Latitudinal Variation in Southern Rocky Mountain Forests

A latitudinal sequence of vegetation diagrams is presented, each relating the forest vegetation of a region of the southern Rocky Mountain Physiographic Province to elevation and moisture gradients. Geographical trends in forest composition and their elevational zonation are described and the importance of possible competitive interaction of dominant tree species is discussed. While description of distinctive vegetation zones which change in elevation with changing latitude is useful, this incorporates neither variation in zonal composition or interaction of elevation with other environmental factors. For example, low-elevation vegetation types of the southern portion of the transect, xeric Pinus edulis, Juniperus monosperma woodlands or mesic ravine forests, change in composition, become attenuated and are eventually lost northward. Neither are species composition or forest structure constant within a zone. Abies lasiocarpa is absent from the timberline forests at the southern end of the transect, whereas it is the dominant timberline species at the northern end. A number of possible cases of competitive displacement are suggested by the vegetation diagrams. The competitive relationship between the two major successional species of the Rocky Mountains, Pinus contorta and Populus tremuloides, has long been unclear. The vegetation diagrams suggest that when Pinus contorta is present in the area, it tends to preempt a central portion of the habitat space of Populus tremuloides. When in competition with Pinus contorta, Populus is confined to peripheral portions of its potential range, whereas in the absence of Pinus contorta, Populus is often the dominant suc- cessional species over a broad range of sites. White pines (Pinus subgenus haploxylon) provide several examples of apparent competitive release. Where Pinus flexilis is the only white pine present, it occupies xeric sites from the montane forests to tree line, but where either Pinus aristata or P albicaulis is present, the species is largely restricted to the low-elevation portion of its potential habitat, and to situations with very rocky substrate. Pinus flexilis also appears to substitute for Pinus contorta or Pinus ponderosa where either species is absent. Many aspects of vegetation composition can best be understood if viewed in geographic perspective; a gradient analytic approach frequently facilitates interpretation of vegetation-environment interactions. The combination of these two approaches provides a useful method for examining aspects of species distribution and community structure.