Effects of native and invasive grasses on the survival and growth of tree seedlings in a neotropical savanna

Invasive grasses have spread over large areas of ancient savannas worldwide and have extensively impacted native landscapes in the neotropics. However, our understanding on how the displacement of native by invasive grasses may affect tree-grass coexistence in neotropical savannas is still poor. The present study tested the imposed effects of an invasive grass (Urochloa brizantha) and a native grass (Paspalum atratum) on survival rates, stem growth and biomass of seedlings of six native tree species representative of savanna physiognomies of the Cerrado biome. We conducted experiments under field conditions subjected to experimental manipulations of light, water supply and fertilization. Considering that grass performance might be reduced by shade, and that irrigation and fertilization might relax competition between grasses and tree seedlings for water and nutrients, respectively, we postulated that these treatments would modulate the effects of the grasses on tree seedling survival and growth. We found that both grass species reduced tree seedling survival, and the effects were not alleviated by shade, irrigation, or fertilization. The presence of either native or invasive grasses reduced seedling stem length, irrespective of fertilization and shade. Irrigation alleviated the negative effects of both grasses on stem length. Both grass species reduced seedling biomass, irrespective of light conditions, irrigation, or fertilization. The impacts imposed by the invasive grass on the tree seedlings were larger than those imposed by the native grass, indicating that the displacement of native by invasive grasses can strongly affect recruitment potential of tree species. Due to the extent of grass invasion in neotropical savannas, and its potential impacts on recruitment patterns of native trees, adequate policies and management plans are needed to control invasive grass spread to conserve native biodiversity and ecosystem services.