Determinant factors to the dominance of African grasses in two tropical seasonally dry ecosystems

Abstract

In a biological invasions context, discussions about the relevance of competition to the community structure appear in the importance of biotic resistance and abiotic variables to the invasibility. Many African grasses have substituted grass species from neotropical savannas, through multiple mechanisms. Although this severe effect, the typical heterogeneity of savannas might have influence in their susceptibility to the invasions, and also in the outcome of interactions between native and exotic species. The objective of this study is identity the main biotic and abiotic caractheristics that have influence in the invasion of the vegetations types from the Estação Ecológica de Itirapina (EEI) by the African grasses Melinis minutiflora, Urochloa decumbens e Rynchelitrum repens. In each vegetation type from the EEI (cerrado sensu stricto, campo cerrado, campo sujo, campo limpo e campo úmido) we will make four 100 x 100 m plots divided in 10 x 10 m subplots. In five subplots from each plot we will observe monthly the phenology of the three Africans species and of the three most common native species at EEI (Andropogon bicornis, Loudetiopsis chrysothrix, Tristachya leiostachya). In the same plots we will evaluate the seed bank and seed rain composition. We also will obtain the community structure in 2 x2 m subplots, and will measure monthly the water table depth and the water available into the soil. We also will introduce seeds from each African grass into two subplots from each plot. In order to evaluate the disturbance effect, the will submit the plots to three different treatments: clipping the aboveground biomass, remove of both biomass and litter layer, and control without manipulations. Like a control we will use plots without introduction, submitted to the same disturbance treatments. In a controlled greenhouse environment we will evaluate the growth of each native species in monoculture or associated to each African species. In order to evaluate the water stress effect, we will repeat the experiment in five replicates to three different soil moisture levels. We will use a Factorial Analysis of Variance in order to estimate the vegetation type and disturbance effects under the African grasses establishment. We will apply a simple regression analysis in order to evaluate the competitiveness of each grass species under different densities, and a Covariance Analysis to assess the soil moisture effect in the interactions outcome. Keywords: Invasibility, invasiveness, competition, stress, disturbance, Melinis minutiflora, Urochloa decumbens. (AU)