1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland
In the past, headwater catchments have often been studied to elucidate the effect of forests on the water cycle. This has been of interest since centuries and we recall here this historical context. We review the quantitative effects found in numerous studies around the world, especially those from paired-catchment experiments. As a rule, they indicate a lower water yield of forests compared to shorter vegetation types, which can be explained by their high evapotranspiration. Discharge peaks are thereby less affected by the vegetation type than low flows, while erosion is prevented by forests and other permanent soil cover. The quality of water obtained from forested catchments is high, in average better than under other land-uses. This high water quality is mainly the result of low inputs of pollutants rather than of an active cleaning. In the future, the effect of forests on the water cycle will change on a regional to global scale mainly because of three ongoing processes: land-use changes, climatic changes and changes in the deposition of pollutants. Long-term monitoring of headwater catchments will continue to help us understanding such changes. In many cases, however, single disturbances like windstorm, fire or pest outbreaks can have more impact than progressive changes. Especially in this regard, paired-catchment experiments and monitoring will remain, a powerful tool to assess the impact of environmental changes at a scale large enough to take many interactions into account.