Photosynthetic light environments in a lowland tropical rain forest in Costa Rica

(1) Measurements of photosynthetic photon flux density (PPFD) in four sites within a lowland tropical rain forest were compared. The habitats investigated were a 0.5 ha clearing, a 400 m2gap, a 200 m2gap, and heavily shaded understorey. (2) Measurements were made during both wet and dry seasons under a variety of weather conditions. Quantum sensors were used to monitor continuously 10-min average PPFD over a 3-10 day period at each location. Daily average PPFD, total daily PPFD, and daily frequency distributions were analysed for two adjacent sensors per site on each sampling day. (3) Daily total PPFD in the understorey, 200 m2gap, and 400 m2gap were 1-2%, 9%, and 20-35%, respectively, of PPFD in the clearing. Daily total PPFD in the 400 m2gap during the dry season was, on average, 2.4 times greater than in the 200 m2gap, and 20-25 times those in the understorey. In the 200 m2gap, daily total PPFD was nine times greater than in the understorey during the dry season. (4) In the clearing, PPFD was significantly different between seasons with 24% higher PPFD during the dry season. In the 400 m2gap and understorey, PPFD was not significantly different between seasons. (5) The percentage of available PPFD reaching the understorey was highest on cloudy, overcast days and lowest on sunny days. No correlation was found between daily total PPFD in the 400 m2gap centre and in the adjacent understorey measured on the same days. (6) In the clearing, a high proportion of 10-min averages were greater than 500 μ mol m-2s-1; in the 400 m2gap, a high proportion of 10-min averages were between 100 μ mol m-2s-1and 500 μ mol m-2s-1; in the understorey over 70% of the 10-min averages were below 10 μ mol m-2s-1. The clearing exhibited the greatest diurnal variation in PPFD and the least day-to-day variation, whereas the understorey exhibited the least diurnal variation and the greatest day-to-day variation in PPFD.