What are the effects of wooded riparian zones on stream temperature?

Abstract

Background: Predicted increases in stream temperature due to climate change will have a number of direct and indirect impacts on stream biota. A potential intervention for mitigating stream temperature rise is the use of wooded riparian zones to increase shade and reduce direct warming through solar radiation. To assess the effectiveness of this intervention, we conducted a systematic review of the available evidence for the effects of wooded riparian zones on stream temperature. Methods: We searched literature databases and conducted relevant web searches. Inclusion criteria were: subjectany stream in a temperate climate; intervention -presence of trees in the riparian zone; comparator -absence of trees in the riparian zone; outcome measure -stream temperature. Included studies were sorted into 3 groups based on the scale of the intervention and design of the study. Two groups were taken forward for synthesis; Group 1 studies comparing water temperature in streams with and without buffer strips/riparian cover and Group 2 comparisons of stream temperatures in open and forested landscapes. Temperature data were extracted and quantitative synthesis performed using a random effects meta-analysis on the differences in mean and maximum temperature. Results: Ten studies were included in each of Groups 1 and 2. Results for both groups suggest that riparian wooded zones lower spring and summer stream temperatures. Lowering of maximum is greater than lowering of mean temperature. Further analysis of environmental variables that might modify the effects of the intervention was not possible using the limited set of studies. Conclusions: Wooded riparian zones can reduce stream temperatures, particularly in terms of maximum temperatures. Because temperature is known to affect fish, amphibian and invertebrate life history, the reported effect sizes are likely to have a biological significance for the stream biotic community. Consequently investment in creation of wooded riparian zones might provide benefits in terms of mitigating some of the ecological effects of climate change on water temperature. Considerable uncertainty lies in the environmental variables that may modify the cooling effect of wooded riparian zones, and therefore it is not possible to identify when the use of this intervention for cooling would be most valuable.

The influence of riparian vegetation shading on water temperature during low flow conditions in a medium sized river

“…The unshaded areas showed up to 2.6°C higher daily WT max in the extreme heatwave days (Kþ) than fully or patchy shaded ones (VSI 6000 above 0.4) in the processed area did. This is consistent with Bowler et al (2012) who concluded that mean WT max are up to 4.9°C lower at forested riversides than at pasture riversides.…”

“…However, WT max and WT range had peaks at unshaded reaches, of up to approximately 4°C difference to shaded ones, which followed a steadily increasing curve. These findings correlates well with the findings of Bowler et al (2012), Johnson (2004) and Webb et al (2008) which show that if a riverine reach is fully clear cut, the daily WT max and WT range are particularly affected.…”

Do changes in riparian zones affect periphyton growth and invertebrate colonization on rocky substrates in Atlantic Forest streams?

“…The warm water temperatures in this stream are probably related to changes in the riparian vegetation that allowed more sunlight to reach the stream channel. Bowler et al (2012), working in European streams, observed that water temperatures were positively correlated with light input.…”

Forest clearfelling effects on dissolved oxygen and metabolism in peatland streams

“…Canopy removal eliminated the shading effect of the trees naturally implying a change in lighting conditions in the open stretches of the impact stream following clearfelling. Solar radiation is the predominant contributor of energy for summer warming in streams with no canopy (Bowler, 2012). Gomi et al (2006) suggest riparian areas along streams protect the stream from increased thermal variability, with effects varying to some degree with buffer width.…”