Zoo New England has announced the launch of a major tree-planting initiative in Boston's Dorchester and Roxbury neighborhoods. It aims to plant 2,000 trees over the next three years to combat urban heat islands and promote public health. Before the end of the year, the zoo will construct a greenhouse to propagate and grow additional trees for planting in the soil of Dorchester and Roxbury. Efforts to expand the tree canopy to parts of Boston like Dorchester and Roxbury aren't new. Determining how to acquire and manage tree canopy on various types of land — including city-owned, state-owned, and private lands — presents unique sets of challenges.