Effectiveness of trees and forests in reducing flood risk

Trees and forests have become vital components of flood mitigation policies at national and international levels as their role in reducing flood risk has become more evident. This has led to their wide-spread use to complement traditional structural defences in flood prone landscapes. The establishment of forests and planting of trees changes surface and subsurface hydrology, often delaying peak discharge and reducing overland runoff and flood risk. There is a significant amount of research that demonstrates flood reduction by forests (plantation and native), and small strips of trees or shelterbelts established in agricultural landscapes. Studies from the United Kingdom, New Zealand and Latin America have shown that the attenuation of floods by trees and forests is circumscribed to their position in the landscape, growth season, age, root structure, canopy architecture, geology and climate. At the same time, there is research indicating a negligible impact of trees and forests when catchment scale is large and rainfall events are extreme. Flood incidences are still on the increase in regions with large areas of forested land. Much emphasis has been placed on the ability of trees and forests to reduce peak flow and mitigate floods through interception loss, high transpiration rates, and amelioration of soil properties such as infiltration, hydraulic conductivity and bulk density at the field or small catchment scale. Less emphasis has been put on identifying the climatic and geological areas where trees are likely to deliver significant flood mitigation services. Furthermore, the stand characteristics that are optimal in delivering this service where appropriate have seldom been reported. This study presents a review of the relationship between trees/forests and flooding. The investigation has focused on how the interaction is affected by different species, age, position in the landscape, scale, soils and magnitude of rainfall events under changing geology and climatic conditions. The hydrological role of trees and forests is also assessed under inherently near or fully saturated conditions in other ecosystem types such as wetlands. This will clarify how trees and forests can be effectively used to manage the risk of flooding, identify areas that may require future research and help in making decisions on their conservation.