This chart illustrates the percentage of land that is covered by trees based off of 2005 aerial photography. Not surprisingly, there aren't many trees in the desert or the plains. No information was available for Alaska because it's really big and no one wanted to fly over all of it. What's really interesting about this data set (and not illustrated in this chart--sorry!) is that states with overall high tree coverage tend to have less dense tree coverage in cities while states with overall low tree coverage tend to have more dense tree coverage in cities. More information can be found here.