represented Castanopsis-Ostodes community; Korphu - Trong site represented Lithocarpus-Quercus community while that at Tangsibji-Langthel exhibited Pinus-Quercus community.
Table 1.2 Tree species structure summary of four sites in JSWNP
No of sampled plots
Total Stem count (â‰¥10 cm DBH)
No of species ha-1
No of families ha-1
Total Mean DBH (cm)
Density (no of individuals ha-1)
Mean height (m)
Mean Basal Area (m2/ha-1)
Simpson's Index (D)
Shannon Index (H)
Structural characteristics of tree species in JSWNP
Results showed approximately 34.2% of the trees with dbh class 15.1 to 25.0 cm and included maximum trees in this class (not shown in table). The highest number of individuals with a dbh range of 15.1 to 25 cm was found in Ada-Pataley site (about 13%) while Korphu - Trong and Tangsibji-Langthel site showed least percentage of trees within the dbh range of 35.1 to 45.0 cm. Similarly, maximum trees were represented in the height range of 5.1 m to 10.0 m with 35.7% of the total trees. Only 5.7% of the trees were represented in the height range of less than five meters (Table 1.3).
Table 1.3 Height and dbh class for trees of four sites along the altitudinal gradients in JSWNP, Inner Himalayas, Bhutan. Percent of the total number of individuals of trees in parenthesis
Species distributions and impact of environmental factors
The results of the direct gradient analysis (CCA) showed that there is successive decrease in the eigenvalues along the first four axes. The eigenvalues for the first axis showed highest value, which indicates that, the representation of first axis with highest proportion of the species composition and distribution as a function of environmental factors (Table 1.4). The CCA showed a low cumulative percentage variance of species data that was explained on the first two CCA axes (Table 1.4). However, the species -environment correlation was higher for the first two canonical axes (0.967 and 0.831), which indicates a strong influence of environmental factors to species composition and distribution. The ï¬rst axis explained about 31% of the variance of species-environment relationship, and the second about 13%. The Monte Carlo test showed high association between elevation and species distributions (p<0.01).
In the Intra-set correlation of the environmental factors, the first axis was positively correlated with Elevation, soil depth A and Carbon content of the soil. The second and the fourth axis were negatively correlated with plot location. Similarly, third axis was positively correlated with plot slope but negatively correlated with depth of the top soil and plot aspect. Fourth axes showed negatively correlated with depth of the top soil. Therefore, elevation was the most important environmental factors and played important role in species composition and distribution, followed by soil depth, Carbon content and plot location.
Based on the CCA ordination diagram the species were divided into four groups (Fig 1.2). The group I included some of the species such as Persea clarkeana, Aesculus glabra., Acer hookeri, Viburnum erubescens and others showed positive correlations to depth of the top soil (A). Group II included some of the species with positive correlations to the ratio of soil Calcium and Magnesium and the species include Garcinia pedunculata, Juglans regia, Sloanea dasycarpa, Eugenia formosa and others. Group III species showed negative correlations elevation and soil depth such as Eleocarpus lanceifolius, Ostodes paniculata, Pinus roxburghii and others and these species showed positive correlations to plot slope because these species usually grow at the edges and steep slopes on the hills and also at the steep slopes of the narrow valleys. Group IV represents high elevation flora such as Rhododendron arboretum, Daphniphyllum himalayense,Lyonia ovalifolia, Quercus lamellosa and others (Fig. 1.2).
Fig. 1.2 The CCA ordination diagram for the influence of environmental gradient on species composition. Arrows represent the environmental factors and their lengths indicate the strength of the environmental effect on species composition. Arrows to the right and left of the y axis represent positive and negative correlations respectively.In the fig 1.2 above only the most correlated species are shown with inclusion rules of 5% through project settings in CANODRAW.
