Modeling harvest and biomass removal effects on the forestcarbon balance of the Midwest, USA

The objective of this study was to use an ecosystem process model, Biome-BGC, to explore the effects of different harvest scenarios on major components of the carbon budget of 205,000 km of temperate forest in the Upper Midwest region of the U.S. We simulated seven harvest scenarios varying the (i) amount of harvest residue retained, (ii) total harvest area, and (iii) harvest type (clear-cut and selective) to assess the potential impacts on net biome production (NBP), net primary production (NPP), and total vegetation carbon. NBP was positive (C sink) in year 1 (2004) and generally decreased over the 50-year simulation period. More intensive management scenarios, those with a high percentage of clear-cut or a doubling of harvest area, decreased average NBP by a maximum of 58% and vegetation C by a maximum of 29% compared to the current harvest regime (base scenario), while less intensive harvest scenarios (low clear-cut or low area harvested) increased NBP. Yearly mean NPP changed less than 3% under the different scenarios. Vegetation carbon increased in all scenarios by at least 12%, except the two most intensive harvest scenarios, where vegetation carbon decreased by more than 8%. Varying the amount of harvest residue retention had a more profound effect on NBP than on vegetation C. Removing additional residue resulted in greater NBP over the 50-year period compared to the base simulation. Results from the seven model simulations suggest that managing for carbon storage and carbon sequestration are not mutually exclusive in Midwest forests. # 2012 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +1 608 265 5628; fax: +1 608 262 9922. E-mail addresses: [email protected], [email protected] (S.D. Peckham). 1 Present address: Department of Botany, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA. 2 Present address: Department of Geosciences, North Dakota State University, Stevens Hall 218, Fargo, ND 58108, USA. Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/envsci 1462-9011/$ – see front matter # 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envsci.2012.09.006 farm woodlots to the expansive wilderness of northern Minnesota, and have been utilized by humans in most areas since the 1800s. The forests’ tree species composition is diverse ranging from central hardwoods in the south, to coldtemperate northern hardwoods and conifer forests, including transitional boreal forests in the far north. In summary, the past and current use(s) of Midwest forests are diverse and sometimes competing. These same forests are also a potential source for feedstock for bioenergy as the U.S. attempts to develop sustainable bioenergy systems that will reduce national dependence on foreign fossil fuel (Perlack et al., 2005). In anticipation of greater demand for woody biomass, forest managers and policy makers are developing harvest guidelines to ensure sustainable forest management practices. Options to increase woody biomass harvest include increased removal of residue (i.e. cull trees, tops of trees etc.) normally left in the forests, increased harvest frequency, and increased harvested area. However, there are extremely few long-term field studies that can be used to guide management and policy decisions. It is unclear how greater biomass utilization of the forest resource will affect the long-term soil carbon storage, nutrient availability, and productivity (i.e. carbon sequestration) of future forests. It is imperative to quantify the effects of harvest regimes on carbon pools with fast to moderate residence times (e.g. vegetation) and especially carbon pools with slow residence times (e.g. mineral soil). Recent studies have shown that forest disturbance is an important driver of ecosystem C balance (Euskirchen et al., 2002; Thornton et al., 2002; Law et al., 2004; Bond-Lamberty et al., 2007b; Amiro et al., 2010; Peckham et al., 2012). Ecosystem process models allow scientists to simulate effects of different management practices on forest sustainability, growth, and carbon dynamics at scales ranging from stand to region. Unlike empirical growth and yield models, process-based ecosystem models simulate water, nitrogen, and carbon cycles, and their interactions, and they account for soil and detritus carbon dynamics (Peckham and Gower, 2011). Modeling the C balance of a heterogeneous forestscape, such as the Upper Midwest, is challenging because the spatially and temporally explicit disturbance history is not well documented. Hence, most modeling studies covering this region do not account for disturbance history in C balance or net ecosystem production (NEP) (e.g. Lu and Zhuang, 2010; Wang et al., 2011). However, previous modeling studies have shown that management regime is the most important determinant of forest C balance (Euskirchen et al., 2002), for individual forest stands (Peckham and Gower, 2011), and at the regional level (Peckham et al., 2012). Landscape-level effects of management choices on the future forest C balance over the Midwest are poorly