Gas Exchange in Six Tropical Semi-Deciduous Forest Canopy Tree Species During the Wet and Dry Seasons

Wet and dry season rates of photosynthesis and stomatal conductance were compared for upper canopy leaves of six species of tropical semi-deciduous forest trees. Photosynthetic rates showed no clear relationship to leaf form (simple/ compound), specific leaf area, or seasonality (evergreen/deciduous). Photosynthetic rates for three of five evergreen species were similar in wet and dry seasons. The rates (in ,mol m-2 sec-') were 14.5 (wet season) and 15.5 (dry season) for Didymopanax morototoni, 13.8 (wet season) and 12.2 (dry season) for Ficus obtusifolia, and 10.4 (wet season) and 10.2 (dry season) for Luehea seemannii. In the dry season, stomatal conductance for all five evergreen species was half or less than in the wet season, except in Didymopanax, for which conductance was high in both seasons. For Anacardium excelsum and Cecropia longipes, photosynthetic rates in the dry season (4.71 and 6.38 ,mol m-2 sec-', respectively) were lower than in the wet season (8.62 and 12.4 umol m-2 sec-'). For these species, the lower photosynthetic rates in the dry season were observed at internal CO2 concentrations (Cl) as high or higher than wet season Cl, suggesting that photosynthetic activity was reduced by water stress. Pseudobombax septenatum, a dry season deciduous species, sometimes showed low levels of photosynthesis and stomatal conductance as well as low C, levels, suggesting that stomatal closure restricted photosynthetic activity. Higher stomatal conductance was associated with higher rates of photosynthesis among species and between seasons.