Extreme drought facilitates shift to non-native C4 perennial grass dominance in a temperate grassland in Central Europe

Climate change is expected to facilitate the spread of many alien C 4 grasses, which perform better in warmer climates than C 3 species. Furthermore, extreme climatic events may promote plant invasions by suppressing native species. Despite several well-documented cases of C 4 perennial grass invasions, the role of extreme events in facilitating plant invasions is rarely documented. Sporobolus cryptandrus , a North American C 4 perennial bunchgrass, has recently been established in several European locations, including mass invasions in natural grasslands. Given the early stage of S. cryptandrus invasion and its potential for future spread, a better understanding of the dynamics and mechanisms of its invasion is urgently needed. In a natural sand steppe in Hungary, an EU priority habitat type with one of the largest S. cryptandrus populations in Europe, we monitored S. cryptandrus invasion and changes in species dominance in permanent plots annually from 2022 to 2024, which included an extreme drought in 2022. In the post-drought years, we found a gradual increase in both the frequency of S. cryptandrus and the number of plots where it was the dominant species. Following the drought-induced dieback of the dominant native grass, Festuca vaginata , the cover of S. cryptandrus almost doubled from 2023 to 2024 and approached that of F. vaginata . This expansion was positively related to F. vaginata dieback. Our study demonstrates that S. cryptandrus can spread and become dominant rapidly in natural grasslands. Furthermore, our results provide empirical evidence on how extreme events can accelerate an ongoing plant invasion process.