Response of sediment concentration and load to removal of juniper woodland and subsequent establishment of grasslands – A paired experimental watershed study

Abstract

Encroachment of woody plants into grasslands is a global phenomenon of environmental concern. Mechanical removal is often necessary to re-establish herbaceous dominance for heavily encroached watersheds, but its impact on water quality and quantity of runoff into streams and reservoirs has not been vigorously studied. The sediment concentration and load following mechanical removal of juniper (Juniperus virginiana, L.) woodland and subsequent re-establishment of tallgrass prairie or switchgrass (Panicum virgatum L.) biomass production were quantified at the experimental watershed scale in the south-central Great Plains, USA. Impact analysis was used to evaluate the effects of watershed treatment and phase of land use conversion. The annual sediment yield from juniper woodland watersheds averaged<0.10 t ha−1 yr−1 before treatment and increased to 0.28 t ha−1 yr−1 after juniper were cut and left on site. In the second year, removing the dried trees increased the annual sediment yield to 1.14 t ha−1 yr−1 in the prairie restoration watershed and 13.29 t ha−1 yr−1 in the switchgrass watershed that was sprayed with herbicide in preparation for no-till planting. The annual sediment loads averaged 0.44 t ha−1 yr−1 from the restored prairie and 0.29 t ha−1 yr−1 from the established switchgrass, comparable to 0.73 t ha−1 yr−1 from the intact juniper woodland during the third and fourth years after initiation of treatments.While restored grassland watersheds had elevated peak flows and longer flow duration leading to greater runoff, lower sediment yields were due to reduced mean and peak sediment concentration compared to the juniper watershed.Therefore, restoring juniper woodland to native prairie or switchgrass biomass production systems may increase water yield without increasing sediment yield, especially in years with extreme storm events.