Landscape structure and diversity of fleshy-fruited species at forest edges

This study investigates correlations between mean alpha-diversity of woody species (fleshy-fruited and non-fleshy-fruited) at forest edges and the attributes of the sites and of the adjacent landscape that might influence it. For a total of 45 forest edges in three areas in northern Switzerland species composition and structure of the edges were recorded. Based on aerial photographs, maps and a ground survey, the adjacent landscape within a radius of 100 m was mapped with GIS for two time periods (1952-1954 and 1994-1997). Diversity of fleshy-fruited species was higher at forest edges adjacent to nutrient-poor grassland or small roads compared with fertilized meadows. A positive correlation existed between diversity of fleshy-fruited species and density of isolated trees in the surroundings, although some trees have disappeared since 1952. Multiple regression analyses revealed that the diversity of fleshy-fruited species correlated positively with edge height and depth, and negatively with mean patch size in the adjacent landscape; a significant fraction of the variation among sites was also explained by the geological bedrock. Only a few other attributes of the sites and of their surroundings had a significant effect. Mean alpha-diversity of non-fleshy-fruited species correlated with only two landscape attributes, i.e. total forest area and geology. The results are discussed on the basis of dispersal of fleshy-fruited species by passerine birds. The resulting hypothesis is that an increase in species diversity along forest edges is partly controlled by bird-mediated landscape effects.