Research paperCellulosic ethanol production: Landscape scale net carbon strongly affected by forest decision making
Introduction
The future development of industrial scale production systems for cellulosic ethanol could help meet the renewable energy goals of the Energy Independence and Security Act (EISA) of 2007. This legislation mandated that in the USA 49.0 million m3 of renewable fuel would be blended with gasoline by 2010 and 136 million m3 of renewable fuel would be blended into gasoline by 2022. These renewable fuels were mandated to include 60.6 million m3 of advanced biofuel production, including cellulosic ethanol [1]. Biofuels are included in the Act because grasses and woody crops fix carbon (C) as they grow, and the displacement of fossil fuels with ethanol from biomass has the potential to lower net C emissions over the cycle of plant growth, fuel conversion, and combustion.
However, woody biomass sources like forests also play a large role in the global scale exchanges of C between the land and the atmosphere and have the potential to mitigate the effects of rising atmospheric CO2 by removing atmospheric CO2 and storing C as forests aggrade [2], [3]. If forests are left to aggrade, C accumulates not only in the wood growth but also in the annual production of foliar and fine root litter. Through ecosystem processes that limit decomposition or stabilize C in soil, forest floor and soil C pools continue to increase at high rates for decades after initiation of a new forest stand [4], [5], [6]. Many strategies are being assessed to manage forest C balance at scales from individual forest stands to large regions. These include reforestation, avoided degradation and deforestation, forest aggradation (unharvested growth), and silvicultural management to promote forest C storage [7].
In this context, if a biomass fuel system relying on forest biomass is considered as a strategy for mitigating rising atmospheric CO2, it is worthwhile to compare the proposed biomass fuel system against the aforementioned other potential uses of forests to mitigate rising atmospheric CO2 [8], [9]. However, to rigorously assess such life cycle net C gain of a biomass fuel system, careful definition of system boundaries is needed, including conceptual, spatial, and temporal boundaries. One such choice of boundary is to include the C balance associated with the land use for land on which the feedstock is produced. This has been a controversial topic in the assessment of net C emissions from biofuel systems [10], [11], [12], [13], [14].
Other considerations, such as the constraints on ethanol biorefineries must be taken into account when judging the effectiveness of cellulosic ethanol as a C mitigating option. For an industrial scale biorefinery to obtain forest biomass much of the feedstock would need to come reliably from landowners over a series of harvest rotations. This need for supply puts small forest landowners in an important position. The more feedstock they are willing to harvest and sell, the lower the distances over which biomass must be transported to fuel the biorefinery. The larger the size of a biorefinery, the greater flow of biomass needed, thus the area over which biomass needs to be transported scales directly with biorefinery size [15], [16]. In economic terms, this is a negative return to scale because average transport costs increase with distance. It is also likely to be a negative return to scale for C emissions because this transportation requires energy (and thus C emissions). Small forest landowners with a history of selling their wood to pulp mills or to other wood industries in decline would be in a good position to benefit from and support the success of the cellulosic ethanol industry, and, in conjunction, the EISA mandate [1], [15], [16], [17]. If these landowners are concentrated in sufficient numbers near the biorefinery, they could also help to minimize economic costs for a biorefinery [18]. Yet, a growing number of private forest landowners in the north central USA are choosing to make management decisions geared toward aesthetics and recreation, maintaining their growing forests, rather than harvesting for timber sales [19].
A novel aspect of our analysis is that we address forest management decision making by forest landowners, specifically nonindustrial private forest (NIPF) owners in northern Michigan, in relation to cellulosic ethanol production. In our analysis, willingness to harvest trees for feedstock affects both the distance over which biomass is transported and the amount of aggrading forest that remains within the transport radius. We addressed the following research question: To what extent are NIPF owners in northern Michigan willing to harvest their forests for bioenergy feedstock, and how do different levels of such private biomass sales impact the system net C balance of an industrial scale biorefinery?
Here we also address an important aspect of the land use and renewable fuels debate by comparing the net C balance of a cellulosic ethanol system from forest biomass at the landscape scale versus forest aggradation as an alternative in the identical landscape. Many analyses in the current literature address the question of how effective, from either a C or an economic perspective, is a given biofuel or C sequestration policy [20], [21], [22], [23], [24], [25], [26], [27]. A second question we indirectly address here is stated differently: How does overall net C balance compare in the uses of forest land for either aggradation or rotation harvests for biofuel feedstock, when both the biofuel production system and feedstock source area are considered at the appropriately large spatial scale and relevant time horizon? Such place based analyses have been done before as a way to assess the land use impact of a particular biofuel production chain [13].
Section snippets
Scope of the project
We focus on forested landscapes and ecosystems of northern Michigan, USA, which includes the Upper Peninsula and the northern areas of the Lower Peninsula. We consider a hypothetical, industrial scale, cellulosic ethanol biorefinery that would use forest tree biomass from short-rotation aspen forests in this region as its feedstock. The model does not intend to capture the full diversity of forest stands over northern Michigan, nor does it try to capture all silvicultural methods available or
Survey of nonindustrial private forest owners
Of the 1203 copies of the survey instrument mailed, 106 were returned undelivered and 505 were returned with responses, yielding an effective response rate of 46%. The land area managed by the NIPF owner respondents was concentrated in Michigan's Upper Peninsula (Fig. 1). When asked about the purposes for which they used their forest land, 82% responded that they used the land for hunting, fishing or trapping; 70% used the land for conservation purposes; 69% used the land for timber or firewood
Cellulosic ethanol
Several previous studies have emphasized the importance of considering land use or land use change and its effects on C accounting when assessing net C emissions related to biofuel production. Melillo et al. [54] used a combination of economic and terrestrial bioscience models to predict the impact of future potential expansion of biofuel production. As anticipated demand for biofuels increased it was predicted that unused, forested land would be converted for biofuel production. Searchinger
Conclusions
In Michigan, the willing participation rate of NIPF owners to harvest trees as a feedstock for cellulosic ethanol was quantified to be 47%. At this rate, over a 40 year period, the cellulosic ethanol biorefinery has a positive system NCB of 0.024 kg/m2, when averaged over the feedstock source area but not including ecosystem C balance in the source area landscape (Table 1). Our sensitivity analysis revealed that changes to biorefinery technology and the energy needed to control moisture content
Acknowledgments
We are grateful to the hundreds of survey respondents who provided critical information on their decision making for private forest management and sales of forest biomass. Valuable feedback from Sarah Kiger, Jason Martina, Seta Chorbajian, Lisa Fouladbash, and three anonymous reviewers helped focus and improve our research questions and analysis. This research was partially funded by the School of Natural Resources and Environment at the University of Michigan and by a U.S. Forest Service
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