Assessing the relative vulnerabilities of Mid-Atlantic freshwater wetlands to projected hydrologic changes

Wetlands are known to provide a myriad of vital ecosystem functions and services, which may be under threat from a changing climate. However, these effects may not be homogenous across ecosystem functions, wetland types, ecoregions, or meso-scale watersheds, making broad application of the same management techniques inappropriate. Here, we present a relative wetland vulnerabilities framework, applicable across a range of spatial and temporal scales, to assist in identifying effective and robust management strategies in light of climate change. We deconstruct vulnerability into dimensions of exposure and sensitivity/adaptive capacity, and identify relevant measures of these as they pertain to the attributes of wetland extent and plant community composition. As a test of the framework, we populate it with data for three primary hydrogeomorphic wetland types (riverine, slope, and depression) in seven small watersheds across four ecoregions (Ridge and Valley, Piedmont, Unglaciated Plateau, and Glaciated Plateau) in the Susquehanna River watershed in Pennsylvania. We use data generated from the SRES A2 emissions experiment and MRI-CGCM2.3.2 climate model as input to the Penn State Integrated Hydrologic Model to simulate future exposure to altered hydrologic conditions in our seven watersheds, as expressed in two hydrologic metrics: % time groundwater levels occur in the upper 30 cm (rooting zone) during the growing season, and median difference between spring and summer mean water levels. We then examine the spatial and temporal scales at which each of the components of vulnerability (exposure and sensitivity/adaptive capacity) shows significant relative differences. Overall, we find that relative differences in exposure persist at a very fine spatial grain, exhibiting high variability even among individual watersheds in a given ecoregion. For temporal scale, we find strong seasonal but weak annual relative differences in exposure resulting from a magnification of summer dry-down combined with winter and spring wet periods becoming wetter. Sensitivities/adaptive capacities show significant differences among wetland types. A comparison between our anticipated hydrologic alterations under climate change and historical changes in hydrology due to anthropogenic disturbance indicates potential shifts in hydrologic patterns that are far beyond anything that wetland managers have experienced in the past.