Is planting trees the solution to reducing flood risks?

Abstract

Flood risk and associated impacts are major societal and policy concerns following widespread flooding in December 2015, which cost the UK economy an estimated £5 billion. Increasing advocacy for alternatives to conventional hard engineering solutions is accompanied by demands for evidence. This study provides a systematic review and meta‐analysis of direct evidence for the effect of tree cover on channel discharge. The results highlighted a deficiency in direct evidence. From 7 eligible studies of 156 papers reviewed, the results show that increasing tree cover has a small statistically significant effect on reducing channel discharge. Meta‐analysis reveals that tree cover reduces channel discharge (standardised mean difference −0.35, 95%CI, −0.71 to 0.00), but the effect was variable (I2 = 81.91%), the potential for confounding was high, and publication bias is strongly suspected (Egger Test z = 3.0568, p = .002). Due to the lack of direct evidence the overall strength of evidence is low, indicating high uncertainty. Further primary research is required to understand reasons for heterogeneity and reduce uncertainty. A Bayesian network parameterised with data from the meta‐analysis supports investment in integrated catchment management, particularly on infrastructure density and water storage (reservoirs), for effective responses to flood risk.

Land cover effects on hydrologic services under a precipitation gradient

“…Further, increases in median discharge are higher with higher annual precipitation amounts. The magnitude of the observed change is similar to that reported by Farley et al (2005) for catchments located in northern Europe and higher than those obtained from hydrological modelling by Carvalho-Santos et al (2016) and Morán-Tejeda et al (2014) for catchments in the Iberian Peninsula.…”

“…an increase in native forests (Table 4). This implies that high flows are attenuated under land cover combinations with high percentages of exotic plantations, which is favourable for flood regulation (Carvalho-Santos et al, 2016). Under high precipitation amounts, the situation remains practically unchanged for all land cover combinations.…”

“…Under high precipitation amounts, the situation remains practically unchanged for all land cover combinations. In this sense Carrick et al (2018), after a meta-analysis of 156 papers, concluded a weak direct influence of the effects of tree cover on flood risk, due to the high uncertainty found in results.…”

Communicating (nature‐based) flood‐mitigation schemes using flood‐excess volume

“…Alternatively, more attenuation features can be built, or more trees and peat restoration in combination with controlled ponding. Co‐benefits of tree planting and peat coverage can be taken into account in cost assessments, such as carbon sequestration and recreational value (Denjean et al, ), but all these measures and decisions demand a clear quantification, especially given the weak evidence for the effect of tree coverage on channel discharge (Carrick et al, ). The above analysis and in particular its graphical presentation in Figure suggests that our FEV‐based protocol can aid in more quantifiable and rational decision making.…”

“…Tree and peat planting requires enormous areas before mitigation effects may become significant for larger flood events. Even then, there is a lack of evidence to suggest that increased tree coverage reduces flood risks (Carrick et al, ). Besides showing beneficial effects in small‐scale NFM pilot studies (Environment Agency, ), it is important to obtain first estimates of NFM's effectiveness and reliability on grander catchment scales for extreme‐flood events.…”