This impressive image, released a few months ago, shows the tree density in the United States (click to enlarge)
According to the NASA Earth Observatory website, the map has been obtained from a variety of sources, from ground data collection to space-based radar, requiring 6 years of work. At the highest resolution, ten pixels correspond to an hectare of land. It shows how little of the territory is actually covered in trees, either because of territory constraints (height, climate) or because of agriculture needs. As I said in a previous post, trees are fundamental (and cheap) carbon dioxide gatherers, but they also tend to require a lot of water and proper climate.