Variation in stand structure, light and seedling abundance across a tropical moist forest chronosequence, Panama

We asked whether forest structure and understory light environments across a tropical moist forest chronosequence followed predictions of a 4-phase mo del of secondary succession (establishment, thinning, transition and steady -state) and whether seedling density and diversity were functions of light availability as predicted by this model. Using aerial photographs, we ident ified eight second-growth stands (two each aged ca. 20, 40, 70, and 100 yr) and two old-growth stands within Barro Colorado Nature Monument, Panama. T rees and seedlings were sampled in nested, contiguous quadrats in 2 160-m t ransects in each stand. Light was measured as percent transmittance of diff use photosynthetically active radiation (TPAR) at each seedling quadrat and by estimation of percent total incident radiation during the growing seaso n from hemispherical canopy photographs. Basal area, tree density, and cano py height followed predictions of the 4-phase model. Percent total radiatio n, but not TPAR, declined with stand age as did seedling density. While see dlings were more likely to occur in quadrats at higher light levels, much v ariation in seedling density was not related to light availability. Seedlin g patch sizes were small irrespective of light patches, estimated as semiva riance ranges. Seedling species richness was a function of seedling density ; estimates of species diversity unbiased by density did not vary systemati cally as a function of stand age. Proximate seed sources, efficient dispers al mechanisms, and appropriate establishment conditions can promote establi shment of species-rich communities early in successions of heterogeneous tr opical moist forest.