Determinants of invasion by single versus multiple plant species in temperate lowland forests

Invasions of alien plants pose a serious threat to native biodiversity and ecosystem processes. Forests are considered more resistant to invasion due to limited light availability in understories. However, disturbance and abiotic stress may open tree canopies and promote invasion. Their combined effects together with the resistance of resident species may determine the numbers and abundances of invasive species. Here we explore how canopy openness, water stress, and taxonomic and functional properties of resident communities affect the invasion by a frequent single invasive species (Aster lanceolatus and Impatiens parviflora) compared to that by multiple invaders in Central European lowland forests. Different abiotic factors and species-specific mechanisms of invasiveness determined the success of single versus multiple invaders. The massive spread of A. lanceolatus was associated with the long-distance seed dispersal and exploitation of available resources by fast growth resulting in formations of compact clonal patches in disturbed, open-canopy floodplain forests. The success of I. parviflora was caused by avoiding competition via tolerating less favorable conditions under the dense tree canopy on drier sandy soils. A. lanceolatus thus suppressed resident species richness, while I. parviflora spread in communities of higher functional and phylogenetic diversity. Multiple invasive species, mostly represented by subordinate species with low cover, colonized forests that were rich in resident species. We conclude that a combination of intense disturbance and stress favor the invasion of single dominant species that act as drivers of changes in native communities. Multiple invasive species colonize forests with less extreme conditions acting more as passengers who increase rather than decrease forest diversity.