Groundwater-Supported Vegetation Refugia as a Mechanism of Forest Recovery in a Rocky Mountain Watershed Impacted by Disturbances

Authors

Xiaonan Tai¹, Brent Ewers², Ye Zhang²* (yzhang9@uwyo.edu), Andrew Parsekian²

Institutions

¹New Jersey Institute of Technology, Newark, NJ; ²University of Wyoming, Laramie, WY

Abstract

Forest disturbances are increasing globally due to the anthropogenic impact on land use and climate. For a rocky mountain watershed impacted by bark beetle kills and wildfire, this research investigates groundwater-supported vegetation refugia as a mechanism of forest recovery at local to landscape (i.e., disturbance) scales. In the project’s study area, two priority forest sites have been identified where active post-disturbance regeneration was observed, each representing an endmember in climate, dominant tree species, topography, and disturbance type/severity. The headwater site (Glacier Lakes Ecosystem Experiments Site) is an Engelmann spruce (Picea engelmannii)–subalpine fir (Abies lasiocarpa) forest recovering from a spruce beetle epidemic from 2008 to 2010. For this site, researchers have developed an integrated ecohydrological model of plant hydraulics coupled to surface/subsurface hydrology (ParFlow-TREES) to investigate the mechanisms of forest water and carbon flux recovery using 20 years of site historical eddy covariance flux data as constraints. Results suggest that during wet years, high soil moisture supported by a shallow water table resulted in enhanced spruce and fir evapotranspiration (ET) and gross primary productivity (GPP). However, during dry periods with a deep water table, tree recovery was slowed down significantly. While this is consistent with the regeneration observations at the site since the spruce beetle disturbance, the model has also identified the important role of understory growth in facilitating forest recovery. The second site lies at the opposite end of the topography at Chimney Park, where a 2008 to 2010 bark beetle epidemic of lodgepole pine (Pinus contorta) was followed by the 2018 Badger Creek wildfire. For this site, the team is developing a preliminary model as well. In the coming summer field season, researchers will collect joint climate, vegetation, soil moisture, and groundwater data at both priority sites and incorporate an understory regeneration module into ParFlow-TREES as well. With the updated model, the team will simulate 20 years of ET, GPP, and soil volumetric water content and water table position at the two sites, with and without the new recovery module and with and without lateral flow. Model results will then be compared against both historical and newly collected field measurements to evaluate the importance of hydrology as well as understory in driving forest flux recovery after disturbance(s).