Abstract

--- - In the Sierra Nevada, CA, the Mediterranean climate exposes montane forests to water stress during the summer drought. Normally, spring snowmelt alleviates summer water stress, especially in riparian ecosystems that receive subsurface lateral inputs from groundwater. However, snow drought could potentially eliminate these beneficial effects. This research investigates how subsurface lateral redistribution mediates hillslope-scale vegetation responses to snow drought. We apply a spatially-distributed ecohydrologic model (Regional Hydro-Ecologic Simulation System (RHESSys)) to a snow-dominated watershed in the Sierra Nevada. We incorporate observed sap flow data along a hillslope to estimate relative differences in water stress for upslope and riparian sites, and constrain RHESSys drainage parameter uncertainty. Our model results show that subsurface lateral inputs buffer riparian water stress against snow drought. For all drought types, both upslope and riparian sites experience substantial losses of net primary productivity (NPP), and on average upslope sites are more adversely affected (upslope loss of NPP = 45% vs. riparian = 28%). Dry droughts from a lack of rain or snow induce substantial loss of biomass for upslope and riparian trees because of low annual precipitation, but a small snowpack retained during cold, dry years slightly alleviates these effects and delays water stress by 2 weeks in riparian areas and 3 weeks upslope. While riparian forests are often buffered against drought stress, our study also shows that riparian forests can suffer from water stress. We found 21% of drought scenarios where riparian trees experience greater water stress than upslope trees that were mostly associated with wet and warm snow droughts. - Plain Language Summary Climate change is increasing the frequency and severity of droughts. Combined with higher temperatures, droughts are causing widespread forest mortality. Drought-induced tree death is especially relevant to the California Sierra Nevada, which experiences wet winters and dry summers. The mountain snowpack is essential for storing winter snow that becomes spring snowmelt, which in turn fills soil water stores for trees before the summer drought. However, rising temperatures are preventing the snowpack from accumulating even in wet years, causing warm snow droughts. Here we seek to understand the consequences of changing snowpacks on forest water stress by using a computer model to explore how a tree's position on a hillslope influences its drought response. Riparian trees near the river benefit from access to lateral inputs from groundwater that provide supplemental water during seasonal drought, as compared to upslope trees that do not receive lateral inputs. Our results show that generally these lateral water inputs buffer riparian trees against drought stress though, due to loss of the snowpack, there are cases where riparian trees experience greater water stress than upslope trees. As droughts become more frequent and severe, riparian trees may be the only source of refuge in a parched landscape.