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 hectares have been lost in recent decades. However, various economic, social, and ecological factors often lead to temporary or permanent agricultural land abandonment, creating opportunities for passive restoration. Objectives: In this study, we identified the different vegetation states and transitions following agricultural cessation in old-fields of the Eastern Plains -one of the most agriculturally transformed areas of the Río de la Plata Grasslands- in order to evaluate their potential for passive restoration. Methods: We constructed a space-for-time chronosequence using 61 old-fields with varying time since agricultural abandonment (ranging from 2 to over 20 years). Phytosociological surveys were conducted in each old-field and reference natural grassland, and soil samples were collected from a subset of sites. Vegetation states were identified using NMDS ordination and compared to reference grasslands based on vegetation attributes. Results: We found five distinct vegetation states. Four of them formed a successional trajectory in which vegetation gradually converged with reference grasslands as abandonment time increased. 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 surveyed were found in a novel steady state which deviated from the restoration trajectory, driven primarily by the dominance of the invasive exotic species Cynodon dactylon. Conclusions: Although our findings demonstrate that passive grassland restoration is possible in this region, the process of species replacement that re-establishes the dominance of native perennial C4 grasses takes over twenty years. During this time, certain conditions may favor invasions, disrupting the successional trajectory and potentially leading to an alternative state from which recovery becomes difficult or unlikely. Implications: Recognizing the risk of transition to an invasive-dominated state in these old-fields highlights the need for both preventive measures and targeted restoration strategies. The findings of this study provide a conceptual foundation for adapting restoration efforts and informing decision-making aimed at promoting desirable ecological transitions in landscapes increasingly affected by land-use change.