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Review

Towards a Carbon Accounting Framework for Assessing the Benefits of Biogenic Wood Carbon to Net Zero Carbon Targets

by
Oludunsin Arodudu
1,*,
Obste Therasme
1,
Timothy Volk
1,
Robert Malmsheimer
1,
Paul Crovella
1,
René Germain
1,
Danielle Kloster
1 and
Deepak Kumar
2
1
Department of Sustainable Resources Management, State University of New York College of Environmental Science and Forestry, Syracuse, NY 13210, USA
2
Department of Chemical Engineering, State University of New York College of Environmental Science and Forestry, Syracuse, NY 13210, USA
*
Author to whom correspondence should be addressed.
Forests 2023, 14(10), 1959; https://doi.org/10.3390/f14101959
Submission received: 24 August 2023 / Revised: 22 September 2023 / Accepted: 25 September 2023 / Published: 27 September 2023
(This article belongs to the Special Issue Utilization of Forest Products for Sustainable Growth)
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Abstract

:
Carbon stored in harvested wood products (HWPs) can play an important role in climate change mitigation and needs to be accounted for accurately and consistently. This study reviewed the features of previous HWP carbon accounting frameworks and discussed potential improvements for a more complete assessment of all HWP contributions to net zero carbon targets at subnational levels. The basic features include the components, the methods, the approaches, and the modeling principles. A key recommendation is to expand previous HWP C accounting framework components to include other climate change mitigation benefits such as local or regional substitution effects (i.e., material replacement, fossil fuel displacement effects, energy efficiency gains, recycling effects, and cascading use impacts) of all produced and consumed HWPs. Another area for improvement is the need for subnational unit-specific activity data and conversion factors. Adopting variants of the domestic origin-stock change approach will also help account for relevant production and consumption activities within the subnational unit. These recommendations will enhance the accuracy and/or precision of HWP accounting frameworks at the subnational level and help capture all potential benefits of HWPs as a carbon sink for climate change mitigation and a valuable contributor to subnational net zero carbon targets.

