Tree plantations can contribute to achieving several Sustainable Development Goals (SDGs) by creating job opportunities, storing carbon, and providing wood products that relieve pressure on natural forests. However, their impacts on SDGs are largely contingent upon the land uses they replace. Here we present a framework for understanding the development trade-offs associated with tree plantations and we introduce a new remote sensing technique to detect their expansion and pre-conversion land use. Using this approach in northern Mozambique, we found that 70% of 2001-2017 tree plantation expansion occurred on cropland, potentially exacerbating poverty, food insecurity and rural unemployment. The remainder occurred on natural vegetation, adversely affecting climate change mitigation efforts and life on land. Forty-one percent of plantation expansion occurred on land not designated for this land use. As tree plantations become more prevalent globally, monitoring their trajectories and expansion is critical to maximizing their benefits for people and the planet.