Deforestation is a large contributor to climate change. Controlling deforestation is difficult, especially because there are 484 million of hectares of forest that are community owned. In principle this arrangement could lead to a tragedy of the "commons", however there could be economies of scale on monitoring outsiders' deforestation. We study the effect of communal titling on deforestation in Colombia, using a difference-in-differences strategy at the border. We find that deforestation decreased only in small communal titles, even when the control area are national parks. This suggests that communal titling can be effective on reducing deforestation if inhabitants can monitor the area titled.