Commercial afforestation can deliver effective climate change mitigation under multiple decarbonisation pathways

Afforestation is an important greenhouse gas (GHG) mitigation strategy but the efficacy of commercial forestry is disputed. Here, we calculate the potential GHG mitigation of a UK national planting strategy of 30,000 ha yr−1 from 2020 to 2050, using dynamic life cycle assessment. What-if scenarios vary: conifer-broadleaf composition, harvesting, product breakouts, and decarbonisation of substituted energy and materials, to estimate 100-year GHG mitigation. Here we find forest growth rate is the most important determinant of cumulative mitigation by 2120, irrespective of whether trees are harvested. A national planting strategy of commercial forest could mitigate 1.64 Pg CO2e by 2120 (cumulative), compared with 0.54–1.72 Pg CO2e for planting only conservation forests, depending on species composition. Even after heavy discounting of future product substitution credits based on industrial decarbonisation projections, GHG mitigation from harvested stands typically surpasses unharvested stands. Commercial afforestation can deliver effective GHG mitigation that is robust to future decarbonisation pathways and wood uses.


Introduction
Achieving the ambition of The Paris Agreement to limit global warming to 1.5 o C above pre-industrial levels requires rapid implementation of negative emissions technologies 1 , in particular increasing forest carbon sinks 2 . Afforestation on land spared from agriculture 3 is proposed as an integral element of greenhouse gas (GHG) mitigation 4,5 , but mitigation efficacy is highly context-specific and depends on, inter alia, climate, tree species, management, and product substitution. In tropical regions, fast growing commercial forests managed for wood production have been unfavourably compared to restoration of natural forest 6 but these conclusions are not transferable to a temperate context. There remains a gap in evidence determining the most effective type of temperate forestry for achieving GHG mitigation. The role of sustainable wood harvesting is recognised in GHG mitigation strategies 5,7,8 and could be enhanced through cascading use of wood 9 , but recent modelling indicates that future mitigation from product substitution will diminish owing to decarbonisation of substituted materials and energy 10,11 . In coniferous temperate forests, bioenergy harvest breakeven times can range from 30 to 70 years assuming coal is being replaced, depending on the scale of the assessment and whether the stands are damaged 12 . Meanwhile, dynamic life cycle assessment (LCA) studies indicate that it can take up to 100 years for CO2 released from current bioenergy generation to be fully compensated by forest regrowth 13,14 .
To fully understand the role of new commercial forests in future GHG mitigation, there remains a need for coherent modelling of GHG mitigation across the four life cycle stages (production, use, cascading use and end-of life) of hierarchical wood value chains 15 in the context of decarbonised material and energy systems 4 . We fill this gap through comparison of 100-yr mitigation achieved by newly planted commercial and conservation (i.e. unharvested) forests in the UK, a country with high afforestation potential 3,16,17 . Although prioritising GHG mitigation in afforestation strategies will involve complex trade-offs with other ecosystem services [18][19][20][21] , such as biodiversity conservation, the purpose of this paper is not to explore these. Rather, it aims to generate robust evidence on the efficacy of alternative afforestation options for the primary strategic aim of mitigating GHG emissions. We apply dynamic consequential LCA 22 to 33 what-if scenarios representing a range of commercial Sitka spruce (Picea sitchensis) forests with different harvested wood product value chains, and conservation forests, across representative growth rates, and in the context of different decarbonisation pathways (Table 1).
Under typical conditions, one hectare of newly-planted commercial forest could achieve cumulative GHG mitigation of up to 2.27 Gg CO2e ha -1 within a core decarbonisation pathway projected for the UK 4 by 2120 -up to 269% more mitigation than delivered by newly planted conservation forests, and 17% more than achieved by leaving a newly planted fast-growing conifer forest unharvested.
Harvesting reduces long-term terrestrial carbon storage by 61%, but this is more than offset by harvested wood product (HWP) carbon storage, and concrete and energy substitution -even with near total decarbonisation of the energy sector within the 100 year study horizon. Projected wide deployment of carbon capture and storage (CCS) 4 contributes up to 45% of cumulative 100-year mitigation benefit, but also reduces the energy substitution credits realised by future wood energy.
Consequently, the efficacy of mitigation is surprisingly similar between hierarchical and bioenergy uses of wood, and remarkably resilient to CCS deployment assumptions. To contextualise the results in a national planting strategy, we model the UK Committee on Climate Change recommended planting rate of 30,000 ha yr -1 from 2020 to 2050. If this strategic target is met with commercial forests, it would cumulatively mitigate up to 1.64 Pg CO2e emissions by 2120, compared with 0.54 Pg CO2e for a semi-natural broadleaf conservation forest, or 1.09 Pg CO2e for an equal planting rate of 15,000 ha yr -1 of each forest type. For UK forests and projected decarbonisation pathways, largescale commercial afforestation could contribute strongly to GHG mitigation targets.

