Floristic Diversity and Important Value Indices of Tree Species in Lower Kanchenjunga Singhalila Ridge Eastern Nepal

The present paper is based on the finding of floristic diversity inventory research work conducted in Lower Kanchenjungha Singhalila Ridge, in Nepal side of Eastern Himalaya. Knowledge on floristic diversity of an area can reflect the total resources, their use patterns and conservation status which have a key role for making conservation strategies and policies. Analysis of vegetation helps to develop detailed picture of plant communities of that region. A total of 299 plant species belonging to 184 genera and 86 families were found in the Lower Kanchenjungha Singhalila Ridge. Dicotyledonous flora includes 69 families, 150 genera and 229 species whereas Monocotyledonous flora includes 15 families, 32 genera and 39 species. On the basis of floristic analysis Rosaceae was found to be the largest family with 23 species followed by Ericaceae 17, and Lauraceae 9 species. Current study furnishes three new addition to the flora of Nepal viz. Begonia flaviflora H. Hara (Begoniaceae), Carex cruciata Wahlenberg var. argocarpa C. B. Clarke (Cyperaceae), and Strobilanthes helicta Anderson (Acanthaceae). Of this total, 30 woody tree species with 551 individuals and 23 genera were recorded from the altitudinal range of 2100 3000 m of study area. The highest relative frequency was recorded by Lithocarpus pachyphylla (18.48%), Symplocos ramosissima (16.30%), Rhododendron falconeri (13.95%), Symplocos dryophylla (10.20%). Eurya acuminata and Symplocos species were found to be dominant in lower elevation whereas Lithocarpus pachyphylla and Rhododendron species were found to be dominant in upper elevation.


Introduction
Flora refers to the brief taxonomic treatment of all plants occurring in a geographical location which generates a comprehensive account. A complete flora of country is necessary to reflect the whole plant diversity of that country. Absence of flora severely hinders any scientific inquiry into plants [1]. The Eastern Himalaya is recognized as a global center of species diversity [2] [3]. The high floristic diversity of this area is due to the presence of diverse ecological habitats as well as climatic variations. It is also interesting phytogeographically because of Sino-Japanese and Sino-Himalayan elements. Species richness and endemism are very high in Eastern Himalaya due to the presence two biological niches viz. Indomalayan and Palaeartic realms [4]. The Kanchenjungha complex mostly covers the Taplejung district, the northeastern part of the Nepal, bordered by Sikkim (India) in the east and Tibet (China) in the north. The climate ranges from upper subtropical to alpine. Out of 6500 species of higher plants in Nepal flora, about two-third of the species are expected to occur in this area and the number of species are generally decreased from Central to Western Nepal [5].
Vegetation is an overall expression of various environmental factors which operates gradually or in a cyclic manner. Analysis of vegetation helps to develop detailed picture of plant communities of geographical location. More deliberatively vegetation refers to the expression of total plants cover in an area which may be made up of one or more plant communities or aggregation of plants usually forming a mosaic or complex [6]. The structure of tree species diversity in hill forest varies greatly from place to place due to variation of altitude, orientation of slope, nature of soil and type and intensity of disturbance [7].
Change in vegetation type occurs not only with respect to altitude, latitude, slope and soil but also with respect to rainfall pattern and human impacts such as natural disturbances like forest fire, soil erosion, landslide, volcanic activity etc. The climatic change determines forest dynamics and tree diversity [8]. Disturbance may increase species richness in old growth forest and may maintain species diversity [9]. Higher species richness is maintained in intermediate level of disturbance [10]. N. B. K. Chhetri, K. K. Shrestha The climate of this area is characterized by high rainfall, cool and humid atmosphere. Figure 1 reveals the map of the study area.

Data Collection
Two extensive field visits were conducted from June to July, 2007 as pre-monsoon and during September to October 2007 as post-monsoon. Pre-monsoon collection was focused for collection of specimens in flowering stages and other collection was focused in fruiting stages. Plants specimens were collected by two botanical expeditions. Triplicate specimens of each species were collected as far as possible. The route of collection and area has been shown in the GIS map. The collected voucher specimens were properly tagged in the field during collection with appropriate field notes. The collected specimens were dried and mounted on herbarium sheets of (28 × 44 cm). The specimens were prepared and managed using techniques mentioned in the Herbarium handbook [11]. The identification of collected plant specimens were done by using relevant literatures such as [12] [13] [14]. Similarly, plants were identified in the field by consulting the books [15] [16] under the supervision of senior taxonomist Professor Doctor Krishna Kumar Shrestha Central Department of Botany Tribhuvan University Nepal. The specimens were further confirmed by cross tallying with the specimens deposited at National Herbarium and Plant Laboratories (KATH), Godawari and Tribhuvan University Central Herbarium, Kirtipur (TUCH). The dried plant specimens were mounted in (28 × 44 cm) sheet. The process of herbarium preparation is based on the herbarium technique developed by Bridson. The

Data Analysis
Vegetation analysis of the selected forest stands along an altitudinal gradient was carried out by using square quadrats by following stratified-random sampling method [19]. The quadrat of 10 m × 10 m sized were laid down for sampling of trees. In each 10 m × 10 m quadrats the number of individual trees DBH [diameter at breast height-1.37 m, dbh ≥ 10 cm] of each species was noted. The height of each tree was estimated with the help of Clinometer. Herbarium specimens were prepared for collected plants. Quantitative data were gathered with the help of field data sheet and quantitatively analyzed for abundance, density and frequency according to the formulae given by Mishra [20]. The relative values were summed up to represent importance value index (IVI) as reported by Curtis [21]. The diversity index (H') was computed by using Shannon-Wiener information Index [22]. The concentration of dominance (CD) was computed by Simpson's Index [23]. For prediction of regeneration behavior of tree species size class distribution was developed. All the trees were divided into dbh classes of 10 cm interval and density of tees in each diameter class was calculated and size class distribution diagram was developed. Similarly, Gymnosperms represented two families, two genera and two species.  (Table 1). Eleven threatened and one endemic plant species mentioned ( Table 2).

