Roadside invasion of Dittrichia graveolens in Slovakia and prediction of its potential future distribution

The aim of this study was to determine the current distribution of Dittrichia graveolens along motorways in Slovakia, including adjacent sections in the Czech Republic, Austria, and Hungary, to assess to what extent the density of the species depends on the age of operation of the studied motorways, and to predict the potential distribution of the species in Slovakia for the period 2021–2040. In total, 1,440 occurrence points of the species were recorded along 419.9 km of surveyed motorways (out of 826.5 km of surveyed roads). Of the 1,157 occurrence points, the highest proportion (1,065 points) was recorded in the western part of the country, with occurrence decreasing towards the east. The generalized additive mixed model showed that the occurrence density of D. graveolens was highest on older motorway sections, specifically those constructed in the late 1970s and late 1990s, while the most recently built sections (post 2012) exhibited significantly lower invasion densities. The MaxEnt model for D. graveolens under the SSP370 scenario demonstrated robust predictive performance (AUC = 0.85, TSS = 0.61). Focusing on the primary study region, the model revealed that over 24,000 km² in Slovakia are climatically suitable for future invasion. However, given the species’ strict concentration along immediate road edges, the actual potential area of occupancy represents 32.9 km² (0.07% of the total national area) localized within road verges. While current occurrences are heavily concentrated in the western lowlands, our predictions indicate a significant potential for continuous eastward expansion into the central and eastern regions along the road network. The primary climatic constraint is the minimum temperature of the coldest month (Bio06), which limits the species at higher altitudes. Our findings confirm that the D. graveolens invasion in Slovakia is driven by a combination of road operational age and climatic suitability. These road verges serve as critical corridors, facilitating the species’ predicted eastward expansion into new regions.