Measurement and modeling of spatially explicit variation in light transmission through interior cedar-hemlock forests of British Columbia

We have characterized canopy geometry and light transmission by the nine do minant conifer and broad-leaved tree species of the interior cedar-hemlock (ICH) forests of northern British Columbia. Our field data were used to par ameterize a spatially explicit model of light transmission through mixed-sp ecies forests. That model, a component of the forest dynamics simulator SOR TIE, was developed for eastern deciduous forests, and this paper presents a test of that model in a very different ecosystem. Our results show that in dividual crowns of the ICH forests intercepted much more light than species of eastern deciduous forests but that the canopy as a whole allowed greate r light penetration, largely because of openings between the relatively nar row, conical crowns of the western conifers. Light transmission by individu al crowns was correlated with shade tolerance among the conifers (as in eas tern deciduous species), but crown depth was not (in contrast with eastern species). Despite the fundamental differences in the nature of light transm ission in the two ecosystems, the SORTIE light model developed for eastern deciduous forests was effective at predicting spatial variation in understo ry light levels in these western coniferous forests. The goodness of fit of such a simple model suggests that the most important factors regulating sp atial variation in understory light levels in these forests are simply the sizes and distribution of nearby trees, and the local sky brightness distri bution. Discrete canopy gaps represent a special case in which a region of the canopy is not occupied by crowns.