**1. Introduction**

In the United States (U.S.), transportation accounts for 69.8% of U.S. petroleum consumption [1] and 27% of total greenhouse gas (GHG) emissions in 2013. The transportation sector is the second largest contributor of U.S. GHG emissions following the electricity sector [2]. Increasing concerns that are associated with depletion of fossil fuels and global warming have imposed pressure on companies in the U.S. transportation sector and stimulated the evaluation of different alternative energy resources [3,4]. Bioethanol and biodiesel from lignocellulosic biomass (e.g., agricultural residues, woody biomass, and energy crops) could serve as partial replacement for petroleum based gasoline and diesel, respectively, and therefore, would help to minimise GHG emissions and achieve environmental goals.

There is a vast literature on biomass potential analysis worldwide. Hernandez et al. [5] assessed the theoretical and technical potential of available woody biomass for energy use with a regional case study in the north and central-south part of Mexico. Crawford et al. [6] conducted a spatial assessment of potentially available biomass for bioenergy in Australia in 2010, 2030, and 2050 for different types

of biomass sources, including pulpwood and residues etc. Zhang et al. [7] explored the quantity and distribution of forest biomass in China based on forestry statistics data. Woch et al. [8] studied the potential of forest woody waste biomass for energy use in eastern Poland.

The State of Michigan (MI) offers significant potential for supplying forest-derived feedstocks [9] with annual growth far exceeding removals plus mortality in most timberland areas [9,10]. At the same time, a recent decline in traditional roundwood industries (e.g., pulp and paper, lumber) have revealed a new opportunity for the sustainable use of forest resources [11]. However, the estimates of feedstock availability suffer substantial uncertainties [12,13], such as the landowners' willingness and acceptance to harvest [14–16], the accessible with roads to harvest [17], the economic performance of feedstock supply chain (consisting of feedstock harvesting, transportation, and storage, etc.), as well as the delivered feedstock price [9,18]. All of these constraints should be considered when approximating long term biomass availability and estimating the corresponding biofuels potential.

Jakes and Smith [19] predicted Michigan's timber yields between 1980 and 2010. Sherrill and MacFarlane [20] assessed potential availability of urban forests, including wood residues and saw timber, for a 13-county region of Lower Michigan in 2007. MacFarlane [21] extended prior research in 2008 by examining potential urban tree biomass availability and the associated implications for energy production, carbon sequestration, and sustainable forest management. Mueller et al. [22] provided a snapshot of Michigan's woody biomass supply in 2010. Brunner et al. [23] assessed cellulosic ethanol production from the perspective of landscape scale net carbon, which is the tradeoff between the displacement of fossil fuel carbon emissions by biofuels and the high rates of carbon storage in aggrading forest stands. Gahagan et al. [24] evaluated carbon fluxes, storage, and harvest removals through 60 years of stand development in red pine plantations and mixed hardwood stands in Northern Michigan. Other studies revealed the availability of timber and residue in the Lake States region of Minnesota (MN), Wisconsin (WI), and Michigan (MI) [3,19]. Kukrety et al. [3] assessed sustainable forest biomass availability, likely harvest levels over a 100-year period, and bioenergy implications for the northern Lake States region. Becker et al. [19] examined current and projected resource needs for forest biomass in the Lake States.

There are also extensive scientific studies investigating GHG emissions mitigation potential of biomass resources worldwide. For example, Weldemichael and Assefaab [25] reported 11–15% of GHG emissions reduction by 2030, with the utilization of agricultural and forest biomass resources for energy production in Alberta. Veronika et al. [26] developed a GHG emission mitigation supply curves assuming a large-scale biomass use in Poland. Winchester and Reilly [27] assessed the contribution of biomass to emissions mitigation (16% less in basic policy case than the reference case) under a global climate policy.

In view of these studies, it is found that prior published research lacks a comprehensive study approximating long-term forest biomass availability and the associated uncertainties in the State of Michigan. In this study, the long-term (2015–2065) timber availability was derived from previous harvest trends for the Michigan. Harvest residues associated with growing stock volume cut or knocked down during harvest (including branches and tops) were approximated using the developed formulas considering a few of influential factors, including harvest types of all merchantable timber and the associated residue managemen<sup>t</sup> options, and residue collection rate etc. Then, the potential forest biomass utilization for ethanol production and GHG reduction were also examined using the developed methods.
