Modelling the Ecological Niche of a Treeline Tree Species (Betula utilis) in the Himalayas—A Methodological Overview

Abstract

Mountains are fascinating habitats, characterized by steep ecological vertical gradients and corresponding altitudinal vegetation zonation. Alpine treelines as upper boundaries of more or less contiguous tree stands are the most conspicuous vegetation limits; they have always attracted great research interest. Globally, alpine treeline elevations in the mountains are caused by heat deficiency. At landscape and local scales, however, multiple interactions of influencing factors and mechanisms determine treeline position, spatial pattern and dynamics. In the course of climate change, it is postulated that treelines will shift to higher elevations. To be able to quantify potential shifts, an analysis of the underlying factors and a correct modelling of the treeline ecotone under current climatic conditions are of great importance. For this purpose, statistical models are used to calculate the ecological niche of species based on climatic factors. These models serve as a baseline for models that project the distribution under future climatic conditions. The Himalayas are the largest mountain range in the world, yet they are often under-represented in the scientific literature. This holds particularly true in relation to modelling studies. Modelling treeline species in remote high-altitude regions faces several challenges, especially the availability of occurrence data and high-quality environmental variables. This book chapter summarizes recent results modelling the ecological niche of the Himalayan birch (Betula utilis) under present climatic conditions in the Himalayan mountain system. B. utilis represents a favourable target species for modelling studies, since it is widespread as a treeline-forming species along the entire Himalayan arch. Due to less distinctive habitat requirements and high adaptation potential, it is gaining importance as a pioneer tree species for possible succession developments at treelines under future climate conditions. In a synergistic approach, a detailed study on comparing the underlying climatic, topographical and plant phenological factors was undertaken to model the potential and the actual distribution of the focal species. The present results provide a new starting point for further investigations aimed at modelling the distribution of the species under past or future climate scenarios. Simultaneously, the presented approaches can also be transferred to other treeline species in high mountains.