PARENT SESSION
Poster Session #39: Light Relations.
Wednesday, August 7. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Fine-scale temporal variation of photosynthetically active radiation above the canopy of a tropical dry forest .

POSADA, JUAN^*,1, ¹ Botany Department, University of Florida, Gainesville, FL

ABSTRACT- Recent studies revealed that tropical forest productivity may be limited by low light availability during the rainy season. However, determining how reductions in photosynthetically active radiation (PAR) influence forest productivity requires careful evaluation because of the non-linear response of photosynthesis to light. To obtain accurate estimates of carbon gain we need to describe temporal distribution of PAR due to cloud cover. The objectives of this study were: (1) determine how PAR frequency distributions change in days with different degree of cloud cover; and (2) develop the basis for a model to predict PAR frequency distributions as a function of daily cloudiness. I took measurements of PAR (mPAR) above the canopy of a Panamanian tropical dry forest. A data logger registered incoming light from two sensors, every two seconds, for 302 days. Second, I calculated theoretical PAR (tPAR), that is, light intensity in cloudless conditions; daily cloudiness was calculated as the ratio daily integrated mPAR / daily integrated tPAR. The results indicate that distribution of mPAR (mol m^-2 s^-1) is related to the daily cloud cover in a complex but predictable pattern. Distributions of ratios mPAR / tPAR also changed consistently with cloudiness, and showed that the proportion of direct vs. obstructed sunlight in a day explained most changes in PAR frequency distributions. Thus, frequency distributions of light can potentially be estimated from single measurements of integrated daily PAR.

KEY WORDS: photosynthetically active radiation, par, tropical forest, primary productivity