|HOME SCHEDULE AUTHOR INDEX SUBJECT INDEX|
Long-term changes in species richness, diversity and eveness in a northeastern Oregon riparian area.
Green, Douglas*,1, Johnson, Douglas2, 1 Arizona State University East, Mesa, AZ, USA2 Oregon State University, Corvallis, OR, USA
ABSTRACT- Species richness and diversity of undisturbed sites have been suggested as one of many metrics to evaluate the health or condition of managed ecosystems. We compared species richness, species diversity (H'), and species evenness (J') over a twenty year period from eight late season grazed and ungrazed plant communities in a northeastern Oregon riparian zone. Communities measured were dry meadows, moist meadows, cheatgrass (Bromus tectorum) Douglas hawthorn (Crataegus douglasii), ponderosa pine (Pinus ponderosa), black cottonwood (Populus tricocarpa), thinleaf alder (Alnus incana), gravel bars. Six stands of each community were measured with thirty 25X25cm quadrats in late June or July of 1979, 1980, 1987, 1989, and 1999. Mean species richness was greatest on gravel bars (33.0), moist meadows (28.4), Douglas hawthorn (24.9), thinleaf alder (24.9), dry meadow (21.0), ponderosa pine (21.0, black cottonwood (18.9) and cheatgrass (17.0). Gravel bars and moist meadows had the highest mean H' (3.03 and 2.75 respectively) and mean species evenness (J') 0.8732 and 0.8640 respectively). Lowest mean diversity and evenness was found in black cottonwood (H' = 2.36, J' 0.8229) and cheatgrass (H' 2.22, J' 0.8196). With the exception of dry meadow communities no community had a consistent grazing effect over the study period. This may be due to application of sustainable stocking rates. Large year-to-year variation in species richness, diversity, and evenness independent of the grazing treatment may be due to study design, proper stocking rates, climatic variations or random error. The large year-to-year variation observed raises questions about the usefulness of the selected indicators to evaluate ecosystem health in this riparian system.
Key words: oregon, long-term, riparian, grazing