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 Washington, 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 environmental and biotic factors which influence diversity at local and landscape scales. Plant species richness and diversity were generally lower in communities characterized by environmental extremes (i.e., excessive or scarce moisture and severely cold high elevation) than in mesic environments at low-to-middle elevations. Evenness among plant species was greatest at higher elevations, where severe climate limited the ability of any single or group of species to dominate. Species turnover rates were also higher near the environmental extremes. High turnover rates among mountain hemlock associations were attributed to highly variable topography and local microclimates which resulted in substantial geographical isolation and species specialization among site types. Moisture appeared to have the most influence on species richness (alpha diversity) and sequential turnover rates (beta diversity) at high elevations, where available water is seasonally limited by low temperatures. Temperature had the greatest influence on overall species turnover (gamma diversity) throughout the landscape. Species richness of the landscape (epsilon diversity) was quite accurately predicted (within 7% for temperature and moisture gradients) by a computational method which uses independent measures of alpha, beta and gamma diversity. Patterns of forest plant diversity appear to be the result of environmental conditions at larger scales and the complex interactions among biological and physical variables at smaller scales, within an historical context of stochastic disturbance events. Managers wishing to restore and sustain the high diversity characteristic of these old-growth coniferous forests will need to consider the respective roles of various biophysical factors and the disturbance dynamics in these unique ecosystems.