Study area

The present study was carried out on forest species compostion and structure along altitudinal gradient at three different altitudes of woody tree species in the forest near Dhanaulti in Garhwal Himalaya of Uttarakhand (Figure 1. Location Map of the study area). Dhanaulti is located in the Garhwal Hills between 30° 27’ 0” N78° 15’ 0” E and the study area falls between N30° 24’ 434” E078° 17’ 894” N30°25’ 336” E078°17’ 811” (Table 1). The place is about 2 hour’s journey from capital city of Dehradun and is situated at an altitude of 2286 m. The forests have thick trees covers of Cedrus deodara, Quercus leucotrichophora, Rhododendron arboreum Pinus roxburghii, etc. The moist Cedrus deodara Deodar forest (Type –12/C₁C; Champion and Seth 1968) is found between altitudes 1750-2150 m a.s.l. Cedrus deodara is mainly observed in pure patches, while few scattered individuals of other associated species such as Quercus leucotrichophora, Pinus wallichiana, Cupressus torulosa and Rhododendron arboreum are also found.

Figure 1. Location map of the study area in Uttarakhand.

Three sites (Figure 2) on the basis of altitudes were categorized i.e., upper altitude, middle altitude and lower altitude (Table 1), selected for the study. The soils of this region vary according to aspect, altitude and climate, and are generally young and thin. Due to sharp variations in geoecological conditions, the soils of this region do not form a complete group. Soils of Dhanaulti area belong to mollisols and of Satengal to inceptisols. All these polypedons are members of fine sandy loam, mixed, messic family. These soils developed from different parent materials are in equilibrium with geogenic factors. All pedogenic processes are active in the study area. The soils are generally acidic in nature with pH increasing with depth.

Figure 2. Google earth view of the study area (LA = Lower altitude; MA = Middle altitude and UA = Upper altitude.

The climatic data of the study area are given in Figure 3. The temperature in this town is cool throughout the year. The summer months are cool. The winters are not very freezing but provide a misty view of distant mountains. The summer temperatures range from 31 °C to 7.5 °C while the winter temperature ranges from 7 °C to 1 °C.

Figure 3. Meterological data of the study area.

Vegetation composition, structure and diversity

The vegetation analysis was carried out in the year of 2012-13. Vegetation analysis in study area was carried out in the tree layer by using 10×10 m² quadrats at three different altitudes. The size and number of quadrats were determined by the species area curve (Misra, 1968) and the running mean methods (Kershaw, 1973). In each altitude 10 randomly placed quadrats of square shapes were used for vegetation analysis. Between each site, along the elevation gradient there was 150 m of separation. Due to steep slope and difference in terrain the quadrats were selected randomly to avoid the missing information of the area and problem to move in single direction of transect.

The categories of trees, in each quadrat was based on individuals >30 cm at gbh (girth at breast height) were considered as trees, 10-30 cm gbh as saplings and <10 cm gbh as seedlings. The vegetation data were quantitatively analyzed for abundance, density and frequency (Curtis & McIntosh, 1950). The two categories of trees i.e., 10-30 cm cbh and <10 cm gbh were considered for regeneration. The trees were further divided into twelve girth classes i.e., 31-50, 51-70, 71-90, 91-110, 111-130, 131-150, 151-170, 171-190, 191-210, 211-230, 231-250, and 251-270 cm for density and diameter distribution of trees.

Frequency expresses the distribution or dispersion of species in a community and was calculated as follows:

The density is defined as the number of individuals per unit area or volune and calculated as:

Abundance refers to density of individuals of a species in those sampling units only, in which a given species occurs and was calculated as follows:

Abundance to frequency ratio (A/F ratio) for different species was used to determine the distribution pattern in terms of regular (<0.025), random (0.025-0.05) and contagious (>0.05), as described by Curtis and Cottam (1956).

Girth at breast height (GBH) was measured at 1.37 m above ground level.

Total basal cover (TBC) refers to the mean basal cover multiplied with respective density to obtain total basal cover (TBC m² ha^-1). Total basal cover (TBC) = Mean basal area of species × density of species.

The importance value index (IVI) was determined as the sum of the relative frequency, relative density and relative dominance (Curtis, 1959).

