Competition and Abiotic Stress Among Trees and Shrubs in Northwest Colorado

Abstract

We tested several hypotheses about the relationship of competition to abiotic stress, using the vegetation of the semiarid Piceance Basin of northwestern Colorado. We studied competition among the shrubs Amelanchier utahensis, Artemisia tridentata, and Symphoricarpos oreophilus, and between the trees Pinus edulis and Juniperus osteosperma, in 10 sites. We calculated several indices of biotic moisture stress, based on the slope, aspect, and evaluation of each site. Competition was measured by regression of the distance separating neighboring plants vs. the sum of their canopy areas. The slope of such a regression (if significant and positive) measures the intensity of competition, and its coefficient of determination (r 2 ) measures the importance of competition, between the plants involved. We detected competition among these plants in all but one combination of species and in most sites. No significant differences in the intensity of competition were found within species combinations. Significant differences in the importance of competition were found in one of three interspecific combinations of shrub species, and in two of three combinations of tree species. Neither the intensity nor the importance of competition showed any consistent relationship with any index of abiotic moisture stress. Thus, no hypothesized relationship between abiotic stress and competition is supported. Our data also show no consistent relationship between the importance of competition and its intensity, supporting our hypothesis that the intensity and the importance of competition are independent.