GRADIENT ANALYSIS OF LATITUDINAL VARIATION IN SOUTHERN ROCKY-MOUNTAIN FORESTS

Vegetation samples collected near Taos, New Mexico, Wet Mountain Valley, Colorado, and Rocky Mountain National Park, Colorado, were used to analyse latitudinal variation in Southern Rocky Mountain forests. Objectives were: (1) to examine latitudinal shifts in species and community positions with respect to local environmental gradients; (2) to compare between regions the factors correlated with local gradients in species composition; and (3) to illustrate the power of gradient-based methods for the analysis of large-scale geographic variation in vegetation. Dominant compositional gradients at the three localities were related to elevation, site moisture status, and potential solar radiation. The average elevation of floristically similar stands declined with increasing latitude between Taos and Wet Mountain Valley (-46.5 m per degree latitude), and between Wet Mountain Valley and Rocky Mountain National Park (-106.6 m per degree latitude). Elevation shifts between localities varied with the reference stand's position along the elevation and moisture gradients. Smaller elevation shifts between localities were found for widespread species. Species were, in general, most widely distributed along the elevation gradient at Rocky Mountain National Park and along the moisture gradient at Taos. High-elevation Picea engelmannii/Abies lasiocarpa forests were comparatively similar in the three regions, although the number of recognizable communities per region increased with latitude. In contrast, there was considerable latitudinal change in the composition of low-elevation forests. Several commonly accepted geographic patterns showed considerable complexity when examined with change in latitude. Although species richness (species per 0.1 ha) is generally considered to decline with increasing latitude, highest species richness was found at Wet Mountain Valley, the middle locality on the latitudinal sequence. However, relative species richness patterns along the elevation and moisture gradients were similar for all three localities.