We monitored mortality and recruitment annually for six years for all trees Ն5 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 abandonment 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 (Ն25 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 Niño 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 Ն10 cm dbh compared to non-ENSO years. Annual mortality rates for trees Ն10 cm dbh decreased significantly with increasing dry-season rainfall, but not with total annual rainfall during the census periods. Tree mortality (particularly for trees Ն25 cm dbh) in these secondary forests was highly sensitive to dryseason rainfall, even during non-ENSO years. Increased tree mortality during ENSO years is likely to accelerate forest dynamics and successional change in wet tropical secondgrowth forests.

Summary1 Chronosequence studies have found that shrubs and lianas are generally more abundant in early stages of tropical forest succession, whereas canopy trees and palms become more abundant and species-rich in older stages and mature forests. 2 We analysed changes in woody seedling communities over 5 years in four secondgrowth forests (initially 13-26 years after pasture abandonment) in Costa Rica. We recorded community-level changes in woody seedling density, species density, species richness and composition in six woody life-forms: canopy trees, subcanopy trees, canopy palms, understorey palms, shrubs and lianas. We evaluated these changes in relation to annualized recruitment and mortality rates for each life-form. 3 Seedling density declined in all four sites over the 5 years, whereas Shannon diversity and the proportion of rare species increased. Species richness and evenness increased in all but the oldest site. 4 Canopy palm, understorey palm and canopy tree seedlings increased in species richness and relative abundance, whereas shrub and liana relative abundance declined. Canopy trees accounted for 34-42% of all new recruits. Detrended correspondence analysis showed that species composition was initially highly distinct within each forest site and remained distinct over the 5-year period. 5 Shifts in life-form were correlated with declining light availability during succession. Across sites, median light availability at the end of the study period in 2003 was positively correlated with recruitment rates of understorey palms, shrubs and lianas, and was negatively correlated with mortality rates of canopy trees and palms. 6 Observed changes among seedling communities mirrored those described in chronosequence studies on plants in larger size classes, lending support to the assumptions of chronosequence studies in Neotropical forests. 7 The results demonstrate the importance of seedling recruitment and mortality in determining the course of succession. Convergence occurs in some community properties, such as relative abundance within life-forms, but not in others, such as species composition. Finally, the results illustrate the value of studying plant community dynamics at the level of woody life-forms, especially in hyperdiverse systems such as tropical forests.

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