How fire at different times a year affect the relationship between invasive grasses and the Cerrado native vegetation?

African grasses are one of the major threats to Cerrado’s biodiversity conservation due to their broad distribution inside nature preserves. Melinis minutiflora and species from the genus Urochloa, like U. brizantha and U. decumbens, are the most common invasive species, being able to alter disturbance regimes, like natural fires. Fire is a recurrent element in savannic ecosystems, and thus, savannic plants are resilient and adapted to fire regime. This study evaluated 1) the impact of Melinis minutiflora and Urochloa brizantha on native Cerrado vegetation; 2) the effect of fire season on their control; and 3) modifications of diversity in invaded communities due to fire season. The study area is located inside the Estação Ecológica de Itirapina, and until 1998, was used as Pinus caribea plantation, since when fire and cattle grazing were excluded. The experimental design is composed by 32 plots (15x15m) dominated by one of the two invasive species (minimum cover of 50%) randomly submitted to three fire treatments: Early- (May), Mid- (July) and Late-Dry (October) fire season, in addition to unburned controls (4 plots/treatment/invasive species = 32 plots). All fire experiments were conducted in 2014 (May, July and October) and vegetation sampling occurred before burning and during two years after its execution. Biomass and percentage cover data was collected at subplots inside the experimental plots (15x15m) due to spacial heterogeneity in the post-fire environment. Aiming to answer our questions, we used 1) the Cohen’s D impact index; 2) mixed additive models to analyze the temporal variation of biomass from native and invasive species; 3) multivariate analysis (PERMANOVA and PCoA) of native and invasive cover data to investigate diversity modifications on burned and unburned plots. Both invasive species respectively impacted more and less the native graminoids and forbs. Melinis minutiflora exerted more pronounced effects than Urochloa brizantha due to its huge biomass production, and there is also a seasonal variation of its impact. In general, invaded and non-invaded communities differed in structure, as well as communities invaded by distinct species. Fire in any season reduced M. minutiflora abundance, while U. brizantha was not affected by fire, despite its total biomass had been temporarily reduced until the end of the next rainy season after burning. On communities initially invaded by M. minutiflora, Early- and Late-Dry fire season favored a new invasion by U. brizantha due to this species propagule availability. Biomass of forbs was increased only in communities initially invaded by M. minutiflora burned at the Mid-Dry season. Overall, the invasive species identity was influential on the effect of fire season. On communities invaded by M. minutiflora, Early- and Mid-Dry fire season increased diversity in relation to unbuned controls, while Late-Dry fire season resulted in less diverse communities after two years. On communities invaded by U. brizantha, fire in any season decreased diversity, but two years after burning there is a tendency to resume values from pre-fire conditions. The regeneration of the initially dominant invasive species influences the community response to fire season. M. minutiflora was reduced by fire in any season, creating open gaps inside the vegetation and leading to stochastic colonization process, mainly guided by propagule availability from native species and from U. brizantha. On the other hand, on communities dominated by this species, its fast regeneration had inhibited the establishment of native species due to space colonization and overlapping of the temporal niche. Considering the results found by this study, we recommend the use of prescribed burnings at July (Mid-Dry season) to control Melinis minutiflora and to maximize the success of Cerrado restoration projects. However, the use of prescribed burnings to control U. brizantha should only be used to temporarily reduce its biomass, favoring the implementation of other management actions like direct seeding of native species. (AU)