LIGHT ENVIRONMENT, GAS-EXCHANGE, AND ANNUAL GROWTH OF SAPLINGS OF 3 SPECIES OF RAIN-FOREST TREES IN COSTA-RICA

Light environment, leaf physiological characteristics, and growth were compared for forest-grown saplings of three species of tropical trees with known life histories. Light environment was assessed both by hemispherical canopy photography and a quantitative visual index of crown illumination. Leaf gas exchange characteristics were measured by infrared gas analysis. The species tested included Lecythis ampla, a species tolerant of understorey conditions, Pithecellobium elegans, a species found in relatively bright sites, and Simarouba amara, a fast-growing, light-demanding species. Annual height and diameter growth did not significantly differ between the three species, but highest average rates were found for Simarouba. Likewise, saplings of the three specics were found in similar low light environments although Simarouba saplings were found in slightly brighter sites and Lecythis saplings were found in the lowest light environments. Despite similar light regimes, the species differed markedly in leaf area and gas exchange. Leaf areas of Lecythis saplings were five and ten-fold greater than Simarouba and Pithecellobium saplings, respectively. Light-saturated leaf photosynthesis and leaf dark respiration rates of Lecythis were about half those of Simarouba; rates of Pithecellobium were intermediate. Lecythis had the highest leaf photosynthesis at understorey diffuse light levels. Measures of annual growth were positively correlated with estimates of both direct and diffuse light with the strongest correlations between sapling performance and diffuse light.