Results
One hundred year snapshots following a single planting event: Terrestrial carbon sequestration in semi-natural broadleaf, mixed conifer-broadleaf and fast-growing conifer conservation forests planted in 2020 equates to cumulative mitigation by 2120 of 0.62 (0.41-0.82), 1.30 (0.94-1.64) and 1.94 (1.30-2.58) Gg CO2e ha -1 , respectively (Fig. 1). Mitigation performance across these conservation forest types is proportionate to yields (Table 1). On average over 100 years, commercial Sitka spruce forests store 61% and 42% less terrestrial carbon than unharvested conifer and mixed conservation forests, respectively, but still 24% more than slow-growing semi-natural broadleaf forests (Fig. 1). However, in addition to terrestrial carbon storage, commercial forests realise ca. 1.7 Gg CO2e ha -1 mitigation by HWP carbon storage, fossil fuel substitution, BECCS and concrete substitution in the Core context, at average growth rates (Fig. 1). Relatively small processing emissions of 0.20 (Bioenergy prioritised wood use) to 0.28 Gg CO2e ha -1 (Hierarchical wood use) are based on the conservative assumption of a very gradual decline in the emission intensity of processing operations (Table 2 and Supplementary Data 1). Overall, one hectare of YC 18 commercial forest planted in 2020 will achieve cumulative GHG mitigation of between 2.0 and 2.27 Gg CO2e by 2120, varying by less than 8% and 10% between wood use strategy and decarbonisation context, respectively. Yield variability extends the mitigation range to 1.36-2.96 Gg CO2e ha -1 by 2120. Thus, commercial Sitka spruce forests support between 3% (relative to 100% conifer) and 269% (relative to 100% broadleaf) more GHG mitigation than conservation forests over a 100-yr time horizon from establishment. Figure 1 -Contribution of major processes to cumulative GHG fluxes from one hectare of afforested land (planted in 2020) by year 100, for commercial and conservation forests. Default yield class 18 (m 3 ha -1 yr -1 ) for the commercial and unharvested conifer conservation forests, 4 for the semi-natural broadleaf conservation forest and 11 for the mixed conifer-broadleaf conservation forest. Error bars represent full yield ranges presented in Table 1. Commercial forest results are presented for Core and Further Ambition (FA) decarbonisation contexts, and for Hierarchical (Hier.) and Bioenergy (Bioen.) wood use strategies, with differing mitigation contributions from concrete and fossil fuel substitution and carbon storage in harvested wood products (HWP) or long term geological stores via carbon capture & storage.

