The influence of forest harvesting on stream temperatures

Abstract

There is considerable interest in the role of riparian shading in controlling stream temperature. In North America, research has primarily focused on understanding the effects of forest harvesting due to the potential negative effects of increasing maximum temperatures for salmonids. In the UK there is increasing interest in the use of riparian woodland to protect streams from high stream temperatures under climate change. Both these research areas require estimates of stream temperature differences associated with the presence of riparian forest over other land uses that provide less shade. Here we present the findings of a study that assesses stream temperature changes associated with forest harvesting in Scotland. Five harvesting events were examined in three catchments. The area of forest removed ranged between 13 and 39%. Changes in daily temperature, mean, minimum, maximum and range were assessed using a BACI experimental design. A novel application of Bayesian generalised additive mixed models allowed for an assessment of felling effects with associated variances. The influence of harvesting varied seasonally, with the greatest effects observed during the summer. The greatest changes were observed for maximum temperatures. The ability to detect statistically significant changes was substantially affected by the precision of temperature data. The magnitude of temperature responses varied depending on the location of harvested forest relative to the stream channel and the proximity to temperature monitoring. Where felling occurred to the north of the streams there was no detectable change in temperature. The findings of this study provide useful insights for assessing the likely effects of forest harvesting on stream temperature, but also the potential for riparian woodland to reduce maximum temperatures. The statistical approach applied here would be useful for future studies of stream temperature that employ a BACI experimental design.