Forest plant diversity is threatened by Robinia pseudoacacia (black-locust) invasion

The effects of black-locust invasion on plant forest diversity are still poorly investigated. Vascular plants are likely to be influenced by increasing nutrient availability associated with the nitrogen-fixing activity of black-locust, whereas it is not clear if, along with stand aging, black-locust formations regain forest species. The main aim of the present study was to test whether the increase of black-locust stand age promoted a plant variation in mature stands leading to assemblages similar to those of native forests. Therefore, plant richness and composition of stands dominated by native trees were compared with pure black-locust stands of different successional stages. Our study confirmed that the replacement of native forests by pure black-locust stands causes both plant richness loss and shifts in species composition. In black-locust stands plant communities are dominated by nitrophilous species and lack many of the oligothrophic and acidophilus species typical of native forests. Plant communities of native forests are more diverse with respect to pure black-locust stands, suggesting that black-locust invasion also causes a homogenization of the plant forest biota. We did not detect differences across the successional gradient of black-locust stands, and mature stands do not recover the diversity of plant species which are lost by the replacement of the native forests by black-locust. Accordingly some efforts in reducing the negative impacts of black-locust invasion on plant forest biota should be focused at least in those areas where conservation is among management priorities, such in the case of habitats included in the Habitat Directive (92/43 ECE).