Ancient mounds, modern refuges: Out-of-production sites on kurgans support rare weeds in agricultural landscapes

Rare weeds (RW) are disturbance-tolerant plants that enhance agrobiodiversity and related ecosystem services without adversely affecting agricultural production. However, due to agricultural intensification, RWs are declining across Europe, and their conservation requires targeted measures addressing their special ecological requirements. We aimed to explore the potential of out-of-production sites for maintaining populations of RWs characteristic of arable lands, old-fields, and grasslands. In an intensive field survey, we collected data from 216 sites in Hungary, including sites covered with spontaneously recovering grassland vegetation and reference grasslands. We aimed to identify site- and landscape-specific factors (i.e., geographic position, landscape transformation, habitat area, environmental heterogeneity, soil properties, age of the vegetation, and other vegetation attributes) influencing the occurrence and species richness of RWs. We recorded 38 RW species, including 15 red-listed and two protected ones. RWs occurred on 50.9 % of the study sites, indicating that out-of-production sites can provide refuge for RWs associated to croplands, old-fields, and grasslands. Environmental heterogeneity was the most important factor supporting the occurrence of RWs, particularly grassland-related RWs. Poisson models revealed that the total number of RW species was lower in northern sites. The number of arable RWs was higher in sites with high soil CaCO₃ content, while high soil phosphorus content supported fewer grassland RW species. Sites with diverse vegetation harboured more old-field and grassland RWs. Land sparing through the maintenance of out-of-production sites and supporting environmental heterogeneity and establishment gaps can contribute to the conservation of RWs in agricultural landscapes.