Species diversity (H) was calculated using following formula as described by Shannon and Wiener (1963):

Where ni = number of individuals of each species, N = total number of individuals of all species.

Concentration of dominance (CD) was determined as described by Simpson’s index (Simpson, 1949) as:

Where, ni and N are same as for Shannon Wiener information function.

The equitability (EC) was calculated as per method described by Whittaker (1975) using density values and the following formula:

Where S = number of the species in the site, and Ni and Ns are the density values of the most and least important species respectively in the site. Beta diversity (BD) was calculated following Whittaker (1975) as follows:

Where, Sc = alfa diversity (species number or species richness), and

Dominance diversity (D-D) curves for trees were plotted by a co-ordinate point method by placing IVI on y-axis, and its position in the sequence of species from highest to lowest IVIs on x-axis (Whittaker, 1975).

Index of similarty (IS) has been estimated for species with the help of Bray-Curits Cluster Analysis using SPSS-16.0 version software. Statistical analysis was carried out by General statistical-32 with one way and two way anova is used.

Community composition and structure

The composition and structure of selected forest sites were studied at three different altitudes for tree, sapling and seedling layers. The details of each layers at different altitude is shown in Table 2.

Tree layer

In the upper altitude, based on IVI values Cedrus deodara was reported dominant species followed by Quercus leucotrichophora and Q. floribunda. The maximum values of frequency and density were also recorded for Cedrus deodara followed by Quercus leucotrichophora and Q. floribunda. The maximum and minimum values of total basal cover were again observed for Cedrus deodara and Q. floribunda respectively. All the tree species in this altitude were distributed contagiously (Table 2).

In middle altitude, Cedrus deodara was again reported dominant with highest IVI value followed by Quercus leucotrichophora, Q. floribunda, Rhododendron arboreum, Lyonia ovalifolia, Benthamidia capitata and Neolitsea pallens. The maximum value of frequency and density was recorded for Quercus leucotrichophora, however, the highest and lowest values of total basal cover was recorded for Cedrus deodara and Neolitsea pallen respectively (Table 2). The distribution pattern of Cedrus deodara, Quercus floribunda, Quercus leucotrichophora and Rhododendron arboreum was random, whereas, Benthamidia capitata, Lyonia ovalifolia and Neolitsea pallens were distributed contagiously (Table 2).

In lower altitude, Quercus leucotrichophora was reported as dominant species with highest value of IVI. The maximum frequency, density and total basal area was also recorded for Quercus leucotrichophora while minimum frequency, density and total basal area for Benthamidia capitata. The distribution pattern of most of the species i.e., Quercus leucotrichophora, Q. floribunda, Rhododendron arboreum and Lyonia ovalifolia was reported random while Benthamidia capitata was distributed contagiously and the distribution pattern of Pinus roxburghii was regular (Table 2). Tree showing variation in frequency, total basal cover and A/F ratio with different altitudes are significant at (p<0.05), while density and IVI were insignificant with different altitudes (Table 3).

Sapling layer

In upper altitude, the saplings of Quercus leucotrichophora were dominant with highest value of IVI, followed by Q. floribunda, Benthamidia capitata and Rhododendron arboreum. Maximum values of frequency and density were recorded for Quercus leucotrichophora however, maximum and minimum values of total basal area were recorded for Quercus leucotrichophora and Rhododendron arboreum respectively. All the saplings in this altitude were distributed contagiously.

In middle altitude, the most dominant species was Quercus leucotrichophora and the least dominant was Lyonia ovalifolia. The maximum density was reported for Quercus leucotrichophora followed by Quercus floribunda Rhododendron arboreum, Benthamidia capitata, Neolitsea pallen and Lyonia ovalifolia. The highest and lowest total basal cover was recorded for Quercus floribunda and Lyonia ovalifolia respectively. Quercus floribunda, Benthamidia capitata, Lyonia ovalifolia, Neolitsea pallens were distributed contagiously while, Quercus leucotrichophora and Rhododendron arboreum were distributed in regular and random patterns respectively.

