Permeability of ancient forest edges for weedy plant species invasion

Within highly fragmented forest landscapes, the flux of matter, energy and species from the landscape matrix into the forest fragment interior is an important ecological phenomenon. We studied the invasion of ancient forest edges by weedy plant species, which are normally confined to the agricultural landscape matrix. We related this to edge orientation and to the occurrence of natural and anthropogenical environmental edge gradients like microclimate and soil chemistry. The plant community composition of the forest edge zone differed significantly from the community composition of the forest core area, and is characterized by a relatively high number of competitive and light demanding species. The maximal width of this edge zone is more extensive at south facing (maximum ca. 20-23 in) than at north facing edges (ca. 0-3 in). However, most of the weedy plant species were unable to penetrate the forest and were confined to the boundary between forest and arable land. We found no negative edge effects on the more stress tolerant 'true' forest species going further than 0-3 m (only at south facing edges). The invasion pattern generally fitted the edge gradient for photo synthetic ally active radiation (PAR). The negative effects of fertilizer misplacement from adjacent arable land on forest soil chemistry (and community composition) are also restricted to the boundary of the forest edge zone. Based on the derived penetration distances of the edge effects and on the shape index of a forest patch it is possible to derive the minimal required forest patch area for forest plant species conservation. (C) 2002 Elsevier Science B.V. All rights reserved.