Abstract

Materials and Methods

The mass of stored biogenic carbon (in metric tonnes, i.e., tC) contained in a reported product volume (m³) or mass (t) was multiplied by its product-specific substitution factor (tC/tC) to yield that product’s substitution impact (converted to tCO₂; Hurmekoski, et al. 2023). The sum of the substitution impacts for the full array of products was used to estimate the total substitution impact, expressed as TgCO₂e.

Substitution factors

Substitution factors were calculated as the GWP differential relative to the difference in biogenic carbon content of the materials being compared. This is the same as the calculation done by Sathre and O’Connor (2010), who referred to the product as “displacement factor”:

Substitution factors were calculated for wood options compared with their alternative material, or with the market-share-weighted average of their options, in some cases where we identified more than one option. Data were taken from the most recent LCA studies or environmental product declarations (EPD) conducted in the United States, as available. Exceptions are noted below. These data included consideration of assumed differences in the service life of the wood and nonwood options. Biogenic carbon was excluded and life-cycle stages from cradle to gate (stage A in International Organization for Standardization [ISO] standard 21930 [ISO 2017]) or cradle to grave (stages A–C in ISO standard 21930 [ISO 2017]) were included depending on data availability. The identification of the substitute products is the most important yet the most uncertain assumption. Following previous literature (e.g., Rüter et al. 2016), we selected substitute products on the basis of expert judgment trying to ensure functional equivalence between the substituted services. Specifics for the various wood products and assumptions regarding their alternatives are described below; the resulting substitution factors are listed in Table 1.

Table 1. End use, volume, and estimated substitution impacts of wood products harvested from forests in the United States in 2020.

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Results and Discussion

The total substitution impact of wood products was 188 TgCO₂e in 2020 (Table 1). Note that this is based on a comparison of single cases covering a fraction of all end uses and functional units. Thus, the estimate only aims to explore the approximate scale of the substitution and it should be interpreted with caution. Moreover, substitution impacts are not directly comparable with absolute emissions and removals because the comparison point (i.e., the baseline scenario) is different. For example: for harvested wood product (HWP) emissions and removals, the baseline is the stock in the last time period. However, for substitution, it is the market condition that would have prevailed if wood had not been used (i.e., zero harvest). However, to help put these impacts for substitution (137 to 188 TgCO₂e/yr) into perspective, they represent avoided emissions equivalents to about 2% to 4% of total GHG emissions from the United States over the years evaluated here (approximately 6,000 to 7,500 TgCO₂e/yr; EPA 2022a).

The most important uses of wood by volume in the United States are for construction lumber in primary construction and for pulp for packaging papers (e.g., cardboard boxes). These two uses also have a dominating influence on the total substitution factors of wood products harvested, given their relatively high substitution factors (0.96 and 1.12 tC/tC, respectively) and large production volumes (Table 1). These substitution values are derived from single case studies, causing notable uncertainty. Future studies incorporating a larger number of cases for these two major wood uses may thus change the overall estimate, perhaps considerably so.

Total substitution impact varied over time because of changes in the volume and types of wood products (Table 2). The relatively low total impact in 2010 reflects reduced construction activity associated with the global economic crisis around 2008, and therefore fewer substitution opportunities for wood materials. Total substitution impacts over time have increased in part because of reduced demand for writing paper and newsprint and an increase in demand for packaging paper (Latta et al. 2016). The portion of paper production for packaging in the United States increased from 52% in 1990 to 70% in 2020 (FAOSTAT 2023) and this has contributed to an increase in the weighted average substitution factor for wood products during that time (from 0.62 to 0.71 tC/tC; Table 2).

Table 2. Total substitution impact and average substitution factors of wood harvested in the United States by year.

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Overall, the weighted average substitution factor—for products that were assumed to have a substitution benefit—was 0.71 tC displaced per tC in the wood product in 2020. When expressed in terms of the total wood harvest, the average substitution factor was 0.52. These factors are comparable with the average substitution factors reported in recent reviews (e.g., 0.55 in Hurmekoski et al. 2021). When expressed in terms of units of roundwood volume harvested, the fossil carbon emissions avoided is 440 kgCO₂e/m³.

The substitution impacts for paper products used in this study were, on average, higher than with those used by Hurmekoski et al. (2023) for Finland. This is primarily because the end use of pulp products in the United States is relatively more in packaging (which has a high assumed substitution factor) than in tissue and graphic papers (which are assumed to have no substitution factor). If the United States’ estimates had been compiled with the end-use distribution of the pulp and paper products produced in Finland, the overall impacts would be reduced to about 117 TgCO₂e for 2020, about 43% lower. In contrast, the substitution factors used recently for a study in Sweden, a country with forests and forest industry similar to Finland (Skytt et al. 2021), were relatively high compared with those used in the United States’ analysis. If the Sweden substitution factors were applied to the United States’ production estimates for 2020, the estimated total impact would be approximately 34% higher. This range in impact estimates highlights the uncertainty created by adopting LCA case studies for individual products to generalize the results for an entire heterogeneous market.