1. Introduction

Carbon stored in harvested wood products (HWP) is an important component of global and national greenhouse gas (GHG) inventories and can mitigate climate change by offsetting carbon emissions through carbon storage, materials substitution, and fossil energy displacement [1,2]. HWPs include all wood-derived products, in the form of raw materials, semi-finished products, finished products, or final use [3,4]. Raw materials are principally sawlogs and pulpwood (round or split) but could also include bark, branches, and other residues. Semi-finished products can be classified into three classes: sawnwood (e.g., timber, beams, planks, joists, wooden poles, lumber, posts, etc.), wood-based panels (e.g., particle boards, hardboard, plywood, fiberboards, veneer sheets, etc.), and paper and paperboard (e.g., graphic paper, cardboard, containerboards, etc.). Finished products or uses include products such as building components (windows, walls/wall panels, frames, roofs, etc.), indoor furniture and house fittings (doors, bookshelves, tables, chairs, etc.), outdoor infrastructures (telephone/electric poles, signpost, billboards, etc.), domestic and office utilities (wooden knife handle, wooden chopping board, books, newspapers, paper box, posters, magazines, etc.), and wood-based fuels (energy pellets, energy chips, charcoal, briquettes, etc.) (Figure 1).
Carbon stored in HWP (HWP C) can be found in use in buildings and other infrastructures, in solid waste disposal sites (SWDS), or stored in wood fuels before or during use [5,6]. Examples of HWPs in use include building components, indoor furniture, house fittings, outdoor infrastructures, domestic and office utilities, and wooden houses [7,8]. Examples of HWPs in solid waste disposal sites include those in landfills, dumps, and composting sites [9,10]. Carbon stored in wood fuels are, however, usually burnt off and oxidized, hence, they may be accounted for as carbon savings from displacement of fossil fuel energy or as short-term storages (with less than 1 year half-life) or excluded because they are not long-term carbon storages [11,12]. The exclusion of wood fuels often leads to the adoption of only three semi-finished product classes (namely, sawnwood, wood-based panel, and paper/paperboard) in most HWP carbon accounting frameworks [5,7]. This is because it is easier to trace all HWPs back to these groups, and because data collection for HWP C accounting of finished products is very susceptible to uncertainties [3,4].
Estimates for HWP C in use and solid waste disposal sites (SWDS) have increased steadily by 23% and 81% respectively, since 1990 in the US [2,13]. In a similar manner, at global levels, the HWP C pool continues to be a net sink, offsetting a fraction of greenhouse gas emissions from industrial processes [14,15]. According to [15], HWP C pool offset about 0.9% of emissions globally between 1990 and 2016. In Ref. [14], the authors further predicted a rise of about 32% in the global HWP C pool between 2015 and 2030. Recent increases in the contribution of HWPs to the global HWP C pool have been attributed to the growth in the global economy over the last several decades [16,17]. In the US, HWPs have long been a net carbon sink [13,18]. While economic downturns such as the 2008 Great Recession turned HWP pools in the US briefly into a source rather than a sink, production and use of HWPs quickly rebounded during the recovery [2,19]. Despite challenges to the forest products industry during the 2008 Great Recession (including mill closures and downsizing), the role of HWPs in the US GHG inventories continued to increase from 3.7% in 1990 to 4.8% in 2019 [2,20]. As consumers continue to use HWPs, especially as replacements for more GHG-intensive materials, the carbon benefits of those products will continue to be valued into the future [5,21]. Carbon stored in HWPs is not only an integral part of US forest carbon estimates, but it also helps demonstrate the potential carbon benefits of HWPs and the entire forest wood product sector as a whole.
