Tree growth and field vegetation in forest edge zones on a property in southeast Sweden

Abstract: Summary  In the 1990s a new way of thinking about forestry started. One of the effects of the changing silvicultural practices was that the size of clearcuts became smaller. As the felling areas were reduced in size, the areas where young and older forest meet grew, with this also the occurrence of edge zones increased. An edge can be defined as an interface between different ecosystems, and a forest edge as an interface between forested and non-forested ecosystems, or between two forests of contrasting composition or structure. This edge results in a transition zone in which the adjacent contrasting ecosystems interact - the edge zone. In the edge zone - both abiotic and biotic - processes result in a detectable difference in composition, structure or function of the vegetation near the edge, as compared with the ecosystem on either side of the edge.  This study aimed to increase understanding of occurrence and types of edge zones and their composition on a property, their effect on tree growth, the species diversity of tree and field vegetation. The study was performed on a property of approximately 60 hectares located in southeastern Blekinge. The sampling procedure was based on four transects ranging from 527 to 860 meters in length, which were laid out in south-north direction over the property. Wherever the transect crossed a forest edge, a corridor perpendicular to the edge was laid out. In each corridor a measuring grid dividing the corridor in 10 distance classes of one meter, measured from the forest edge, was laid out. In each distance class tree species were determined, stems counted, and the following variables were measured per tree: height, distance from edge and the diameter at breast height. Data gathering on forest field vegetation was conducted in 1x1 meter plots laid out in the middle of the corridor. Within each sample area, the occurrence of species was noted according to field vegetation type index. For comparing the edge zones to the rest of the estate, reference plots were laid out along the transects between the forest edges. On the property of 60 hectares an estimated 4,2 hectares were influenced by edge effects. All edges were created by anthropogenic processes, there were forest edges with roads and edges created by forestry. Where thinning stands were bordering roads, growth was found to decrease closer to the road. In pre-commercial thinning stands bordering a road the opposite effect was visible. Where old and young productive forest stands bordered, on the young side there was an increase of both tree height as well as diameter further away from the edge. On the old side no edge effect on tree growth was seen. Herbaceous plants, mosses and lichens were more abundant close to the edge in both old and young stands. Borders with roads had the highest diversity of tree and plant species. The field vegetation was found to be more diverse and abundant closer to the edge. The edge zone showed to be of great importance for natural regeneration. In about half the corridors positive, negative or neutral edge effects were found on one or more of the variables: tree height, diameter, basal area, stems per hectare, plants per hectare. These effects were found up to eight meters into the stand. The inventory of tree species and field vegetation showed that different kinds of edge zones give rise to different species compositions. A higher diversity of tree species was found in the edge zone compared to the reference plots. One could use the edge zone for nature conservation purposes as these areas favor biological diversity and constitute interesting environments for species that would otherwise be at risk of being outcompeted in neighboring, more closed spruce forest stands. Keywords: tree growth, diversity, field vegetation, edge zones, edge effect