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 year in the understorey of a tropical wet forest along a transect extending from a treefall gap to the closed canopy at La Selva Biological Station, Costa Rica. Quantum sensors attached to data loggers were operated continuously. Hemispherical photographs were taken monthly above the sensors. An additional quantum sensor was used to monitor PPFD outside the canopy. Sensor measurements show large differences between gap and closed-canopy locations and high daily and seasonal variability at each sensor location, with notable increases in solar radiation in gap stations during September and April. The photographs demonstrate that seasonal variation in PPFD results primarily from shifts in the solar angle relative to canopy openings and secondarily from variation in PPFD levels outside the canopy (cloudiness). Photographs also demonstrate an overall decrease in PPFD with time, for the gap stations, owing 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 photography to permit estimation of PPFD with a high degree of reliability.