EFFECTS OF ROBINIA PSEUDOACACIA COVERAGE ON DIVERSITY AND ENVIRONMENTAL CONDITIONS OF CENTRAL-NORTHERN ITALIAN QUERCUS PUBESCENS SUB-MEDITERRANEAN FORESTS (HABITAT CODE 91AA*): A THRESHOLD ASSESSMENT

The invasive alien species Robinia pseudoacacia may lead to species homogenization in high invaded forest ecosystems. Its invasive behaviour is poorly known in dry forest habitats and low cover-abundance conditions. We have investigated central-northern Italian Quercus pubescens forest habitats (code 91AA* - EU Habitat Directive) without and with the presence of R. pseudoacacia and in respect of R. pseudoacacia dominant communities that are present in the same forest potential areas. Using levels of R. pseudoacacia cover-abundance values (Braun-Blanquet scale) we classified the vegetation releves in five groups: from 0 (total absence) to 4 (dominant conditions). Through the calculation of some indices, we highlighted the relations between groups of releves and the variation in term of ecological features. A threshold has been identified at low coverage values of  R. pseudoacacia. We detected a significant difference in the lower invaded sites in term of presence of nitrophilous and alien species, Shannon diversity index and changes in nutrient, moisture, light and reaction Ellenberg indicator values. These results inform about the early alarm level to set in monitoring Q. pubescens forest habitats in sub-Mediterranean areas.