National planting programme
In our scenario of a commercial forest planting programme involving 30,000 ha planted every year from 2020 until 2050 (Fig. 2), only 15% of the 100-year mitigation will be achieved by 2050. A 30-year commercial forest planting strategy could achieve cumulative net mitigation of up to 1.64 Pg CO2e at default yield (Fig. 2), with a range across yields and scenarios of 1.13 to 2.0 Pg CO2e for YC 14 Hierarchical (Further Ambition) and YC22 Bioenergy (Core), respectively (shown in S3 & S4). This compares with mitigation of between 0.54, 1.15 or 1.72 Pg CO2e from equal planting of semi-natural, 50:50 conifer-broadleaf mixed or unharvested conifer forests, respectively (Fig. 2), at default yields (Table 1)   (FF) substitution (negligible in the Further Ambition context), concrete substitution and bioenergy carbon capture and storage (BECCS). Also shown (lines) are cumulative CO2e mitigation trends for conservation forests planted at the same rate, ranging from semi-natural 100% broadleaf (YC 4), through a 50:50 mixed conifer:broadleaf forest (YC 11) to an unharvested 100% conifer forest (YC 18). These lines represent terrestrial forest carbon storage only, and the C100-18 forest provides a direct comparison of cumulative mitigation that could be achieved by YC 18 conifers if they are not harvested.

Wood use and decarbonisation scenarios
For a national planting programme of 30,000 ha yr -1 , more rapid decarbonisation of the wider economy represented in the Further Ambition (vs Core) context could reduce cumulative national mitigation achieved by Hierarchical or Bioenergy wood use in 2120 by 5% or 2%, respectively (Fig. 2).
Meanwhile, a Bioenergy wood use strategy rather than Hierarchical wood use strategy could increase cumulative GHG mitigation achieved in 2120 by 5% (Core context) to 8% (Further Ambition context) (Fig. 2). In the Further Ambition context, future process emissions and fossil fuel substitution credits are minor, concrete substitution credits are diminished relative to the Core context, and mitigation by BECCS becomes prominent through time (Fig. 2). In fact, BECCS accounts for 33% of cumulative mitigation by 2120 in the Bioenergy wood use strategy, whilst HWP carbon storage accounts for just 5% of cumulative mitigation (Fig. 2), reflecting the fact that 65% of harvested biomass immediately goes to bioelectricity generation (Table 1) both Bioenergy and Hierarchical scenarios, the BECCS pool will also continue to grow as biomass and cascaded products reach their final use.

Sensitivity analysis
In the national planting program with Hierarchical wood use, product substitution (concrete) accounts for 0.12-0.32 Pg CO2 (8-20%) of cumulative GHG mitigation by 2120 for Core and Further Ambition contexts (YC18), respectively. Reducing product substitution by 50% would reduce cumulative GHG mitigation by 5% or 11% for Core or Further Ambition contexts, respectively (Supplementary Data 5). Counter-intuitively, concrete substitution makes a greater relative contribution to GHG mitigation in the Further Ambition context than the Core context, because net mitigation is lower and concrete decarbonisation lags energy decarbonisation under the Further Ambition decarbonisation pathway. Alternatively, introducing glulam production (substituting structural steel) alongside development of fibres to substitute oil-derived textile fibres could increase cumulative GHG mitigation achieved in 2120 by 10% or 7% for a national planting program in a Core or Further Ambition context, respectively (Supplementary Data 5). Potentially, viscose fibre could substitute cotton fibre instead of synthetic fibres, in which case one ha of cotton production could be avoided for every ha of commercial forest planted -which could lead to significant further afforestation and associated GHG mitigation overseas (beyond the scope of this study to quantify).
In the pessimistic event of no deployment of CCS technology during the 100-year study time horizon, 100-yr mitigation achieved by a commercial forest planting strategy in the UK would be reduced by just 8% (Core) or 6% (Further Ambition) for Bioenergy wood use (Supplementary Data 7) and 6% (Core) or 4% (Further Ambition) for Hierarchical wood use (Supplementary Data 6), respectively. In the absence of CCS, energy and concrete substitution credits are larger than in the base case, counteracting the effects of reduced BECCS and thus reducing the sensitivity of net GHG mitigation to CCS deployment. Across all these alternative product substitutions, commercial forestry remains an effective GHG mitigation strategy, still achieving up to 162% more GHG mitigation than seminatural conservation forestry in the worst case scenario of reduced concrete substitution.

