Stream Temperature Response to Riparian Buffer Configurations: A Replicated Experiment Across Oregon's Coast Range

Riparian buffers provide many important functions for streams, including shade to limit water temperature increases after forest harvest. Conventional buffer designs often designate a minimum width, but alternative configurations such as hydrologically adaptive variable retention or canopy gaps buffers may enhance stream productivity by introducing heterogeneity in stream light whilst still providing thermal buffering, shade, and large wood recruitment. Therefore, alternative buffer designs may be an important climate adaptation strategy. To better understand stream temperature responses to conventional and novel alternative riparian buffer configurations, we conducted a large-scale before-after control-impact experiment across 28 streams in western Oregon. We established six replicate stream blocks that each included five streams: an uncut reference stream and four streams with different riparian buffer designs (Fixed-Width, Standard Practise, Variable Retention, and Canopy Gaps). Prior to treatment, all streams were well shaded (mean effective shade = 96.2%), post-treatment percent shade was reduced by up to 31.4 percentage points. Mixed effect model estimates indicated a 7 day moving average of daily maxima temperatures (T7Max) increased post-treatment by 0.89°C, 0.94°C, 0.79°C, and 0.42°C in Fixed-Width, Standard Practise, Variable Retention, and Canopy Gaps treatments, respectively, but did not differ amongst treatments. Stream temperature response was explained by both percent reduction in effective shade and bankfull width, indicating stream temperature was more sensitive to reductions in shade in smaller streams. We found Variable Retention and Fixed-Width buffers, but not buffers with Canopy Gaps, increased longitudinal temperature variability. Our findings suggest that novel alternative buffers were as protective of temperature as conventional buffers whilst allowing flexibility in application. Therefore, these hydrologically adaptive Variable Retention or Canopy Gaps alternatives may be viable tools to both limit temperature increases and enhance variation in light conditions.