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Woody plant encroachment is an ongoing global issue. In the Southern Great Plains of the United States, the rapid encroachment and coalescence of woody plants are transforming herbaceous-dominated rangelands into woodlands with a detrimental impact on water quality and quantity. In this study, we conducted modeling simulations to assess how converting juniper (Juniperus virginiana) woodland and low to moderately productive grassland into switchgrass (Panicum virgatum) biomass production system would affect streamflow and sediment yields in the Lower Cimarron River, Oklahoma. First, the grassland areas in the basin were divided into productivity classes suitable for rangeland activities based on the soil productivity index. Next, the Soil and Water Assessment Tool was used to develop the basin hydrologic model, calibrated and validated for streamflow in five gaging stations with a percent bias of <10%, Nash–Sutcliffe Efficiency index of >0.76, and R2 of >0.77. Then, the model was used to simulate evapotranspiration (ET), streamflow, groundwater recharge, and sediment loads under different land use conversion scenarios. Results showed that converting existing juniper woodlands, ~4% of the basin, to switchgrass had limited impacts on the water budget and sediment yield. A hypothetical scenario of converting low to moderately productive rangeland to switchgrass increased annual ET by 2.6%, with a decrease in streamflow by 10.8% and a reduction in sediment yield by 39.2% compared to the baseline model. Results indicated that switchgrass could be considered a potential land use alternative to address the juniper encroached grassland with minimal loss in streamflow but a substantial reduction in sediment yield in the southcentral region of the Great Plains. [ABSTRACT FROM AUTHOR]

The southern Great Plains of the USA has great potential to produce biofuel feedstock while minimizing the dual stresses of woody plant encroachment and climate change. Switchgrass (Panicum virgatum) cultivation, woody biomass captured during removal of the encroaching eastern redcedar (Juniperus virginiana) to restore grasslands and thinning of the native oak forest can provide an integrated source of feedstock and improve ecosystem services. In north‐central Oklahoma, we quantified productivity and ecosystem water use of switchgrass stands and degraded ecosystems encroached by eastern redcedar and compared these to native oak forest and tallgrass prairie ecosystems. We measured aboveground net primary productivity (ANPP) using allometric equations (trees) and clip plots (herbaceous), and evapotranspiration (ET) using a water balance approach from gauged watersheds, and calculated water use efficiency (WUE = ANPP/ET) from 2016 to 2019. Among vegetation cover types, ANPP averaged 5.1, 5.4, 6.0, and 7.8 Mg ha−1 year−1 for the prairie, oak, eastern redcedar, and switchgrass watersheds and was significantly greater for switchgrass in 2018 and 2019 (2 and 3 years post establishment) when it reached 8.6 Mg ha−1 year−1. Averaged across 2017–2019, ET was significantly greater in the forested watersheds than the grassland watersheds (1022 mm year−1 for eastern redcedar, 1025 mm year−1 for oak, 874 mm year−1 for prairie, and 828 mm year−1 for switchgrass). The mean WUE was significantly greater (9.47 kg ha−1 mm−1) for switchgrass than for the prairie, eastern redcedar, and oak cover types (6.03, 6.02, and 5.31 kg ha−1 mm−1). Switchgrass offered benefits of greater ANPP, less ET, and greater WUE. Our findings indicate that an integrated biofuel feedstock system that includes converting eastern redcedar encroached areas to switchgrass and thinning the oak forest can increase productivity, increase runoff to streams, and improve ecosystem services. [ABSTRACT FROM AUTHOR]

• Pulsed increase in sediment load follows mechanical removal of juniper. • Sediment load declines rapidly during the first growing season. • Converting juniper to grasslands increases water yield without sediment increase. 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. [ABSTRACT FROM AUTHOR]

