Structure and floristics of secondary and old-growth forest stands in lowland Costa Rica

We characterized stand structure and floristic composition of woody life forms in three, 16-18 yr old secondary stands that regenerated after pasture abandonment, and three nearby old-growth stands of tropical rain forest in lowland Costa Rica. Basal area and stem density for each of four plant size classes (seedlings, saplings, treelets, trees) were similar among stand types, but density of adult canopy palms (individuals greater than or equal to 10 cm DBH), was lower in the secondary stands. We estimate that 15% of the basal area of stems greater than or equal to 10 cm DBH correspond to remnant trees in our secondary stands. The observed rapid woody regrowth compared to other published studies in the lowland neotropics, can be attributed to moderate land use and possibly, to the influence of nutrient-rich volcanic soils in the study area. Overall, plant species richness was lower in the secondary stands, but this difference was less pronounced in the smallest size classes (seedlings, saplings). Median percent similarity of all pairwise stand comparisons showed that floristic composition of saplings (stems greater than or equal to 1 m tall and greater than or equal to 5 cm DBH) was more similar between secondary and old-growth stands than for trees (stems greater than or equal to 10 cm DBH). Because the potential value of secondary forests in conserving woody plant diversity appears highest for the young size classes, we suggest that further studies on floristic composition, especially those addressing the dynamics of the understory component, are needed to refine our understanding of the role of this natural resource in the maintenance of plant biodiversity in disturbed landscapes.