Zhou Shen

Lorie Hamelin

Ligia Tiruta-Barna

Converting carbon vulnerable lands to wood plantations for use as building materials: overall environmental performance and time-dependent assessment of carbon dioxide removals

Keywords: soil organic carbon, consequential life cycle assessment, dynamic life cycle assessment, climate change mitigation, biomass, negative emissions

Bioeconomy addresses the triple balancing challenge posed by the Paris Agreement: inducing additional GHG mitigation alongside biophysical carbon dioxide removals, while keeping fossil carbon in the ground. The use of woody plants as construction materials could simultaneously address this challenge: CO2 is absorbed through photosynthesis, while wood-based products provide mitigation by replacing either petrochemical materials such as plastic boards or energyintensive materials like bricks. This study combines, at the scale of France, static and dynamic life cycle assessment with soil carbon simulations to investigate the long-term environmental performance of using, as input for the building sector, black locust (Robinia pseudoacacia L.) planted on "carbon vulnerable lands". These are areas specifically selected for their low soil organic carbon (SOC) levels and low potential to interfere with food security. After the consecutive cultivation of three rotations, black locust was shown to stock an additional 92000 kg ha -1 of carbon in soils as SOC. The mitigation effect from the plastic board replacement is the major contributor to the assessed environmental impacts, including climate change, leading the black locust system to generally perform better than the reference system where the carbon vulnerable areas are left as they are with their initial vegetation. Considering that cultivation starts in 2022, the dynamic evaluation of the global mean temperature change (GMTC) shows that the black locust system results in net negative emissions in the near-and mid-term, but with a GMTC overshot in the second half of the century. Even if a considerable carbon stock is created through time (e.g., 81% and 39% of the carbon captured by photosynthesis, in 2050 and 2100 respectively), the GHG emissions stemming from manufacturing and end-of-life processes were shown to offset the benefit of carbon capture.

List of Abbreviations

Achieving the climate objectives of the Paris Agreement and national policies in terms of emissions reduction and carbon neutrality by 2050 implies the implementation of drastic greenhouse gas (GHG) emission reductions and carbon dioxide (CO2) capture and storage actions. To this end, the concept of Carbon Storage in Arable land and Anthropogenic Products (CSAAP) has recently been proposed (Shen et al., 2022b), consisting in cultivating plants, also called biopumps, that can induce a net increase of soil organic carbon (SOC) stocks while providing biomass for long-lasting anthropogenic products.

Putting CSAAP into practice needs solving two issues, namely defining the type of suitable biopump species and areas to grow them. According to previous research (Shen et al., 2022b), there are up to 2,400,000 ha of suitable land in France alone that could be available for planting biopumps for climate mitigation purposes. These lands are called 'carbon vulnerable lands (CV-lands)' as they present a high potential for SOC sequestration due to their current SOC stock being <50 t C ha -1 [START_REF] Minasny | Soil carbon 4 per mille[END_REF]. They were also selected to respect a set of specific sustainability criteria defined in Shen et al. (2022b). Among the list of 14 suitable woody biopumps presented in Shen et al.(2022b), black locust (Robinia pseudoacacia L.) was highlighted not only because it could capture 31.5 kg CO2 per tree per year (Supplementary material, SM, section 2.1), but also for its capacity to convert part of the captured C in the soil as SOC [START_REF] Wang | Effect of black locust (Robinia pseudoacacia) on soil chemical and microbiological properties in the eroded hilly area of China's loess plateau[END_REF]. Furthermore, woody products have a relatively long lifetime, hence delaying the return of C to atmosphere as gaseous emissions (CO2, and eventually methane and carbon monoxide) .

When analyzing how wood value chains affect climate change, the conventional theory considers biogenic CO2 as neutral (balanced), thus neither CO2 absorbed nor CO2 emitted from biomass is included in climate change impact calculations [START_REF] Chen | Life Cycle Assessment (LCA) of Cross-Laminated Timber (CLT) produced in Western Washington: The role of logistics and wood species mix[END_REF][START_REF] Da Costa | Environmental impacts of forest biomass-to-energy conversion technologies: Grate furnace vs. fluidised bed furnace[END_REF]. This approach is suitable for annual or short rotation coppice, while not proper in estimating forest mitigation potential because the effect of C storage and delayed emission is ignored. [START_REF] Guest | Global Warming Potential of Carbon Dioxide Emissions from Biomass Stored in the Anthroposphere and Used for Bioenergy at End of Life[END_REF] studied the relationship between the wood rotation and C storage in products on the biogenic global warming potential. They concluded that in order to consider biogenic CO2 as neutral, the lifespan of wood products should be at least half of the plantation rotation. Another common accounting approach is considering the forest as a permanent C sink, where the CO2 captured through photosynthesis is counted while the emission of biogenic CO2

