Influence of Managed Forest Structure on Hillslope-scale Post-thinning
Recovery and Water Yield
Abstract
Forest management can enhance forest resiliency against natural
disturbances such as fire, drought, or disease. Mechanical thinning,
followed by a prescribed burn, is a useful technique to achieve a
desired forest structure, usually maximizing large tree basal area or
decreasing fuel loads, meant to protect against wildfire or reduce water
stress in the western US. Changing forest structure can alter ecosystem
function by reducing competition and exposing soil, modifying
microclimates and creating suitable conditions for shrubs and grasses to
encroach. Typically, forest treatments are expected to make the
remaining trees more productive through competitive release, and an open
canopy helps the understory to thrive. This enhanced plant water use
often contradicts the expected result of increased streamflow following
thinning. In mountainous terrain, water yield is further complicated by
hillslope-scale processes of subsurface lateral flow and groundwater
recharge. This research seeks to understand how management-derived
forest structure influences hillslope-scale forest regrowth and water
yield. We apply a spatially-distributed ecohydrologic model (RHESSys) to
an experimental hillslope in the Sierra Nevada, CA. We incorporate
multi-temporal Lidar observations and U.S. Forest Service Forest
Inventory & Analysis (FIA) survey data to estimate post-thinning
regrowth in treated plots in the watershed, which is used to verify
RHESSys accuracy of vegetation regrowth. Then, we run long-term virtual
thinning experiments to understand how the combination of thinning and
prescribed burns in upslope and riparian sites separately and
concurrently influences regrowth and water fluxes in these sites. We
expect that an intermediate forest density will yield the most
co-benefits in terms of carbon sequestration and water yield. However,
these patterns will likely be modified along a hillslope, such that
riparian forest stands will be less sensitive to the competitive release
that thinning provides, whereas dense upslope forests will be highly
sensitive to treatment since they are more water-limited. Water yield is
likely to be confounded by multiple factors, including topography,
whether a burn follows thinning to remove understory fluxes, and
interactions between upslope thinning and processes of lateral flow and
groundwater recharge when increased riparian water use compensates for
additional upslope subsidies.