Grazing management effects on environmental quality of riparian and upland grassland ecosystems.

摘要

Grazing cattle in grasslands can impact many ecosystems services including the movement of sediment and nutrients to water bodies, biodiversity, and wildlife habitat. In riparian grassland ecosystems congregation of cattle in or near streams may increase the sediment, nutrient, and pathogen loading of surface water resources, however the impact of cattle on water bodies may be limited through pasture characteristics or management practices that reduce congregation of cattle in or near streams. The first study in this thesis was designed to determine the effects of pasture size, stream access, and off-stream water on the presence of cattle near pasture streams. In the first study the effects of an off-stream water site or limiting the stream access of cattle to stabilized sites on the presence of cattle in or near a pasture stream was measured in small (4.0 ha) and large (12.1 ha) pastures. Limiting stream access of cattle to stabilized sites reduced presence of cattle in or near streams. However, providing off-stream water sites affected congregation of cattle in or near streams relatively little. Regardless of management treatment, presence of cattle in and near the pasture stream was reduced in pastures with a larger proportion of grazing land outside of the riparian zone. As temperatures increased, the probability of cattle spending time in and near the pasture stream or tree shade increased, with a greater probability of presence in riparian shade occurring in small pastures. In upland grassland ecosystems, cattle grazing at elevated stocking densities has the potential to improve plant diversity, carbon sequestration, and wildlife habitat through soil disturbance, incorporation of plant litter into the soil profile, and removal of aboveground forage. A second study was designed to determine the effects of a single spring grazing event at two stocking densities with or without subsequent rotational grazing on plant community properties, soil characteristics, and wildlife habitat in upland grasslands. Soil structural characteristics, proportion of plant species, and wildlife habitat were measured following no grazing or a single grazing event at elevated stocking densities with or without subsequent rotational grazing. Grazing at elevated stocking densities during periods of heavy rainfall reduced the proportion of cool season grass species for 14 months allowing succession of annual grass followed by legume species. However, after 14 months the proportion of cool season grass species returned to pre-grazing levels. The maximum height with 50% visual obstruction from vegetation was reduced for 12 months following grazing, but there were few subsequent differences. Although a single spring grazing event at either a high or moderate stocking density during periods of heavy rainfall increased soil bulk density, penetration resistance to a depth of 10 cm, and bare ground, grazing at a higher stocking density had less impact on soil structural characteristics likely because of a shorter stocking duration. Further research is necessary to determine if shade can be used to influence cattle distribution in pastures and rangelands in addition to more comprehensive research on the effects of periodic grazing at elevated stocking densities on soil aggregate stability, soil organic carbon, soil erosion, and wildlife habitat in Midwest grassland ecosystems.