The Signature of Snow Drought: A Spatially-Connected Approach to Understanding Forest Water Stress

Globally, and especially in Mediterranean-type ecosystems (MTEs), forests are increasingly vulnerable to drought stress, leading to high rates of mortality. Global climate models project increased drought frequency and severity, with higher temperatures leading to snow droughts. Warm snow droughts are produced by warming temperatures that prevent precipitation from accumulating on the landscape as a snowpack. Dry snow droughts have very little rain or snow. Any drought reduces water availability and increases vegetation water stress. But in the complex topography of mountain environments, spatial patterns of drought stress and subsequent tree mortality are influenced by snowmelt and subsurface lateral redistribution. Along a hillslope, snowmelt induces hydrologic connectivity, enhancing groundwater recharge and lateral flows. Riparian vegetation benefits from these upslope subsidies, which makes riparian trees more productive but also more sensitive to climate variability. This research seeks to understand, what is the implication of snow drought for hillslope ecohydrology? Using observed sap flow data taken along a topographic gradient in an experimental watershed in the Sierra Nevada, CA, we calibrate an ecohydrological model (RHESSys) to consider the effects of climate, geology, and forest management on riparian water stress. We demonstrate that riparian trees are buffered against drought stress by lateral inputs at our groundwater-dominated study site. But riparian forests without a significant groundwater influence will not be so fortunate. I am proposing an international collaboration between US and EU ecohydrologists to understand the conditions that lead to greater riparian water stress in Mediterranean ecosystems and determine potential solutions to protect these sensitive hydrological microrefugia.