Environmental Drivers of Species Composition and Tree Species Density of a Near-Natural Central Himalayan Treeline Ecotone: Consequences for the Response to Climate Change

Climate warming is expected to facilitate alpine treeline advance to higher elevations. However, empirical studies in diverse mountain ranges give evidence of both advancing alpine treelines and rather insignificant responses. In this context, we aim at analysing environmental drivers of species composition and tree species density in the near-natural treeline ecotone in Rolwaling Himal, Nepal, in order to infer the sensitivity and responsiveness to climate warming. We differentiated plant communities and analysed population densities of tree species along the treeline ecotone from closed forest stands via the krummholz belt to alpine dwarf shrub heaths (3700–4300 m). We determined vegetation–environment–soil relationships, i.e. the effects of changing environmental conditions (e.g. nutrient and thermal deficits, plant interactions) on plant communities and stand structures across the ecotone by means of multivariate statistics. In particular, we focus on explaining the high competitiveness of Rhododendron campanulatum forming a dense krummholz belt and on its relation to climate change. We identified five plant communities, belonging to two different classes. Soil temperature, nitrogen supply and availability, and soil moisture content mainly differentiate species composition of the identified communities. Results indicate that trees in the ecotone show species-specific responses to the influence of site conditions, and that juvenile and adult tree responses are modulated by environmental constraints in differing intensity. In general, the analysed vegetation–environment relationships in the treeline ecotone suggest that the dense Rhododendron krummholz belt largely prevents the upward migration of other tree species and thus constrains the future response of Himalayan krummholz treelines to climate warming.