HWPs remain in use for different lengths of time, ranging from a piece of paper with a short lifetime compared to a structural lumber in a building that can serve its use for up to a century or more [4,11]. Mass timber buildings lock up the carbon in trees and store them for decades or even centuries [22,23]. Their substitution effects when compared to those of buildings made with other types of materials such as concrete or steel can be between −2.3 and 15, with most lying between 1 and 3, and an average displacement factor value of about 2.1 [24,25]. Within the context of life cycle carbon accounting for HWPs, carbon benefits of HWPs vary according to how fast or slow they decay or if they are subjected to consumptive energy use, i.e., how quickly they release carbon back to the atmosphere [26,27]. The half-life is a commonly used metric to indicate the number of years it takes to lose one-half of the material from a pool [3,4]. According to the IPCC, the half-life of paper in use is up to 2 years; it is up to 25 years for wood-based panels and up to 35 years for sawnwood [5,28]. The half-life of paper and wood in a landfill is 14.5 years and 29 years, respectively [20,29].
Despite having HWP carbon reported over the years nationally and globally under the Kyoto Protocol (KP) and the United Nations Framework Convention on Climate Change (UNFCCC), carbon stored in HWPs is still either poorly accounted for and/or under-reported [3,30,31,32]. While carbon stored in harvested wood products was previously assumed to be oxidized in the biomass harvest year [3,32], others do not have sufficient data to appropriately quantify the carbon stored during HWP use phase and in HWPs discarded in SWDS [10,19]. To improve previous HWP carbon accounting frameworks, the Intergovernmental Panel on Climate Change (IPCC) has successively provided guidance for HWP carbon accounting both within the Kyoto Protocol and the UNFCCC reporting [1,4]. IPCC first recognized HWPs in their Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories, even though it erroneously assumed “all carbon biomass harvested is oxidized in the biomass harvest year” [9,33]. This was adopted during the first Kyoto Protocol commitment period (2008–2012) [23,34]. In the 2006 Guidelines, the IPCC revised their treatment of HWPs and proposed accounting guidelines that allowed Annex I parties to account for HWP carbon benefits [22,35]. In 2011, members of the 17th Conference of Parties (COP17) concluded that only HWPs made from domestic harvests can contribute toward national GHG inventories [7,14]. The IPCC subsequently published the 2013 Revised Supplementary Methods and Good Practice Guidance after initial reporting under the Kyoto Protocol, adding HWPs produced and consumed in a region, as well as those produced and exported to other regions for consumption as a mandatory pool to be reported within land-use, land-use change, and forestry (LULUCF) activities, hence, updating the reporting framework for Annex I parties during the second Kyoto period (2013–2020) [3,36]. Moving forward, countries are expected to report their HWP carbon pool against nationally determined contributions under the Paris Agreement [15,37]. There were also 2019 refinements to the 2006 IPCC guidelines with regard to the estimation of uncertainties related to service lifetimes of HWPs and obsolescence [4,38].
Greenhouse gas inventories at subnational levels must be as accurate and as complete as possible. They can contribute to improvements of national greenhouse gas inventories but also support the increasing number of subnational climate policies and laws, which often have different measurement baselines, system boundary requirements, and ambition levels from set national targets [39]. Some are more ambitious than those set at national levels, while others are less ambitious. An example of this disparity is the New York State GHG reduction emission target of 40% below 1990 levels by 2030 and no less than 85% below 1990 levels by 2050, according to its Climate Leadership and Community Protection Act (CLCPA). This is in contrast with the national US GHG reduction emission targets of 50%–52% below 2005 levels by 2030 and net zero by 2050. Measuring and monitoring the progress towards these targets will require different carbon measurement, reporting, accounting, and verification approaches, techniques, standards, and frameworks (part of which is a HWP carbon accounting framework that is suitable for application at the state or any other subnational unit level). Within this context, this paper reviews the current literature and recommends a HWP carbon accounting framework suitable for that purpose. To arrive at this, the objectives of this paper are (1) to synthesize the main features of HWP carbon accounting frameworks as defined by the components, methods, approaches, and modeling principles applied, and (2) to identify possible improvements to individual elements of the main features of HWP carbon accounting frameworks, to make them suitable for application at subnational unit levels.

