More than half of the world's population inhabit urban areas, causing significant environmental changes. In underdeveloped countries this fact is expressed in a chaotic manner, generating social and structural problems. The city of Rio de Janeiro is a megalopolis known worldwide for its natural beauties, including beaches and forests and because of favelas (subnormal clusters of houses of economically poor people).The favelas are composed of a mosaic of cultures which reflects even in urban afforestation. In this work we characterize the afforestation structure of a favela to understand its ecological and social functionalities. The survey was conducted in the favela Morro da Formiga, with an area of approximately 30 ha. It was conducted through a census and using the list of species, a classification based on 11 parameters was carried out. 1,042 individuals were found, belonging to 110 genera, 148 species and 42 families. The most abundant species shows that the planters intend is, leastwise partially, generating an extra-support for community in terms of feeding fruits (62% of the subjects had this purpose). In relation to the nature thereof, the natives species represents 54% of the total, while the exotic species 39%. About 50% of the trees are in the backyards of houses and a few in squares and streets. It was found that 70% of individuals were planted by residents. Studies are needed to identify which native species can replace community demands currently supplied by exotic species that have been used for decades. The "forestation" in favelas is quite distinct from that implemented by the city hall in more urbanized areas. In favelas, the species reflect more about the knowledge of the local population, their backgrounds and needs.