Seasonal gas exchange and resource-use efficiency in evergreen versus deciduous species from a tropical dry forest

Tropical dry forests (TDFs) experience a long dry season in which plant species are subject to several months of water deficit. However, TDFs maintain a diverse group of plant life forms, growth forms and leaf phenology, and it is not clear how they vary in their mechanisms for coping with seasonal drought. We studied seasonal changes in leaf water potential (Psi(leaf)), gas exchange, photochemical activity and functional traits in evergreen and drought-deciduous species from a TDF to determine if leaf phenology mediates plant responses to drought. We found seasonal decreases in Psi(leaf), stomatal conductance (g(s)) and transpiration rate (E), and increases in both intrinsic and instantaneous water-use efficiency. We did not find seasonal differences in photosynthetic rate (A(area)) and carbon isotope composition (delta C-13); however, these traits differed between leaf phenology groups, with drought-deciduous plants having higher A(area) and delta C-13 than evergreen plants. We also found that plants with high leaf nitrogen concentration (N-area) also had low mass-based photosynthetic rate (A(mass)), photosynthetic-nitrogen-use efficiency and specific leaf area, contrary to the expected relationships given by the leaf economics spectrum. Despite higher N-area, sclerophyllous leaves maintained lower A(mass), and this increased structural toughness of leaves may be imposing a stronger limitation for CO2 diffusion and hence photosynthesis. Overall, we found more water-conservative traits in deciduous than in evergreen plants, contrary to what is known about these two leaf phenology groups in other seasonal sites both at tropical and temperate latitudes.