Ranking Site Vulnerability to Increasing Temperatures in Southern Appalachian Brook Trout Streams in Virginia: An Exposure-Sensitivity Approach

Abstract

Models based on simple air temperature–water temperature relationships have been useful in highlighting potential threats to coldwater-dependent species such as Brook Trout Salvelinus fontinalis by predicting major losses of habitat and substantial reductions in geographic distribution. However, spatial variability in the relationship between changes in air temperature to changes in water temperature complicates predictions. We directly measured paired summer air and water temperatures over 2 years in a stratified representative sample of watersheds (<1–274 km2) supporting wild Brook Trout throughout Virginia near the southern edge of the species distribution. We used the temperature data to rank streams in terms of two important components of habitat vulnerability: sensitivity (predicted change in water temperature per unit increase in air temperature) and exposure (predicted frequency, magnitude, and duration of threshold water temperatures). Across all sites, sensitivity was substantially lower (median sensitivity = 0.35\textdegreeC) than the 0.80\textdegreeC assumed in some previous models. Median sensitivity across all sites did not differ between the 2 years of the study. In contrast, median exposure was considerably greater in 2010 (a particularly warm summer) than in 2009, but exposure ranks of habitat patches were highly consistent. Variation in sensitivity and exposure among habitat patches was influenced by landscape metrics (percent forested riparian corridor, patch area, and elevation), but considerable unexplained variation in sensitivity and exposure among sites was likely due to local-scale differences in the extent of groundwater influence. Overall, our direct measurement approach identified significantly more Brook Trout habitat patches with low sensitivity and low exposure that may persist under warming air temperatures than did previous large-scale models. Our sensitivity and exposure classification should provide a useful general framework for managers in making investment decisions for protecting and restoring Brook Trout habitat.