Understanding the patterns and drivers of species distribution has remained a central theme for biogeographical, conservation, and ecological research. This study aims to investigate the elevational patterns of plant species richness and compare the observed species richness with the predictions of the mid-domain effect (MDE) null model. By combining information from field observations and the published literature, we compiled a comprehensive database of the elevational distribution of plant species for three protected areas in the Western Himalayas. We used generalised linear model (GLM) and null model simulations to explore the elevational patterns of plant species richness. Our study revealed simple linear to complex non-linear patterns depending on the location and range of the elevational gradient. While non-linear unimodal patterns were common, a linear decreasing pattern was also observed. The observed species richness showed consistent deviations from the predictions of the mid-domain effect null model, suggesting that factors beyond the range constraints shape species richness patterns. These observations indicate that richness patterns are not solely generated by random processes, rather climatic gradients, ecological interactions, and topographic heterogeneity can shape these patterns. Understanding these factors can aid in predicting and managing the impacts of ongoing environmental changes on Himalayan biodiversity.

Mountain biodiversity faces significant threats from climate change with uncertain consequences on species distributions and ecosystem integrity. Understanding the patterns and dynamics of species distribution is critical for managing and safeguarding biodiversity in a changing environment. Therefore, this thesis investigates plant distribution patterns, determinants, and dynamics along elevational gradients in the Western Himalayas. Following a macroecological approach, this research analysed a database for the elevational distribution of over 1000 plant species from three protected areas in the Western Himalayas. Using an ensemble MaxEnt modelling, this study predicted future shifts in suitable habitats for ten dominant tree species under various climate scenarios. The results showed that elevational patterns of species richness varied across study sites and indicated substantial deviations from the mid domain effect null model predictions. The interacting effects of climate, energy, and water availability emerged as key determinants of species richness. Direct effects of climate were stronger than indirect effects, and water energy dynamics mediated the indirect effects more strongly than the energy availability. About half of the investigated tree species can face range contractions, while others may move upslope. All studied species, except Shorea robusta, are predicted to lose their suitable habitats under future climates. These findings pave the way for prioritising conservation efforts and developing management plans to mitigate the impacts of climate change. Further, insights into future trajectories can inform targeted conservation strategies and develop plans to sustain the Himalayan trees in a changing environment.

Background Understanding the patterns and processes of species distributions has long remained a central focus of biogeographical and ecological research. While the evidence for elevational patterns in species richness is widespread, our understanding of underlying causes and mechanisms remains limited. Therefore, this study aimed to entangle the influence of environmental variables on plant species richness along elevational gradients in the Western Himalayas.
Methods We compiled elevational distribution for about 1150 vascular plants using the published literature and available database. The species richness was estimated in 100-m elevational bands using the range interpolation method. We used the generalised linear model and structural equation modelling (SEM) framework to identify the direct and indirect effects of climatic factors on species richness.
Results Our results indicated that primary environmental correlates of species richness varied with elevational gradients. Climatic variables combined with energy and water availability were more important than the topographic heterogeneity. Further, the direct and interaction effects of climatic variables were more substantial than their indirect effects. The indirect effects of climate are more strongly mediated by water-energy dynamics than the energy alone.
Conclusions Overall, our findings emphasise the importance of considering direct effects and interactions among environmental variables while studying the underlying mechanisms governing elevational biodiversity gradients. Species richness appeared to be shaped by climatic tolerances rather than habitat heterogeneity at regional scales. This information can have implications for biodiversity dynamics under environmental change.