Different post-fire stages encompass different plant community compositions in fire-prone grasslands from Southern Brazil

Fire is among the major forces shaping patterns in nature. Although sometimes it is only acknowledged for its destructive power, it was one of the evolutionary drivers that produced present-day species and biome distributions, being inextricably linked to fire-prone ecosystems such as grasslands. Knowing how fire influences grassland biodiversity is therefore fundamental to conserve it. Here we analyzed the taxonomic and functional structure of grassland plant communities from Southern Brazil under different post-fire successional stages. We sampled 12 sites that were fire-suppressed for varying amounts of time (3 to 300 months), categorized in three post-fire stages: freshly-burnt (FB), intermediate-burnt (IB), and old-burnt (OB). We compared these stages regarding taxonomic composition, species richness, diversity (partitioned in different spatial components), and functional composition (based on plant life forms and metabolism). We sampled 307 plant species from 52 families. Species richness was lower in the OB stage, and did not differ between FB and IB stages. Species composition was markedly different between stages. Forty-five percent of the sampled species were exclusive to one post-fire stage, and only 23% were shared among them. Old-burnt sites share only up to 5% species with other stages and concentrated most of the trees and tussocks. Because long-term fire suppression allowed for the entry of different floristic elements in the community, taxonomic diversity (beta and gamma levels) was higher in old-burnt sites. Our results provide empirical evidence that fire suppression is likely to shift the fire-prone grasslands from Southern Brazil towards different ecosystem states. However, whether this process is leading towards a closed forest system or to a different system with coexisting grassland and forest elements, remains an open question. The sharp floristic differences and exclusive species in the different post-fire stages reinforce the need to include fire in the conservation framework for fire-prone grasslands.