Floristic heterogeneity of the last grassland remnants in an agricultural landscape: a phytosociological study in the Uruguayan Eastern Plains.

Aims. The Eastern Plains of Uruguay are part of the Río de la Plata Grasslands (RPG), one of the largest grassland areas globally. Due to the spread of rice agriculture, this region has experienced significant fragmentation and grassland loss. Relictual grasslands represent an underexplored source of native species diversity. Our study aims to describe the floristic heterogeneity in these remnants and recognize the determinants of plant variation in the area based on their association with different environmental variables. Study area. The Eastern Plains in Uruguay, with an extension of 743,600 ha, is located in the Southeast region of the RPG in South America. Methods. We conducted phytosociological relevés of natural grasslands in areas that were carefully verified to have no agricultural history. We performed agglomerative hierarchical analyses to identify relevé clusters, and indicator species analyses. We characterized and compared relevé groups based on species richness and Shannon diversity index, indicator species, composition and cover of Plant Functional Types, and structural attributes of the vegetation cover. We carried out a two-dimensional NMDS and its correlation with different environmental variables to identify the drivers of the main floristic gradients. Results. Our analysis revealed two communities that constitute an intricate vegetation mosaic. Changes in floristic composition are observed over short distances, with one community characterized by hydrophytic species and the other by mesophytic species. The differences in the two communities respond to subtle micro-topographical differences in the landscape mosaic. Both communities are dominated mainly by native C4 perennial grasses with very low presence of exotic species, and some degree of internal heterogeneity that could be explained by grazing management. Conclusions. Our results demonstrate the resilience of Eastern Plains grasslands, despite their advanced state of loss and fragmentation. However, overgrazing and the presence of a strong invasive exotic species pose significant threats to them. [ABSTRACT FROM AUTHOR]