Removal of European alder Alnus glutinosa - an active method of mire conservation.
Tree-less peatland communities can subject to vegetative succession, which often progresses toward a recovery of woodland or forest dominated by Salix, Betula or Alnus glutinosa. Since this process provides a threat to already established combinations of species and habitats of nature conservation value, there is a need to elaborate an efficient and environmentally friendly method of managing invading species. We investigated the effectiveness of removing invading black alder, Alnus glutinosa (∼18 years old; up to 4 m tall), from a non-forest mire by applying a single cut. We tested whether alder removal was positively related to the height of the cut and the season of the year. Most mortality occurred in the first year after the cut. Three years later, the response to cutting showed a moderate increase in dead trees, especially among trees that had been cut in autumn. Stump height significantly affected tree mortality. A cut at breast height eliminated 22% of the trees, a cut 5-10 cm above the ground at the base of the trunk eliminated 36%, and a cut at or just below the ground surface eliminated 100%. The success of this Alnus elimination method is likely to depend on a combination of mechanical treatment and shallow inundation. Otherwise, Alnus removal will require additional measures, including herbicide treatment.