Forestry

ABSTRACT: United States (U.S.) energy policy includes an expectation that bioenergy will be a substantial future energy source. In particular, the Energy Independence and Security Act of 2007 (EISA) aims to inaease annual U.S. biofiiel (secondary bioenergy) production by more than 3-fold, from 40 to 136 billion liters ethanol, which implies an even larger increase in biomass demand (primary energy), from roughly 2.9 to 7.4 EJ yr“ ^ However, our understanding of many of the factors used to establish such energy targets is far from complete, introducing signiiicgant uncertainty into the feasibility of current estimates of bioenergy potential. Here, we utilized satellite-derived net primary productivity (NPP) data—measured for every I km ^ of the 7.2 million km ^ of vegetated land in the conterminous U.S. — to estimate primary bioenergy potential (PBP). Our results indicate that PBP of the conterminous U.S. ranges from roughly 5.9 to 22.2 EJ yr~ \ depending on land use. The low end of this range represents the potential when harvesting residues only, while the high end would require an annual biomass harvest over an area more than three times current U.S. agricultural extent. While EISA energy targets are theoretically achievable, we show that meeting these targets utilizing current technology would require either an 80 % displacement of current crop harvest or the conversion of 60 % of rangeland productivity. Accordingly, realistically constrained estimates of bioenerg