Gradient structure of forest vegetation in the central Washington cascades
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Forest vegetation located in three areas of the central Washington Cascades, arrayed along a gradient of increasing continentality and decreasing rainfall, were compared using ordination methods. Within each region, lower and upper elevation sites were analyzed separately and for each set of sites, trees and ground story vegetation were analyzed independently. Principal components analysis, reciprocal averaging, weighted averaging, and polar ordination were applied to each set. The characteristics of the data determined which method gave the most readily interpretable results, but RA and WA usually best ordered the stands along a complex, combined coenocline, while PO often decomposed the gradient into moisture and temperature components. PCA was of little use, even with relatively low beta diversity.
Results are presented in the form of stand by species tables for each data set and the most appropriate two dimensional ordination. Both are correlated to the classification. A mosaic diagram for each region is synthesized from these analyses and habitat data. These diagrams indicate that community types occupy a smaller portion of the habitat as continentality increases.
The ordination results were in close agreement with our earlier classification of these stands. The maritime (west) region contains stands of low richness and with relatively little coenocline differentiation. In contrast, the continental (east) region has high alpha and beta diversity. Species in the west tend to occupy a broad portion of the available habitat range, while species in the east do not. The analyses reveal that ordinations by different strata may produce similar stand sequences if both strata are responding directly to the same factors, but that the correspondence degenerates where the understory responds primarily to the nature of the canopy dominant species. Thus stand sequence correlations are highest in the lowland eastern region and lowest in the lowland western region.
This study demonstrates that none of these metric ordination methods is fool-proof and that none should be used exclusively or in isolation. RA and PO are demonstrated to be useful general methods; WA gave results similar to those of RA. PCA never produced uniquely superior results. Analyses with too few species and moderate beta diversity often produce distortions as pronounced as those with many species and high beta diversity.
KeywordsConiferous forest Ordination Polar ordination Principal components analysis Reciprocal averaging Vegetation pattern Vegetation structure
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