TROPICAL FOREST LITTER DYNAMICS AND DRY SEASON IRRIGATION ON BARRO-COLORADO ISLAND, PANAMA

Moisture seasonality may control forest floor decomposition rates in t ropical forests. We used a mass balance model and 5 yr (December 1986 through December 1990) of weekly litterfall and monthly forest floor m ass measurements from control and dry season irrigated plots to test t his hypothesis on Barro Colorado Island, Panama. Litterfall and forest floor mass were greater in the dry season than in the wet season. Irr igation affected neither the timing nor the quantity of litterfall. In contrast, dry season irrigation reduced forest floor mass throughout the year, not just during the dry season. Forest floor decomposition d uring the dry season was enhanced by irrigation. During the dry season , net decomposition (in grams per square metre per day) and exponentia l decay coefficients (per day) averaged 48 and 42% greater, respective ly, in irrigated plots than in control plots. As a consequence, season al differences in decomposition rates were more pronounced in the cont rol plots than in the irrigated plots. Net decomposition rates, for ex ample, averaged 105 and 22% greater during the wet season than during the dry season on control and irrigated plots, respectively. Net decom position was positively correlated with rainfall in the control plots: but not in the irrigated plots. These results support the hypothesis that moisture seasonality controls forest floor decomposition in tropi cal moist forests.