Successional dynamics of woody seedling communities in wet tropical secondary forests

1 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 second-growth 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.