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, r ichness, diversity, evenness, and livestock utilization in 8 plant com munities, Livestock grazed the study area from late August until mid S eptember 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 a nd moist meadow communities had significantly lower (P less than or eq ual to 0.1) species richness and diversity when compared to grazed cou nterparts. In the most heavily grazed communities, ruderal and competi tive ruderal species were favored by grazing disturbance, In exclosure s of the same communities, competitive or competitive stress tolerant species were favored, Both height and density of woody riparian specie s were significantly greater in ungrazed gravel bar communities, Our r esults indicate that influences of herbivory on species diversity and eveness varies from 1 community to another and basing management recom mendation on 1 component ignores the inherent complexity of riparian e cosystems.