Spatial autocorrelation of solar radiation in a tropical moist forest understory

Several indices of solar radiation were estimated from hemispherical canopy photographs of mature forest on Barro Colorado Island, Panamá. The photographs were taken during two wet seasons at 400 uniformly spaced stations on two 1-km transects. In an El Niño year, mean daily global radiation was positively autocorrelated from 2.5 to 12.5–22.5 m when light penetration was abnormally high due to drought-induced leaf fall in the previous dry season. In a more typical year, weak positive spatial autocorrelation occurred only between adjacent sites 2.5 m apart. Other radiation indices, including the proportion of unobstructed sky in the canopy on the solar path, showed correlograms nearly identical to that of global radiation in the El Niño year. Spectral analysis revealed a period of 125 m in mean daily global radiation along a transect in the El Niño, but not in the normal year.