Discussion
New evidence presented in this paper emphasises the efficacy of commercial forestry for GHG mitigation, and its resilience to future decarbonisation pathways and wood uses, highlighting the effective and reliable contribution that commercial forestry could make towards the Paris Agreement 1 even with projected decarbonisation of marginal materials and energy that are likely to be substituted by harvested wood products in the future. These results counter the conclusions of recent studies questioning the climate credentials of commercial forestry 6,10,45 . Our study demonstrates the importance of considering multiple stages of hierarchical wood use, considering, inter alia, consistent time-dependent substitution factors and CCS deployment based on published projections 4,29 . However, we do find that at equivalent growth rates, commercial forests lag unharvested conservation forests in cumulative GHG mitigation until just before the second harvest cycle (90 years after establishment for typical UK forests).
The magnitude of GHG mitigation achieved by commercial forestry is remarkably resilient to the uses of wood and wider decarbonisation pathways over a 100-year timeframe. Indeed, across all the variables considered, it is clear that tree growth rates have the greatest influence on GHG mitigation, in line with previous studies focussed primarily on terrestrial carbon storage by forests 46,47 . In this study, the vast majority of product substitution credits attributable to new forest planting are heavily discounted because they arise over 50 years into the future (after first main harvest) when decarbonisation projections indicate much lower emission intensities for substituted energy and materials (Supplementary Data 1). Had current substitution credits been applied throughout, the GHG mitigation achieved by commercial forests would have been considerably greater, and more sensitive to variations in wood use 15,44,48 . Nonetheless, it remains clear that hierarchical use of timber relies on proven technologies to deliver the majority of 100-year mitigation via HWP carbon storage and displacement of mineral construction materials which are unlikely to fully decarbonise for many decades 4,9 (Table 2), maintaining effective mitigation 100 years after first plantings (Fig. 2).
Demand for wood is expected to grow as the circular bio-economy expands and new products are developed (e.g. engineered and modified timber construction products, silvi-chemicals, textiles and biocomposites) 47,4 . Our results show that commercial forests could supply a large share of these future demands whilst delivering comparable or more long-term GHG mitigation compared with conservation forests.
Somewhat surprisingly, both Hierarchical and Bioenergy value chain GHG mitigation was robust to assumptions about CCS deployment. BECCS features prominently in many climate stabilisation scenarios 1,4,5 but depends on commercially unproven technology 51 . Conservatively, our Core decarbonisation pathway involves widespread deployment of BECCS only after 2085, whilst our Further Ambition pathway assumes widespread deployment from 2050 (Table 2) -though with limited effects until the first main harvests from 2070 onwards (Fig. 2). Most climate change mitigation projections indicate a substantial role for dispatchable bioenergy (with CCS) at least up to the end of this century 4,5,17,52 . Global requirements for primary energy from biomass to meet Paris Agreement goals range from circa 100 to 400 EJ yr -1 , and BECCS is expected to play an important role in achieving net zero carbon emissions 5,51-53 . Associated global land requirements have been estimated at between 3 and 7 million km 2 , with possible negative side effects for biodiversity and food security 5,52,54 . The UK Climate Change Committee recommends installing circa 67 TWh of BECCS electricity generation by 2050 55 . Based on our Bioenergy value chain, over 7 million ha of land would need to be afforested by 2070 in order to fully meet BECCS ambitions exclusively with forest harvests (excluding high-quality sawn wood) -requiring a rate of planting 4.6 times higher, and continuing 20 years longer, than the 30,000 ha yr -1 over 30 years considered in our study based on UK Committee on Climate Change recommendations 4 . Although a large area, this would bring total UK forest cover to around 42%, close to the European average of 40% 56 . Whilst purpose-grown bioenergy crops could deliver more rapid fossil fuel substitution using less land 55 , commercial forests combine the provision of high-quality materials into the circular economy with the delivery of multiple ecosystem services 5,20 , including medium-term climate regulation by terrestrial C storage 57 and HWP C storage in the decades before BECCS is anticipated to become widely deployed 4,53 ( Table   2). Unlike many purpose-grown bioenergy crops, Sitka spruce can also produce reliable growth on marginal land that could be spared from extensive livestock production in the UK 3 .
