The CO₂ balance of tropical forests is an important component of the global terrestrial carbon balance, but quantifying it by direct measurement has been challenging. Processed-based modeling presents an attractive approach to overcoming logistical issues of measuring ecosystem fluxes. Modeling the CO₂ uptake component requires understanding of the photosynthetic processes among species across the vertical canopy gradient, across the landscape, and over time. We recently completed studies of the vertical canopy gradient with replicated samples across the landscape, here we consider temporal variation in leaf photosynthesis of canopy leaves. To test if leaf-level photosynthetic capacity in the canopy exhibits significant temporal variation, we made monthly measurements of light-saturated photosynthesis (A_max) over a two-year period at the La Selva Biological Station in Costa Rica. Five replicate leaves were measured for two tree and two liana species accessible from our two canopy towers. Maximum photosynthetic rates were measured mid-morning with a LI-6400 portable photosynthesis system equipped with a red-blue LED light source. Subsamples of the measured leaves were analyzed for N and P concentration.

Results/Conclusions

Average A_max among the four study species ranged from 9.6 to 17.6 µmol m^-2 s^-1. All four species showed significant intra-annual variation, and for the three evergreens the patterns of variation were concordant. The  exception was Stryphnodendron microstachyum, a leaf exchanging tree species with short-lived leaves. Coefficients of variation over the two years ranged from 22 to 36%, with the greatest variation in the liana Dendropanax arboreous. Lowest A_max tended to occur during the mild dry season. Possible causes for this surprising variation include vapor pressure deficit, soil moisture, air temperature, and leaf age.