Background/Question/Methods   Light penetration through tree crowns is a key determinant of understory regeneration patterns and competitive interactions within forest communities.  Mechanistically, the penetration of light through the canopy is driven by the density and geometry of optical elements (leaves and branches), and the optical properties of these elements.  Recent survey results have found large size-related decreases in leaf area index and light penetration in some canopy tree species, suggesting that gradual senescence, rather than gap formation and closure, may drive forest dynamics in some late-seral forest communities.  

Results/Conclusions     I present data to evaluate the generality of size-dependent light penetration and its determinants in a temperate forest ecosystem, using a combination of ground-based surveys of canopy openness, high-density LiDAR remote sensing, and, for select species, analysis of leaf spectral optical parameters.  Species show variable patterns of size-dependent canopy openness (and derived estimates of leaf area index): more shade-tolerant species, such as Acer saccharum, commonly show increased canopy openness with tree size, while some less shade-tolerant species show the opposite pattern.  Relative crown depth (crown depth over tree height) decreased with tree size in most species, but slopes of this relationship vary greatly.  Leaf spectral transmittance of photosynthetically active radiation decreased with tree size, though this effect is relatively small. Linear model analyses indicate that although the species terms are larger, size and size by species interactions are highly significant and account for ~1/3 of explained variance in canopy openness, crown depth, and leaf transmittance. Current “mechanistic” forest simulation models may be over-generalizing species-specific parameters for light extinction effects by not accounting for these important and predictable size-dependent effects.