• Paired watershed is effective in studying runoff response to woody encroachment. • Rooting zone soil moisture increased after converting juniper woodland to grassland. • Juniper removal in central Great Plains resulted in fivefold increase in runoff. Mechanical removal of encroached juniper (Juniperus spp.) is a common practice to restore native grasslands. However, the hydrological responses to grassland restoration remain mostly unquantified for the climate transition zone in the southern Great Plains of the USA where ecosystem evapotranspiration is highly sensitive to the change of vegetation functional type. We used a paired watershed approach to directly quantify the impact of mechanical removal of eastern redcedar (Juniperus virginiana L., redcedar) and restoration to native prairie or establishment of switchgrass (Panicum virgatum L.) on root zone soil moisture and event-based runoff for eight years including three main phases - calibration, transition, and restored grassland in north-central Oklahoma, USA. Results showed that the root zone soil water storage on average increased 1.6 and 1.9 times for restored prairie and switchgrass, respectively, after juniper removal. The regression model estimation based on the relationships established in the calibration phases between event-based runoff from control watershed and the treatment watersheds found that the restored prairie and switchgrass production system increased annual runoff by 4.46 and 4.54 times, respectively. These results indicated that both soil moisture and runoff are highly responsive to land use change in the southern Great Plains. Reestablishment of herbaceous dominance by mechanically removing encroached woody species is closely followed by restoration of soil moisture dynamics and watershed runoff regime. [ABSTRACT FROM AUTHOR]

The meteorological droughts in the climate transition zone of the Great Plains of the USA are projected to intensify, potentially leading to major shifts in water provisioning services in rangelands. To understand how meteorological drought interacts with vegetation to regulate runoff response, we collected precipitation, root zone soil moisture, and runoff data from experimental grassland and juniper (Juniperus virginiana L., redcedar) woodland watersheds for five years encompassing a drought year to pluvial year cycle. We contrasted the frequency distribution of precipitation intensities and applied wavelet analysis to reveal the coherence between precipitation and root zone soil moisture patterns. Compared with grassland, the root zone soil moisture in woodland had a narrower range, with the peak frequency skewed to a lower soil moisture content. The conversion of herbaceous vegetation to evergreen juniper woodland results in a delayed response of runoff to precipitation due to reduced antecedent soil moisture. The reduction of streamflow from the woodland watershed was greater in the normal and pluvial years than in the drought year. Thus, conversion from grassland to evergreen woody vegetation prolongs the impact of meteorological drought on soil moisture and streamflow. Restoring prairie that is heavily encroached by woody species may serve as an adaptive measure to mitigate the climate change impact on water resources and other ecosystem services provided by rangeland. [ABSTRACT FROM AUTHOR]

In the Great Plains of the central United States, water resources for human and aquatic life rely primarily on surface runoff and local recharge from rangelands that are under rapid transformation to woodland by the encroachment of Eastern redcedar (redcedar; Juniperus virginiana) trees. In this synthesis, the current understanding and impact of redcedar encroachment on the water budget and water resources available for non-ecosystem use are reviewed. Existing studies concluded that the conversion from herbaceous-dominated rangeland to redcedar woodland increases precipitation loss to canopy interception and vegetation transpiration. The decrease of soil moisture, particularly for the subsurface soil layer, is widely documented. The depletion of soil moisture is directly related to the observed decrease in surface runoff, and the potential of deep recharge for redcedar encroached watersheds. Model simulations suggest that complete conversion of the rangelands to redcedar woodland at the watershed and basin scale in the South-central Great Plains would lead to reduced streamflow throughout the year, with the reductions of streamflow between 20 to 40% depending on the aridity of the climate of the watershed. Recommended topics for future studies include: (i) The spatial dynamics of redcedar proliferation and its impact on water budget across a regional hydrologic network; (ii) the temporal dynamics of precipitation interception by the herbaceous canopy; (iii) the impact of redcedar infilling into deciduous forests such as the Cross Timbers and its impact on water budget and water availability for non-ecosystem use; (iv) land surface and climate interaction and cross-scale hydrological modeling and forecasting; (v) impact of redcedar encroachment on sediment production and water quality; and (vi) assessment and efficacy of different redcedar control measures in restoring hydrological functions of watershed. [ABSTRACT FROM AUTHOR]