Forest–flood relation still tenuous – comment on ‘Global evidence that deforestation amplifies flood risk and severity in the developing world’ by C. J. A. Bradshaw, N.S. Sodi, K. S.‐H. Peh and B.W. Brook

Abstract

In a recent paper in this journal, Bradshaw and colleagues analyse country statistics on flood characteristics, land cover and land cover change, and conclude that deforestation amplifies flood risk and severity in the developing world. The study addresses an important and long-standing question, but we identify important flaws. Principal among these are difficulties in interpreting country statistics and the correlation between population and floods. We review current knowledge, which suggests that the removal of trees does not affect large flood events, although associated landscape changes can under some circumstances. Reanalysis of the data analysed by Bradshaw and colleagues shows that population density alone already explains up to 83% of the variation in reported flood occurrences, considerably more than forest cover or deforestation (o10%). Feasible explanations for this statistical finding -whether spurious or causative -are not difficult to conceive. We, therefore, consider the conclusion of Bradshaw and colleagues to be unsupported. However, their study is a valuable first step to show how these or similar flood data might be used to further explore the relationship between land cover and flooding.

The impact of forest regeneration on streamflow in 12 mesoscale humid tropical catchments

“…Although previous work on the relationship between land-cover change and Q using lumped meso-and macro-scale catchment data has experienced some difficulty demonstrating unequivocal results (e.g. Van Dijk et al, 2012), possibly stronger conclusions may be obtained by selecting catchments within similar regions (in this case central Puerto Rico; cf. Peña-Arancibia et al, 2010).…”

“…Van Dijk et al, 2012), or in time, due to land-cover changes altering the climate (Van der Molen, 2002;Pielke et al, 2007). Several mesoscale atmospheric modelling studies have produced conflicting results regarding the existence of such a relationship for Puerto Rico (Van der Molen et al, 2006;Comarazamy and González, 2011).…”

“…Using data for 278 Australian catchments Van Dijk et al (2012) showed that the introduction of noise to long-term means of observed PET, P , and Q reduced the likelihood of detecting a land-cover signal. Here, as different methodologies were employed to derive the land-cover maps for the different years, and because the maps had different spatial resolutions and classification schemes (see Sect.…”

“…The influence of forest cover change on flooding is particularly contentious (e.g. FAO, 2005;Bradshaw et al, 2007;Van Dijk et al, 2009) whereas the effect of forestation on tropical dry-season flows is also under debate (Calder, 2005;Scott et al, 2005). The general contention is that the net effect of an increase in forest cover on dryseason flow depends on the "trade-off" between increases in Q due to enhanced soil water recharge on the one hand (as forestation generally increases soil macro-porosity and infiltration characteristics; Ilstedt et al, 2007;Bonell et al, 2010;Zimmermann et al, 2006Zimmermann et al, , 2010Hassler et al, 2011), and decreases in soil water reserves and Q on the other hand due to the higher water use of trees compared to crops, pasture, or scrubs (Bruijnzeel, 1989(Bruijnzeel, , 2004Jackson et al, 2005;Scott et al, 2005).…”

Mycorrhizal Interactions for Reforestation: Constraints to Dryland Agroforest in Brazil

“…As forests can reduce flooding by acting as sponges (trapping water during heavy rainfall, and releasing it slowly into streams), which decreases the severity of floods and maintains stream flows during dry periods, several nations have invested in forest protection or reforestation [32][33][34] (Figure 1). However, some authors suggested that removal of trees does not affect large flood events [35]. Thus, the need for large-scale forest protection and more reforestation to help reduce the frequency and severity of floods to protect human wellbeing was pointed out [32].…”