Photosynthetic Light Environments in a Lowland Tropical Rain Forest in Costa Rica

Daily total photosynthetic photo flux density (PPFD) in the understorey, 200 m2 gap, and 400 m2 gap were 1-1%, 9%, and 20-35% respectively, of PPFD in the clearing. Daily total PPFD in the 400 m2 gap during the dry season was, on average, 2.4 times greater than in the 200 m2 gap, and 20-25 times those in the understorey. In the 200m2 gap daily total PPFD was 9 times greater than in the understorey during the dry season. In the clearing, PPFD was significantly different between seasons with 24% higher PPFD during the dry season. In the 400 m2 gap and understorey, PPFD was not significantly different between seasons. In the clearing, a high proportion of 10-min averages were greater than 500 mu mol m-2s-1; in the 400 m2 gap, a high proportion of 10-min averagegs were between 100 mu mol m-2s-1 and 500 mu mol m-2s-1; in the understorey over 70% of the 10-min averages were below 10 mu 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.-from Authors