Long-term trajectories of impacts and recovery of headwater stream temperatures after forest harvest in interior British Columbia

Abstract

This study examines the impact-recovery trajectory of stream temperatures following different riparian harvest treatments. We assessed the seasonal and annual changes in thermal sensitivity and temperatures across four different riparian management treatments in interior British Columbia using a Before-After-Control-Impact design across 6 headwater streams and 28 years. Riparian treatments included combinations of two buffer widths and two levels of retention of merchantable trees. Harvest periods included a pre-harvest period (1995–1996), short-term (1997–2009) and long-term post-harvest (2019–2024) periods. We found thermal sensitivity and stream temperatures responded rapidly to forest harvest similar to previous studies. Although stream temperatures peaked during months with the warmest air temperatures (July and August), the effect of riparian harvest on thermal sensitivity and mean daily temperatures had a strong seasonal component, whereby the largest harvest effects occurred during May and June. The largest short-term effects of forest harvest were observed in the stream with the least protective riparian management treatment and little to no effect was observed in the most protected stream. Forest harvest impacts on stream temperatures persisted through the short-term post-harvest period but recovery appeared to be underway in the long-term post-harvest period. Collectively, this work characterizes a multi-decadal impact-recovery trajectory for headwater stream temperatures across seasons and different riparian management treatments.