Forest structure, canopy architecture, and light transmittance in tropicalwet forests

Identifying factors that influence variation in light availability within f orested ecosystems represents an important component in our understanding o f the complex determinants of tree seedling regeneration. We assessed the i nfluence of forest structure and canopy tree architecture on spatial hetero geneity of understory light availability in three old-growth and three seco nd-growth forests in lowland Costa Rica. Forest structure and understory li ght availability were measured within forest types using contiguous 10 x 10 in quadrats along three 130-160 m transects in each stand. Two 20 x 60 m p lots in each forest type were sampled more intensively, including vertical profiles of light availability from 1 to 9 in height. Mean diffuse light tr ansmittance increased from 2% at 1 in height to over 10% at 9 in height and did not differ significantly between forest types at any height. However, the relationships among height classes differed between forest types. Secon d-growth plots showed a negative spatial autocorrelation for light measurem ents separated by vertical distances over 4 m. Differences in the vertical distribution of light and foliage suggest that old-growth and second-growth stands differ in vertical organization of the vegetation. The most pronoun ced structural differences between forest types were found in trees between 10 and 25 cm in diameter at breast height (dbh). In second-growth stands, trees in the 10-25 cin dbh size class were more abundant and differed in al lometry. They were taller for a given stein diameter and had narrower crown s for a given height than old-growth trees. Within forest types, we did not find strong relationships between measures of forest structure and light a vailability, although the strength of these relationships differed between forest types. In both old- and second-growth forest, understory light avail ability at 0.75 in decreased with increased sapling and shrub density, but was not significantly influenced by local tree density or basal area. From 1-m to 9-m heights, tree density was a significant, but weak, predictor of light availability in old-growth plots. In second-growth plots, tree densit y showed little or no influence on light availability at heights below 9 m. Our findings challenge the view that, within a forest, canopy and subcanop y vegetation directly influence light transmittance near the forest floor. Instead, we argue that spatial patterning of the light environment occurs t hrough complex interactions among canopy, subcanopy, and understory vegetat ion.