Source water, phenology and growth of two tropical dry forest tree species growing on shallow karst soils

Seasonally dry tropical forests are dominated by deciduous and evergreen tree species with a wide range of leaf phenology. We hypothesized that Piscidia piscipula is able to extend leaf senescence until later in the dry season due to deeper and more reliable water sources than Gymnopodium floribundum, which loses leaves earlier in the dry season. Physiological performance was assessed as timing of leaf production and loss, growth, leaf water potential, depth of water uptake determined by stable isotopes, and leaf stable isotopic composition of carbon (δ¹³C) and oxygen (δ¹⁸O). P. piscipula took water primarily from shallow sources, whereas G. floribundum took water from shallow and deep sources. The greatest variation in water sources occurred during the onset of the dry season, when G. floribundum was shedding old leaves and growing new leaves, but P. piscipula maintained its leaves from the previous wet season. P. piscipula showed greater relative growth rate, greater leaf expansion rates, and more negative predawn and midday water potentials than G. floribundum. P. piscipula also exhibited greater leaf organic δ¹³C and lower δ¹⁸O values, indicating that the decrease in photosynthetic carbon isotope discrimination was associated with greater stomatal conductance and greater photosynthesis. Our results indicate that the contrasting early and late dry season leaf loss phenology of these two species is not simply determined by rooting depth, but rather a more complicated suite of characteristics based on opportunistic use of dynamic water sources, maximizing carbon gain, and maintenance of water potential during the dry season