CAUSES AND CONSEQUENCES OF RESOURCE HETEROGENEITY IN FORESTS - INTERSPECIFIC VARIATION IN LIGHT TRANSMISSION BY CANOPY TREES

We have analyzed the light transmission characteristics of the nine de ciduous and coniferous species that dominate the transition oak - nort hern hardwood forests of southern New England. Maximum likelihood tech niques were used to estimate species-specific light extinction coeffic ients, using fish-eye photography combined with data on the locations and geometry of trees in the neighborhood around each photo point. Qua ntum sensors were also used to quantify interspecific variation in the importance of sunflecks and beam enrichment. Variation in light extin ction was closely correlated with shade tolerance and successional sta tus of the species. The most shade-tolerant species (Fagus grandifolia Ehrh. and Tsuga canadensis (L.) Carr.) cast the deepest shade (<2% of full sun), while earlier successional species such as Quercus rubra L . and Fraxinus americana L. allowed greater light penetration (>5% ful l sun). These differences were more closely related to differences in crown depth than to differences in light extinction per unit depth of crown. Sunflecks contributed relatively little radiation beneath late successional species (<10% of total understory photosynthetically acti ve radiation), but represented a major fraction (40-50%) of radiation beneath less shade-tolerant species. Using growth and mortality functi ons for the same species developed in a related study, our results ind icate that saplings of all of the species have high survivorship in th e shade cast by conspecific adults. However, only the three most shade -tolerant species have low rates of sapling mortality under the low li ght levels characteristic of stands dominated by late successional spe cies. Our results are consistent with previously reported models, whic h propose that secondary succession is driven by interspecific differe nces in resource uptake and tolerance.