GAS-EXCHANGE IN 6 TROPICAL SEMI-DECIDUOUS FOREST CANOPY TREE SPECIES DURING THE WET AND DRY SEASONS

Wet and dry season rates of photosynthesis and stomatal conductance we re compared for upper canopy leaves of six species of tropical semi-de ciduous forest trees. Photosynthetic rates showed no dear relationship to leaf form (simple/compound), specific leaf area, or seasonality (e vergreen/deciduous). Photosynthetic rates for three of five evergreen species were similar in wet and dry seasons. The rates (in mu mol m(-2 ) sec(-1)) were 14.5 (wet season) and 15.5 (dry season) for Didymopana x morototoni, 13.8 (wet season) and 12.2 (dry season) for Ficus obtusi folia, and 10.4 (wet season) and 10.2 (dry season) for Luebea seemanni i. In the dry season, stomatal conductance for all five evergreen spec ies was half or less than in the wee season, except in Didymopanax, fo r which conductance was high in both seasons. For Anacardium excelsum and Cecropia longipes, photosynthetic rates in the dry season (4.71 an d 6.38 pmol m(-2) sec(-1), respectively) were lower than in the wet se ason (8.62 and 12.4 mu mol m(-2) sec(-1)). For these species, the lowe r photosynthetic rates in the dry season were observed at internal CO2 concentrations (C-i) as high or higher than wet season C-i, suggestin g: that photosynthetic activity was reduced by water stress. Pseudobom bax septenatum, a dry season deciduous species, sometimes showed low l evels of photosynthesis and stomatal conductance as well as low C-i le vels, suggesting chat stomatal closure restricted photosynthetic activ ity. Higher stomatal conductance was associated with higher rates of p hotosynthesis among species and between seasons.