Graduate Thesis Or Dissertation
 

An analysis of montane forest vegetation on the east flank of the central Oregon Cascades

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  • Montane forest vegetation as it occurs on the east flank of the central Oregon Cascades has provided excellent conditions for a "natural experiment" in the use of various methodologies in studying vegetational distribution. This "experiment" has reflected on some theory and practice for the discipline of plant synecology. Detailed descriptions, analysis, and interpretation of the data also document present conditions in relation to the past and allow prognostication of future changes, which in turn may be of silvicultural importance for a portion of this forest type found extensively on the east flank of the Sierra Nevada-Cascades cordillera. A major objective of this study was to determine the relative merits and deficiencies of attempting to transpose to this vegetation the methods of analyzing vegetation based on the individualistic or continuum philosophy of phytosociology that has developed and been practiced principally in the north-central United States. These attempts have been contrasted to strengths and short-comings of poly-climax theory, the most widely used basis of vegetational classification in the Pacific Northwest, in relation to the analysis of this and other vegetation types. The influences on and of these varying interpretations have been outlined and presented in tabular form. The study area is nearly ideal for posing these questions because edaphic and topographic factor complexes remain surprisingly uniform. The vegetation is superimposed in apparent primary response to the condensed gradient of total precipitation due to the orographic "rain shadow" effect from the Cascades intervening in this region of prevailing westerly winds. The stability of the vegetation concomitant with this nearly ideal set of "naturally controlled" physical conditions, minimized variations in successional status, except that due to fire exclusion. This latter variation, however, paralleled the complex gradient studied, a happenstance which allowed silviculturally important interpretations to be made. Stable vegetation occurring in general contiguity allowed stands to be sampled systematically, leaving little doubt concerning the validity of interpolation between stands and the areal representation of the samples. Another main objective of the study was to obtain and analyze data to substantiate the hypothesized influence of light periodic ground fires in initiating and maintaining the characteristic mosaic of size-age class distribution of ponderosa pine. The data also yielded a quantitative indication of the shift in species composition and dominance that is in largest measure due to the continued exclusion of fire by man for approximately the past 50 years. From these data the future appearance of the forest can be surmised, and silvicultural manipulations can be suggested that are best in harmony with the ecology of these forests. The circumstances that provided this "natural experiment" have allowed the author to demonstrate certain limitations to direct transposition of methods widely used in other areas of the United States, or even the Pacific Northwest, when description and analysis in closest feasible parallel to the nature of the patterns of vegetational distribution found in this area are attempted. This finding catalyzed the development of a method incorporating cluster analysis of matrices of relative parameter-weighted coefficients of association into a means of making very objective synecological delimitations. The dendrograms derived from this analysis allow a "sliding scale" of stratification to be made in this vegetation of most realistically intermediate, yet of more continuously variable than unit-association nature.
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