Cloud Effects on Photosynthetically Active Radiation, Tropical Forest Canopy PAR Absorption and Photosynthesis in West Kalimantan, Indonesia

This study combines field measurements of total and diffuse photosynthetically active radiation (PAR, 400-700 nm) with a 3-D model of forest canopy radiative transfer to investigate the effects of atmospheric scattering by clouds on the surface PAR regime, canopy PAR absorption and photosynthetic rates at an observation site in Putussibau, West Kalimantan Province, Indonesia (0.84N, 112.93E). A time-series of PAR measurements collected from December 2006 to July 2007 is characterized by large diffuse fractions and frequent occurrences of enhanced total and diffuse irradiance, occasionally exceeding the extraterrestrial flux, which result from reflectance and scattering by the broken cloud fields that typify this tropical site. Simulations of canopy-absorbed PAR and gross photosynthesis with the Forest Light Environmental Simulator (FLiES) demonstrate the potential role of clouds in controlling forest canopy light-use efficiency and rates of photosynthesis and vegetation- atmosphere carbon exchange in this moist tropical environment.