Document: STE-3-40-53

The extent and composition of conifer invasion of aspen dominated sites on the San Francisco Peaks of Arizona in relation to variations in slope, elevation, and aspect.

WATHEN, S.* and MEMBERS OF ECOLOGY GRADUATE GROUP

University of California, Davis, CA 95616 USA ¹

Abstract:
Elevation, slope and aspect are known to affect the distribution of vegetation. C. H. Merriam described the vegetation of the San Francisco Peaks in 1890, delineating seven "life zones" that changed in elevation depending upon exposure and the mass of the mountain involved. A more recent soils report described plant associations on the San Francisco Peaks that differed in elevation from those described by Merriam. This study investigates the distribution of conifers on sites moderated by the presence of an aspen overstory and compares this distribution with the distribution of vegetation as described by Merriam and the San Francisco Peaks soil survey. Plots were placed along transects through aspen stands located on the San Francisco Peaks and measurements made for aspen basal area, slope, elevation and aspect; followed by measurements of basal area and stem density, by size class, for each conifer species present. This information was later graphed so as to portray variation in basal area and density by species, slope, elevation and aspect. Slope, elevation, and aspect were found to be important factors in determining the distributions of conifers within aspen type on the San Francisco Peaks. The moderating influence of aspen results in a lowering of the elevation ranges of conifer species, within versus outside aspen type. The results of this study do not support the elevation ranges of conifers as described by Merriam. The results do support the continuum theory of vegetation as described by R. H. Whittaker. The ranges of various species also appear to be present on warmer, and perhaps more xeric sites, at the upper end of their elevation ranges. The data suggests that conifer establishment is more affected by site conditions than competition on the San Francisco Peaks and that conifer species, although depauperate in number due to relaxation since the end of the Pleistocene, are very finely tuned to specific site conditions; exhibiting narrowly defined niches, at least as far as establishment within aspen stands is concerned.

Keywords: white fir, aspen, ponderosa pine, Douglas-fir, corkbark fir, bristlecone pine, Englemann spruce, white pine, San Francisco Peaks, Merriam, Whittaker, continuum, site conditions, elevation, aspect, slope, Whittaker

This abstract is being presented at: 3:30 PM in session:
Poster Session #17: Vegetative Analysis.