Riparian forest as a management tool for moderating future thermal conditions of lowland temperate streams

Abstract

Predictions of future climate suggest that stream water temperature will increase in temperate lowland areas. Streams without riparian forest will be particularly prone to elevated temperature. Planting riparian forest is a potential mitigation measure to reduce water temperature for the benefit of stream organisms; however, no studies have yet determined the combination of shading (% canopy cover) and stream length required to obtain a timely temperature decrease. We measured the temperature in 5 small Danish lowland streams from June 2010 to July 2011, all showing a sharp transition between an upstream open reach and a downstream forested reach. At each site we also measured canopy cover and a range of physical variables characterising the stream reach to analyse differences in mean daily temperature and amplitude per month among forested and open reaches, annual temperature regimes, and the influence of physical conditions on temperature changes. In July, water temperature increased over the entire length of the open reach in 3 of the 5 streams, reaching temperatures higher than the incipient lethal limit for brown trout (Salmo trutta). In 4 of the streams, July temperature decreased within 100 m of the stream entering the forested area, and in 3 of our study streams the temperature continued to decrease within the forested reach, without reaching a plateau. Daily temperature variation was greater in the open reaches than the forest reaches. Regression analysis indicated that temperature changes along the forested reach in July were significantly related to canopy cover, width:depth ratio, and temperature of the water entering the forested reach. We conclude that even relatively short stretches (100-500 m) of forest alongside streams may reduce water temperature and so combat the negative effects of temperature increases on stream ecology. Although forestation can be a useful mitigation measure, many lowland streams are historically rich in macrophytes, which are important hotspots for fish and macroinvertebrates. If managers leave open reaches interspersed in the riparian canopy, loss of diversity provided by these macrophyte beds could be diminished while still leaving cooler water refugia.