A comparison of stream water temperature regimes from open and afforested moorland, Yorkshire Dales, northern England

Despite the known importance of water temperature for river ecosystems, the thermal regime of streams and rivers can be heavily modified by afforestation. Although the nature of the heat budget affecting streams in forested catchments shows high variability in space and time, most of the studies of stream temperature response to afforestation have lacked replication among streams. This study examined the impacts of coniferous forest plantations on stream water temperature at six sites (three forested and three open moorland) in the Yorkshire Dales, northern England. Our aim was to test the hypothesis that afforestation would alter the thermal regime of streams, leading to reduced year-round thermal variability, and cooler summer/warmer winter water temperatures, relative to streams flowing across open moorland. Data collected from April 2007 to March 2009 showed similar thermal dynamics among all six streams over the study period, although variability in forested streams was markedly lower as expected. Mean and maximum daily water temperatures were significantly higher in open moorland streams for much of the year but while some forested streams were warmer than individual moorland streams during winter months (November to February), there was considerable overlap in water temperature between moorland and forest streams. Most stream temperature records showed evidence of low/no winter flow and freezing. These results contrast with many previous studies that have reported warmer temperatures in forested versus open moorland streams during winter, a finding that most likely reflects site-specific hydrological, geomorphological and climatological influences on water temperature in addition to afforestation. This study demonstrates the need for replication of hydrological monitoring when examining the effects of basin-scale management practices and provides further evidence for changes in stream thermal regime following afforestation, a practice that is likely to increase in future due to growing demands for increased forest cover in the UK uplands.