Abstract
The entire western US is in the midst of a megadrought. Combined with
high temperatures, increasingly severe droughts are causing widespread
forest mortality. In the Sierra Nevada, CA in particular, the
Mediterranean climate exposes montane forests to water stress due to the
summer drought. Normally, the slow melting of the winter snowpack helps
to alleviate summer water stress, especially in riparian ecosystems that
benefit from subsurface lateral inputs along a hillslope. However, the
loss of the snowpack due to snow drought could potentially eliminate
these buffering effects. This research aims to address the role of
subsurface lateral redistribution in mediating vegetation responses to
drought along a hillslope. We apply a spatially-distributed
ecohydrologic model (RHESSys) to an experimental hillslope in a
snow-dominated watershed in the Sierra Nevada, CA. We incorporate
observed sap flow data from the experimental hillslope to estimate the
relative differences in onset of water stress for upslope and riparian
sites, which is used to constrain RHESSys drainage parameter
uncertainty. Then, we run hypothetical multi-year drought experiments to
investigate how climate variability translates to water stress on a
hillslope. Our results challenge the common assumption that riparian
forests are buffered against drought stress by subsurface lateral
inputs. For all drought types, both upslope and riparian sites
experience severe losses of net primary productivity (NPP), and on
average upslope sites are more adversely affected (upslope loss of NPP =
50% vs. riparian = 35%). But even in a wet year, as temperatures rise
and the snowpack disappears (i.e., warm snow drought), vegetation
approaches a threshold response that destabilizes the riparian buffering
effect. Our results show that for 12% of all scenarios, riparian NPP
decreases more than upslope NPP, as a consequence of earlier snowmelt.
Interactions between climate variability and ecophysiological
uncertainty produce scenarios that exhibit the riparian threshold
response. By recognizing the conditions that determine riparian
sensitivity to drought, management actions can be proactive in
preserving this important hydrological refugia.