Seasonal Drought and Leaf Fall in a Tropical Forest

Peak rates of leaf fall almost always occur during dry seasons in low—latitude, low—elevation tropical forests. The hypothesis that plant water stress is the proximal cue for leaf fall was tested by augmenting water supplies during the 4—mo dry season over two 2.25—ha plots of tropical moist forest on Barro Colorado Island (BCI), Panama. The manipulation maintained soil water potentials at or above field capacity throughout the dry season but did not affect atmospheric conditions in the canopy (i.e., relative humidity, temperature, windspeed, incident radiation). The manipulation ameliorated plant water status; for most species, dry—season leaf water potentials in the manipulated plots were similar to wet—season values and both were consistently greater than dry—season values in the control plots. The manipulation delayed leaf fall for 2 of 9 species of trees for which qualitative data are available and possibly for 2 of 20 species of trees and lianas for which quantitative data are available. The timing of leaf fall was indistinguishable in manipulated and control plots for the remaining 25 species. We conclude that plant water status is rarely the proximal cue for leaf fall on BCI. Atmospheric conditions may be important for some species, but there is no reason to presuppose that a majority of tropical plants are responsive to any single cue.