is not [START_REF] González-García | Environmental assessment of black locust (Robinia pseudoacacia L.)-based ethanol as potential transport fuel[END_REF]. This approach shows the effectiveness of biomass as a C sink on a limited time scale, however, it is not correct under time scales encompassing the life cycle of biomass from cradle-to-grave. Albers et al. (2019b) modeled the trees' growth and residue decomposition, showing that the tree's lifetime highly influences the mitigation potential, the shorter the lifetime, the lower the mitigation effect of converting forests to biofuels. However, since biofuels are used at once with almost all C emitted immediately, there is no technosphere C sink. [START_REF] Lan | Dynamic life cycle carbon and energy analysis for cross-laminated timber in the Southeastern United States[END_REF] tracked the carbon flows from one rotation of pine forest cultivation to the end of wood products on a 100-year scale, and highlighted the difference between using residue for wood products or energy. They treated biogenic CO2 as neutral, only considering the influence of fossil CO2 emissions. [START_REF] Zieger | Impact of GHGs temporal dynamics on the GWP assessment of building materials: A case study on bio-based and non-bio-based walls[END_REF] counted both biogenic CO2 absorption by timber and wheat, and degradation of biobased walls including incineration and compost, in a time-dependant manner. However, the belowground part of plants and the SOC changes were not included in these studies. Soil is an important C sink, not taking it into account results in C flows that are not correctly balanced [START_REF] Hamelin | Modelling the carbon and nitrogen balances of direct land use changes from energy crops in Denmark: A consequential life cycle inventory[END_REF].

Moreover, because aerial crops are commonly harvested, soil and root carbon storage is a key determinant of climate change mitigation potential, and tends to be higher with greater plant diversity, according to experimental results [START_REF] Yang | Soil and root carbon storage is key to climate benefits of bioenergy crops[END_REF].

The most existing LCA studies use a static approach and CO2-eq as an aggregation metric for all GHG, independently of the time scale considered, which could mislead the conclusions [START_REF] Barna | A climate goal -based , multicriteria method for system evaluation in life cycle assessment[END_REF][START_REF] Saez De Bikuña | A comparison of land use change accounting methods: seeking common grounds for key modeling choices in biofuel assessments[END_REF][START_REF] Zieger | Impact of GHGs temporal dynamics on the GWP assessment of building materials: A case study on bio-based and non-bio-based walls[END_REF]. Because wood and wood-based products have a relatively long lifespan, and biomass residues decay at different rates, the temporal factor is essential in assessing wood product systems. Recent studies demonstrated a considerable difference between static and dynamic LCA approaches for woodbased products (e.g., [START_REF] Cardellini | Temporalis, a generic method and tool for dynamic Life Cycle Assessment[END_REF][START_REF] Göswein | Dynamic life cycle assessment of straw-based renovation: A case study from a Portuguese neighbourhood[END_REF], especially for the quantification of biogenic CO2 capture and release effects. A recent study pointed out that for woody products, the static LCA overestimates the long-term cumulative impact without providing information on short-term impact [START_REF] Cordier | Dynamic LCA of the increased use of wood in buildings and its consequences: Integration of CO2 sequestration and material substitutions[END_REF]. Although the study built the results on a temporal inventory of C flows, the climate change impact was calculated by a metric based on CO2-eq which induces a bias in the results.

This work explores the mitigation potential for global warming and for overall environmental impacts, of an application case of the CSAAP strategy with perennial plantations, selected in the light of earlier work (Shen Zhou; Ligia Tiruta-Barna Shivesh Kishore Karan;

Lorie Hamelin, 2022). The case is the implementation of a 35-y plantation of black locust on CV-lands in France, using the biomass for bio-based products intended for the building sector.

Objectives are to: 1) evaluate the C sinks induced over time, 2) investigate the overall environmental consequences of replacing current CV-land and fossil-based products with the black locust system and the related bio-based products, and particularly the global warming mitigation potential in order to contrast it with the sinks, and 3) investigate the effect on global mean temperature change in time. In the general endeavor to uncover environmentallyperformant solutions towards both carbon dioxide removals and GHG mitigation, this work proposes an original, comprehensive framework with adapted modeling tools to address all these objectives, and to fill gaps in the existing literature. The C flows were modeled from the tree cultivation to products manufacturing and end of life (for a cradle-to-grave scope), and the C pools in soil (SOC) and materials (biomass) were estimated in function of time. The time period encompasses the tree cultivation (three rotations) but also the product lifetime and end of life period with slow material degradation. The dynamic C balance, but also all GHG temporal emissions over the life cycle were integrated in dynamic LCA for global warming impact, evaluated by the global mean temperature change indicator. The overall environmental consequences of changing the initial land use to black locust cultivation and utilization were evaluated with the consequential LCA approach. Finally, sensitivity and uncertainty analysis were performed to explore the robustness of the results obtained.

Methods

Scenarios description

The point of departure of this analysis is one hectare of average French CV-land, to be either converted for black locust cultivation from the year 2022, over three rotations of 35-y each (BL Scenario), or maintained as CV-land for 105 y (reference scenario, from here onwards referred to as REF Scenario). For the static environmental assessment, only 35 years (the time of one rotation) is considered, as further detailed in section 2.5.

