LONG-TERM STUDY OF SOLAR-RADIATION REGIMES IN A TROPICAL WET FOREST USING QUANTUM SENSORS AND HEMISPHERICAL PHOTOGRAPHY

Daily photosynthetic photon flux density (PPFD) was monitored for 1 ye ar in the understorey of a tropical wet forest along a transect extend ing from a treefall gap to the closed canopy at La Selva Biological St ation, Costa Rica. Quantum sensors attached to data loggers were opera ted continuously. Hemispherical photographs were taken monthly above t he sensors. An additional quantum sensor was used to monitor PPFD outs ide the canopy. Sensor measurements show large differences between gap and closed-canopy locations and high daily and seasonal variability a t each sensor location, with notable increases in solar radiation in g ap stations during September and April. The photographs demonstrate th at seasonal variation in PPFD results primarily from shifts in the sol ar angle relative to canopy openings and secondarily from variation in PPFD levels outside the canopy (cloudiness). Photographs also demonst rate an overall decrease in PPFD with time, for the gap stations, owin g to vegetation regrowth. Analyses reveal excellent agreement between sensor PPFD measurements and estimates from hemispherical photographs. Long-term monitoring of PPFD enables calibration of hemispherical pho tography to permit estimation of PPFD with a high degree of reliabilit y.