Explosive spread of sand dropseed (Sporobolus cryptandrus), a C4 perennial bunchgrass, threatens unique grasslands in Hungary (Central Europe)

Sporobolus cryptandrus is a C4 perennial bunchgrass native to extensive areas of North America. As a non-native species, it has been reported from several continents, and it has been described as a transformer species in sand steppes of Central and Eastern Europe. However, its spreading ability across the landscape and within habitats has not been quantified, and factors determining its success have not yet been assessed. In this study, we focused on the largest stronghold of S. cryptandrus invasion in Hungary, where the species was first recorded in 2016, and investigated its present distribution in the landscape by mapping along dirt roads. In a separate local study in a heavily infested sand dune site of 2 km2, we assessed the infestation level and factors affecting the species’ establishment. Our landscape-scale mapping found that in April 2023, the distribution of S. cryptandrus encompassed a largely contiguous 600 km2, with documented presence from 282 1-km2 mapping units. The species occurred more than 5 m away from roads in 71 mapping units, mostly in the centre of its distribution area. Sporobolus cryptandrus presence was negatively related to soil organic matter content and positively related to sand content. At the local scale, we found the species in 39% of vegetation plots in a sand dune site originally covered by Pannonic sand steppes, a priority habitat in the EU Habitats Directive. Sporobolus cryptandrus presence at this site was negatively related to the total cover of resident grassland but, surprisingly, was unrelated to the distance from roads. Collectively, these results suggest that primary spread occurs mostly along roads; these roadside populations likely serve as sources for establishment in neighbouring grasslands, but subsequent mass invasion becomes independent from roads. Together with the species’ broad macroclimatic tolerance in North America and reported mass invasion events in Ukraine and Russia, our results suggest that S. cryptandrus likely poses a broad-scale threat to Eurasian dry grasslands, in particular on coarse-textured sandy soils with low vegetation cover.