Commercial afforestation can deliver significantly greater GHG mitigation than semi-natural afforestation largely due to the higher growth rate of conifers. Commercial investment in afforestation is based on the expectation of timber harvest and, to match current timber markets in the UK and the benefit of earlier harvest of faster-growing tree species, this generally leads to the planting of conifer rather than broadleaved species. Thus, faster growing commercial forests deliver more, and longer-term, GHG mitigation than slower growing semi-natural forests. Depending on future decarbonisation and technology contexts, market demand and environmental risks, high yielding conifer forests could be harvested 58 for hierarchical wood use or for bioenergy (with CCS), or left in the ground to maximise terrestrial C storage. The latter option can be chosen late in the forestry cycle in response to relevant market, environmental and/or policy signals. Commercial forestry therefore provides a robust strategy for GHG mitigation across a wide range of future contexts, at least in temperate zones, and merits a prominent role in climate policies, including nationally determined contributions under the Paris Agreement, and the forthcoming EU Forest Strategy 58 .
Our results pertain to regions with commercial forest rotations of 50 years (or less). In regions (e.g. boreal) with slower tree growth rates and longer forestry cycles, leaving wood unharvested could be a better mitigation option over the next century 18,58,5 . We applied a 100-year cut-off to our analysis to maintain a realistic time horizon for climate policy decision-making, and to avoid deeply uncertain marginal technologies beyond this time horizon. A longer time horizon is likely to favour harvested commercial forests owing to saturating carbon sequestration in conservation forests (Fig. 2). There is some uncertainty about whether old forests may in fact continue to act as significant carbon sinks 19,59,60 . Unexpected losses from pests, pathogens, fires, windthrow and drought-caused mortality pose an increasing risk to GHG mitigation from affected areas, but such losses are not likely to affect commercial forests more than conservation forests. In fact, active management and periodic harvesting of commercial forests can reduce fire risks 61,62 and safeguard carbon stocks within harvested wood products whilst climate change increases environmental risks to old forests.
Whilst results in this paper demonstrate the comparatively high long-term GHG mitigation efficacy of commercial forests compared with unharvested conservation forests, it is imperative to note that commercial and conservation forests deliver different suites of important ecosystem services and should be considered complementary and not conflicting land uses 5 . Rather than limiting the area of commercial forests in favour of semi-natural forests 6 , more emphasis should be placed on rapid deployment of both forest types, and on the cascading use of harvested wood to derive maximum economic and mitigation value. Although our modelled commercial forestry system is 100% Sitka spruce, single species forests are not the only commercial option, and there is increasing support for mixed-species conifer forests (or conifer-broadleaf mixtures), under sustainable forest management to deliver both a high yield of wood products and other ecosystem services 20,63 . Diversification of commercial forestry could also enhance resilience to pests, diseases and climate change effects 64,65,66 . Nabuurs et al. 8 argue for policy changes to encourage such "climate-smart forestry", claiming it could deliver an extra 441 Mt CO2e yr -1 mitigation across Europe by 2050. National afforestation strategies should promote a portfolio of commercial, conservation and agroforestry systems tailored to local conditions and adapted to climate change 5,67 ,in order to deliver the range of ecosystem services required by society, in addition to GHG mitigation 68 . To achieve this there is a strong case to increase the 30,000 ha yr -1 UK afforestation area target of the Committee on Climate Change, which is very modest in the context of international ambitions for increased tree canopy cover, such as the 0.9 billion hectares identified by Bastin et al. 69 , or indeed the EU Forest Strategy objective to plant 3 billion trees by 2030 58 . Successful forest policy will also need to integrate social outcomes, and engage local stakeholders in strategic planning 63 .
We conclude that fast growing tree species should be a major component of national afforestation strategies in order to achieve ambitious GHG mitigation targets. Commercial investment will favour such high production forests of species that produce timber matching market demand. The GHG mitigation potential of commercial forestry is robust against future decarbonisation scenarios, and will continue to deliver mitigation well beyond the 100-yr time frame considered in this study, when conservation systems have reached their peak capacity. Furthermore, commercial forestry can deliver similar long-term mitigation when wood is used in either hierarchical or bioenergy value chains, potentially contributing to future circular bioeconomy, energy security and carbon removal objectives as required at the time of harvest. However, it takes time to establish large areas of new forest. In addition to the time-lag between forest establishment and GHG mitigation, especially pronounced when considering mitigation from commercial forest hierarchical value chains, there is a time lag in implementation of national afforestation policies owing to constrained planting rates 71 .
Whilst sustainable intensification 3 and curtailed demand for livestock products 5,31,53 are projected to free up substantial land for afforestation in the UK and elsewhere over coming decades, our results reveal the urgency of commencing a programme of large-scale tree planting if afforestation is to significantly contribute to Paris Agreement targets for the second half of this century 1,71 .

