Insights into the influence of groundwater and drought in a north temperate forest

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Vegetation may avoid the effects of drought when roots are able to access available shallow groundwater. The critical role of shallow groundwater in reducing drought vulnerability in arid and semi-arid ecosystems is well-studied, but is rarely evaluated in wetter environments. In a water-rich landscape, the North Temperate Lakes LTER monitored a prolonged drought from 2006-2013, when groundwater depths increased on average by 1 m. Here, remote sensing and dendrochronology results are presented to investigate groundwater-tree interactions in the Trout Lake Watershed in northern Wisconsin with attention to the recent drought. At the watershed scale, Landsat ETM+ images tracked the changes in NDVI for each vegetation pixel for a range of years. NDVI dynamics identified potential groundwater-influenced vegetation in regions where NDVI was high during extended dry periods and also exhibited low seasonal and inter-annual NDVI changes between dry and wet years. At the tree scale, annual growth chronologies were created for different species where historic groundwater data was available. Standard regressions were performed based on a model of annual tree growth as a function of groundwater depths as well as climate variables, including temperature, precipitation, irradiance, and potential evapotranspiration. Though preliminary, individual tress in areas of shallow groundwater showed growth influenced by groundwater depth variability. In general, tree growth rates were among the lowest when groundwater was the deepest at an individual site.