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.
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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.
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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???
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.
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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.