2. Features of HWP Carbon Accounting Frameworks

The basic features of all HWP carbon accounting frameworks are the components (also referred to as boundaries), the methods (also referred to as method classifications), the approaches, and the modeling principles (also referred to as models) [7,35]. This is illustrated in Figure 2. Components refer to various carbon stocks stored within HWP, carbon lost from HWPs due to decay over time, or carbon attributable to HWPs (e.g., material substitution effects, fossil energy replacements, energy efficiency gains, circular economy impacts etc.) [12,25]. Methods refer to the entirety of choices regarding HWP activity data (product category and process data), conversion factors, and calculation procedures applied for the estimation of HWP C [9,10]. Approaches are spatial accounting boundaries delineated for HWP C estimation, i.e., they constitute the different ways of delineating HWP carbon stocks and flows, either at national or subnational unit levels [5,19]. Approaches delineate the different components of carbon stocks or flows included in or excluded from HWP C estimation [7,30]. Examples of components included or excluded from different approaches may include in-region or out-region production and consumption of HWPs, emissions associated with HWP production and consumption, and substitution effects of in-region or out-region production or consumption of HWPs, etc. Modeling principles refer to tools or systems applied for HWP C estimation [3,23]. These four features (i.e., components, methods, approaches, and modeling principles) together make up an HWP carbon accounting framework as described in successive IPCC guidelines and other works from the literature on HWP C estimation.

3. Components

Components in HWP C accounting refer to relevant individual C stocks and flows that make up the total HWP C estimate [10,40]. Pre-2006 IPCC guidelines assumed all HWPs had their carbon oxidized within the first year of harvest. Subsequent IPCC guidelines recognized HWP C benefits and identified the following components: carbon flows associated with HWP production, use and disposal, and carbon stored in HWPs in use and in SWDS [3,4]. It is recommended to define carbon stock and flows within region (domestic), to other regions (export), and from other regions (import). The 2019 refinements of IPCC guidelines for HWP carbon accounting [4] recommended further estimation of carbon stocks based on product removal options (i.e., with or without obsolescence). Other important components of HWP C accounting put up for consideration by the IPCC 2019 refinement guidelines [4] include carbon stock and flows at shelf-life, over lifetime, and the frequency of local fire incidences in buildings and elsewhere where HWPs are in use or dumped. Estimating these components will involve local or regional data collection on the average shelf-life and lifetime of different HWP categories and information on the frequency of fire incidence, as well as their respective obsolescence rates (usually at tier 2 level assessment and upward). Obsolescence rates could be influenced by rapid change in technology (e.g., change from paper-based economy to fully digital economy) and the adoption of circular economy principles by enterprises and households (i.e., change in company policy or family traditions to increase or reduce the average lifetime of furniture). Accounting for shelf-life, lifetime, frequency of fire incidences and/or obsolescence however requires special attention to accurately represent the subnational unit conditions.
Forests 14 01959 g002
Figure 2. Graphical illustrations of the key features (components, methods, approach, and modelling principles) of HWP carbon accounting framework.
Figure 2. Graphical illustrations of the key features (components, methods, approach, and modelling principles) of HWP carbon accounting framework.
Forests 14 01959 g002
Several HWP C assessments at national and subnational levels focus on carbon storage stocks and flows [41,42]. However, there are additional HWP accounting components recognized by experts that are not yet officially recommended or made mandatory by IPCC primarily due to limited data. These components are carbon savings from substitution effects such as recycling and cascading uses, direct material substitution effects, fossil energy displacements, and improved energy efficiencies [43,44]. Substitution effects from improved energy efficiencies may result from better insulation and reduction in energy consumption due to internal changes in building properties from concrete and steel to mass timber [12,30]. All of these need to be included in HWP C accounting frameworks for examining HWP contributions to climate change mitigation and net zero carbon targets.

