Do trees in UK-relevant river catchments influence fluvial flood peaks?: a systematic review
This review sought to capture qualitative findings from peer-reviewed studies focussing on the impact of trees on river flooding. It specifically did not focus on the impacts of trees on intermediate processes (e.g. rainfall interception, groundwater recharge or overland flow). Whilst the role of these processes in determining hydrological response is recognised, it was agreed that inferred findings should not be included in the review as these do not necessarily translate to a substantive influence of river flooding. The effect on river flooding of trees or woody debris within river channels was not included.
The findings help to explain why there is continuing debate associated with this topic and how it is possible for strongly held yet differing views on the influence of trees on peak flows to exist. Greater consideration of the influence of effect modifiers in the reviewed studies should help to clarify the results, although important questions are likely to remain. This is especially the case regarding the effects of tree cover on large flood peaks and in large catchments, where there is a paucity of observed data to test and validate models.
Increasing the number of observational studies would help to address the gap but presents a number of significant challenges, including: accurately measuring changes to large peak flows; controlling for land use and management changes, as well as background shifts in weather; and not least, maintaining long-term funding for data collection to capture changes to infrequent, large peak flows. This report presents some initial thoughts on the requirements of future studies and identifies some areas for further research.