Wow. From geek.com:

The research team used information from space-radar stations as well as data from other satellite installations in conjunction with existing ground-based surveys to complete the cartography. The group estimates that along with the large overall view, they also have gathered measurement data for five million individual trees.

The full map is said to have an incredible scale: 4 pixels for every acre in the contiguous United States, according to the release. (link)