Species climate-niche properties suggest that both physiological tolerance and stress dominance shape plant community assembly across a tropical dry ecosystem

Abstract

Climate influences community assembly by constraining the conditions under which species can persist, but the consistency of macroecological processes across different ecosystems remains unclear. Interspecific variation in climatic niche properties that govern community assembly across climate gradients can be gleaned from species distributions. Species-climate associations, niche properties, and resulting assembly remains poorly understood for plants in tropical dry ecosystems, and among different life-forms. We examined how species-climate associations governed community assembly of four plant life-forms across 60,000 km2 of tropical dry ecosystem in peninsular India. In the 1750 km long Eastern Ghats mountain range, we surveyed vegetation in 2500 20 x 50 m plots. Across the north-south climatic gradient from cool, wet and seasonal sites to warm, dry and less-seasonal sites, from presence-absence data of 1601 species in four life forms (trees, shrubs, herbs, climbers), we performed Bayesian Hierarchical Modelling of Species Communities to predict conditions of occurrence for each species. From this, we derived species climatic niche optima and niche width, and examined the local-level prevalence of niche properties across the climate gradient to glean assembly. Most tree, shrub and climber species associated with warm-dry conditions had wider niches than species associated with cool-wet conditions, which also resulted in assemblages having wider niches at warmer sites. Herbaceous species, by contrast, had narrower niches when associated with warmer conditions, where narrow-niche species dominated the assemblage. In all life-forms, however, assemblages in both warm-dry and cool-humid conditions consisted of species primarily associated with and preferring (having their optima in) those conditions, suggestive of stress dominance. Further evidence for stress dominance in shrubs and climbers came from assemblages in the warmest and coolest sites having low richness and comprising mainly of species with wider niches. Tree richness increased at either end of the gradient, while herb richness increased in warm-dry conditions. Overall, our results suggest that plant life-forms differ in the processes driving assembly across broad climate gradients in a tropical dry ecosystem, but many herbs were specialized to warm-dry climates. To understand life-form-specific responses to environmental gradients in tropical dry ecosystems, future work should incorporate traits and evolutionary history.