4. Methods

HWP C accounting methods comprise activity data, conversion factors, and calculation procedure applied [4,5]. They provide information fed into models for estimation of HWP carbon.
The activity data are local product category and process data [8,23]. They often contain material flow information on HWPs such as the amounts of HWPs produced or consumed, HWPs sent to SWDS, or HWPs reused [16,17]. They need to be updated regularly (e.g., annually) to reflect changing realities and understand new trends, as well as to ensure the completeness and accuracy of the HWP C estimates [11,19].
Conversion factors within HWP C accounting frameworks at local, regional, or national levels refer to the amount of carbon per unit of mass or volume of HWPs. Conversion factors at these same levels may also include values of C decomposition rates for HWPs in use or at SWDS. At the product level, conversion factors could include substitution effects of HWPs (mostly from previous LCAs comparing wood products to other materials), and recycling and cascading impacts of different HWPs [22,45]. Conversion factors are usually employed in the calculation of lost carbon from or stored carbon in HWPs [4,30].
Calculation procedures are the specific protocols adhered to in HWP C estimation [3,4]. Calculation procedures are determined by the resolution of the activity data available and conversion factors applied for HWP C estimation. A part of calculation procedure also includes the determination of the choice of assessment base year [46,47]. The IPCC recommends 1991 as base year for HWP and other climate-related assessments when there are data limitations [3,4]. The California Forest Project Protocol (CFFP), on the other hand, applies a 100-year average in the estimation of HWP C stocks. This is because unlike other calculation procedures with data limitations, CFFP applies models with reliable data that help back cast estimates up to over 100 years ago.
According to IPCC [3,4], every HWP C accounting method can be classified based on activity data and conversion factors availability or based on calculation procedures. HWP C accounting methods can be classified based on activity data and conversion factors availability as either direct inventory or flux estimation, or a combined direct inventory and flux estimation method. HWP C accounting methods can be classified based on calculation procedures into steady state, tier 1, tier 2, or tier 3. Direct inventory involves local or regional direct measurements of HWP C stocks and flows, local or regional derivation of half-life equations/curves, and conversion factors. It is often more reliable than flux estimation. Flux estimation involves the adoption of IPCC default values or values from other key works in the literature in the estimation of HWP C stock and flow data, derivation of half-life (decay) equations/curves, and conversion factors. This is adopted when there are little or no local and directly measured HWP C stock and flow data. Combined direct inventory and fluxes estimation can be combined in situations where one set of data is available (e.g., activity data) and the other is unavailable (e.g., half-life) and vice versa. HWP accounting methods classifications are further explained in Table 1.
An added benefit of the HWP C accounting method proposed at a subnational level will be accounting for the substitution effects of HWPs [27,31]. This is not a requirement for national level reporting under Kyoto and UNFCCC protocols [3,24]. Kyoto and UNFCCC Protocols only require the assessment of carbon storage at national levels [4,25]. Much more than accounting for carbon storage is required to accurately account for the contributions of HWPs to net zero carbon targets, hence, the added value of accounting for substitution effects of HWPs [12,31].
The improvement of activity data applied by previous national and subnational HWP C accounting frameworks can be realized via the collection of more detailed subnational material flow information across all value chains involved, including, but not limited to carbon stocks and flows within forests, wood processing plants, and solid waste disposal sites, as well as in between them. This is better than using generalized mill efficiency factors for the estimation of HWP volumes and derivation of decay curves across different product life stages (i.e., roundwood to softwood sawtimber, softwood sawtimber to lumber, lumber to new housing, panels to new housing, and residues to pulp) [42,48]. Collection of more detailed subnational material flow information can be achieved by the implementation of well-designed and exhaustive surveys and stakeholders’ interviews, which collect granular information on local or regional HWP production, consumption, and loss flows across every value chain involved. Material flow information for estimation of the benefits of all substitution activities associated with HWP flows will also be needed. This includes material replacement, fossil fuel displacement, and energy efficiency improvement, as well as recycling and cascading use activities associated with retained production, exports, and imports. Collecting this will greatly improve the resolution of activity data at the subnational level.
Conversion factors are either derived from local or regional carbon content measurements of different components or those provided in successive IPCC guidelines (also drawn from the literature). However, due to the need for accurate HWP C estimations at subnational levels, local and regional measurements and estimations are preferred for all conversion factors. This preference applies also for the estimation of product half-lives, shelf life, lifetimes, obsolescence, and fire incidence rates of all products in use and in SWDS, as well as displacement factors due to different substitution effects (e.g., material replacements, fossil fuel displacement, energy efficiency gains, recycling, and cascading use activities, etc.) [7,47].
The most widely adopted calculation procedure for national and subnational HWP C accounting is that recommended by the IPCC [46,47]. The IPCC recommends accounting for carbon storage stocks and flows across the different life cycle stages of HWPs (harvest to timber products to primary products to end use and disposal) using either generalized mill efficiency and/or local or regional material flow ratios and half-life coefficients [4,7]. Conversely, a subnational calculation procedure adopted by the US Forest Service Northern region, the California Forest Project Protocol (CFPP) applies simple storage ratios to harvest rather than tracking carbon through the life cycle stages of HWPs using either generalized mill efficiency and/or local or regional material flow ratios and half-life coefficients as recommended by the IPCC [4,47]. Also, while IPCC recommends estimation of carbon storage stocks and flows annually, CFPP prescribes estimation of carbon stocks using the 100-year average from the current year’s harvest [46,47]. The CFPP is prone to errors for years within the 100-year period where sufficient data were not collected, hence, back-casting of HWP volumes using regression models was applied [7,47]. For both national and subnational HWP C accounting frameworks, applying IPCC-recommended calculation procedures using more detailed local/regional activity data and conversion factors should be pursued.