Landfill was the only end-of-life scenario included in this analysis, and only the reported wood and paper component of solid waste that is incinerated to provide energy was included. In Finland, almost all wood and paper products are collected at disposal and incinerated for energy production, thus providing a substantial additional substitution impact (by displacing need for fossil fuels); Hurmekoski et al. (2023) found that end-of-life practice in Finland increased the total substitution impact by almost 50%. In the United States, most of the wood and paper that is disposed of ends up in landfills and provides no substitution benefit. However, the wood and paper disposed of in landfills is included in calculations of carbon storage and this represents a large carbon sink in the United States: 63.6 TgCO₂e in 2020 (Domke et al. 2023). As the assumed substitution factor used here for combustion of this wood and paper is 0.68 tC/tC (i.e., <1.0), then the reported carbon storage estimate may be greater than its potential substitution benefit if it were combusted for energy. This assumes that much of the wood (77%) and paper (44%) disposed of in landfills does not decay and thus represents “permanent” carbon storage (EPA 2022b).

Substitution factors are highly uncertain and contentious (Harmon 2019; Leturcq 2020). Uncertainty results in general from the lack of representative LCA case studies across the thousands of end uses, poor understanding of market dynamics, and the hypothetical nature of the exercise—trying to judge how consumer needs would have been satisfied in the absence of wood-based products. For example, the substitution factor calculation used here assumes that wood-based products substitute nonwood products at a rate of 1:1 and compares attributional LCA studies of products’ average emissions. However, if the substitution were only partial, the overall impact would be reduced because the additional supply would substitute only part of the other product, with the remainder merely adding to overall supply (e.g., Chalmers et al. 2015). In addition, the products added to or removed from the market could have different emissions than the average for those products. Consequential LCA would be required to explore the impact of marginal changes in product use in response to carbon taxes or other incentives. Also, substitution factors are likely to change in the future (e.g., with the development of new products and processes and with changes in the fossil-fuel intensity of the systems that power the production of alternative materials; Brunet-Navarro, et al. 2021). Thus, this analysis only intends to explore the approximate scale of the substitution effects. Future estimations would benefit from further LCA case studies to increase the coverage of end uses as well as an analysis on market dynamics regarding which nonwood products the wood products substitute for and the direct and indirect consequences of this substitution.

A finding of substantial substitution benefits for forest products in the status quo does not necessarily imply that greater harvest levels would yield proportional carbon benefits. Hurmekoski et al. (2020) observed that increased harvest levels in Finland would result in a proportionately higher reduction in the forest carbon sink that would require products with extremely high substitution factors to offset. This may be different in the United States, having different forest ecosystems and forest product portfolios (Zhang et al. 2023).

Given that the unit change in forest (biogenic) carbon storage is greater than the sum of unit change in wood (biogenic) product storage and avoided (fossil) emissions, increased harvest levels would likely result in a net emission of carbon, at least in the short to medium term (Fig. 1). However, just as there is uncertainty regarding the amount and future potential for substitution impacts of HWPs, there is also uncertainty regarding the capacity of forests to continue to sequester and store carbon at recent rates. After a period of large-scale disturbance, during which forests in the United States represented a major source of carbon emissions, forest growth has rebounded over the past half century, in large part because of fossil fuel substitution for energy sources previously provided by wood (Birdsey et al. 2006). However, it now appears that the future carbon sequestration capacity of United States’ forests is slowing. For example, Zhu et al. (2018) calculated that United States’ forests are at about 80% of their storage potential, similar to much of the global forest resource (Roebroek et al. 2023). Thus, the interpretation of the balance between fossil and biogenic emissions and removals over time is not straightforward. Further research on the relationship of forest carbon stocks, HWP carbon pools, and wood product substitution factors is needed for the United States’ context to judge the overall climate-change mitigation potential of wood-based products.

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Conclusion

The substitution impacts of wood products from forests in the United States ranged from 137 to 188 TgCO₂e annually over the past 2 decades. This is an amount equal to about 2% to 4% of total GWP emissions from the United States during that time. This large estimated impact does not necessarily imply that more HWPs would increase the net carbon benefit; associated forest carbon storage levels would need to be considered. Substitution impact estimates are very sensitive to the substitution factor used. For example, the total substitution impact estimate would have been much lower or higher if the factors used in recent studies in Finland or Sweden, respectively, has been used instead. Estimation of substitution effects involves much uncertainty because of the importance of underlying assumptions and the limited availability of LCA data. Further work to address this uncertainty would help in evaluating the overall climate-change mitigation potential of wood use in the United States.