Aim & scenarios
This study seeks to determine the most effective afforestation strategy for achieving climate change mitigation in a temperate region. Specifically, it aims to generate new evidence on the comparative long-term (100 year) GHG mitigation efficacy resulting from the establishment of commercial forests and conservation forests on grassland in the UK (baseline). We modelled 33 scenarios representing common temperate commercial and conservation forestry typologies, tree growth rates (m 3 ha -1 yr -1 : "yield class") and HWP value chains (Table 1). Rationale for scenario development commensurate with study objectives and conclusions is fully elaborated in S1, and summarised below.

Table 1 -Evaluated forest value chain scenarios, comprising commercial and conservation forests with a mix of conifer and broadleaf species. Harvested wood product (HWP) carbon storage, and greenhouse gas emission displacement via product substitution, depend on choice of Hierarchical or Bioenergy wood use strategies, and the degree of decarbonisation in the wider economy -defined by UK Committee on Climate
Change "Core" and "Further Ambition" pathways (see Table 2 (Fig. 3), i.e. wood is subject to cascading use via a prioritised order: durable wood product manufacturing, extending service life times, re-use, recycling, bioenergy generation, and Ambition" pathways 4 -detailed in Table 2. These pathways involve differing rates of fossil fuel replacement by low carbon and renewable technologies, and differing rates of carbon capture and storage (CCS) implementation across industry sectors, representing different future contexts in which forestry value chains could exist. Note that fossil fuels remain the marginal source of dispatchable energy generation well into the future, especially for industrial heat, despite renewable energy technologies predominating 29 . Very few studies to date consider the timing of GHG emissions 63 and avoided GHG emissions in this way. Green' refer to a quality threshold of acceptable straightness, taper and knots in a log accepted by sawmills, with 'Green' being the higher quality. Carcassing refers to construction grade sawn wood. Data sources include wood harvest volumes from the CBM-CFS3 model, saw-mill product breakouts, UK statistics on wood waste and harvested wood product decay functions, elaborated in relevant sections of Methods and in S1.
Forests may be established for varying conservation objectives, from habitat provisioning to terrestrial carbon storage 5,7 . We represented a diverse range of possible conservation (unharvested) forest types based on different ratios of conifer (Sitka spruce, Corsican pine, Douglas fir) to broadleaf (silver birch, rowan, oak) trees ( Table 1). The 100%-conifer conservation forest (YC 18) represents carbon sequestration prioritisation and provides an unharvested comparator for the equal-YC commercial forests. Meanwhile, the semi-natural 100%-broadleaf forest represents prioritisation of habitat provisioning, and delivers much lower rates of carbon sequestration owing to slow growth (YC 4) -representative of broadleaf forests grown on lower grade land in UK 24 . Among a range of intermediate conservation forests, the default conservation comparator, delivering a mix of carbon sequestration and habitat provisioning, was a 50:50 conifer:broadleaf mix with YC 11. Yield classes for 100% Sitka spruce (18) and 100% broadleaved (4) are for the same site conditions.
Scenarios were analysed at two scales: 1 ha planted in 2020, and a national scenario of 30,000 ha yr -1 planted from 2020 to 2050 as per UK climate recommendations 4,17 on 900,000 ha of lower-quality grassland that could be taken out of livestock production through intensification 3 and/or reduced market demand for livestock products 4,31 . We assume that social and political change will lead to sufficient land being available, therefore, avoided GHG emissions from reduced livestock production or additional emissions from livestock production displacement are not considered. Analyses were cut off at 2120 in both scenarios, representing a maximum time horizon for climate policy. Climate change impacts on forest productivity were not included due to the already high complexity of the scenarios.

