Unexpected hydrologic response to ecosystem state change in tallgrass prairie

Abstract

In grasslands around the world, climate change is occurring in tandem with woody encroachment (the spread of woody vegetation in grass-dominated ecosystems). State transitions from grassland to shrub/woodland have been identified aboveground via changes in species cover and composition, but the hydrological impact of these transitions is not well understood. Shifts from grass- to woody-dominance have the potential to impact evapotranspiration, soil moisture dynamics, and groundwater recharge. Therefore, it is possible that the consequences of aboveground vegetation change may be observable in the hydrological system. We leveraged long-term hydrological records from a tallgrass prairie site in northeastern Kansas, USA, to examine how concurrent changes in climate and land-cover have altered hydrological dynamics over the last century. Stream discharge has declined in recent decades despite > 100 years of climate wetting. The relationship between incoming precipitation and streamflow has weakened over the last 40 years, suggesting that shifts in the physical landscape are altering patterns of hydrological connectivity. Long-term isotope records show a divergence in the isotopic composition of precipitation (no change in δ18O) and stream/groundwater (decreasing δ18O) over the last decade. These results suggest that woody encroachment is accelerating the hydrological cycle, potentially by decreasing groundwater recharge (via increased evapotranspiration) and/or increasing infiltration rates (via creation of macropores). Holistically, these changes illustrate the interdependence of above- and below-ground processes in the local hydrological cycle, and the cascading long-term consequences (decades to centuries) for critical zone function once woody encroachment has occurred.