Table 1.4 The eigenvalues and intraset correlations for CCA of the environmental factors
1 2 3 4 Total inertia
Eigenvalues : 0.709 0.302 0.270 0.264 16.758
Species-environment correlations: 0.967 0.831 0.835 0.808
Cumulative percentage variance
of species data: 4.2 6.0 7.6 9.2
of species-environment relation: 31.2 44.5 56.3 67.9
Sum of all eigenvalues 16.758
Sum of all canonical eigenvalues 2.275
Elevation (m) 0.978 -0.011 0.115 0.091
Soil depth A (cm) 0.540 0.112 -0.431 -0.694
Soil depth B (cm) 0.465 -0.242 -0.378 -0.409
Ca:Mg -0.011 0.333 0.365 -0.178
Carbon (%) 0.508 -0.305 -0.298 0.141
Aspect (Category) 0.075 0.238 -0.469 0.300
Plot Location (Category) -0.195 -0.546 0.211 -0.534
Slope (degrees) -0.061 -0.050 0.492 -0.326
Tree species diversity along the elevation gradient
The Shannon wiener Index (H') was calculated and illustrated against the elevation gradient and the variation showed bimodal curve and the peak value occurred along the elevation zone centered around 1800 m a.s.l. (Fig. 1.3). The diversity indices were lower after 500 m a.s.l.till about 1000 m a.s.l. and then again showed lower diversity around 1300 m a.s.l. then showed constant decrease after 2100 m a.s.l.
Fig. 1.3. Relationship between the diversity index (H') and the elevation gradient.
Effect of forest resources use by local people to species composition
During our household survey, we asked local people to rank the forest resources based on the need and preference based on their daily use. Our intention was to evaluate which among the forest products are used mostly in rural villages inside the park and also to assess which among the forest use is associated most frequently to forest in the park (Table 1.5).
Table 1.5 Forest product use ranks by local people in JSWNP
Leaf litter and fodder
Weaving and basketry
Poles for fencing and prayer flags
*Total responses are more than 264 (N=264) as multiple answers were recorded.
Based on the above mentioned uses of the forest resources, disturbance factors were investigated along the elevation gradients. Results showed that among the disturbance regimes, grazing/trampling accounted for highest frequency registered from sampling plots followed by tree fell and leaf litter collection and others (Fig. 1.4). The maximum grazing incidences were recorded at Korphu-Trong site. Some plots we come across had evidences of Tseri farming. This was confirmed with the help of local guides. Livestock grazing and tree fell indicates that local people are dependent on the park forest for fuel wood, building materials and livestock rearing (see Table 1.5).
Fig 1.4, Frequency of the human disturbance factors (estimated relative factors based on visual observations) across four sites in JSWNP
The disturbance index showed first peak at 750 m a.s.l. This is due to most of the settlements are located at lower altitude within the valleys of JSWNP and they use the nearby forest. Some villages located at higher elevation where the second peak occurred at 1750 m and the third at 2250 m (Fig. 1.5).
Fig 1.5 The relationship between the disturbance index and the elevation gradient.
Differences in forest plots condition (>10 cm DBH) with disturbance intensity
The 92 inventory plots for tree species were laid down across the four sites in JSWNP and the number of stems, average DBH and the density of trees per ha were assessed. Based on the above recorded disturbance regimes the forest plots inventoried were classified, based on the visual observations, as very low disturbed or no disturbed, low disturbed, moderate disturbed and highly disturbed forest plots. Mann Whitney pair wise tests showed that there was a significant difference (p<0.05) in the mean DBH between the highly disturbed plots and the other of low, Moderate or non disturbed plots (Table 1.6).