5. Approaches

HWP C approaches are different implementations of HWP C assessment or accounting boundaries [5,37]. Each approach has the HWP C components (i.e., stocks and flows) they include or exclude clearly stated [10,36]. Approaches usually mark the boundaries of inclusions and exclusions of domestic production and consumption of HWPs, as well as import and export of HWPs for consumption in-region or out-region. There are seven major approaches recognized in successive IPCC guidelines [3,4] and the literature [9,15]. Of the seven approaches, five are pool-based (instantaneous oxidation, stock change, production, stock change approach for HWPs of domestic origin, and domestic origin-stock change), while only two are flux-based (atmospheric flow and simple decay). All approaches are discussed in this section as follows, as well as in Table 2.
The instantaneous oxidation (IO) approach counts all carbon from HWPs as removed and oxidized within one year of harvest. This is also known as the default IPCC approach, and it is often applied when there are no activity data. The stock change (SC) approach estimates change in the HWP C pool (inclusive of logs, pulpwood, and chips) within the consuming region only. It accounts for all wood products produced and consumed within the region and those imported and consumed within the region. However, it excludes HWPs exported to other regions. It is a consumption-oriented HWP C accounting approach. The production approach estimates the HWP C pool within the producing region only. It accounts for all wood products produced and consumed within the region and those produced and exported to other regions. However, it excludes wood products imported from other regions. It is a production-oriented HWP C accounting approach. The atmospheric flow (AF) approach is consumption-oriented. It involves the estimation of CO2 fluxes from and to the atmosphere associated with HWPs within the region of consumption by accounting for all wood products produced and consumed within the region and those imported and consumed within the region, but it excludes those exported to other regions. The simple decay (SD) approach is another production-oriented accounting approach. It involves the estimation of CO2 fluxes from and to the atmosphere associated with HWPs within the region of production. It accounts for wood products produced and consumed within the region and those produced and exported to other regions, while excluding those imported from other regions. Please note that the CO2 fluxes from the atmosphere accounted for in atmospheric flow and simple decay approaches are the net annual CO2 sequestered in trees, as well as CO2 stored in HWPs in use and in SWDS. The domestic-origin-stock change (DOSC) is a hybrid HWP C approach that estimates all HWP C pools associated with the region. This approach accounts for HWPs produced and consumed within the region, those imported and consumed by the region, and those exported to and consumed in other regions. It is a combination of production and stock change approaches. Even though it exhaustively accounts for HWPs pertaining to particular regions, it subjects HWP C accounting under the KP and UNFCCC framework to double counting, as both exporting and importing regions may take ownership of stored carbon at the same time. The stock change approach for HWPs of domestic origin (SCAD) is a unique HWP C approach that estimates only HWP C pools produced and consumed within a region. This approach allocates HWP C stocks and flows imported and consumed by one region to the source regions, and HWP C stocks and flows exported to and consumed by other regions to the destination regions. SCAD is a HWP C accounting approach that encourages local production and consumption to maximize the C benefits of HWP within the region rather than exporting it to other regions or importing it from other regions.
While most of the previous subnational HWP C assessment frameworks adopt the production approach as recommended by the IPCC, it is noteworthy that the production approach as currently constituted does not meet the requirement for assessing other contributions of HWPs to net zero carbon targets. While the production approach as currently constituted accounts for carbon storage, any HWP C assessment framework for assessing the contributions of HWPs to net zero carbon targets also ought to account for substitution effects of HWPs such as recycling and cascading use activities, material replacement, fossil fuel displacement, and energy efficiency gains, hence, the need for a new approach. We recommend adopting a modified DOSC approach because it is the only HWP C approach that allows for accounting for import and export within the same framework. While accounting for carbon storages of export and import within the same framework amounts to double counting, the modified DOSC approach recommended by this study prescribes the exclusion of carbon storages of imports in accordance with IPCC-recommended production approach applied by previous subnational HWP C studies but proposes the inclusion of the assessment of all substitution activities associated with all HWP flows (i.e., retained production, export and import), since they are all associated with the region under consideration.