In lower altitude, the dominant species was Quercus leucotrichophora followed by Lyonia ovalifolia, Rhododendron arboreum, Quercus floribunda, Benthamidia capitata, Pinus roxburghii and Neolitsea pallens. The values of frequency, density and total basal area were recorded maximum for Quercus leucotrichophora while the minimum values of density and total basal area were observed for Neolitsea pallens however, minimum value of frequency was recorded for Neolitsea pallens and Pinus roxburghii. On this altitude all the species were distributed contagiously except Quercus floribunda and Rhododendron arboreum which were distributed randomly (Table 2). The variation in frequency along altidue is significant at (p<0.05) while density and total basal cover of saplings is significant at (p<0.01) among different altitudes while IVI and A/F ratio was insignificant with different altitudes (Table 3).

Seedling layer

In upper altitude, the dominant species was Quercus leucotrichophora followed by Rhododendron arboreum, Benthamidia capitata and Quercus floribunda. The maximum and minimum values of density were recorded for Quercus leucotrichophora and Quercus floribunda respectively. The highest value of total basal area was reported for Quercus leucotrichophora followed by Benthamidia capitata, Rhododendron arboreum and Quercus floribunda. All the seedlings in this altitude were distributed contagiously.

In middle altitude, the dominant species was Quercus floribunda. Other species in decreasing order of IVI were Rhododendron arboreum, Benthamidia capitata, Neolitsea pallens and Quercus leucotrichophora. The value of frequency was 50% for Quercus floribunda and Quercus leucotrichophora and 40% for Benthamidia capitata, Rhododendron arboreum and Neolitsea pallens. The highest and lowest values of density were reported for Quercus floribunda and Quercus leucotrichophora respectively. The total basal cover was reported maximum for Quercus floribunda followed by Benthamidia capitata, Rhododendron arboreum, Quercus leucotrichophora and Neolitsea pallens. Almost all seedlings were distributed contagiously except Quercus leucotrichophora which was distributed randomly.

In lower altitude, the seedlings of Rhododendron arboreum were dominated with highest values of IVI. The maximum frequency was reported for Rhododendron arboreum whereas, minimum value was reported for two species (Benthamidia capitata and Neolitsea pallens). The maximum and minimum values of density were reported for Rhododendron arboreum and Benthamidia capitata respectively. The maximum total basal cover was for Neolitsea pallens and Quercus luecotrichophora followed by Rhododendron arboreum, Bethamidia capitata and Quercus floribunda. The distribution pattern of all the species reported was random except Neolitsea pallens which was distributed contagiously (Table 2). Seedling showing variation in frequency and A/F ratio along altitude is significant at (p<0.001), while density is significant at (p<0.01) and total basal cover and IVI are insignificant with different altitude (Table 3).

Species diversity (H), concentration of dominance (CD), equitability (EC) and Beta diversity (BD)

The species diversity values for trees, saplings and seedlings at different altitudes i.e., upper altitude, middle altitude and lower altitude have been given in Table 4. In tree layer, species diversity was reported equal (H = 2.13) in both middle and lower altitudes which were higher compared to upper altitude where, species diversity reported of H = 0.84. In sapling layer, the reported values of diversity reduced with increasing altitudes as H = 2.40, H = 2.30 and H = 1.24 for upper, middle and lower atitudes respectively. Similarly the species diversity in seedling layer ranged from H = 1.66 (upper altitude) to H = 2.29 (lower altitude).

The CD values for tree layer increased with increasing altitude ranging from 0.26 to 0.69 (Table 4). The maximum and minimum values of CD in sapling layer reported were 0.57 and 0.22 respectively, wereas, in seedling layer the CD values again increased with increasing altitude (Table 3).

In each layer of trees, saplings and seedlings, the EC reduced with increasing altitudes from lower altitude to upper, where the values reported for each layer varied from 2.81 to 4.47 (for tree layer) 3.65 to 7.25 (for sapling layer) and 3.32 to 18.99 (for seedling layer) (Table 4).

The results for beta diversity (BD) shows that in each layer (trees, saplings and seedlings), the values of BD were reported reverse of CD. These values of BD increased with increasing altitudes from lower to upper altitudes (Table 4).