Table 1.6, Comparison of forest diversity and structure in different intensity of disturbance
Trees >10 cm DBH
No of stems
Discussion and Conclusion
Among the four sites in JSWNP, Korphu - Trong site showed the highest mean dbh while that of Dovan - Jigmechhoeling exhibited highest number of species ha-1 and Ada- Pataley site exhibited the highest stem density ha-1. Most of the mountain slopes and valleys along the Ada - Paraley site are directed towards south and most of the slopes receive relatively higher rate of radiation. This result is supported by the fact that the structural characteristics of species at Ada - Pataley site showed higher stem density with shorter height and this could be probably the soils are drier relative to other sites. Korphu - Trong site showed largest number of trees (76%) with the height range of 20 m and more. Korphu - Trong site is represented by moist soil and with most of the slopes are directed towards the northeast, north and northwest and relatively soil moisture content is higher than the Ada-Pataley site. However, Dovan -Jigmechhoeling showed higher diversity compared to the other three sites. Researchers (Bratton, 1976; Yadav and Gupta, 2006) reported the micro site heterogeneity like soil moisture and soil nutrient concentration due to landscape heterogeneity that affects species distributions. The effect of slope is reported by Yadav and Gupta (2006), in the Sariska Tiger Project, India where the forest floor is found to be different on slope facing different directions. The slopes facing west were slightly xeric condition and represented maximum woody species. Similarly most of the east facing slopes in JSWNP are generally drier and represented by Pinus rocburghii and Woodfordia fruticosa where as valleys were usually dominated by Ostodes paniculata, Duabanga grandiflora and Alnus nepalensis in the lower altitudes.
In terms of overall ecological dominance with across sites the high importance value species are different, although, the altitudinal variation is almost similar on all sites. The plot directions and plot positions across site might have affected the species dominance and codo-minance. The findings of Pinus roxburghii as the dominant species are consistent with the findings of the Wangda and Ohsawa (2006) in their studies in dry valleys slopes of the Bhutan Himalaya. Pinus roxburghii could occur as low as 500 m to 2500 m elevation in the inner Himalayas. The difference in important species across site showed similarity with the studies by Reddy et al., (2008) in Andhra Pradesh, India and Zhu et al., (2007) from the secondary forests in montane region of northeastern china.
Over all analysis of the species distributions in JSWNP showed that Castanopsis hystrix, the most dominant species followed by Pinus roxburghii, Duanbanga gramdiflora, Pinus wallichii,and Ostodes paniculata respectively. Relatively cooler environment with most of the slopes being north facing along the Korphu - Trong and Dovan - Jigmechhoeling provided conducive environment for the cool broad leaved forest species like Castanopsis hystrix. Therefore the most frequent occurrence as recorded was Castanopsis hystrix (22%). However, frequent valleys and complex land forms have created micro-environment allowing the species of lower elevations to occur even at the higher altitude. Ostodes paniculata (17%), Lithocarpus elegans and Schima wallichii both accounted for 16% in their distributions.
Environmental variables showed significant effects to tree species distributions in JSWNP. A Monte Carlo permutation test showed elevation as the highly significant among all the environmental variables (Table 1.4). The species richness is affected by elevation gradient. There was an occurrence of higher stem density and the higher diversity at the middle elevations (Fig 1.3 and 1.6). The CCA analysis showed six environmental factors that is affecting the vegetation distribution and the factors would be: elevation > soil depth > Carbon content > Plot location >aspect. The change in vegetation types along the altitudinal gradient, based on the CCA result, is much greater than other environmental factors, although the contribution of the other factors to the distribution in vegetation types is apparent (Fig. 1.2). Therefore, the altitude could be affecting on the vegetation distribution at regional scale. The other environmental factors however tended to have small impact on vegetation distribution and it could be in localized landscape scales and is consistent with the finding of Kang et al., (2007) on the Helan Mountain, China. This could be because of complex landforms and microclimate heterogeneity as described by Ohsawa (1987) as "Inner Himalaya" which is described as the complex heterogeneous parameters where similar environment could be noted at varied elevation. The spatial heterogeneity could be due to difference in the soil moisture and nutrients.
