Vegetation Patterns and Competitive Dynamics along Elevation Gradients: Interactions between Environmental Factors and Vegetation in the Central Himalayas

Abstract. Elevation gradients are generally characterized by a steady reduction in temperature with altitude, resulting in differences in growth conditions that often produce clear patterns of vegetation zonation. A south-north transect of the central Himalayas spans from a tropical climate in the south to alpine conditions in the north, offering an opportunity to investigate the relative roles of abiotic stress and competitive interactions in shaping plant community assembly. We hypothesise a shift from vegetation composition and productivity being characterised by realised niches, defined by competitive interactions at lower elevations, to physiological niches, shaped by stress (freezing temperature) at higher elevations. To investigate how these niche transitions influence community dynamics and ecosystem processes, we used a dynamic vegetation model with regional plant functional types (PFTs) parameterised with trait data, including allometric relationships. The model effectively captured spatial and temporal variability in vegetation structure and productivity along the gradient, with simulated patterns closely matching observed vegetation zonation across the transect. The establishment and performance of the PFTs were dependent on their climatic niche and the local abundance of competing PFTs, with persistence shaped by specific traits and adaptation strategies. At low elevations, where competitive interactions dominate, tropical shade-intolerant raingreen and tropical shade-tolerant evergreen PFTs dominated carbon mass production and vegetation cover. In contrast, shorter stature, evergreen phenology, and cold-tolerant types were favoured at high elevations, reflecting reduced interspecific competition and physiological adaptation to low temperature stress. Along the elevation gradient, PFT functional diversity declined with elevation, but evenness in composition increased. Conversely, low-elevation communities supported higher functional diversity, yet vegetation structure and function (e.g., LAI, FPC, and carbon mass) were dominated by a few competitively superior PFTs. We conclude that vegetation dynamics along the temperature gradient are governed by a trade-off between competitive ability and stress tolerance, as reflected in shifts in structure, composition, and productivity, which are shaped by environmental conditions, functional traits, and adaptive strategies of the vegetation.