Spatial and covariate-varying relationships among dominant tree species in Utah

The presence and establishment of a tree species at a particular spatial location is influenced by multiple physiological and environmental filters such as propagule pressure (seed availability), light and moisture availability, and slope and elevation. However, a less understood environmental filter to species-specific establishment is competition or facilitation between dominant tree species. For example, certain tree species may compete for resources at spatial locations where such resources are scarce while less competition may occur at resource-rich areas. Using data from the Forest Inventory and Analysis (FIA) program of the United States Department of Agriculture (USDA) Forest Service, we develop a multivariate spatial Bernoulli model to investigate the space-varying relationship between extant tree species in Utah. Additionally, we propose a novel modeling strategy that explains the spatially varying relationships by regressing the associated between-species correlation matrix on available covariate data. Positive definite conditions of the covariate-varying correlation matrix are ensured by defining the regression in terms of the unique partial correlation matrix. Results indicate that correlations between species are dependent upon elevation.