Stream Temperature Response to Timber Harvest in the Oregon Coast Range Two and Five Years Post-Harvest

We examined the efficacy of timber harvest regulations at protecting Oregon Coast Range stream temperatures at 33 sites over seven years.  Although Pacific Northwest states began enacting riparian timber harvest regulations decades ago to protect salmonid habitat quality and human health, we lack information regarding regulatory sufficiency.  Characteristic inter-stream and inter-annual variability in stream temperature has posed a major obstacle in our understanding of general harvest effects magnitudes.  To address this issue, the Oregon Department of Forestry developed a manipulative study in 2002 that offered temporal and spatial control including two years of pre-harvest data measurement, five years of post-harvest measurement, and data collection at each sites’ unharvested upstream control reach.  Data collection in the control and harvested treatment reach included measurements of shade, channel characteristics, and extensive riparian vegetation quantification.  By the second year post-harvest stream temperatures increased by an average of 0.7 °C and at some sites by as much as 2.5 °C.  The observed increase was a function of pre- and post-harvest shade, and shade itself was best predicted by riparian tree basal area and height.  At five years post-harvest sites that initially registered temperature increases generally maintained the increase.  We interpret the findings to indicate that riparian management standards that result in more post-harvest shade over streams are more likely to prevent increases in stream temperature. Current Oregon riparian regulations for private forests may not result in sufficient tree retention as to prevent stream temperature increases for two to five years after harvesting.