The effect of the various hill slopes on the species is also different. The slope range with 6 - 10 degrees showed the maximum stem density with 401.8 stems ha-1 and slopes with 10 - 15 degrees slopes showed maximum number of species in average of 8 species per plot. Plot aspect play important role in shaping the plant distribution and composition as it is associated with the solar radiations and the moisture content (Yadav and Gupta, 2006). In JSWNP stem density South -east facing slopes had the highest mean number of plant species (10) followed by south - west (8) and then west facing slopes. However, the west facing slopes had the highest stem density (500 stems ha-1) followed by south - east facing slopes (412.5 stems ha-1). The north and the south facing slopes had almost the same number of species which is consistent with the findings of Yadav and Gupta (2006).
In JSWNP, most of the east facing slopes are drier and mostly dominated by chir pine forest whereas west facing slopes are usually cooler with moist environment. In plot location the plots in the back slope recorded the higher mean number of species than others (mean number of 7 spp) however, plots located on the shoulder were recorded the highest stem density (431.9 stems ha-1). Thus, the diversity pattern of the plant species is determined by the topography and the slope direct which is apparent in JSWNP, Bhutan. The JSWNP is located in the region of inner Himalayas as classified by Ohsawa (1987), which is characterized by the deep valleys and mountains with slopes of varied degrees and complex landforms. The vegetation distribution shows distinct regions of sub-tropical and temperate types and usually larger spaces of ecotone regions.
The tree diversity showed higher peak at the mid elevations (Fig. 1.3). The peak for diverse species might have occurred due to transitional zone of vegetation that starts changing from sub-tropical or warm temperate to cool temperate (See Ohsawa, (ed) 1987). Another possibility could be likelihood of slight human disturbances along this elevation range. The tree diversity is observed low at around 900 m a.s.l because at this elevation usually covered by Chir Pine forest. Also most of the locations around 800 m a.s.l. and 1300 m a.s.l are inhabited by human settlements where nearby forests are highly utilized and due to this the forest have degraded and in some cases the monotypic species were observed. For instance, plots located in sokshing area comprised mainly of Quercus griffithii and in extensively grazed areas have non-palatable species.
JSWNP is located in the central Bhutan surrounded by settlements on all sides. Most of the people are dependent on the natural resources of the park in addition to the local people living inside the national park. Local people are not only depending on the forest for grazing and leaf litter collection but also on fuel wood and timber. Our household survey on the forest resources use by local people indicated that timber for building houses is not used as regularly as fuel wood but that better quality and more wood is required for this purpose. But other uses such as prayer flags and building temporary huts, for fencing the crops in order to guard it from wild life depredation and to construct temporary sheds for livestock. Therefore, substantial numbers of young trees are felled seasonally. There were also reports of felling too many trees for use of for prayer flags and has become one of the forest related issues in Bhutan (Kuenselonline, 2009). This use of the forest resources by local community creates disturbances to tree species diversity and structures. The tree diversity is lower in the plots near to the settlements where it is highly disturbed. However, the diversity is higher in the areas with history of Tseri cultivation. The plots with record of Tseri cultivation showed diverse species with smaller sized stems and shorter heights. This observation agrees with the findings of Schmidt-Vogt's findings where the greater species diversity was observed in forests of northern Thailand with incidences of swidden farming (1998). Other than the Tseri cultivation in JSWNP, the community uses extensive forest resources (see Table 1.5). The Intermediate disturbances hypothesis of Connell (1978) suggests that the diversity of species might be higher in the areas with slight disturbances.
