Dg. Brockway, FOREST PLANT DIVERSITY AT LOCAL AND LANDSCAPE SCALES IN THE CASCADE MOUNTAINS OF SOUTHWESTERN WASHINGTON, Forest ecology and management, 109(1-3), 1998, pp. 323-341
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.