How robust is the evidence for beneficial hydrological effects of urban tree planting?

Sustainable urban water management initiatives are increasingly required to combat rapid urbanization and climate pressures. Initiatives include the role of tree planting, for which there is a need for strong evidence of benefits and drawbacks to support effective future planning. We report on the robustness of evidence from an assimilated database of urban hydrological impact studies which often had differing primary purposes. Consistent impacts were found at the local level, with trees reducing runoff and infiltration. Despite the consistency of evidence, much is undermined by the studies being somewhat lacking in robustness and scientific rigour. Many studies lack adequate controls, and models are often not strongly tested against observations. Moreover, evidence of impact at larger scales is lacking. Effects of tree characteristics were also investigated, such as maturity and species (for which evidence is consistent and detailed) and arrangement (for which there is less evidence). Realizing the full potential of trees in urban water management decision-making would benefit from more rigorous evidence.