Impact of invasive grasses on Cerrado under natural regeneration

Cerrado is the Brazilian neotropical savanna threatened by invasive African grasses. We aimed to quantify the impact of invasive Melinis minutiflora and Urochloa brizantha on the cover of different functional groups (native graminoids, forbs, shrubs) and the structure (bare soil and the cover of natives' and invasives' dead biomass) of regenerating plant communities. We hypothesized that the impact of invasives would be negative and more pronounced in the rainy period, given the great influence of seasonality in savannas. In three community types (non-invaded, invaded by M. minutiflora and invaded by U. brizantha) we evaluated the cover of functional groups and structural components by sampling 120 1mx1m plots, four times a year. Using the Cohen's D impact index, we showed that both invasive species reduced the cover of all native functional groups, decreased bare soil and increased total dead cover. Greatest effects occurred when M. minutiflora was present. M. minutiflora's impact on total graminoids varied from positive in the Early-Dry season to negative in the Mid-Dry season, while the negative impact of U. brizantha on bare soil became more pronounced from the dry to the rainy season. Differences in the impacts caused by the invasive species are probably due to the large biomass produced by M. minutiflora versus the fast colonization and soil occupancy by U. brizantha. Overall, invaded versus non-invaded communities differed in structure, as well as both invaded communities differed from each other. Our results demonstrate the need to control these species for conservation and restoration of Cerrado ecosystems.