Enemy Behind the Gates? Predicted Climate Change and Land‐Use Intensification Likely Speed Up <scp>C4</scp> Grass Invasions in Europe

ABSTRACT Aims Climate change is expected to bolster opportunities for alien species' establishment and spread. In Europe, alien C4 grass species have the potential to benefit from a changing climate, being better adapted to higher temperatures and heat stress. Our aim was to compile an up‐to‐date inventory of alien C4 grass species in Europe, to find information about their distribution and country‐ and region‐wise statuses, to identify the most widespread and high‐risk species, and the most invaded habitat types, and to draw the temporal trends of their European establishment. Location Europe. Methods We used the standard systematic literature review methodology to identify relevant publications and online plant distribution databases to get an up‐to‐date list of alien C4 grass species in Europe. We aimed also to gather information on (1) their current country‐ and region‐wise distribution; (2) alien status in each country (casual, naturalised, invasive); (3) the most widespread and high‐risk species; (4) native climatic zones and habitat preferences of each species in European countries; and (5) temporal distribution patterns of the identified species. Results We detected 133 alien C4 grass species across Europe, with the highest number of species in western European countries (103 species) and the lowest in northern Europe (69 species), with considerable overlap amongst regions. Southern Europe stands out with the highest number of naturalised (55) and invasive (21) species compared to other regions, while northern Europe has the highest number of casual species (53). We identified the most widespread and high‐risk species, including those from tropical and subtropical climatic zones, which pose the highest invasion risks. These species predominantly occur in ruderal and anthropogenic habitat types, but also in natural habitat types, especially in grasslands. Regarding temporal trends, we detected an alarming increase in the establishment of tropical C4 grasses in Europe in recent decades. Conclusions Many European countries already harbour a high number of C4 grass species, but with different alien statuses. Besides climate and invasion potential, human activities are also important factors in the ongoing invasion process of alien C4 grass species. Collective actions are required to halt this alarming biodiversity problem.