Plant species diversity in alien black locust stands: A paired comparison with native stands across a north-Mediterranean range expansion

Black locust (Robinia pseudoacacia L.) is a widespread alien tree species commonly thought to influence plant assemblages. The aim of this study was to compare the plant diversity between black locust and native recent secondary stands within the European Mediterranean Mountains environmental zone. Spontaneous reforestation was detected by comparing historical aerial photographs and the most recent images. Distributed throughout a 2700 km2 hilly and piedmont area, 32 black locust and 32 paired native stands were selected and all vascular plant species were surveyed in a 100 m2 area. Analyses of the α and β-diversity were performed separately for six identified plant groups. Despite a clear difference in the tree diversity between the black locust and native recent secondary stands and a homogenisation of the tree layer by the black locust stands, we found only inconsistent hints for homogenisation of the ground-layer vegetation by the black locust stands. There is no evidence to suggest that the presence of black locust in recent secondary stands plays a major role in shaping the diversity of the understory plant groups compared to native stands.