Southwestern riparian plant communities: Site characteristics, tree species distributions, and size-class structures

Abstract

Elevation, stream direction, stream gradient, and valley cross-sectional area affect the distribution of riparian tree species and community types in the southwestern United States. Riparian plant communities in Arizona and New Mexico were examined at 153 sites and classified into 28 community types. Canonical discriminant analysis was used to examine variability of physical site characteristics both within and among community types. Elevation was the most significant factor along the first canonical axis in analyses for all community types, all riparian forest community types, and all riparian scrub community types. Both stream gradient and stream direction also correlated significantly, but on a more limited and local level. Tree population size structures and tree species distribution were studied both on sites where species were dominant and at other sites where they occurred. The dominance and/or codominance of some species, such as Arizona walnut ( Juglans major (Torr.) Heller), was directly related to site characteristics such as elevation and direction of stream flow. For example, on west-flowing streams, Arizona walnut was codominant with Arizona sycamore ( Platanus wrightii Wats.), while on east-flowing streams, sycamore was absent and walnut dominated stands.