This paper aims to discuss an essay by the anarchist writer Rafael Barrett - Lo que son los yerbales paraguayos, published in 1908. The essay constitutes a strong denouncement of work conditions in the plantations of yerba mate on the triple border between Paraguay, Argentina and Brazil, as strong as the Casement report on Congo, released in 1904. Barrett himself acknowledged his proximity to the Casement report, noting that it was necessary for the world to know that atrocities were not only practiced in the African colonies, but also occurred in the young South American republics. Reading the essay, it will highlight first the concept of the forest as a prison, a concept crucial to the maintenance of the debt peonage on which the yerba economy developed - a system that Barrett did not hesitate to classify as enslavement. Nevertheless, the author envisages in the forest, in the impassible reproduction of nature an instance of resistance to the necropolitics which presided the yerba mate explotation of labour. The paper argues that such views gain full inteligibility within the framework of the libertarian biopolitical project which Barrett embraced.