FOREST PLANT DIVERSITY AT LOCAL AND LANDSCAPE SCALES IN THE CASCADE MOUNTAINS OF SOUTHWESTERN WASHINGTON

Old-growth forests in the Pacific Northwest are known to support high levels of diversity across the varied landscapes they occupy. On 1200 plots distributed over the Cascade Mountains in southwestern Washingto n, climatic, physiographic, edaphic and floristic data were collected to evaluate the ecological characteristics of these coniferous forests and develop a classification framework useful in land management. The resulting abundance of data provided a unique opportunity to analyze plant diversity relationships at multiple levels and identify the envi ronmental and biotic factors which influence diversity at local and la ndscape scales. Plant species richness and diversity were generally lo wer in communities characterized by environmental extremes (i.e., exce ssive or scarce moisture and severely cold high elevation) than in mes ic environments at low-to-middle elevations. Evenness among plant spec ies was greatest at higher elevations, where severe climate limited th e ability of any single or group of species to dominate. Species turno ver rates were also higher near the environmental extremes. High turno ver rates among mountain hemlock, associations were attributed to high ly variable topography and local microclimates which resulted in subst antial geographical isolation and species specialization among site ty pes. Moisture appeared to have the most influence on species richness (alpha diversity) and sequential turnover rates (beta diversity) at hi gh elevations, where available water is seasonally Limited by low temp eratures. Temperature had the greatest influence on overall species tu rnover (gamma diversity) throughout the landscape. Species richness of the landscape (epsilon diversity) was quite accurately predicted (wit hin 7% for temperature and moisture gradients) by a computational meth od which uses independent measures of alpha, beta and gamma diversity. Patterns of forest plant diversity appear to be the result of environ mental conditions at larger scales and the complex interactions among biological and physical variables at smaller scales, within an histori cal context of stochastic disturbance events. Managers wishing to rest ore and sustain the high diversity characteristic of these old-growth coniferous forests wilt need to consider the respective roles of vario us biophysical factors and the disturbance dynamics in these unique ec osystems. (C) 1998 Elsevier Science B.V.