Response of riparian areas of the shortgrass steppe to release from cattle grazing using exclosures: Comparison at different spatial and temporal scales

Background/Question/Methods

Human impact on grasslands through cattle grazing has had a profound influence on plant structure and composition, but little work has been conducted on the shortgrass steppe to determine how riparian areas respond to cessation of cattle grazing.  To investigate this question, a seasonal and a multi-year study were conducted on the Pawnee National Grassland.  Species composition and functional composition were compared for areas exposed to grazing and areas released from grazing.  Plant richness, evenness and diversity was calculated for each treatment and compared over the study period.  In addition, soil nutrient levels and biomass levels were compared over one grazing season.  We hypothesized that species and functional composition would differ in areas released from grazing and that overtime differences will become greater.  We also predicted that areas released from grazing would have greater aboveground biomass and species diversity but display less evenness.  We did not think soil nutrient levels would differ over one grazing season.

Results/Conclusions

Results from the seasonal study showed areas exposed to grazing had statistically less aboveground biomass and greater heterogeneity than non-grazed areas. However, no differences in diversity were observed from this short-term study. The multi-year study showed that species composition varied comparing grazed to un-grazed areas from the onset but demonstrated similar directional movement for diversity and species composition.  This suggested other environmental factors, perhaps hail, had a greater impact on the vegetation community than grazing.  Release from grazing caused the species composition to change more than continual grazing and contributed to an increase in species evenness.  Functional composition started and remained different for the treatments, but within treatments did not change significantly.  This study demonstrates variability of species composition from year to year on the shortgrass steppe and supports the idea that grazing has minimal impact in grasslands adapted to low levels of precipitation.