In response to the important benefits forests provide, there is a growing effort to reforest the world. Past policies and current commitments indicate that many of these forests will be plantations. Since plantations often replace more carbon-rich or biodiverse land covers, this approach to forest expansion may undermine objectives of increased carbon storage and biodiversity. We use an econometric land use change model to simulate the carbon and biodiversity impacts of subsidy driven plantation expansion in Chile between 1986 and 2011. A comparison of simulations with and without subsidies indicates that payments for afforestation increased tree cover through expansion of plantations of exotic species but decreased the area of native forests. Chile’s forest subsidies probably decreased biodiversity without increasing total carbon stored in aboveground biomass. Carefully enforced safeguards on the conversion of natural ecosystems can improve both the carbon and biodiversity outcomes of reforestation policies.