| Interspecific competition and the availability of resources are important forces in the development and maintenance of biotic communities. The relationships between competition and resource availability to produce the current structure and composition of communities have been a source of opposing views in ecology. Few empirical studies have addressed this issue at the scale of the community in and systems. Knowledge of the processes (including species interactions) that affect plant communities is essential for large-scale management of species, communities, and ecosystems.; I sought evidence of a relationship between shrub association and the degree of water stress to which communities are subject in an and environment. My research hypothesis was that association between species in similar resource-use guilds should be less pronounced in xeric than in more mesic sites. This was expected if past competition was most intense under conditions of high water deficit (i.e., where the resource was least abundant).; Nonparametric, bivariate linear correlations, and a multivariate direct gradient analysis were used to compare patterns of association between shrub species on a gradient of water deficit across 49 sites. In the bivariate analyses, the hypothesis was supported by significant results for only 1 of 38 pairs of species. Correlations were significant for two additional pairs for which correlations with water deficit were opposite to the relationship hypothesized. The majority of species pairs showed no significant relationship between degree of association and site water deficit. In the multivariate analysis, an overall significant relationship was found between the first ordination gradient, which was strongly and positively related to water deficit, and levels of association. Environmental factors other than water deficit were also moderately correlated with the first axis and other directions of variation in the data set, but variation for most species pairs was not well-explained by the first axis. Considered together, the study results showed that while a strong relationship between association and water deficit exists for a few species, relationships are, overall, neither consistent in the direction of correlation, nor do they hold generally for most pairs of species classified into the same resource-use type. |
