Evaluation of different methods to estimate understorey light conditions in tropical forests

The suitability of several methods for estimating light conditions in the understorey of tropical forests, and of different sampling schedules was evaluated. Light conditions at 16 understorey sites in a Panamanian lowland forest were continuously measured for 9 mo with quantum sensors and photodiodes. Light conditions at the sites were also assessed indirectly with hemispherical fisheye photographs, plant canopy analysis, 38-mm photographs, 24-mm photographs and a spherical densiometer. Estimates from all indirect methods, except the spherical densiometer, were highly correlated with the direct measurements. Short-term direct light measurements for a day or a week also correlated with long-term light conditions. The indirect measures differed by up to c. 70% from the direct measures relative to single site measurements. Hence, the indirect methods are inadequate where single site light conditions have to be assessed accurately. However, because light conditions encountered in the understorey varied up to 13-fold, the indirect methods were found to be well suited to rank understorey light conditions among a large number of sites. The results from frequent and infrequent sampling schedules differed only slightly, suggesting that taking indirect measures at the beginning and the end of a study offers a reasonable compromise between accuracy and sampling effort.