The influence of edge effects on evapotranspiration of fragmented woodlands. Der Einfluss von Randeffekten auf die Verdunstung fragmentierter Waldbestande

The water use of forests has been the subject of many studies in the past decades. They were mostly carried out in extensive areas of woodland and achieved consistent results. However, there is as yet a large uncertainty about the role of fragmented woodlands in the catchment water balance, since water losses from small patches of woodland have rarely been measured. In the framework of the "Lowland Catchment Research" (LOCAR) programme, a 7-months field measurement campaign has been carried out in southern England in order to measure the transpiration of a mixed deciduous forest in various distances from the forest edge by means of the sap flux techniqure. The annual transpiration per unit ground area near the forest edge equalled potential evaporation and was about 60% higher than in the forest interior and similar to the transpiration of hedgerows as determined in a corresponding study. Interception evaporation was not affected by the proximity to an edge. Based on these results it is shown that the edge effect dominates the water use of small forests (<10 ha) and becomes negligible only for woodlands larger than 100 ha.