Similarity index

Similarity among species at different stages of vegetation was observed in different clusters as per Bray-Curtis analysis (Figure 4a, 4b, 4c). At tree stage maximum numbers of individuals were recorded for Cedrus deodara (94) and Quercus leucotrichophora (89) followed by Lyonia ovalifolia (38), Rhododendron arboreum (30) and Quercus floribunda (25) cluster, while a separate cluster of Benthamidia capitata (4) and Nelolitsea pallens (1) indicated minimum number of individuals at tree stage (Figure 4a). In the sapling stage, Quercus leucotrichophora (75) showed highest number of individuals. Quercus floribunda (27) and Rhododendron arboreum (25) showed almost similar number of individuals at sapling stage, while lowest individuals (3) were recorded for Pinus roxburghii at the sapling stage (Figure 4b). At seedling stage Rhododendron arboreum (49) and Quercus leucotrichophora (48) showed maximum number of individuals forming a separate cluster followed by Quercus floribunda (39) and Nelolitsea pallens (35), while for Cedrus deodara only 1 individual was recorded at seedling stage which indicates poor regeneration of this species in the study area (Figure 4c).

Figure 4a. Cluster analysis of trees.

Figure 4b. Cluster analysis of saplings.

Figure 4c. Cluster analysis of seedlings.

Forest composition and structure

In the present study, tree density ranged from 850 to 1210 trees ha^-1 and total basal cover of trees ranged between 43.68 to 137.4 m² ha^-1. In the world forests, Duvigneaud & Denayer-De Smet (1970) and Dabel & Dey (1977) reported total basal cover values ranging from 12.2 to 83.8 m² ha^-1 for montane temperate forest of world. Valencia et al. (1994) reported the density values between 716 to 1440 ha^-1 for Kurupukari forest in Guyana and density of terra firm forest as 1561 ha^-1 respectively. Kunwar & Sharma, (2004) carried out a study in Dolpa district of mid-west Nepal, where the density values reported from 2090 to 2100 ind ha^-1.

In Kumaun Himalayan forests in Uttarakhand, Bargali et al. (1987) described the density values from 490 to 1640 ha^-1 in different pine forest. Ralhan et al. (1982) reported mean density value of 416 trees ha^-1 and mean total basal cover value of 32.71 m² ha^-1. Saxena & Singh (1982) carried out a study in Quercus floribunda forest and reported the density and total basal cover values of 1300 ha^-1 and 29.40 m² ha^-1 respectively. Bharali et al. (2011) reported tree density ranging between 707 to 963 ind. ha^–1. Saxena & Singh (1982) reported density values ranging from 420 to 1640 ind. ha^–1 for temperate forests. The values carried out in the present study fall within the range of values reported by researchers in the Kumaun Himalayan forests.

In a study of Kumar & Bhatt (2006) in Garhwal Himalayan temperate oak mixed forest reported density and total basal cover values ranging from 760 to 1110 trees ha^-1 and 38-56.62 m² ha^-1 respectively. Kumar et al. (2009) also reported the density and total basal cover values of 652 to 1028 m² ha^-1 and 11.38 to 31.70 m² ha^-1 respectively in a temperate forest. Sharma et al. (2011) studied in temperate forest on various slope aspects showing variation in stem density, tree biomass, where stem density was lowest on north east aspect in moist Cedrus deodara forest type (380 ± 34.2 ind. ha⁻¹) and highest on north east aspect in Quercus leucotrichophora forest type (1360 ± 105.6 ind ha⁻¹). Bhandari & Tiwari (1997) reported density values from 1570–1785 ind ha^-1 in Himalayan temperate forests. These values reported by above experts for Garhwal Himalayan forests are fluctuating above and below the values of the present study, due to different levels of disturbances in the forests. Many researchers (Baduni & Sharma, 1996; Ghildiyal et al., 1998; Sharma & Baduni, 2000; Ram et al., 2004) reported basal area values ranging from 17.9 to 180.1 m² ha^–1 in the temperate forest of Himalaya. In different forest types of Garhwal Himalaya, Raturi (2012) reported highest value of density (1980 plant ha^-1) in the temperate mixed forest and minimum value of density (1090 plant ha^-1) in sub-tropical forest. However, among the other forests, total basal area was maximum in Pinus roxburghii (43.62 m² ha^-1) of sub-tropical mixed forest and minimum in Q. glauca (3.182 m² ha^-1) temperate forest. Dhaulakhandi et al. (2008) reported density value of 820 trees ha^-1 in natural forest of Gangotri. Devlal & Sharma (2008) reported total density of trees ranging from 1166 trees ha^-1 to 1826 trees ha^-1. This study revealed that Quercus leucotricophora was the most dominant species of all stands. Oak forests are most extensively distributed between the altitudes 1000m to timberline and represent the climax stage, throughout the Central Himalaya (Champion & Seth, 1968; Upreti et al., 1985).

