Drought effects on seedling survival in a tropical moist forest

The amount and seasonality of rainfall varies strongly in the tropics, and plant species abundance, distribution and diversity are correlated with rainfall. Drought periods leading to plant stress occur not only in dry forests, but also in moist and even wet forests. We quantified experimentally the effect of drought on survival of first year seedlings of 28 co-occurring tropical woody plant species in the understory of a tropical moist forest. The seedlings were transplanted to plots and subjected to a drought and an irrigation treatment for 22 weeks during the dry season. Drought effects on mortality and wilting behavior varied greatly among species, so that relative survival in the dry treatment ranged from 0% to about 100% of that in the irrigated treatment. Drought stress was the main factor in mortality, causing about 90% (median) of the total mortality observed in the dry treatment. In almost half of the species, the difference in survival between treatments was not significant even after 22 weeks, implying that many of the species are well adapted to drought in this forest. Relative drought survival was significantly higher in species associated with dry habitats than in those associated with wet habitats, and in species with higher abundance on the dry side of the Isthmus of Panama, than in those more abundant on the wet side. These data show that differential species survival in response to drought, combined with variation in soil moisture availability, may be important for species distribution at the local and regional scale in many tropical forests.