When you take the total annual cellulose from woodlands in the U.S. & do the numbers it doesn't go very far. Especially when you are trying to replace more than 22,000,000 barrels/day of petroleum products. A lot of other energy sources (solar, wind, biomass, geothermal, hydro, methane hydrates, nuclear, etc.) are needed to make much of a dent. Conservation needs to play a very important role also. Still once peak oil production comes it won't be pretty.