Application of non-equilibrium ecology to rangeland riparian zones

Traditional theories of plant succession leading to a single equilibrium community are being re-evaluated. Alternative theories involving multiple steady states, and state-and-transition processes have been postulated to more adequately reflect the dynamics of rangeland ecosystems. The ecological literature provides examples of apparent thresholds in arid and semi-arid plant communities, however the literature is void of discussion of the applicability of non-equilibrium ecological theory to riparian areas contained within the rangelands of the world. In arid and semi-arid environments the availability of soil water is critical in the determination of the composition of the plant community. In this study we hypothesized that the relationship between soil moisture and depth to groundwater within the riparian zone controlled the composition of the associated plant communities. These soil water, groundwater, plant community composition relationships were used to test the applicability of state and transition models to riparian zones. Water table levels within an irrigated eastern Oregon riparian valley were monitored for 2 consecutive summers. The study area was mapped into 4 distinct plant community types on the basis of dominant graminoids. We measured depth to the water table, soil moisture content, relative species composition, litter, percent bareground and percent relative basal cover of key plant species and life forms. Relationships between water table levels, soil moisture content and plant communities were analyzed. Results indicated the 4 plant communities contained within this study area can be segregated on the basis of soil moisture content and/or depth to groundwater during the growing season. Ecological states and transition zones based on soil moisture content and/or water table depth were determined.