Light occlusion at forest edges: an analysis of tree architectural characteristics

We examined the phenomenon of tree canopy closure by lateral branch expansion at the edges of temperate forest fragments bordering fields, in southwestern Ontario, Canada. We evaluated the relationship between tree crown characteristics and the degree of light reduction (a measure of canopy closure) among six tree species of differing shade tolerance (Fagus grandifolia Ehrh., Acer saccharum Marsh., Quercus rubra L., Prunus serotina Ehrh., Juglans nigra L. and Populus grandidentata Michx.). Tree species differed in their capacity to occlude light: the most effective were the most shade tolerant species, F grandifolia and A. saccharum, while the most shade intolerant trees, J. nigra and P grandidentata, had the highest percentage of light transmission. The most shade tolerant species presented most of their branches and foliage towards the field and resulted in the most voluminous crowns with the largest values in branch density, branch clumping, canopy depth and canopy width. The best model to predict light occlusion was obtained when we considered the effect of each species as a factor, and foliage biomass facing the field as covariates; this explained 80% of the total variation in light transmission. The ability of shade tolerant trees to enlarge their crowns in response to an increase in light availability allows them to occlude light better. To maximize the protection of microclimates in forest fragments, shade tolerant trees should be encouraged at forest edge communities by favoring recruitment or by selective thinning of the least shade tolerant species. (C) 2001 Elsevier Science B.V. All rights reserved.