Distribution Pattern (%)

The distribution pattern of all the species in upper altitude showed contagious distribution, however, in middle altitude majority of the species were distributed contagiously and few species were randon in distribution whereas no species reported showing regular distribution. In lower altitude, random distribution was reported dominant. In saplings and seedlings layers majority of species were distributed contagiously.

The present study revealed that majority of species in tree sapling and seedling layers were distributed contagiously. Odum (1971) also suggested that contagious distribution is the commonest pattern in nature, whereas, random distribution occurs only in a very uniform environment, and regular distribution occurs when severe competition occurs between individuals. Many workers (Greig-Smith, 1957; Kershw, 1973; Singh & Yadav, 1974; Kumar & Bhatt, 2006) have also reported contagious distribution for Himalayan forests. Rawat & Chankhok (2009) reported distribution pattern of tree, sapling and seedling layers indicating that most of the species are distributed randomly, while some species of trees and saplings were regular in distribution, however, in tree layer few of them were distributed contagiously with absence of contagious distribution in saplings. Raturi (2012) studied four forest types of Himalaya and reported tree distributed randomly, while shrub layer revealed random and regular distribution patterns.

Species richness

In tree layer, species richness ranged from 3 species for upper altitude and 7 species for middle altitude, whereas, 6 species reported for lower altitude. Middle site has indicated highest number of species which favour higher number of species compared to both upper and lower. Parthasarathy (1999) reported species richness of 144 species in a tropical wet evergreen forest of Kalakad National Park in Western Ghats. Bhuyan et al. (2003) reported species richness of 54 species in a tropical wet evergreen forest of Arunachal Pradesh. Semwal et al. (2010) reported maximum tree richness (10 species) in Rhododendron arboreum dominated forest and minimum (8 species) in Q. leucotrichophora forest. Raturi (2012) recorded the species richness between 2-13 species for trees. Nath et al. (2005) recorded that the tree species richness decreased with increase in the intensity of disturbance. Although present study has quite lower number of species richness which showed negative relationship with altitude. Sharma et al. (2009) also reported negative relation of tree species richness index with increasing altitude. The decrease in diversity and species richness at high elevation strata could be due to ecophysiological constraints, such as reduced growing season, low temperature and low productivity (Korner, 1998; Gairola et al., 2008). Other factors such as soil fertility and topography may also affect the patterns of species richness along altitudinal gradient (Gairola et al., 2008). Rahbek (1995) presented a critical literature review on species richness patterns in relation to altitude and showed that approximately half of the studies detected a mid-altitude peak in species richness (Grytnes & Vetaas 2002).

Grytnes & Vetaas (2002) compare different null models for species richness patterns in the Nepalese Himalayas. Species richness is estimated by interpolation of presences between the extreme recorded altitudinal ranges. The number of species in 100-m altitudinal bands increases steeply with altitude until 1,500 m above sea level. Between 1,500 and 2,500 m, little change in the number of species is observed, but above this altitude, a decrease in species richness is evident.

Singh et al. (1994) reported that Pinus roxburghii-mixed broadleaved forests had the highest species reichness, while high elevation forests had the lowest. Other researchers such as Burns (1995) and Austin et al. (1996) have found that the total species richness was greatest at lower elevation and warmer sites. The overall pattern of species richness showed a sharp decline as the altitude increased beyond 3000 m asl (Sharma et al., 2009).

