Seedling Trait Variation and Functional Space Occupation Responses to Topographic Water Shifts in A Tropical Dry Forest

Abstract

Water availability can change substantially at local scales due to topography (i.e., valleys and ridges), filtering out species depending upon their ecological strategies. Under drier conditions on ridges, species are expected to exhibit denser tissues, increased hydraulic safety, and greater water exploration, while simultaneously reducing their functional space, as low water availability acts as an environmental filter. However, it is commonly observed that generalist species are able to perform across a wide range of habitats, and it remains unclear how topographic heterogeneity influences their traits. In this study, we assessed intraspecific trait variation between valleys and ridges for 11 tree generalist species. We focused on ten morphological, physiological, and biomass allocation traits. To account for integrated plant responses, we also measured functional space occupation. We found higher leaf drought tolerance (lower turgor loss point), greater soil exploration (root mass fraction), and less dense tissues (leaf thickness and root tissue density) in drier ridges. Remarkably, π_tlp was the only trait that consistently varied across all species studied, indicating a strong sensitivity of physiological traits to local-scale abiotic changes. Additionally, we found that generalist species exhibited a broader range of functional traits at drier topographic positions, raising new questions about trait optimization in species with these resource use strategies. Overall, the level of intraspecific trait variability enables species to cope with variations in water availability at small spatial scales. We emphasize the importance of considering small-scale abiotic variation and physiological traits to enhance our understanding of the underlying mechanisms of community assembly.