Understanding the effects of spatially variable riparian tree planting strategies to target water temperature reductions in rivers

Climate change is increasing river temperature globally, altering the thermal suitability for iconic cold-water adapted fishes. In regions with low tree cover, the impacts of projected climate change on river temperature will be particularly pronounced due to limited shading of the channel. Reforestation of the riparian corridor is thus increasingly being used to shade rivers and offset projected increases in water temperature. However, tree planting can be expensive and logistically challenging, meaning that there is a need to develop guidance to prioritise tree planting where it can deliver greatest benefits.In this study, we use a process-based stream temperature model to simulate the likely effects of a real-world tree planting scheme recently implemented on the Baddoch Burn, a tributary of the Aberdeenshire Dee, Scotland. Our results show that, when mature, ∼3 km of recent tree planting will increase effective shading in the lower reaches of the Burn from 22% to 47%, delivering a ∼1.5 °C decrease in maximum summer stream temperature in comparison to the present-day baseline. We subsequently systematically simulate riparian tree planting in different locations and configurations to determine how and where riparian planting produces and optimal stream temperature response. Our results highlight that different spatial configurations of planting (in terms of length, number, location upstream and spacing between planting zones) can have a considerable impact on stream temperature outcomes, but optimal temperature reductions are generally achieved through planting longer and/or more numerous strips of woodland in upstream reaches, where effective shade is maximised (due to reduced channel width) and where water volumes and residence times mean that impact of reduced solar radiation is greatest.Our investigation not only highlights the extent to which a real-world tree planting scheme will likely deliver summer stream temperature reductions, but also underscores the importance of planting configuration for delivering a temperature reduction in a desired location. Overall, our results provide useful information for river managers and practitioners to develop appropriate riparian shading schemes to combat climate change-driven stream temperature warming.