Forested watersheds mitigate the thermal degradation of headwater fish assemblages under future climate change

Abstract

Certain habitat features of stream ecosystems can reduce their sensitivity to climate change and help protect the integrity of cold-water aquatic resources. Identifying such features is imperative for conserving the climate refugia of cold-water species. Using a combination of stream temperature and fish assemblage data, we quantified thermal sensitivities of 192 headwater streams in Northwest Pennsylvania to identify which landscape features best explained stream susceptibility to temperature change. We then projected changes in native brook trout (Salvelinus fontinalis) distributions, non-native brown trout (Salmo trutta) distributions and declines in cold-water thermal integrity under future climate warming and land use change scenarios. Brook trout were predicted to become increasingly relegated to smaller streams under future stream warming. However, we found that streams with intact forest cover at the watershed level had low thermal sensitivities, which slowed rates of projected warming. As a result, streams with forested watersheds were predicted to have smaller declines in thermal integrity and lower extirpation probabilities of brook trout. Additionally, non-native brown trout were not predicted to expand distributions under projected warming, suggesting minimal synergistic effects between non-native species and climate change. Forest cover buffers headwater streams from the effects of global change, similar to how groundwater inputs reduce the rate of stream warming. Forest restoration at riparian and watershed levels should help mitigate thermal-induced degradation of cold-water aquatic resources.