Species diversity (H)

In the present study the species diversity of trees, saplings and seedlings at different altitudes ranged from 0.84 to 2.13 (for trees), 1.24 to 2.40 (for saplings) and 1.66 to 2.29 (for seedlings). The results showed that diversity of trees, saplings and seedlings generally decresed with increasing altitudes. Semwal et al. (2010) reported species diversity (H) in different forest types varying between 0.75 to 0.96 (for tree), 1.22 to 2.46 (for shrub) and 1.96 to 3.12 (for herb). Overall, Q. semecarpi­folia forest showed maximum diversity (H = 0.96) and Rhododendron arboreum mixed forest exhibited mini­mum diversity (H = 0.75) values. The values of species diversity of trees are comparatively lower than those of the present study. Singh & Singh (1987) also reported diversity values (H = 1.19 to 2.15) in chir pine mixed forest of Central Himalaya. Rawat & Chandhok (2009) reported species diversity ranging from 1.9 to 2.24. Gairola et al. (2008) carried out study in Pindari, Lata and Tungnath areas at <3000m, 3000-3200m and >3200m elevations and reported highest diversity in 3000-3200m and lowest >3200m and reported diversity values for Pindari (H = 0.44 to 1.55), Lata (H = 0.56 to1.09) and Tungnath (H = 0.693-0.87), which have not shown any specific trend with altitutde. Raturi (2012) reported species diversity ranging between 0.78 to 3.45 for tree stratum. Maximum diversity (H = 3.45) was recorded for temperate mixed forest and minimum (H = 0.78) for sub-tropical forest.

Kharkwal et al. (2005) carried out a study on phytodiversity and growth form of plants in relation to altitudinal gradient in the Central Himalayan region of India and reported that along the altitude the geographic and climatic conditions change sharply. Here total number of species, including all growth forms were higher near low altitude to mid altitude of tropical/subtropical belt but with further increase in altitude it decreased consistently due to decrease in atmospheric temperature with increase in altitude.

Gairola et al. (2008) reported variation in species diversity (H) along environmental gradient as a major topic of ecological investigation and has been explained by reference to climate, productivity, biotic interaction, habitat heterogeneity and history (Givnish, 1999; Willig et al., 2003; Currie & Francis, 2004; Gonzalez-Espinosa et al., 2004; Qian & Ricklefs, 2004).

Concentration of Dominance (CD)

In the present study the concentration of dominance ranged from 0.26 to 0.69 (for trees), 0.22 to 0.57 for saplings and 0.21 to 0.35 for seedlings. Raturi (2012), reported highest concentration of dominance for subtropical forest (CD = 0.63) and the lowest (CD = 0.09) for temperate mixed forest. Risser & Rice (1971) reported values between 0.10 to 0.99 for tropical forest. Ahmed (2012) in a study in Ranikhet forest of Uttarakhand the value of concentration of dominace for Oak, Oak-pine, Oak-scrub and chir pine were 0.237, 0.246, 0.349 and 0.56 respectively. Sharma et al. (2009) in a study on temperate mixed forest along altitudinal gradiant reported concentration of dominance values as 0.135, 0.138 and 0.321 at upper, middle and lower altitudes respectively.

Beta Diversity (BD)

In present study the values of beta diversity ranged between 2.18 to 3.12 for trees, 2.41 to 4.0 for saplings and 0.14 to 3.84 for seedlings. Semwal et al. (1999) in a study on Garhwal Himalayan region reported beta diversity values of 1.304 for trees, 1.53 for saplings and 1.41for seedlings. Kumar et al. (2004) reported the the range of values of beta diversity from 1.75 to 15.24 in a oak forest in Garhwal Himalaya. Sharma et al. (2009) reported beta diversity values from 2.89 to 3.50 in a temprate forest of Garhwal Himalaya. Raturi (2012) reported the values of beta diversity between 0.40 to 16.90 in four types of forests in Garhwal Himalaya. The reported values of researchers are close to the present study.

Density-diameter distribution

The density-diameter graph of trees in different altitude is shown in Figure 5a, 5b, 5c. Density-diameter distribution has often been used to represent the population structure of forest (Khan et al., 1987; Kumar et al., 2009). The nature of distribution curve can also be used to interpret the character of vegetation. In general, trees with higher girth classes have lower density value. Kumar et al. (2009) also reported lower density values with increasing girth classes.

