Brigham Young University Science Bulletin, Biological Series


An analysis has been made of the basic ecology of aspen forests of the mountainous areas of Utah and northern Arizona. Forty-nine study areas have been examined in respect to density, height and age of the tree cover, tree reproduction, and composition and biomass production of the understory. A graphic model of interspecific assocations among prevalent understory species has been prepared by a cluster-analysis procedure based on presence and absence of species in 1,225 small quadrats (0.25 m2) uniformly distributed among the 49 stands. The model separates species of relatively stable as opposed to rapidly seral aspen forests. It also tends to separate species known to respond differently to grazing.

Gross environmental variables as well as soil profile characteristics were recorded at each site. Several soil characteristics were determined in the laboratory. Analyses were performed to determine which, if any, environmental variables differed significantly between sites stratified according to site-quality for aspen. Only the pH of the A1 horizon and aspect were found to be different.

Site-quality was determined for 43 stands using natural site index curves. Average site index for these stands at 80 years of age was estimated to be 15.8 m (52 feet with a range of 30-75 feet). Understory species significantly associated with stands of high or low site-quality were identified by comparing quadrat frequency of 53 herbaceous species in groups of stands stratified according to site-quality. Fifteen species were found to be significantly associated (either positively or negatively) with site-quality.

An index for predicting site-quality for aspen was devised using those species demonstrated to have indicator value. The degree of correspondence between natural site indices and the understory site indices for the 43 stands was evaluated by simple correlation. The correlation coefficient for the test was +0.70 (coefficient of determination, r2, of 0.49). Also, understory production had a significantly positive correlation with site-quality for aspen. Thus, sites having the best site-quality for aspen also produce the greatest amount of understory forage. Prospects are good that the index can be refined to yield more precise estimates.