Modeling the distributions of shrub species in Pacific Northwest forests

In Pacific Northwest forests, understory species are important elements of forest structure and composition, providing habitatand forage for wildlife and contributing to much of the floral diversity. Knowledge of the factors that control the distributionand abundance of understory species is central to understanding the ecology and dynamics of natural forests and how they willrespond to management. Generalized linear models of the binomial family were used, at two spatial scales, to estimate theprobability of occurrence of shrub species as a function of environmental variables and the cover of mature and regeneratingtree species. Graphical analysis and resampling techniques were used to: II) examine whether species show unimodalresponses to environmental gradients: (2) explore the scale-dependence of model forms; and (3) predict areas of highprobability of species occurrence within a multi-dimensional environmental space. Results suggest that the concept ofunimodal response to environmental gradients can be used to improve predictions about species' distributions, and tostrengthen the biological interpretation of statistical models. They also demonstrate the scale-dependence of model equationsand the likelihood that key processes influencing plant distributions may change, or appear different, when viewed at differentscales. (0 1999 Elsevier Science B.V. All rights reserved.