Post‐agricultural restoration of Uruguayan grasslands: successional trajectories and risks of alternative invaded states

Introduction Agricultural conversion is the leading cause of grassland loss worldwide. In the Río de la Plata grasslands, 2.4 million ha have been lost in recent decades. However, economic, social, and ecological factors often result in land abandonment, creating opportunities for passive restoration. Objectives We identified vegetation states and transitions following agricultural cessation in old‐fields of the Eastern Plains—one of the most transformed areas of the Río de la Plata Grasslands—to evaluate their potential for passive restoration. Methods We constructed a space‐for‐time chronosequence using 61 old‐fields spanning 2 to greater than 20 years since abandonment. Phytosociological surveys were conducted in old‐fields and reference grasslands (RG), and soil samples were collected from a subset of sites. Vegetation states were identified using non‐metric multidimensional scaling ordination and compared to RG based on vegetation attributes. Results Five distinct vegetation states were identified. Four formed a successional trajectory characterized by gradual convergence toward RG over time. After two decades, old‐fields were no longer distinguishable from reference sites. Community structure along this trajectory was strongly correlated with soil organic carbon content. In contrast, one‐third of the old‐fields were found in a novel steady state dominated by the invasive exotic species Cynodon dactylon , diverging from the restoration trajectory. Conclusions Passive grassland restoration is feasible in this region, but the re‐establishment of native perennial C4 grass dominance requires more than 20 years. During this period, invasion‐prone conditions may interrupt succession and lead to alternative states from which recovery is difficult or unlikely.