Subsurface Lateral Flows Buffer Riparian Water Stress Against Snow Drought

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.

Canopy structure modulates the sensitivity of subalpine forest stands to interannual snowpack and precipitation variability

“…The dynamic observed by Martin et al (2018)s supports a view of hillslope ecohydrology where subsurface connections increase downslope and this determines water use patterns in a way that may supersede species traits (Brooks et al, 2015). However, it also should be noted that elevational position along the hillslope may be less important to forest water use than sub-hillslope context such as proximity to local convergence zones where surface connections to groundwater are locally critical to support tree water demand (Hoylman et al, 2018;Graup et al, 2022). In contrast, other studies have found limited differences in water use patterns across the hillslope reflecting multiple interacting influences emerging from species traits, stand properties and hillslope hydrology that ultimately mute gradients in sensitivities to changing precipitation inputs (Fabiani et al, 2022).…”

“…The primary goal of this study was to test whether the sensitivity of widespread Rocky Mountain tree species to changing snowpack is related to reliance on snowmelt as a primary water source (Graup et al, 2022;Tague and Peng, 2013;Strange et al). Although many studies have independently tracked water sources by trees with stable isotopes spatially (e.g.…”

“…We consolidate the numerous processes acting across sites by collapsing the measurement clusters along a simple gradient of their canopy density that has been shown to be a useful aggregator of multiple interacting hillslope processes (Metzen et al, 2019). In the absence of disturbance, the density of sapwood to conduct water from an area reflects the long term tree acclimation to available water and sets the peak rates of water use (Graup et al, 2022;Tor-Ngern et al, 2017;Pataki et al, 2000). In snow dominated montane and subalpine areas of the western US, stand density thus may reflect snowmelt access because summer rain alone is insufficient to support high rates of water use and investment in conducting wood tissue (Goodwin et al, 2023).…”

Quantifying hydrological responses to monsoon‐controlled precipitation across the soil‐groundwater‐stream continuum with long‐term high‐frequency hydrometric monitoring

“…Short‐duration heavy rainstorm events driven by typhoon exerted a pulsing control mainly on groundwater level and soil moisture content (Sawyer et al, 2014; Shih et al, 2018; Zhang et al, 2022). Total amount and duration of monsoon‐controlled precipitation shaped water‐table rise, producing vertical linkage with unsaturated zone and lateral linkage with stream water in riparian zones (Graup et al, 2022; Scaife et al, 2020). Soil moisture content variations of middle and deep layers were closely related to groundwater level fluctuations (Figures 3b,c and 9e,f), because water table usually fluctuated near these two layers.…”