LIGHT REGIMES BENEATH CLOSED CANOPIES AND TREE-FALL GAPS IN TEMPERATE AND TROPICAL FORESTS

Beneath intact canopies, analyses of canopy photographs indicate that sunflecks potentially contribute 37-68% of seasonal total photosynthetically active radiation. In all of the forests, potential sunfleck duration is brief (4-6 min), but the frequency distributions of potential sunfleck duration vary because of differences in canopy geometry and recent disturbance history. Incidence angles for photosynthetically active radiation beneath closed canopies are not generally vertical, but there was considerable variation both among and within sites in the contribution of overhead versus low-angle lighting. Calculations of light penetration through idealized single-tree gaps in old growth Douglas-fir - hemlock forests indicate that such gaps have little effect on understory light regimes because of the high ratio of canopy height to gap diameter, but single-tree gaps in the other four forest types produce significant overall increases in understory light levels. There is also significant spatial variation in seasonal total radiation in and around single-tree gaps. There can be significant penetration of light into the understory adjacent to a gap, particularly at high latitudes. As gap size increases, both the mean and the range of light levels within the gap increases, but even in large gaps (1000 m2) the potential duration of direct sunlight is generally <4 h. the major differences in gap light regimes are largely a function of canopy height and latitude. -from Authors