Light regimes beneath closed canopies and tree-fall gaps in temperate and tropical forests

Light regimes beneath closed canopies and tree-fall gaps are compared for five temperate and tropical forests usingfish-eye photography of intact forest canopies and a model for calculating light penetration through idealized gaps.Beneath intact canopies, analyses of canopy photographs indicate that sunflecks potentially contribute 37-68% of seasonaltotal photosynthetically active radiation. In all of the forests, potential sunfleck duration is brief (4-6 min), but thefrequency distributions of potential sunfleck duration vary because of differences in canopy geometry and recent distur-bance history. Analysis of the photographs reveals that incidence angles for photosynthetically active radiation beneathclosed canopies are not generally vertical for any of the forests, but there was considerable variation both among andwithin sites in the contribution of overhead versus low-angle lighting. Calculations of light penetration through idealizedsingle-tree gaps in old growth Douglas-fir - hemlock forests indicate that such gaps have little effect on understorylight regimes because of the high ratio of canopy height to gap diameter. However, single-tree gaps in the other fourforest types produce significant overall increases in understory light levels. There is also significant spatial variationin seasonal total radiation in and around single-tree gaps. Our results demonstrate that there can be significant penetra-tion of light into the understory adjacent to a gap, particularly at high latitudes. As gap size increases, both the meanand the range of light levels within the gap increases, but even in large gaps (ca. 1000 m2) the potential duration ofdirect sunlight is generally brief (