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Groundwater-supported vegetation refugia as a mechanism of forest recovery in a Rocky Mountain watershed impacted by wildfire

Active Dates 9/1/2022-8/31/2025
Program Area Environmental Systems Science
Project Description
Under a warming climate, forests in the Rocky Mountains of North America are experiencing significant ecohydrological shifts including reduced snowpack, decreased soil moisture, and multiple vegetation disturbances from drought, insect outbreak, and wildfire. After a fire, healthy tree stands and their crucial role as a seed source are more likely found in areas with shallow water tables and high soil moisture. These groundwater-supported vegetation refugia have also sheltered trees against multiple disturbances, which compels the need to improve our predictive understanding of their location and function in order to maximize the efficiency of land management resources to conserve them. While shallow water table has been identified as a key contributor to refugium formation at the scale of individual tree stands, processes that control water table dynamics at the landscape (i.e., disturbance) scale remain unclear. In a sloping landscape, water table depth (wtd) is controlled by both local and long-distance groundwater flow within a groundwater flow system (GFS). Refugia supported by local groundwater are sensitive to precipitation variability and are less stable while converging local and long-distance groundwater outflows can support persistently shallow water tables that act as long-term vegetation shelters. These hidden, albeit dynamic, GFSs are hypothesized to drive the trajectory of forest recovery after wildfire and other disturbance: refugia can exhibit a range of stability depending on wtd and its temporal persistence. Besides topography, precipitation, and evapotranspiration, wtd is strongly influenced by subsurface geological heterogeneity which can channel, deflect, and trap groundwater at unexpected locations. Thus, groundwater-supported vegetation refugia are hypothesized to form at both headwater and downstream regions in a landscape. The objective of this research is to test groundwater-supported vegetation refugia as a mechanism of forest recovery after wildfire by jointly examining spatiotemporal drivers that influence groundwater and vegetation. Using a recently burnt Rocky Mountain watershed as a testbed, we propose to collect co-located microclimate, vegetation, and hydrological measurements at five refugium sites along a landscape gradient encompassing a range of wtd and disturbance severity. Field measurements will be used to parameterize, test, and extend a distributed ecohydrological model that couples a new vegetation recovery module to a soil and groundwater model that considers lateral and vertical flow in a GFS. During this model-data integration, parameter and model structural deficiencies will be identified to inform field samplings and model development. Model verification will also be carried out by collecting additional measurements of ecosystem water use and status at a refugium site with vegetation growth data used as an independent test of the ecohydrological model. Given climate change and associated precipitation shifts in the Western mountains, we also hypothesize that existing refugia can be weakened or strengthened depending on how climate change feedbacks drive wtd and its persistence. Therefore, after the model is calibrated against historical (including proxy) and new field measurements, it will be used to project wtd and refugium trajectories in the study area under a range of climate change scenarios. Through quantitative examination of the interaction between GFS and vegetation, the proposed research will improve our predictive understanding of the change, response, and trajectory of forest recovery at the landscape scale. This study will (1) generate insights into groundwater-supported vegetation refugia that can be generalized to other sloping landscapes, (2) test, refine, and enhance wildfire recovery modeling capability of a coupled ecohydrological model, and (3) inform post-fire ecosystem assessments that require knowledge and information at the disturbance scale. Data and models generated in this research will be made available to researchers and other stakeholders. Besides disseminating the study findings through publications and presentations, the participle investigators will present actionable results to government agencies overseeing post-fire remedial actions.
Award Recipient(s)
  • University of Wyoming Laramie (PI: Zhang, Ye)