Effects of Riparian Buffers and Climate Change on Headwater Stream Flow: Shrinking Heads in Western Oregon
Climate change models for the US Northwest have been used to assess changes in stream flow conditions, with altered flow regimes projected to affect aquatic-dependent communities. We addressed the effects of climate on headwater stream amphibian habitats in western Oregon managed forests using a combined approach of retrospective analyses of 16 years of headwater stream flow in 109 stream reaches and associated climate data, and applying results to future climate projections and to landscape projections of consequent headwater stream flow conditions. Using multivariate and simple linear models, percent dry length of spatially intermittent stream reaches was positively related to summer heat:moisture index. Although we included alternative riparian buffer widths with upland thinning in models, these management treatments were not significant in our models. Increases in summer heat:moisture indices with climate change models of future conditions (to year 2080) resulted in ~2% increases in percent dry lengths of headwater reaches (i.e., shrinking heads). We applied our results to the western Cascade Range, examining first-order streams occurring in 135 6th-field watersheds between 0 and 1433 m (0 to 4,700 ft) elevation, the area coincident with the distribution of the sensitive headwater-associated Cascade Torrent Salamander, Rhyacotriton cascadae. The projected ~2% increase in dry length of first-order streams sums to ~358 km across the area known to be occupied by this species. Consequences of shrinking headwaters to both first-order stream habitat conditions and over-ridge head-to-head connectivity distances are potentially pronounced, likely having an isolating effect on salamanders occurring in discrete watersheds. Presented by Deanna Olson, USDA Forest Service, at the 2018 SAF National Convention, Portland, OR