Stemflow is the part of precipitation that flows from the tree crown along the trunk. It occurs mainly in tree species with a smooth bark, e.g. beech and hornbeam. In beech stands, stemflow forms an important part of precipitation under forest cover. Together with the measurements from the rain (3) and snow collectors (4) stemflow measurements allow us to quantify the precipitation under forest cover and the nutrient and pollutant inputs in the forest ecosystem.
On LWF sites with beech and, for a 1-year period in 1997/1998, on one site with oak, we sealed a silicone gutter coiled in a spiral around the stem of selected trees. The stemflow runs down in this gutter and is collected in an automated tipping bucket connected to a counter. Each tipping of the lever increases the counter value and redirects a small fraction of stemflow into a collecting vessel.
The stemflow collectors remain in place all year round. The quantity of stemflow is recorded every other week and samples are brought back to the laboratory for chemical analyses.