Technically this shows "above-ground woody biomass," but in practical terms it is a map of tree density.
Over six years, researchers assembled the national forest map from space-based radar, satellite sensors, computer models, and a massive amount of ground-based data. It is possibly the highest resolution and most detailed view of forest structure and carbon storage ever assembled for any country.Posted at Neatorama by Miss Cellania, who found it at NASA's Earth Observatory (their Image of the Day page is well worth a bookmark).
Forests in the U.S. were mapped down to a scale of 30 meters, or roughly 10 computer display pixels for every hectare of land (4 pixels per acre). They divided the country into 66 mapping zones and ended up mapping 265 million segments of the American land surface...
p.s. - The embedded image enlarges with a click, but for maximum detail enlarge x2 the original at the NASA link.
(Reposted from 2012 for Arbor Day 2015)