Flowering and fruiting phenologies of seasonal and aseasonal neotropical forests:: the role of annual changes in irradiance

The seasonality of both rainfall and solar irradiance might influence the evolution of flowering and fruiting in tropical forests. In seasonally dry forests, to the degree that soil moisture limits plant productivity, community-wide peaks in reproduction are expected during the rainy season, with seedfall and germination timed to allow seedlings to become well established while soil moisture is available. Where soil moisture is never seasonally limiting, seasonal changes in light availability caused by periods of cloudiness or seasonally low zenithal sun angles should favour reproduction during seasons when irradiance levels are high. To evaluate these predictions, we documented the timing of flower and fruit fall for 10 and 15 y at El Verde, Puerto Rico, and Barro Colorado Island (BCI), Panama. At El Verde, rainfall is abundant year-round and solar declination largely determines seasonal variation in irradiance. At BCL rainfall is abundant throughout the 8-mo wet season while drought develops and average solar irradiance increases by 40-50% over the 4-mo dry season. Seasonal variation in the number of species flowering and fruiting at both sites was generally consistent with the hypothesis that seasonal variation in irradiance limits the evolution of reproductive phenologies. Community-level metrics provided no evidence for a similar role for moisture availability at BCL Seasonal variation in irradiance also strongly influenced seed development times at both sites. Thus, community-wide phenologics reveal a strong signature of seasonal changes in irradiance, even in those forests that exhibit some degree of seasonality in rainfall.