Patterns of Plant Species Richness Across the Himalayan Treeline Ecotone

Treeline ecotones, the transition zone between the upper closed forest limit (timberline) and treeless alpine vegetation, are the most conspicuous features of mountain ecosystems around the world. Understanding the patterns of plant species richness in the treeline ecotones is crucial in accurately assessing and monitoring the treeline shifts and vegetation dynamics in the face of climate warming. This chapter, based on a quantitative systematic review of the scientific literature published over the last 30 years (1991-2021), unravels the taxonomic and distributional patterns of plant species richness in the Himalayan treeline ecotone. We found a total of 593 vascular plant species belonging to 232 genera in 70 families reported in the literature on treeline ecotones of the Himalaya. Asteraceae, Rosaceae, Ericaceae, Ranunculaceae and Pinaceae were found to be species-rich families. Overall, the treeline ecotone plant species pool was higher in Western Himalaya as compared to that of Eastern Himalaya and only a small proportion of species was common to both the regions. The majority of the species were of herbaceous growth form and the trend was consistent across the treeline ecotones of Western and Eastern Himalaya. In the ecotone flora of the Himalaya, there are 2 plant species currently recognized by IUCN as near threatened, 4 vulnerable, 7 endangered, and 2 as critically endangered. Based on the insights gained from the present empirical synthesis, we highlight the research knowledge gaps and suggest way forward for better understanding of patterns of treeline ecotone flora in the Himalaya.