Linking the diversity of native flora to land cover heterogeneity and plant invasions in a river valley

Plant invasions and land cover changes are two important threats to biodiversity. River valleys, which are considered as hotspots of biodiversity, have been subjected to the both threats for centuries. Here we examined the impact of river bed proximity and land cover heterogeneity on the species richness of native, red-listed and invasive plants as well as the spatial associations between the three plant groups for alpha-, beta- and gamma-diversity. Surveys were conducted in 140 plots (1 km^{2} each) in the San River Valley (SE Poland). Our study showed that proximity to the river bed and land cover diversity was positively associated with both native and invasive plant species richness. The species richness of all three plant groups in the studied plots (alpha-diversity) was positively correlated across space. However, invasive plant species richness was negatively linked to beta- and gamma-diversity of native and red-listed species. In contrast, native plant species richness correlated neither with beta- nor with gamma-diversity of invasive species, thus, the hampering effect of high species richness on invasions was not confirmed. We conclude that studies of invasive plants should include multiple diversity levels as the effects may be hidden when evaluations are only made at the local spatial scale (alpha-diversity). Our study suggests that maintenance and restoration of forests close to the river may hamper alien plant invasions