Human activities have resulted in novel ecosystems around the globe, but many of these ecosystems are poorly understood, particularly when multiple disturbances interact to create novel combinations of species. The Campos ecoregion of South America is one of the most extensive grasslands globally, yet is unique in the fact that there has been little historical evidence of herbivory or fire that traditionally maintain grassland ecosystems. Livestock were introduced several centuries ago, and afforestation began several decades ago. Introducing afforestation to an already grazed system can further alter light conditions, soil characteristics, and moisture regimes, which may result in decreased species richness, increased plant species introductions, and altered abundances of grasses, herbs, and woody species. We examined agroforestry management cycle effects on vegetation species richness, frequency, and composition in Eucalyptus grandis plantations, studying five phases of the management cycle (grassland, young forest, mid-stage forest, old forest, and post-harvest). At each of 25 sites, we recorded species presence in 40 quadrats (1 m2) and categorized species by origin (native or introduced) and life form (annual graminoid, annual herb, perennial graminoid, perennial herb, woody, or fern). Impacts on species richness and composition were greatest in mid-stage forests. The proportion of introduced species did not differ by management phase. Life form categories responded strongly but differently to management phases. Our research suggests that afforestation initially reduces vegetation richness and alters composition from that in grasslands, although there is considerable recovery following thinning and tree harvest. Factors that enhance recovery in this system include the presence of disturbance-adapted vegetation, proximity to remnant grassland patches, and the short rotation cycles of plantation forests. Further research is needed to understand the effects of multiple rotations, effects of afforestation on taxa other than plants, and consequences of landscape fragmentation across the Campos. While our research suggests that afforestation has created a novel ecosystem that is a variant of the system that evolved under long-term grazing, the extent to which this new condition will evolve over time with the continued presence of grazing and repeated forest management cycles remains unknown.