Wei et al. Carbon Balance and Management
(2023) 18:1
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our estimates. This is mainly due to the longer service life (15 years) for all paper products employed in their estimation. Skog [44] applied the stock change approach and found that the carbon stock in wood products in the United States increased to 44 Tg C in 2005, while our estimate suggested an increase of 56 Tg C from 1961 to 2019. This is because Skog [44] employed a shorter ser- vice life (2 years) for all paper products, did not include the charcoal carbon pool, and applied a faster turnover approach to model the decomposition of waste wood materials in landfills. Zhang et al. [4] reported that the carbon accumulated in end-use wood products in the United States was estimated to be 818 Tg C from 1992 to 2015, which is similar to our result (797 Tg C). Over- all, the WPsCS Estimator can successfully account for the carbon stored in wood products. As a part of the lateral carbon export from forest ecosystems [45], accounting for the carbon storage in wood products is required to reconcile the discrepancy between bottom-up estimates of carbon stock change with top-down estimates of land- atmosphere carbon exchange [46, 47]. This estimator can be widely applied to quantify carbon stock changes. The bucking allocation processes that transfers carbon from primary to secondary and ultimately to end-use wood products are omitted in the estimator. Because the pro- duction of each type of end-use wood products has sig- nificant interannual dynamics, it is a challenge to use a single regression model or a fraction to represent the allocation process over a longer time period [21, 48]. Therefore, the off-the-shelf allocated wood products data is required for the estimator. The service life for each type of wood product is a criti- cal parameter needed for quantifying the carbon stored in end-use wood products. In this study, parameters obtained from studies conducted in the United States are used to quantify the wood products carbon storage for both Maine and the United States. The service life of each type of wood product varies by region. For example, the service life of wood products used for home application is highly correlated with household income [4, 49], and they have longer service lives in developed countries than developing countries. Therefore, region-specific parame- ters are essential to obtaining reliable estimates. Ignoring the recycling process may overestimate the carbon inflow rate for the landfill carbon pool; therefore, is essential to include the recycling process in wood products carbon budget estimations. The WPsCS Estimator uses a time- dependent approach to represent the effect of the Indus- trial Revolution on the waste wood materials recycling. But the processes that use recycled wood products to make new wood products or used as biofuel to generate energy are not modeled in WPsCS Estimator. Thus, the input data should include the wood products made with
Fig. 5 The accumulated carbon storage in wood products made from the timber harvested in Maine, USA, during the period of 1961– 2019 ( a ), and the accumulated carbon storage in wood products consumed in the United States during the period of 1961–2020 ( b )
analysis. In addition, landfill carbon pool stored 4.58 Tg C (12.76%). The carbon stored in wood products consumed in the United State during the period of 1961–2020 was calcu- lated as 2607 Tg C with an average annual accumulation rate of 43.4 Tg C (Fig. 5b). In 2020, 73.1% (1905.2 Tg C) of the carbon was stored in the building wood products pool, representing the largest end-use pool. Charcoal had the least amount of carbon storage (32.7 Tg C) while paper products had a larger size of 42.4 Tg C. Home application wood products and landfill carbon pool had similar sizes at 252.7 and 256.5 Tg C, respectively, accu- mulated in the United States over the period of this analysis. Discussion Li et al. [20] reported a carbon storage of 40.3 Tg C in wood products made from the harvested timber in Maine, USA, in the same period, which is higher than
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