Wei et al. Carbon Balance and Management
(2023) 18:1
Page 2 of 10
a larger carbon sink in global end-use wood products, with an average of 122 Tg C per year during the period of 1992–2015. This annual sink of harvested carbon is heavily influenced by demand and supply in the products market, which is impacted by various social-economic factors such as population and household income, tech- nological advancement in the wood industry, climate and other environmental factors, and forest management strategies [5–8]. Therefore, an accurate accounting for the carbon stored in wood products is essential to assess- ing land-atmosphere carbon exchange by developing car- bon budgets at regional, continental, and global scales. The Intergovernmental Panel on Climate Change (IPCC) provides calculation guidance for estimating the size of harvested wood products carbon pools and their annual stock changes in three tiers of approach that can be used based on the availability of wood products data and the level of aggregation in the pool category defini- tions [9]. This guidance has been widely used to develop a considerable number of harvested wood products car- bon accounting models and frameworks, which have been widely applied to various system boundaries [e.g., 101112]. Brunet-Navarro et al. [13] reviewed 41 wood product carbon accounting models and summarized their characteristics. These models are different in their system boundary, spin-up, bucking allocation method, number of carbon pools, treatment of wood product dis- posal and recycling processes, as well as technological advancement in the wood industry. The 2006 IPCC guidelines describe four approaches to define system boundaries for wood products carbon storage estimation [13, 14]. The stock-change approach estimates the carbon in wood products consumed and physically stored in the study area. The atmospheric flow approach estimates the carbon stored in wood products made from the harvested timber from local forests along with the emissions from wood products consumed in the study area, but the carbon emissions from the prod- ucts exported to and consumed in other regions are not counted. The production approach estimates the carbon stored in wood products made from timber harvested in the study area. The carbon stock and emission from exported products are counted, but the carbon stock in imported wood products is not included in the calcula- tion. Finally, the simple decay approach estimates carbon stored in wood products consumed in the study area. Meanwhile, the carbon stock and emission made from local forests but exported and consumed in other regions are also counted in this approach. Wood products estimation models often use a “spin- up” process to account for the initial size of the carbon pool at the start of the period for reporting. The initiali- zation is not always included in the accounting [e.g., 15],
but the study or reporting time period should be well documented. Another strategy is to run the spin-up for a long enough period using historical wood production data to reach the equilibrium stage [16]. If the harvested timber is not adequately categorized into intermediate and end-use wood products, a bucking allocation process is required, which refers to the allocation of harvested timber to different wood products pools [17]. A carbon pool is typically defined as a group of wood products that have a similar life cycle [18]. Wood product disposal is the time point when products are retired from use and disposed [13]. The recycling process includes the waste wood material reused to make new products or to gener- ate energy at the end of its service life [19]. Technologi- cal advancement in the wood industry may result in more carbon from the forest sector ending up in the wood products by reduce processing residuals, extended ser- vice life of each end-use wood product, and an increase in the recycling rate, which can significantly expand the wood products carbon pool size [20]. To estimate the size of a wood products carbon pool and its interannual stock changes, monitoring carbon inflow and outflow rates is the most popular approach [21]. Carbon in harvested timber initially flows into the overall wood products pool, and then allocated among the different products such as construction material, fur- niture, paper, and biofuel [22]. The end-use wood prod- uct is disposed of when it reaches the end of its service life. A part of the disposed wood products will be recy- cled to make new products or directly burned as biofuel to generate energy, and the remainder will be disposed to landfills. Waste wood materials in landfills will be slowly decomposed and the carbon released to the atmosphere. Therefore, using the life cycle of each wood product is an efficient method to realize the estimation of wood prod- ucts carbon pool size over time. In this study, we developed a life cycle and product type based estimator for quantifying the amount and interan- nual change in wood products carbon storage using the annual production of each product type, a service life based disposal method, a time-dependent recycling pro- cess, and a time-dependent decomposition approach for waste wood materials in landfills. To evaluate the per- formance of this estimator, we applied it to (1) account for the carbon storage in wood products produced by timber harvested in Maine, USA from 1961 to 2019 (system boundary: production approach) and (2) esti- mate the carbon storage in wood products consumed in the United States over the period of 1961–2020 (system boundary: stock-change approach).
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