what percent of us is covered by trees?

Answer 1

Current maps of US forestation

www.fs.fed.us/pnw/corvallis/mdr/mapss/17Color.pdf

Canada has about 4,020,000 km² of forest land.
US has 3,000,000 km² (750 million acres) of forest land much of it in 155 national forests containing almost 190 million acres (297,000 mi² - 769 000 km²) of land.
http://en.wikipedia.org/wiki/U.S._National_Forest