Seeing the forest for the trees: How much woody biomass can the Midwest United States sustainably produce?

Abstract

Widespread interest in advancing goals of energy independence, climate change mitigation, and rural economic development has led to unprecedented growth of the global bioeconomy. Continued development of this burgeoning industry relies upon the long-term availability of plentiful, sustainable sources of biomass. In the Midwest United States, efforts to secure such supplies have focused on herbaceous plants, either from the seeds or residues of annual crops or from perennial lignocellulosic species. Here, we explore the potential for this region to provide woody biomass from forests and short rotation woody crops (SRWC). We compare estimates of current and future availability—as defined by biophysical, technical, or economic potential—from four United States governmental entities. We find that estimates vary widely due to key parameter choices and assumptions, from current annual potential of 19.9–47.6 Mg (Mg, or metric ton) and future (year 2030) potential of 8.1–210.5 Mg. For the largest future estimate, the economic woody biomass potential from SRWC is triple that of forests. To complement these detailed assessments of flows, we introduce a comparison with biomass stocks to assess the long term sustainability of biomass extraction. We find that the average biomass growth rate of Midwestern forests (3.4%) is lower than estimated extraction rates when prices are high (3.7%) and even less sustainable for specific states, such as Minnesota (2.4% growth compared to 8.6% extraction). We recommend that future studies of biomass potential should (1) estimate stocks and flows all three categories (biophysical, technical, and economic) side-by-side, (2) improve the transparency of parameter assumptions, and (3) make models and methods available to the public so that readjustments of parameters can be tested and harmonized. The potential quantities and spatial distribution of the biomass potentials shown here can provide the basis both for planning of regional bioenergy production and for future work that explores the ecosystem services provided by agroforestry ecosystems.