EFFECTS OF CLIMATE AND STAND AGE ON ANNUAL TREE DYNAMICS IN TROPICAL SECOND-GROWTH RAIN FORESTS

Abstract

We monitored mortality and recruitment annually for six years for all trees >5 cm diameter at breast height in four 1-ha plots in second-growth, lowland rain forests in northeastern Costa Rica. Stands initially ranged in age from 12 to 25 years since aban- donment of cattle pastures. In younger stands, abundance decreased 10-20% in the small size class (5-9.9 cm dbh), but increased 49-100% in the large size class (?25 cm dbh). In the two youngest plots, 45.0% and 27.6% of the common tree species showed annual population increases of 3% or more, whereas 10% showed annual decreases of 4% or more. Annual rates of tree mortality for all size classes and recruitment into the 5-cm size class varied significantly among years, and mortality rates of trees 5-9.9 cm dbh were higher in younger than in older stands. Tree mortality was highest during the 1997-1998 El Nifio Southern Oscillation (ENSO) year; on average, 5.6%, 3.5%, and 1.4% of trees died in the small, intermediate, and large size classes, respectively. Mortality rates during the ENSO year were 1.9 times higher for trees ?10 cm dbh compared to non-ENSO years. Annual mortality rates for trees >10 cm dbh decreased significantly with increasing dry-season rainfall, but not with total annual rainfall during the census periods. Tree mortality (par- ticularly for trees -25 cm dbh) in these secondary forests was highly sensitive to dry- season rainfall, even during non-ENSO years. Increased tree mortality during ENSO years is likely to accelerate forest dynamics and successional change in wet tropical second- growth forests.