What are the effects of wooded riparian zones on stream temperature? (systematic review)
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.
We searched literature databases and conducted relevant web searches. Inclusion criteria were: subject – any 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.
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.
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.
Stream temperature, Riparian buffer, Fish, Climate change, Systematic review
The riparian zone is the transition zone between a freshwater and terrestrial ecosystem (Naiman and Décamps 1997). The unique, dynamic and complex nature of riparian habitat means that it can support relatively high biodiversity and hence can be important for conservation management (Pusey and Arthington, 2003). Changes in land use and river regulation have meant that wooded riparian habitats next to rivers and streams have become less abundant over time. Different activities have contributed to this decline, such as logging for timber (Young, 2001) and livestock grazing and trampling (Belsky et al. 1999). Increasingly, there is concern that degradation of this habitat may lead to a reduction in the abundance and diversity of species in the aquatic environment (Pusey and Arthington, 2003).
The riparian zone can play a key role in the functioning of the aquatic ecosystem, affecting chemical, physical and biological processes (Naiman and Décamps 1997; Pusey and Arthington, 2003). Degradation of this environment therefore has implications for many different aspects of river function, such as a loss of bank stability leading to increased erosion and chemical leaching from the surrounding land. One potentially major impact of riparian habitat on stream functioning is on water temperature (Moore et al. 2005; Hannah et al. 2008). Riparian habitat can affect the level of solar radiation received by the stream and so removal of tree cover may lead to an increase in water temperature (Moore et al., 2005). Increased stream temperature can have direct effects on poikilothermic organisms, such as fish, through changes in metabolism and ultimately mortality rates. There has been a particular concern that extreme summer temperatures, which may increase further with climate change, can increase fish mortality (Malcolm et al. 2008). However, changes in riparian habitat and temperature can have a diverse range of impacts on primary productivity and decomposition, and may lead to changes in the structure and functioning of the whole biotic community (Pusey and Arthington 2003; Durance and Ormerod, 2007), which increases the complexity of predicting the biological impact. A number of other factors, including stream flow rate and ground water inflow (Ward, 1985; Caissie, 2006), may also affect stream temperature, and affect the relative importance of riparian habitat for water temperature.
Management interventions have been proposed to limit the consequences of riparian degradation, such as the use of riparian buffer strips (Osborne and Kovacic, 1993; Haycock and Muscutt, 1995) or livestock exclusion fencing (Belsky et al. 1999). Empirical research has been carried out to explore the importance of the riparian habitat and several reviews have been published addressing issues on this topic such as the effects of logging (Young 2001) and livestock (Belsky et al. 1999) on stream and riparian ecosystems, and the relationship between stream temperature and forest harvesting (Moore et al. 2005). This review examines the relationship between wooded riparian cover and stream temperature and the impact on the diversity and abundance of stream biota in temperate zones. Systematic review methodology will be used to comprehensively search for data, critically appraise the quality of study and quantitatively synthesise and compare the outcomes of different studies.