Mounting evidence suggests that geographic ranges of tree species worldwide are shifting under global environmental change, but little is known about forest migration—the shift in the geographic ranges of forest types—and how it differs from individual tree species migration. Here, based on in situ records of more than 9 million trees from 596,282 sample plots, we quantified and compared the migration patterns of forests and tree species across North America between 1970 and 2019. On average, forests migrated at a mean velocity of 205.2 km per decade, which is twice as fast as species-level migration (95.6 km per decade), and 12 times faster than the average of previous estimates (16.3 km per decade). Our findings suggest that as subtle perturbations in species abundance can aggregate to change an entire forest from one type to another, failing to see the forest for the trees may result in a gross underestimation of the impacts of global change on forest ecosystem functioning and services. With the first forest classification and quantification of forest migration patterns at a continental level, this study provides an urgently needed scientific basis for a new paradigm of adaptive forest management and conservation under a rapid forest migration.