An evidence-based review of the effectiveness of riparian buffers to maintain stream temperature and stream-associated amphibian populations in the Pacific Northwest of Canada and the United States

Abstract

We employed a systematic evidence review to evaluate empirical scientific evidence for the effectiveness of buffering headwater (typically non-fish-bearing) streams to maintain stream temperature and stream-associated amphibian populations in the Pacific Northwest of Canada and the United States. To address our synthesis objective, we identified thirteen temperature, seven amphibian, and two temperature/amphibian primary research studies that met objective inclusion criteria. We evaluated external validity for how study treatments inform or were linked to causal factors influencing temperature response (a) and how the sampled population represented or provided inference to an intended target population or landscape (b). The evidence indicated substantial variability in the temperature response to streamside buffers. The effect size for the mean 7-day maximum temperature metric showed a positive association when comparing no-buffers (clearcut) to treatments with wide buffers (≥30 m). However, this effect varied substantially and overlap existed in effect sizes among no-cut buffers, no-cut plus variable retention buffers, and no-cut patch buffers all ≤20 m wide. Large variability in effect size among treatments obscured any potential trend between effect size for the seasonal (summer) mean daily maximum temperature metric and buffer width. Shade was correlated with temperature response within several studies, but direct comparisons of treatment effectiveness among studies as a function of shade was confounded by different measurement methods. The evidence also indicated that variation in temperature response among studies may be associated with multiple factors (geology, hydrology, topography, latitude, and stream azimuth) that influence thermal sensitivity of streams to shade loss. For amphibians, we found mixed evidence for relationships between population responses and buffers maintained along streams after forest harvest. Specifically, we did not find evidence to support the contention that positive population responses are associated consistently with larger buffers. Also, considerable uncertainty exists about which environmental covariates reliably explain variation in amphibian population responses. Collectively, our results indicate that evidence is weak to address questions most relevant to policy discussions concerning effectiveness of alternative riparian management schemes. Future studies should test effectiveness of alternative treatments with either experimental or purposefully structured observational studies to develop tools and derive guidelines for how to achieve management goals based on site and landscape characteristics.