Figure 5. Showing density-diameter distribution of trees under different girth classes in upper, middle and lower altitudes.

Dominance-diversity curve (D-D Curve)

The dominanace-dversity curve for all the vegetation layers i.e., trees, saplings and seedlings have been given in Figures 6a, 6b, 6c. The dominanace-dversity curve fit for the lognormal situation (Khera et al., 2001). The lognormal series describes the partitioning of realized niche space among various species and it is consequence of the evolution of diversity in the niche parameters that is exploits (Whittaker, 1965; Khera et al., 2001). Raturi (2012) repoted the dominance-diversity curve for tree and shrub layers approached geometric series for all forest types constructed on the basis of importance value index. The lognormal distribution of the forest layer was due to highly mixed nature of vegetation. The geometric series for both the layers confirmed the niche preemption hypothesis of Whittaker (1975).

Figure 6. Indicating dominance-diversity curve of trees, saplings and seedlings in different altitudes.

Regeneration status

The regeneration pattern of the species is shown in Table 2. In tree layer, Cedrus deodara was reported on upper and middle altitudes; however no sapling and seedling were reported, which is indication that in future no further growth may appear in the forest. In lower altitudes, the density of Quercus leucotrichophora trees reduced with increasing altitudes as 440 trees ha^-1, 370 trees ha^-1, 90 trees ha^-1 . Similarly Quercus floribunda also reduced with increasing altitudes.Among the altitudes, Benthamidia capitata in tree layer was reported in lower and middle altitudes with sufficient number of saplings and seedlings however, it was interesting to note that enough saplings and seedlings were reported in upper altitude while no single tree was reported in upper altitudes, which indicate the shifting of species towards higher altitude. In tree layer, Rhododendron arboreum also reduced with altitudes but not reported in upper altitude, while saplings and seedlings were reported in upper altitude, this also indicates that Rhodendron arboreum is shifting upper to upper altitude.

Regeneration status of tree species of any forest is determined by recruitment of saplings and seedlings (Dhar et al., 1997; Singh & Singh, 1992). In a study, Pant & Samant (2012) suggested the number of species in seedling and sapling stages varied from community to community.

The density values of seedlings and saplings are considered as regeneration potential of the species or forest. The presence of good regeneration potential shows suitability of a species to the environment. Climatic factors and biotic interference influence the regeneration of different species in the vegetation. Higher seedling density values get reduced to sapling due to biotic disturbance and competition for space and nutrients. Good & Good (1972) have considered three major components which cause the successful regeneration of tree species. These components are the ability to initiate new seedlings, ability of seedlings and saplings to survive, and ability of seedlings and saplings to grow.

In the seedling and sapling layers the density ranged from 340 to 1190 ind. ha^-1 for seedlings and 340 to 920 ind. ha^-1 for saplings. The total basal cover values for seedlings and saplings layers were reported from 0.15 to 1.02 m² ha^-1 and 0.61 to 2.74 m² ha^-1 respectively.

A study by Gairola et al. (2008) for different high altitude Himalayan forests reported that among the study sites, maximum seedling density throughout the altitudinal strata in Pindari suggests that the slope and aspect favour regeneration of tree species. However, low temperature, wind direction and snow depth have also been considered the main factors which determine the seed germination and rate of seedling survival (Qingshan et al., 2007).

Rawat & Chandhok (2009) reported saplings and seedlings layer densities ranging from 90 to 410 ind ha^-1 for saplings and 50 to 510 ind ha^-1 for seedlings. Natural regeneration is a key process for the continued existence of a species in a community. Ability of species to initiate new seedlings, their survival and growth are the three major components of successful regeneration (Good & Good, 1972; Saikia & Khan, 2013). Presence of sufficient number of seedlings, saplings and young trees in a given population indicates successful regeneration (Saxena & Singh, 1984), which is frequently influenced by the biotic interactions and physical factors in the community.

In general, Pinus roxburghii and Quercus leucotrichophora showed fair regeneration in spite of fire and logging. Tree density showed strong correlations with the densities of seedlings and poles, while saplings density was not significantly correlated that of trees, indicating low sapling density. Observations suggest that conversion of saplings to tree strata is a crucial factor to ensure good regeneration of species in these types of forests.