Quantifying Carbon Sequestration in Forest Plantations by Modeling the Dynamics of above and below Ground Carbon Pools

Intensive pine plantation management may provide opportunities to increase carbon sequestration in the Southeastern United States. Developing management options that increase fi ber production and soil carbon sequestration require an understanding of the biological and edaphic processes that control soil carbon turnover. Belowground carbon resides primarily in three pools: roots, necromass (litter, roots), and soil. There is little evidence that intensive management affects mineral soil carbon. Conversely, perennial root systems contribute to carbon sequestration through formation of longlived belowground biomass and carbon in root necromass and woody debris that may persist for years following harvest. Due to their large mass and physicochemical composition, these dead coarse roots require decades to decompose. If the length of the decay process extends beyond the length of the next harvest rotation, it will result in an accumulation of soil carbon. Increasing productivity and shortening rotation length may accelerate carbon sequestration over successive rotations. Further, management activities that retain forest fl oor and slash material or incorporate organic materials into the soil during site preparation may also increase soil carbon. 1Research Biological Scientist and Team Leader, respectively, U.S. Forest Service, Southern Research Station, Research Triangle Park, NC. Citation for proceedings: Stanturf, John A., ed. 2010. Proceedings of the 14th biennial southern silvicultural research conference. Gen. Tech. Rep. SRS-121. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 614 p. INTRODUCTION Forests are being considered as one option for stabilizing or reducing atmospheric carbon dioxide (CO2). Forests can reduce atmospheric CO2 by storing carbon in biomass, soil, and products and can be used as biofuel offsetting fossil fuel (Birdsey and Heath 2001). However, forests grown into perpetuity will provide no long-term CO2 reduction because eventually carbon losses will equal or exceed carbon gain. Management of forest carbon sequestration should be viewed as a temporary mitigation effort spanning 50 to 100 years as new technologies to store carbon or reduce carbon emissions are developed. Carbon capture in forest growth provides a low cost approach for meeting State and national carbon sequestration goals and can be accomplished with available technology. Forests will likely never be managed solely for carbon sequestration (Johnsen and others 2004). However, the potential economic value of emission credits from carbon sequestration might provide a co-benefi t that, depending on fi nancial value, could affect management practices (Birdsey 2006). Intensive pine plantation management may provide opportunities to increase carbon sequestration in the Southeastern United States. An understanding of the biological and edaphic processes that increase and retain soil carbon is required so that management can be modifi ed to increase fi ber production and soil carbon sequestration. MULTIPLE ROTATION CARBON DYNAMICS Managed forests can provide in-situ (biomass and soils) and ex-situ (products) pools for carbon sequestration (Johnsen and others 2001). Intensive management utilizing improved silviculture, fertilization, and genetically superior planting stock has increased aboveground loblolly pine productivity threefold (Borders and Bailey 2001) and decreased rotation lengths. Less is known about how plantation forestry affects the stand carbon balance (Johnsen and others 2001, 2004). Belowground biomass carbon and fl uxes is the weakest link in our understanding of forest carbon cycling. There is little evidence that silviculture and intensive management affects, either positively or negatively, long-term mineral soil carbon (Schlesinger 1990, Richter and others 1999, Laiho and others 2003). This is presumably because of the relatively high decomposition rates of newly input carbon and the low rate of carbon incorporated into organo-mineral complexes (controlled by soil physical properties). Additionally, most studies have been conducted during the fi rst rotation following the abandonment of agriculture (Richter and others 1999) or soil sampling has randomly or even systematically (Laiho and others 2003 Schlesinger 1990,) avoided regions intimately associated with stumps where decomposition rates of large coarse roots are slower. Thus, given little evidence of the potential of forest management to increase mineral soil carbon, we concentrate here on examining the dynamics of root biomass and necromass and their contribution to belowground carbon storage. Along with aboveground pools, we consider the potential of forest management to provide shortor mediumterm carbon sequestration. In-situ plantation carbon dynamics can be conceived as follows: trees are planted, above and belowground biomass grows over time, trees are harvested, root biomass becomes root necromass, trees are replanted and new biomass is accreted as root necromass decomposes (fi g. 1). The varying rates of these processes, the rotation age, silviculture, management, and the period for which these carbon dynamics are assessed all greatly infl uence the estimate of carbon sequestration. For example, intensive management practices that increase aboveground productivity results in increased belowground carbon in tap and coarse root systems (Albaugh and others 2004, Samuelson and others 2004a). Because of their relatively large mass and physicochemical confi guration, these root systems require decades (20 to 60 years) to decompose (Ludovici and others 2002). If the length of the decay process extends beyond the length of the next rotation, it will result in an accumulation