BIRD ASSEMBLAGES IN MOSAIC FORESTS - THE RELATIVE IMPORTANCE OF VEGETATION STRUCTURE AND FLORISTIC COMPOSITION ALONG THE SUCCESSIONAL GRADIENT

We examined the hypothesis that birds choose their habitat on a large scale according to structural features of the vegetation, but that, wi thin homogeneous habitat types, bird assemblages are more structured b y the taxonomic composition of the plants. We studied bird assemblages in riparian forests near lake Grand Lieu, Brittany, France. These for ests are mosaics composed of patches of different vegetation types, wh ich cover the entire range of the forest succession. We used partial c anonical correspondence analysis to investigate the contributions of t he floristic and structural components of vegetation along the success ional gradient, restricting the scale of investigation from the entire range of the succession toward mature forest. Vegetation structure ac counted for the highest individual fraction of variation at the large scale, but the importance of floristics increased at smaller scales. T hese differences in contribution of sources of variation are statistic ally significant. We analysed the individual contribution of each vari able, and it became apparent that this pattern was due to a strong inc rease of the importance of the richness of tree species, in contrast t o the decrease of the importance of variables describing the structure of trees. These results are consistent with the hypothesis.