COS 303-6 - Shrub encroachment decouples ecohydrological linkages and depletes surface water in tallgrass prairie

Abstract: The spread of woody species in grass-dominated ecosystems (woody encroachment) is occurring in grasslands globally. This process leads to reductions in biodiversity and forage availability for grazers and can alter carbon and nutrient cycling. Mesic grasslands in the central USA are primarily experiencing encroachment by clonal shrubs (i.e., Cornus drummondii, Rhus glabra). Shrub encroachment can also impact water cycling, particularly in mesic grasslands where precipitation typically exceeds evapotranspiration and soil water pools recharge during the dormant season. Ecohydrological impacts of woody encroachment have been studied more extensively in grasslands with encroaching trees (i.e., Juniperus sp.), but clonal shrubs have different water-use strategies that will likely result in unique impacts on grassland ecohydrology. In this work from Konza Prairie Biological Station (northeastern KS, USA), our primary goal was to better understand the impacts of shrub encroachment on soil water storage and stream discharge in tallgrass prairie by combining ecophysiological and hydrological approaches. In a series of studies, we (1) assessed plasticity of depth of water uptake and leaf-level physiology in C. drummondii (rough-leaf dogwood) across individual growing seasons, (2) estimated changes in landscape-scale water loss as woody cover increases, and (3) assessed long-term changes in stream discharge and soil moisture as woody encroachment has intensified over the last 30-40 years.

We found that C. drummondii adjusted depth of water uptake within individual growing seasons to take advantage of surface soil moisture when it was available and avoid competition with grasses as surface soils began to dry. Access to deeper soil moisture allowed C. drummondii to maintain remarkably stable photosynthetic rates, even during dry conditions when leaf water potentials declined. This ability, along with generally higher rates of canopy transpiration in shrubs compared to grasses, has likely increased deep soil water use. Using long-term measurements of soil moisture, we report decreased deep soil moisture in shrub-encroached areas. Depletion of soil moisture by shrubs has reduced the amount of water available to recharge groundwater and support streamflow in this system. As encroachment has progressed, correlations between precipitation and streamflow have weakened – despite increased annual precipitation over the last 4-5 decades, stream discharge has declined significantly. These results indicate that shrub encroachment has directly and indirectly decreased soil water storage and streamflow in tallgrass prairie. As this process occurs over large portions of the central USA, shrub encroachment is likely to have substantial negative impacts on local and regional water yields.