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

The suitability of several methods for estimating light conditions in the u nderstorey of tropical forests, and of different sampling schedules was eva luated. Eight conditions at 16 understorey sites in a Panamanian loll land forest were continuously measured for 9 mo with quantum sensors and photodi odes. Light conditions at the sites were also assessed indirectly with hemi spherical fisheye photographs, plant canopy analysis, SX-mm photographs, 24 -mm photographs and a spherical densiometer. Estimates from all indirect me thods, except the spherical densiometer, were highly correlated with the di rect measurements. Short-term direct light measurements for a day or a week also correlated with long-term light conditions. The indirect measures dif fered by up to c. 70% from the direct measures relative to single site meas urements. Hence, the indirect methods are inadequate where single site ligh t conditions have to be assessed accurately. However, because light conditi ons encountered in the understorey varied up to 13-fold, the indirect metho ds 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 betw een accuracy and sampling effort.