Decrease in species richness and diversity, and shrub encroachment in Cerrado grasslands: A 20 years study

Question The Cerrado is a global biodiversity hotspot, highly threatened by changes in land use. Alterations in species composition and the colonization of Cerrado grasslands by woody species can lead to long-term changes in these communities. In this study, we investigated changes over 20 years in three grassland areas, in terms of species richness and diversity and growth-form cover, as well as the effects of shrub and grass encroachment on species richness. Location Cerrado Brazilian savanna, moist grassland and dry shrub grassland. Areas under low and high anthropogenic pressure. Methods Plant species composition and cover were sampled on seven occasions between the years 2000 and 2020, in permanent line-transects. We compared plant cover, richness, diversity and growth-form cover between the periods for each area. Next, we developed mixed-effect linear models to test whether species richness in grasslands at the latest sampling event was altered by species representing 70% of the species cover at this time. Results Grasslands located in areas under higher anthropogenic pressure presented reduced richness and diversity, and cover concentrated on a few species of shrubs and large tussock grasses. Moist grassland undergoing encroachment by the native shrub Trembleya parviflora had lost over 50% of its diversity since encroachment. In the same area, a shrub grassland undergoing encroachment by native shrub Raulinoreitzia tremula, three native grass species and one exotic grass presented significantly reduced species richness. The moist grassland in an area under low anthropogenic pressure showed a reordering of the species cover over time, but a decrease in species richness. Conclusion We observed changes in plant richness, diversity, cover, composition and growth-form cover over a period of 20 years in grassland communities. We found, however, different trajectories in the three grassland areas sampled, mainly due to the encroachment of a few species, leading to floristic homogenization in grasslands located in areas under high anthropogenic pressure.