In case of JSWNP with regard to diversity, the number of woody species at the lower elevations was found to be lower and increased to mid elevations and then again decreased while going to higher elevations. When the disturbances factors were assessed, the result showed that most of the mid-elevations plots were moderately disturbed. It is found that the sampled plots with low and moderately disturbed plots showed higher stem density whereas highly disturbed plots showed lower stem density. Therefore, it could be due to intermediate disturbance compared to low elevations and this is in consistency with the findings of Zhao et al., (2009) in the Baishuijiang river basin China and Yadav and Gupta (2006) at the Sariska Tiger Project in India. It could be argued that the woody species distributions in the JSWNP were the expressions of disturbance - diversity pattern which is called the 'Intermediate -Disturbance Hypothesis' (Robert and Giliam, 1995). In JSWNP most of the forest resources use other than nearby settlements, are used seasonally where migratory grazing is usual practice. Also most of the forest resources use involves seasonal harvests that would encourage some species to establish and facilitate increase in number of species in moderately and low disturbed areas. However, in areas with high disturbance some species may just get extinct that do not tolerate higher disturbances. The diversity and evenness of species showed decreasing trends with increased incidences of grazing, as reported by Alados et al., (2004) in the Mediterranean grazing ecosystems. However, households who were regularly involved in forest resources harvest mentioned that they observed increase in the ground flora and shrub species in the areas with decrease in tree species. The village population of people and livestock could be also associated to forest resources degradation and change. For instance, Karanth et al., (2006) found that larger villages with higher number of households and population densities have higher impacts on the forests. But lack of alternative resources collection area could be rather important to argue with than simply the population densities.
The tree density, species richness and basal area were significantly higher in the inner areas of Bardia National Park, Nepal, where the human interference was relatively less (Thapa and Chapman, 2010). This agrees with the results from JSWNP in Bhutan where the tree density and basal area were significantly different in disturbed plots and also the trees with lower dbh were found to be subjected to higher lopping. However, moderately disturbed and low disturbed forest sampling plots showed more stem density and bigger mean dbh with lower species diversity. The species recorded showed dominant. For instance most of the disturbed plots in Dovan and Korphu site showed mostly presence of Castanopsis hystrix which is tolerant to disturbance. The species which can tolerate human disturbances would become dominant where as other intolerant species will extinct in the areas with frequent disturbances (Sagar, et al., 2003; Yadav and Gupta, 2006; Sapkota et al., 2009). Constant collection of fuel wood, cutting of trees for fencing and building materials, grazing, leaf litter and trampling can substantially alter species regeneration capacity (Pandey and shukla, 1999; Sapkota et al., 2009) and in JSWNP the mean dbh and stem density were found to be relatively lower and significantly different in plots with higher intensities of disturbances, thus, it could be argued that the human impacts with higher intensity has impacted the species regeneration capacity.
The results of the present study indicated that forest in JSWNP is affected by human interference but frequent and simultaneous multiple impacts are responsible for differences in forest condition. Environmental variables obviously have higher proportion of impacts to species richness and density where different stages of plant growth would have specific environmental requirements. These varying characters may also respond differently to human disturbances at different growth stages in plant life. We concluded that the human impacts obviously affected the tree species diversity and densities in JSWNP, however, management should monitor the intensity of human activities so that some management strategy may be required to exclude some deleterious activities. We agree with the peak value of diversity indices at the middle elevations of our study however whether it is purely based on the Intermediate - disturbance hypothesis or the mid elevation effect. Nevertheless, we recommend further studies to narrate the precise decoupling effects of environmental and human influence to species richness, diversity and structural traits on the inner Himalayas forest ecosystems.
Limitations of this study is that our data represent a cross sectional study, a snapshot of forest structural traits and distribution characteristics. This study only provides the first hand basic ideas of species characteristics however it is difficult to conclude where we stand in terms of forest change that existed due to historical disturbance variables. Further investigations on the regional patterns of tree diversity and the effect of disturbances are highly recommended to clarify these relationships and also characteristics of general community attribute as indicators along the disturbance gradient and establishment of disturbance threshold level in JSWNP is highly recommended. Priorities for conservation should not be based on the particular species alone. It is particularly important in the context of the growing concerns with regard to forest resources use by local people and the priorities to protect the species of conservation importance in the national park where the impacts of climate change is also a growing concern.