6. Modeling Principles

The modeling principles for HWP C estimation refer to the different kinds of estimation formats, models, or platforms for estimating HWP C stocks and stock changes [6,49]. The authors in [7] identified and described three major modeling principles: book-keeping or spreadsheet, material flow analysis, and life cycle assessment.
Book-keeping or spreadsheet modeling principles are not synonymous with spreadsheet or Excel-based HWP carbon calculators; they are calibrated using regression equations, half-life equations, and carbon response curves derived for a certain region and transferred for other regions irrespective of the level of conformity or closeness in structure [50,51]. They could be prone to high error levels because they are initialized based on information derived from global conditions or other ecological zones. However, using locally available data for their initialization may help minimize the error [5,7,52,53]. An example of this can be found in the overestimation (by 40%–46%) of HWPs from the Thuringian State Forests of Central Germany by a model originally developed for the Finnish Forest Sector (CO2FIX) [7,52].
Material flow analysis modeling principles allow the systematic assessment and tracking of HWP C stocks and flows of HWP C from the time the wood is harvested in the forest, along the transformation process, the consumption, and end-use of transformed products within a defined region and timeframe [54,55,56]. As previously stated above, verifiable and validated local or regional activity data and conversion factors can improve the precision of HWP C estimates. This task could be daunting due to the rigorous burden associated with collecting such detailed information, as well as the current lack of local/regional measurement, reporting, accounting, and verification standards and frameworks. Consequently, validated inventory data collected could be applied as material flow allocation and conversion factors [11,40]. An example of previous material flow analysis inventory data applied in this manner for the estimation of embodied carbon stocks or fluxes is the human appropriation of net primary production (HANPP) material data [16,17]. The HANPP accounting framework is a database of conversion factors that can be applied for estimating biomass or net primary production (NPP) stocks and flows from establishment to production, through to harvest and to their eventual socio-economic use phases (either linear or circular) [11,57]. HANPP presents a structural framework for accounting for local, regional, or even global biomass or NPP flows, as well as energies, materials, and wastes (carbon emissions inclusive) associated with their establishment, production, harvest, and use in any sector of the local and global economy [17,40].
Life cycle assessment modeling principles are applied in specific cases to estimate the carbon footprint (i.e., embodied carbon) and other environmental impacts of a given HWP, scaled to a functional unit, over its life cycle stage. An LCA of five typical HWPs produced in China reported total carbon footprints of 154.81 kgC/m3, 408.34 kgC/m3, and 228.73 kgC/m3 for construction products, furniture products, and panel products, respectively [58]. Over the past two decades, the Consortium for Research on Renewable Industrial Materials (CORRIM) has published numerous LCAs of HWP production in the US [59,60]. Researchers have also attempted to use LCA to capture the substitution effects of material and fossil energy displacements [12,27], as well as recycling and cascading use activities [6,61,62,63,64,65]. While some researchers estimate substitution factors by directly comparing the life cycle greenhouse gas impact of HWP to other greenhouse-gas-intensive materials [25] or estimating marginal benefits of change in end use [65], others argue that the substitution effect can be ascertained only from economy-wide and consequential LCA [62,63]. Such consequential LCA will rely on historical and future market trends to determine if wood has substituted or will substitute non-wood-based products [64,65,66]. Consequential LCA modeling is however often associated with higher uncertainties compared to attributional LCA modeling [56,63]. Sources of uncertainties in previous LCA-based assessment of HWP substitution effects often include system boundary considerations such as the nature of the forest supplying HWPs, short- and long-term impacts of increased wood harvest on forest and forest soil sink, longevity of buildings, value and duration of fossil carbon displacement, changes in carbon stock of HWPs, post-use of HWPs, etc. [62,63,64,65,66]. Besides estimating substitution effects or factors, more work is still needed to aggregate substitution effects to subnational boundaries. The authors in [67,68,69,70] developed and applied territorial LCA frameworks for assessing and comparing the eco-efficiency of all production and consumption activities within defined French territories. Going forward, similar attempts should be made at applying territorial LCA for assessing all the contributions of HWPs (substitution effects inclusive) to future subnational net zero carbon or life cycle GHG emission targets.

7. Conclusions

To meet the specific needs of HWP carbon accounting at a subnational level, we identified the need to expand previous HWP C accounting framework components to account for other climate change mitigation benefits such as local or regional substitution effects of all produced and consumed HWPs. To ensure improved accuracy (an important motivation for subnational HWP carbon accounting), we recognize the need to attain the highest level of detail in terms of the method applied for accounting i.e., tier 3 or near tier 3 level assessment. This can be achieved by improved local or regional measurements and estimations of all parameters for HWP carbon estimations. To ensure more accuracy and completeness of HWP carbon accounting frameworks at the subnational unit level, we also recommended the adoption of a modified domestic origin-stock change approach to account for all HWP carbon stocks and flows contextually relevant for a full assessment of all climate change mitigation benefits associated with HWPs at sub-national levels, e.g., including substitution effects of retained, imported, and exported HWPs. Lastly, we recommended engendering value chain modeling principles (which combines material flow information and life cycle assessment perspectives) to deal with new accounting requirements expected to accompany the recommended expansion of HWP C accounting framework components at subnational levels, i.e., inclusion of substitution effects etc. The suggested recommendations by this study are expected to form parts of new guidelines for supporting subnational units to assess all potential climate change mitigation contributions of HWPs to their respective net zero carbon targets.

Author Contributions

Conceptualization, O.A., O.T., T.V., R.M., R.G., P.C., D.K. (Danielle Kloster) and D.K. (Deepak Kumar); methodology, O.A., O.T., T.V., R.M., R.G. and P.C.; resources, O.A., O.T. and T.V.; writing—original draft preparation, O.A. and O.T.; writing—review and editing, O.A., O.T., T.V., R.M., R.G., P.C., D.K. (Danielle Kloster) and D.K. (Deepak Kumar); visualization, O.A., O.T. and T.V.; supervision, O.T.; project administration, O.T. and T.V.; funding acquisition, O.T., T.V. and R.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research and the APC was funded by the New York State Department of Agriculture and Markets under the award 0314-AD FP-3886.

