Summer stream temperature changes following forest harvest in the headwaters of the Trask River watershed, Oregon Coast Range

The Trask River Watershed Study in the northern Oregon Coast Range was designed to examine physical, chemical, and biological effects of contemporary forest management practices on aquatic ecosystems. We measured stream temperature for 11 summers in 15 small watersheds, eight of which were harvested in 2012. Three riparian buffer treatments, which varied by landowner, were implemented. Using half-hourly data, we characterized summer water temperature distributions with five percentiles: 5th, 25th, 50th, 75th, and 95th. Each percentile was analysed as a separate response variable using a linear mixed model. After harvest, streams without overstory buffer requirements showed shifts in all the percentiles of the temperature distribution; the largest increase (3.6 degrees C) occurred at the 95th percentile. Sites with narrow riparian buffers showed little to no change. We also calculated changes in duration of thermal exposure above 15 degrees C and 16 degrees C for two species of native stream amphibians; these temperatures occurred 4.7% and 1.3% of the time postharvest in the sites clearcut with no buffer. Analysis of distributions of summer temperatures preharvest and postharvest enabled us to more fully characterize site-to-site variability and responses to forest management.