The distribution and abundance patterns of species are of primary interest in ecology, and the interactions between an organism and its abiotic and biotic environment provide a basis for a better understanding of the mechanisms by which distribution and abundance patterns are governed. Physiological ecology provides an ideal platform for integrating the effects of both biotic and abiotic influences upon species performance, thus conferring a process-oriented insight into ecological patterns. This dissertation considers the physiological characteristics that may be related to Mojave Desert floodplain domination by Tamarix ramosissima, an exotic invasive riparian plant; Four woody riparian species---Tamarix ramosissima, Salix exigua, Prosopis pubescens and Pluchea sericea---were studied along the Virgin River in southern Nevada. Through field observation and manipulative experiments in the field and glasshouse, the relationships between plant water relations, competition, facilitation, and abiotic stress and environment were investigated at both proximate (i.e., process) and ultimate (i.e., pattern) levels; Tamarix dominated the Virgin River floodplain at a previously burned site (Half-Way Wash) within 50 years following fire. Physiological mechanisms for the success of Tamarix include its water use (on both leaf- and whole branch-levels), starch storage, early investment in additional ramets, and slow radial growth. Pluchea, a native, summer-active halophyte, dominates early seres through tolerance to extreme abiotic stresses, as indicated by the general unresponsiveness of physiological characteristics to the imposition of stress. Salix, which is also native, demonstrated radial growth patterns consistent with sensitivity to drought stress, demonstrated sensitivity to heat and drought stresses in many key physiological traits, and was relatively insensitive to the presence of other species; Studies of plant responses to stress yield a great deal of information concerning the relationships, both inhibitive and facilitative, between species. Furthermore, the relationships between abiotic stress, inhibition, facilitation, and chance environmental events (e.g., aseasonal flooding) contributes to further understanding the short- and long-term mechanisms involved in the successful invasion of the Virgin River floodplain by Tamarix ramosissima