Data Availability Statement

The data is available on request from the corresponding author.

Acknowledgments

We acknowledge the support received from the New York State Department of Environmental Conservation.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Forests 14 01959 g001
Figure 1. Classifications of harvested wood products [3,4].
Figure 1. Classifications of harvested wood products [3,4].
Forests 14 01959 g001
Table 1. HWP accounting methods classifications (i.e., based on availability of activity data and conversion factors, and calculation procedures).
Table 1. HWP accounting methods classifications (i.e., based on availability of activity data and conversion factors, and calculation procedures).
MethodActivity DataConversion FactorsCalculation Procedures
Steady-state HWP poolNo activity dataAssumes steady state over gap years when there are no data. No new estimates and no new conversion factors. Rough estimates of steady-state HWP pool may rely on or apply production harvest for a single year or takes an average over five years and assumes the same for a gap year or for gap years with no data.No new calculation and estimates
Tier 1Activity data are availableNo country/region-specific conversion factors/methods, neither of local direct measurements and estimations of stored carbon and half-life are available (default IPCC carbon and half-life fluxes/values are adopted or assumed), no clearly defined system boundaries.Calculation based on activity data and default IPCC conversion factors (i.e., carbon and half-life fluxes/values)
Tier 2Activity data are availableSome country/region specific conversion factors/methods are available. A combination of direct inventory and flux estimation is applied i.e., either of local direct measurement or estimations of carbon and half-life is available (default IPCC carbon and half-life fluxes/values are adopted or assumed when not available locally). System boundaries are more clearly defined than Tier 1.Calculation based on activity data and a mix of locally measured and default IPCC conversion factors (i.e., carbon and half-life fluxes/values)
Tier 3Activity data are availableCountry/region-specific conversion factors/methods are available. Mostly based on direct inventory (i.e., local direct measurement or estimations of carbon and half-life are mostly available). Local measurements are sometimes supplemented with default IPCC carbon and half-life fluxes/values if they are consistent with country/region-specific values or under advanced modelling scenarios (for comparison). Well-defined system boundaries.Calculation based on activity data and mostly locally measured and estimated conversion factors (i.e., carbon and half-life fluxes/values).
Source: Adapted from IPCC, 2014 and IPCC, 2019.
Table 2. HWP accounting approaches based on their respective included and excluded boundaries.
Table 2. HWP accounting approaches based on their respective included and excluded boundaries.
HWP Approaches Components and Boundaries
Pool-Based ApproachesFlux-Based ApproachesForest Land Carbon Pool/FluxesHWP Pool Domestically Produced and UtilizedHWP Pool Exported and Utilized in Other RegionsHWP Pool Imported and Utilized Domestically
Instantaneous oxidation (IO)-√
Stock change approach for HWP of domestic origin (SCAD)-√√
Production (P)Simple decay (SD)√√√
Stock change (SC)Atmospheric flow (AF)√√ √
Domestic origin-stock change (DOSC)-√√√√
Source: Adapted from [9,15] (√-components included within the boundary).
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Arodudu, O.; Therasme, O.; Volk, T.; Malmsheimer, R.; Crovella, P.; Germain, R.; Kloster, D.; Kumar, D. Towards a Carbon Accounting Framework for Assessing the Benefits of Biogenic Wood Carbon to Net Zero Carbon Targets. Forests 2023, 14, 1959. https://doi.org/10.3390/f14101959

AMA Style

Arodudu O, Therasme O, Volk T, Malmsheimer R, Crovella P, Germain R, Kloster D, Kumar D. Towards a Carbon Accounting Framework for Assessing the Benefits of Biogenic Wood Carbon to Net Zero Carbon Targets. Forests. 2023; 14(10):1959. https://doi.org/10.3390/f14101959

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Arodudu, Oludunsin, Obste Therasme, Timothy Volk, Robert Malmsheimer, Paul Crovella, René Germain, Danielle Kloster, and Deepak Kumar. 2023. "Towards a Carbon Accounting Framework for Assessing the Benefits of Biogenic Wood Carbon to Net Zero Carbon Targets" Forests 14, no. 10: 1959. https://doi.org/10.3390/f14101959

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