4.1. Forest structure and tree species diversity pattern
Species richness and diversity are well established parameters of community structure and any changes in these parameters can be used as an indicator of change in community dynamics (Korner 2007; Brinkmann et al., 2009; Zhang and Dong 2010). Spatial variations in species diversity showed significant characteristics of vegetation patterns in the studied area. The results exhibited that patterns of tree species diversity are related to both large-scale (climate) and small-scale variables (anthropogenic and soil). These variables together determined assemblages of local communities and distribution of species in given space. The humped shaped responses of tree species richness along altitudinal gradient is in line with previous studies from the Himalayan region (Grytnes and Vetaas, 2002; Bhattarai and Vetaas, 2003; Chawla et al., 2008; Dar and Sundarapandian, 2016; Saikia et al., 2017; Bhatta et al. 2018; Sharma et al., 2019; Ahmad et al., 2020). The variation in species diversity and richness might be due to spatial heterogeneity of habitats within different study plots. Low species diversity at lower altitudes might be attributed to higher anthropogenic disturbances and localization of non-native species (Zhang et al., 2016; Ahmad et al., 2018; Rawal et al. 2018). Significant influence of anthropogenic disturbance on species diversity is well established (Newbold et al., 2015, Ahmad et al., 2020).
Low species diversity towards higher altitudes is attributed to harsh environmental conditions that cause physiological stress to the plants (i.e., low temperature, low rainfall), limiting plant growth and their regeneration (McCain, 2007; Körner et al., 2011; Lee et al., 2013; Gómez-Díaz et al., 2017). Further, plants at higher altitudes are reported to have short growing season, and low ecosystem productivity as compared to the plants at lower altitude (Körner, 2003; Körner, 2007). Occurrence of maximum species at mid-altitudes is significantly correlated with availability of water, optimum temperature, rainfall, etc., which is required for better survival and growth performance (Grytnes and Vetaas, 2002; Bhattarai and Vetaas, 2003; Zhang et al., 2016).
4.2. Classification of community composition
The knowledge of plant functional types is very important for differentiating forest communities. This study quantitatively classiﬁed and grouped forest community along altitudinal gradients. The community were divided into four groups includes subtropical, lower temperate, upper temperate and subalpine. NMDS analysis showed marked significant differences in tree composition along altitudinal gradients. A clear succession shift in community composition from subtropical to subalpine zone was observed. These four forest communities corresponded to the altitudinal gradient, hence, reiterated the importance of altitude as a covariate to determine the composition of tree species in the region (Fig. 2). This suggests that tree species composition changes with altitude, and demonstrate the role of various environmental factors in dispersion of these forest communities. Similar results have been reported from eastern Himalaya (Sharma et al., 2018). The changes in forest composition might be due to the fact that 'the time needed to recover species composition is longer than to recover species richness' (Lebrija-Trejos et al. 2011; Rozendaal et al. 2019).
4.3. Influence of environmental variables on communities composition
As reflected by RDA analysis, variables such as climate, soil and anthropogenic disturbance together played a significant role in determining species composition along altitudinal gradient. Subtropical and temperate forests gets negatively impacted by environmental variables such as bulk density, disturbance index, mean Diurnal Temperature Range (Bio2) and Solar Radiation (SR), whereas upper temperate and subalpine forest types get positively influenced by soil organic carbon, precipitation of the Driest Quarter (Bio17) and pH. For example, the RDA analysis showed that subtropical and temperate forests preferred low bulk density (Zheng et al., 2017), which might be due to the variation in bulk density, that indicates low soil porosity and soil compaction. This has been reported to cause restrictions to root growth, and poor movement of air and water there by affecting tree species in tropical forests (Sarvade et al., 2016).
These forests in the region experienced high level of human pressure. Lopping of tree for leaf fodder and fuelwood collection were the main anthropogenic pressure which cause large-scale disturbance in these forests; however, other practices such as logging, cutting and grazing also exist. Earlier studies have reported implications of fuelwood collection and fodder harvesting on species richness of the sub-tropical and temperate forests in western Himalaya (Rawal et al. 2012; Negi and Maikhuri 2017, 2018a) and across the globe (Ramı́rez-Marcial et al. 2001;Williams-Linera & Lorea 2009; Gibson et al. 2011). Anthropogenic disturbance is likely to regulate the community distribution pattern and certainly modifies natural ecosystem process by changing the land use pattern (Newbold et al. 2015; Panda et al. 2019). The vulnerability of a particular species or forest is primarily dependent on the intensity of disturbance (Thakur et al. 2020). Composition of tree species in subtropical and temperate forest gets affected by disturbance index and Bio2.
The range of diurnal temperature is an important indicator of climate change (Karl et al., 1991; Braganza et al., 2004; Qu et al., 2014; Yang et al., 2016) as it influence physiological attributes such as photosynthesis, respiration, reproduction, etc. Mean diurnal temperature range in the present study was found significantly associated with species composition in subtropical to temperate forests. The altitude, precipitation and temperature variables were found highly collinear. Diurnal temperature variation associated with solar radiation and photoperiod, has been reported to regulate plant functions, including central carbon metabolism, stomatal opening, and photoperiodism (Michael et al., 2003; Hu et al., 2019).
The soil organic carbon was important factor that impacted plant species composition of temperate and subalpine forest in the study area. As such, plant functional traits are highly related to soil organic carbon stock and carbon sequestration that influences plant diversity and composition (Jobbágy and Jackson, 2000; De Deyn et al., 2008; Stein et al., 2014; Stark et al., 2017). The intensity and durability of rainfall is reported to have impenetrable effect on local plant diversity and vegetation composition. Bio17 (Precipitation of the Driest Quarter) showed a negative relationship with tree species composition towards high altitude. The water stress during dry season also played a critical role in tree species composition. Earlier studies from the region have indicated that precipitation is important for determining plant species richness and composition in west Himalaya (Panda et al., 2017, 2019). Annual rainfall and its seasonal variation is reported to regulate temporal conditions of plant functional traits i.e., leaf flushing and flowering (Eamus 1999). Higher impact of mean annual precipitation on species richness pattern in the tropics is well reported (O’Brien 1993; Leigh et al. 2004; Davidar et al. 2005). Close association of soil pH with Upper temperate and subalpine forest types can be explained with the reported implications of soil pH in leaching of micronutrients, which changes the nutrient balance among plant tissues (Drenovsky et al., 2004; Partel et al., 2004 ; Fontaine and Barot, 2005; Clark et al., 2007). Cation exchange capacity under the influence of soil pH affects plant diversity through changing N availability (Partel et al., 2000, 2004; Stein et al., 2014).