Diversity and Community Structure Underlie Divergence in Thermal Strategies Across Tropical Elevations

We assessed how thermal tolerance strategies are associated with community diversity and phylogenetic context along a tropical elevational gradient, integrating community- and population-level perspectives by examining a widespread focal species together with its co-occurring species, and evaluated whether biotic and evolutionary context accounts for variation in thermal niches beyond elevation alone. Between 2023 and 2024, we surveyed vascular plant assemblages across seven sites in the Brazilian Atlantic Forest, spanning sea level to ~2200 m elevation, with emphasis on the bromeliad Pitcairnia flammea and co-occurring monocots, dicots, and ferns. Community diversity, species cover, and phylogenetic structure were quantified for each assemblage, and photosynthetic heat and cold tolerance (T50) and leaf functional traits were measured for P. flammea populations and dominant sympatric species. This two-level design allowed community-wide patterns among species to be assessed alongside intraspecific variation among populations of the focal species. We used phylogenetic analyses, multivariate analysis, and best model selection to assess the relative roles of elevation, diversity, and phylogenetic context in shaping thermal strategies. Cold tolerance and leaf area showed strong phylogenetic conservatism across taxa, whereas heat tolerance exhibited little phylogenetic structure. Mid-elevation assemblages, characterized by peak species richness, showed greater divergence in thermal tolerance among species. In contrast, P. flammea populations displayed their broadest thermal tolerance at high elevations, coinciding with high monocot diversity and phylogenetic clustering, as well as harsher abiotic conditions. Lowland populations were the most abundant in assemblages with lower representation of closely related taxa, but exhibited reduced heat tolerance. Overall, community diversity and phylogenetic structure explained variation in thermal strategies better than elevation alone, indicating that biotic context, captured here through patterns of community diversity and phylogenetic relatedness, is an important correlate of thermal niche differentiation along tropical elevational gradients.