Succession and Livestock Grazing in a Northeastern Oregon Riparian Ecosystem

Comparisons of vegetation dynamics of riparian plant communities under livestock use and exclusions over a 10 year period were quantified in a Northeastern Oregon riparian zone. We measured species frequency, richness, diversity, evenness, and livestock utilization in 8 plant communities. Livestock grazed the study area from late August until mid September at a rate of 1.3 to 1.8 ha/AUM. Utilization varied from > 70% in dry meadows to < 3% in cheatgrass dominated stands. Ungrazed dry and moist meadow communities had significantly lower (P lesser than or equal to 0.1) species richness and diversity when compared to grazed counterparts. In the most heavily grazed communities, ruderal and competitive ruderal species were favored by grazing disturbance. In exclosures of the same communities, competitive or competitive stress tolerant species were favored. Both height and density of woody riparian species were significantly greater in ungrazed gravel bar communities. Our results indicate that influences of herbivory on species diversity and eveness varies from 1 community to another and basing management recommendation on 1 component ignores the inherent complexity of riparian ecosystems.