Life cycle assessment goal & scope
We applied dynamic consequential LCA 22 Table 1 (S1). Material flows were derived using UK data from a combination of forest carbon modelling 34 , harvest data from over 2,000 ha of commercially managed forests, data form a commercial sawmill that maximises sawn-wood output, national recycling data 35 and timber-use statistics 36 -elaborated in S1. Given the focus of this paper on GHG mitigation, only the global warming potential (GWP100) impact category was evaluated, expressed as kg CO2e. We discuss limitations later. Main processes arise during each stage of the value chain, including: forest establishment (and associated land use change from grassland); wood processing (debarking, milling, wood panel and paper production from primary and secondary wood flows); storage of carbon in HWP (primary and secondary wood 23 products); recycling and disposal of HWP (primary and secondary wood products), including incineration (with energy recovery) and landfill; bioenergy generation (with bioenergy carbon capture & storage (BECCS) in later years); substitution of fossil fuels for energy generation; substitution of mineral construction materials.

Terrestrial carbon
Forest growth, decay and harvest volumes were calculated using the Carbon Budget Model for the Canadian Forest Sector (CBM-CFS3) 34

Process emissions and carbon storage
All emissions associated with the production and transport of forestry inputs and all primary and secondary wood processing phases were extracted from Ecoinvent v.3.5 39 41 . As products retire from the HWP carbon pool, they are recycled or disposed of (by incineration or landfill) in proportions calculated according to UK recycling statsistics 35 (Fig. 3).
Retired tertiary products (i.e. products that have already been recycled once) are conservatively assumed to be disposed of. Horticultural mulch is assumed to decay at a rate similar to composted municipal solid waste 42 . Wood fuel is not included in the HWP carbon pools owing to rapid oxidation; return of this biogenic carbon to the atmosphere from the forest ecosystem is represented in the deficit between ecosystem carbon loss at the point of harvest, HWP carbon storage gain, and loss from recycled materials.

Substitution credits
Fuel-to-energy conversion factors (for coal, natural gas and wood chips) were taken from Ecoinvent unit processes 39 (Table 2). Assumptions about substitution are elaborated in S1, and reflect marginal changes associated with additional wood supply (e.g. an increase in timber frame construction, hence substitution of mineral construction materials). CCS deployment is gradually increased on a decadal basis, at a faster rate for the Further Ambition context, and applied equally to marginal (substituted) fossil fuel and wood electricity generation (Table 2 and S1).

Sensitivity analysis
Recent papers have highlighted the uncertain assumptions inherent in wood product substitution estimates 15,44 . This study presents a methodological approach that addresses key limitations by considering different value chains and dynamic product counterfactuals under different decarbonisation pathways. To further explore the complexity and uncertainty of future wood value chains, we modelled alternative product substitution assumptions to assess the sensitivity of model results to product substitution. Details of these are provided in S1 and Supplementary Data 5, summarised in Table 3. We also modelled an alternative decarbonisation scenario in which there is no deployment of CCS during the study time frame (Supplementary Data 6 and Supplementary Data   7), to evaluate the dependency of results on this technology.

Data availability
The authors declare that the data supporting the findings of this study are available within the paper and its supplementary information and data files: S1 for elaboration of scenarios and assumptions, Background data were generated using the publicly-available CBM-CFS3 model and extracted from the Ecoinvent v.3.5 database. All subsequent calculations were undertaken using standard MS Excel functions.