Variation in plant traits along a tropical wet-dry gradient: a test of ecophysiological theories

Abstract Many ecophysiological theories have been proposed as universal rules to calculate plant photosynthesis given their living environment, where temperature and vapor pressure deficit (VPD) are commonly considered. Although these theories claim universality and have been applied in global modeling, they are often developed and tested using global datasets that confound temperature and VPD and lack data from tropical Africa where there is minimal temperature variation. Here we confront these theories with tree traits collected along a VPD gradient in West Africa. Widely used ecophysiological theories were used to derive predictions about the variation in key plant traits along the wet-dry gradient. Most photosynthetic traits measurements along the gradient show trends consistently predicted by optimality-theory, including higher net CO2 assimilation rate and greater photosynthetic capacity at drier sites, and an association of higher photosynthetic capacity with greater respiration rates and greater water transport. Hydraulic traits show less consistency with theory or global-scale patterns, especially predictions based on xylem efficiency-safety tradeoff. Nonetheless, the link between photosynthesis and water transport still holds: species (predominantly deciduous species found in dry sites) with both high sapwood-to-leaf area ratio (AS/AL) and high potential hydraulic conductivity (Kp) (i.e. high transpiration), tend to have both high photosynthetic capacity and low leaf-internal CO2.