Spatial and temporal patterns of light and canopy structure in a lowland tropical moist forest

Computer-assisted image processing of hemispherical photographs of the forest canopy was used to study the light environment of a mature lowland moist tropical forest in the Republic of Panama. Wet and dry-season measurements were made in 1983 and 1984 near ground level at 2.5 m intervals along two 1-km long transects; additional measurements were made at a single site at 5 heights from forest floor to above-canopy, and in 10 experimentally produced treefall gaps in which understory herb responses to gap formation were also monitored. Mean daily global radiation along the transects showed no distinct dichotomy corresponding to "gap" versus "understory" habitats. Light measurements showed the effect of seasonality in rainfall, and year-to-year correlation was low, indicating the need for frequent assessment of the light environment for long-term studies of plant responses. Various measures of total irradiance (e.g., canopy openness, minutes of potential sunflecks, total solar energy) were highly correlated. However, the correlation of canopy openness in the eastern and western octants of the hemispherical images was low, indicating that the time of day in which direct sunlight occurs may differentiate microsites with similar daily total light levels. In treefall gaps, the light environment was more heterogeneous one year after treefalls; but, two years after the treefall, light levels were both lower and more homogeneous. Both spatial and temporal complexity in the light environment suggest that "gap" versus "understory" characterizations are too simplistic.

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