Local, forest stand and landscape-scale correlates of plant communities in isolated coastal forest reserves

In mixed oak stands situated within two isolated forest reserves in NE Italy, we investigated how plant communities are modulated by local conditions, forest structure and landscape attributes. Species richness and functional dispersion increased towards canals, whereas soil salinity, canopy density or landscape heterogeneity were less relevant. Mean nutrient indicator values increased near canals and with higher proportions of agriculture around. Functional dispersion decreased at wet, nutrient rich sites. Also, the proportion of salt tolerant species increased towards canals, but was unrelated to measures of soil salinity. At sites with more modified landscapes around, widely distributed species were more prevalent, at cost of plants with restricted distributional ranges. Hence, biotic homogenization is fostered inside the reserves through landscape modification in their surroundings. In contrast to species richness, composition turned out to be markedly modulated by environmental variation, with local site factors, forest stand structure and landscape attributes contributing to roughly the same extent. Conservation practices should therefore not only focus on managing local conditions, but also take landscape structure into account. For coastal forests, dry and open, nutrient poor sites are of special conservation concern, which are believed to most closely resemble the original diverse vegetation of these Mediterranean habitats.