Seeing the climate through the trees: observing climate and forestry impacts on streamflow using a 60-year record

Paired watershed experiments involving the removal or manipulation of forest cover in one of the watersheds have been conducted for more than a century to quantify the impact of forestry operations on streamflow. Because climate variability is expected to be large, forestry treatment effects would be undetectable without the treatment-control comparison. New understanding of climate variability provides an opportunity to examine whether climate variability interacts with forestry treatments, in a predictable manner. Here we use data from the HJ Andrews Experimental Forest, Oregon, USA, to examine the impact of the El Niño-Southern Oscillation (ENSO) on streamflow linked to forest harvesting. Our results show that the contrast between El Niño and La Niña events is so large that, whatever the state of the treated watershed in terms of re-growth of the forest canopy, extreme climatic variability related to ENSO remains the more dominant driver of streamflow response at this location. Improvements in forecasting inter-annual variation in climate might be used to minimise the impact of forestry treatments on streamflow by avoiding initial operations in La Niña years.