The vegetation gradient along the longitudinal profile of a braided stream: a case study in central Italy

The aim of this study was to evaluate the importance of the variation of environmental variables (geomorphological, topographical and climatic factors) along the lon gitudinal profile of a braided stream for the distribution of plant species and communities at a local scale. This was achieved by means of a case study in central Italy. Vegetation data from the river bed were grouped according to floristic and ecological affinities as follows: semi aquatic, helophytic, hygrophilous pioneer grasses, synantropical grasses, mesohygrophilous and pioneer grasses, garigues, hygrophilous shrubs, thermophilous shrubs, xerothermophilous shrubs and grasses, hygrophilous forest and thermophilous forest. A clear division between herbaceous and woody communities was observed along the first DCA axis and was inter preted as a transversal gradient of the stream due to decreasing flooding. Canonical Correspon dence Analysis suggested that altitude, distance from sea and bed width are the most important environmental variables explaining community distribution along the longitudinal profile. However, on this local scale, partial CCA showed that the abiotic variables explained 49.6% of the species data variation. This result, together with the lack of a correlation between environ mental factors and DCA axes 1 and 2, suggests that factors other than those included in the analysis, and probably only marginally linked to the longitudinal gradient, influenced the dis tribution of communities as well. On a local scale, we observed that there was no variation in the floristic composition of the coenoses along the longitudinal gradient. Consequently there is no vicariance of communities in terms of climate and/or biogeographic region. Along the longitudinal profile we found, however, the presence of coenoses that were ecologically (hard wood and soft wood) or structurally (dominance of woods, garigues, or pioneer coenoses) different in terms of geomorphological and topographical variables.