Floristic Composition
The study includes 11 threatened species as follows: Taxus wallichiana

Community Structure in Three Zones
The whole study side was site which includes temperate Zone was resolved in three sub zones on the basis of elevation ranges.
Lower temperate zone (2100 -2400 m) In this range Symplocos ramosissimashowed the highest frequency (63.64%) and relative frequency 14.29%. The lowest frequency and relative frequency were 9.09% and 2.04% respectively. Likewise, the density of individual tree species ranged from 263.64 plant/ha -9.09 plant/ha. The total density of all tree species was 1109.09 plant/ha. The dominance of the individual tree species ranged from 15.31 m 2 /ha -0.07 m 2 /ha. Similarly the important value index was found to be highest for Eurya acuminate (39.72) and followed by Symplocos. ramosissima (38.72), while the other species likes Lindera, Rhus showed least IVI (2.94). In this range canopy was dominated Quercus lamellosa, Castanopsis hystrix, Lithocarpus pachyphylla and Persea odoratissima but sub-canopy was well dominated by Eurya acuminata and Symlocos species. Altogether 19 tree species were recorded from this zone. Table 3 shows the quantitative vegetation analysis of lower temperate zone.  Altogether 20 tree species were recorded from this site. Table 4 shows quantitative vegetation analysis of middle temperate zones.  Table 5 shows quantitative vegetation analysis of upper temperate zone.

Species Richness and Diversity Indices
The

Beta (β) Diversity and Similarity Index
The calculation of Beta (β) diversity helps to know the extent of species turn over between the sites. The Whittaker's β diversity (β w ) was calculated using the following formula [25]. The similarity index ranges from 0% to 100% to quantify the range from no similarity to complete. It is mentioned in Table 6.
Similarity, the value for tree species richness was achieved highest for lower temperate zone (5.27) and lowest in middle temperate zone (3.29) while upper  Table 7.

Size class distribution
In each sites the tree species were classified into ten size classes with an interval of 10 cm dbh. Then the density diameter curves were developed to assess the general population structure of tree in three study sites. As shown in the given Figures 3-5 respectively.

Floristic Composition
Flora governs the key position among the natural resources of any geographical area. Study on floristic composition of any region gives the clear picture of floristic content, which in turn can be useful for developing strategy for conservation and management of biodiversity. Eastern Nepal is unique in species diversity as it as the supplementary zone for Eastern Himalaya. The Eastern Himalaya stands out as being one of the globally important sites representing the important hotspot of South Asia. Eastern Himalaya of Nepal has been identified as one of the rich biodiversity hot spot in the world with high species diversity and high level of endemism. Floristic composition has been carried out to enumerate and prepare the comprehensive description of the vascular plants from the Lower Kanchenjungha Singhalila Ridge Ilam district east, Nepal. The variation in altitudinal range and climatic conditions favours the diversity of flora. The genera Rhododendron and Rubus both consisted of 8 species were found to be largest genera and Begonia with 6 species followed by Sorbus, Hypericum, and Berberis with 5 species in each. The floristic analysis showed that the dicotyledonae was dominant in comprision to the monocotyledoneae. The ratio of Monocotyledoneae to Dicotyledoneae was found to be 1:5.95 for species, 1:4.56 for genera and 1:5 for families. Siwakoti and Varma [26] recorded 743 species of flowering N. B. K. Chhetri, K. K. Shrestha

Structure of the Vegetation
Forest is a among study sites may be due to difference in altitude, species composition, age of trees, and degree of disturbance and succession stages of the stands. The value obtained for basal area in the present study is comparable to the Indian tropical forests [27]. Importance value index of Lithocarpus pachyphylla as the present study area is located at altitude ranged from 2100 m -3000 m. The change in IVI of Lithocarpus pachyphylla among the study sites is due to the change in species composition, disturbance and altitude. Other studies elsewhere have reported a similar range of species richness [28]. Importance value index (IVI) of tree species indicated that Lithocarpus pachyphylla and Eurya acuminata were the dominant species at all the study sites in the tree and sapling layers of tree serve followed by Rhododendron, Lyonia, Acer species, etc.

Size Class Distribution
In the study the size class distribution doesn't indicate different population structure, which may be related to differences in environment and disturbance regimes. The reverse J-shaped size class distribution of trees in a community indicates sustainable regeneration [29]. Although a clear inverse J-shaped graph is absent in present work, but the population structure of middle temperate zone shows somewhat continuous regeneration than others two range. The regenera-tion potential of trees in the study area was somewhat in continuous. The different shape of density-diameter shows the extent of effect of disturbances on the density dbh classes [30]. In a montane rain forest in Mexico, Ramierz-Marcial et al. [31] found that stem density decreases with disturbance intensity. Our study also found that the stem density declined with increasing disturbance such as grazing damages sapling through trampling and browsing [32]. Dbh distribution of tree species among elevation is largely controlled by the density of over storey species and the pattern of regeneration can be described by the size distribution [33] Figure 4 shows the total density of tree species ranged between 1009 trees/ha (UTZ) to 1203 trees/ha (MTF). Study revealed that Quercus leucotricophora is the most dominant species of all stands.
Oak (Qurecus spp) forests are most extensively distributed between the altitudes 1000 m to timberline and represent the climax stage, throughout the central Himalaya [35]. The result of the present study is pronounced that as well as the altitude is increase the tree diversity is also increase which is the result of above biotic disturbance and invention by new species on these lands.