Changes to plant traits and productivity of two functional types across a soil water content gradient in a tropical dry forest

Abstract1. Specific differences in water-use strategies may critically shift the structure and productivity of plant communities in monsoonal climates. Understanding variation in functional traits in plant species across soil water content gradients and their contributions to community productivity can help us to understand the capacity of communities to cope with water stress, and predict community change under climate variability.2. We conducted a study across 45 fragments of tropical dry forest distributed across a soil water content (SWC) gradient within a 50 km radius, to understand shifts in site-level community-weighted trait means and ranges of all trees with DBH > 10 cm. Species were classified into two functional types representing different water-use strategies, drought avoiders and drought tolerators, based on their leaf hydraulic behaviour (isohydric versus anisohydric). We compared how water safety traits, leaf carbon capture traits and canopy extent changed across the SWC gradient in the functional groups. We further measured site-level stem counts and growth rates for the two groups over a 5-year period across the SWC gradient, which we related to soil water content and functional group trait means and ranges that were calculated from measurements taken annually across the five years.3. Changes in trait means across the SWC gradient were markedly different between the functional types, especially for water safety and leaf carbon capture traits, with larger differences towards the drier end of the SWC. Towards drier sites, drought tolerators showed increased wood and leaf density and reduced carbon capture capacity. By contrast, drought avoiders had increased leaf carbon capture capacity towards drier sites. Drought avoiders had greater stem numbers and aboveground biomass at drier sites, but drought tolerators increased biomass non-linearly across the SWC gradient and were dominant at wetter sites.4. Synthesis. We found non-linear shifts in community biomass that appear to be related to trait-determined constraints on drought tolerators. We observed these patterns over a small spatial scale, indicating that landscape edaphic variability substantially alters the balance between drought tolerators and drought avoiders. The patterns further suggest that future climate shifts will alter the spatial distribution of drought tolerators and drought avoiders.