Initial vegetation scenario on CV-land (REF Scenario)

This scenario considers that CV-lands remain CV-lands throughout the time covered by the analysis. The vegetation considered on average French CV-land stem from the work of (Shen et al., 2022;Shen Zhou;Ligia Tiruta-Barna Shivesh Kishore Karan;Lorie Hamelin, 2022). Accordingly, four types of initial land cover are considered with different shares in terms of the area they cover, namely intensive grasslands (53%), natural grasslands (20%), rapeseed lands (20%), and woody moorlands (7%) (details in Shen et al., 2022b). Ryegrass (Lolium perenne L.) and rapeseed (Brassica napus L.) represent the initial vegetation on intensive grassland and rapeseed lands, respectively, while heather (Calluna vulgaris) is selected to represent both natural grasslands and woody moorlands vegetation. On CV-lands remaining CV-lands, ryegrass and rapeseed are harvested annually and used for animal feed and oil production respectively, while heather is considered to decay on-site [START_REF] Pehme | Environmental performance of manure codigestion with natural and cultivated grass -A consequential life cycle assessment[END_REF]. The rapeseed cake co-produced with rapeseed oil is also considered to be used for animal feed.

Black locust scenario (BL)

In this scenario, CV-lands are ploughed prior to black locust planting. Black locust is a nitrogen-fixing species and tolerates droughts [START_REF] Nicolescu | Ecology, growth and management of black locust (Robinia pseudoacacia L.), a non-native species integrated into European forests[END_REF][START_REF] Seserman | Simulating climate change impacts on hybrid-poplar and black locust short rotation coppices[END_REF].

For this reason, and also in accordance with the CSAAP vision to favor systems with minimal inputs (Shen et al., 2022), no fertilization or irrigation is considered. To remove defective trees, selective thinning is conducted twice, in the 10 th year and 20 th year of the plantation's lifetime, where whole trees are removed [START_REF] Keresztesi | Breeding and cultivation of black locust, Robinia pseudoacacia, in Hungary[END_REF][START_REF] Nicolescu | Ecology, growth and management of black locust (Robinia pseudoacacia L.), a non-native species integrated into European forests[END_REF]. After the second selective thinning, extra branches are pruned to provide enough space for stem growth. After harvesting, black locust is delimbed and debarked. Barks are incinerated to provide heat for downstream manufacture [START_REF] Silva | Life cycle assessment of medium density particleboard (MDP) produced in Brazil[END_REF], while branches are chopped into wood chips (Fig. 1a). Stumps are crushed and left on the ground as residues during thinning operations when black locust is logged.

The processes after logging are depicted in Fig. 1b. First, there is the processing of stems.

After delimbing and debarking, the stems are dried on open fields to meet a moisture content of ca. 15% (Żelazna et al., 2019). The dried stems are trimmed and sawn into panels, then laminated into several layers [START_REF] Sahoo | Life cycle assessment of forest-based products: A review[END_REF]. These high-value laminated panels, called 'cross-laminated timber' (CLT), are widely used in the building sector, as a more sustainable alternative than carbon-intensive materials [START_REF] Cadorel | Life cycle analysis of cross laminated timber in buildings: a review[END_REF][START_REF] Cowie | Applying a science-based systems perspective to dispel misconceptions about climate effects of forest bioenergy[END_REF].

Herein CLT is assumed to be used for exterior walls, with 50 years lifespan [START_REF] Dodoo | Lifecycle primary energy analysis of low-energy timber building systems for multi-storey residential buildings[END_REF].

After demolition, 45% of CLT is incinerated, 45% is recycled, while the remaining 10% is lost

(regarded as open-landfills) [START_REF] Jayalath | Life cycle performance of Cross Laminated Timber mid-rise residential buildings in Australia[END_REF]. The recycled CLT is chopped into wood chips for further treatment.

Second, the branches from stem's trimming are cut into chips (to produce medium density fiberboard, MDF), then wood chips are washed to remove the dust. The clean chips are mixed with other wood chips obtained from branches, along with the woody material from 1 st and 2 nd thinning and recycled CLT. All these materials are softened in a steam-pressurized digester (the heat to generate steam commonly comes from burning barks), then mixed with urea-formaldehyde resin and additives in the resonating process, to improve the strength [START_REF] Puettmann | Cradle to gate life cycle assessment of north American cellulosic fiberboard production[END_REF]. The mixture is pressed into boards (MDF) at 170 °C for ca. 5 minutes using a mechanically controlled oil-heated press [START_REF] Yuan | Do green wooden composites using lignin-based binder have environmentally benign alternatives? A preliminary LCA case study in China[END_REF]. The produced MDF has multiple usages. Having a lower mechanical strength than CLT, it is more suitable for furniture and interior architecture [START_REF] Piekarski | Life cycle assessment of medium-density fiberboard (MDF) manufacturing process in Brazil[END_REF]. After 12 years of use, most MDF in Europe is incinerated with heat recovery, while a minor part is landfilled [START_REF] Couret | Extraction and characterization of cellulose nanocrystals from post-consumer wood fiberboard waste[END_REF]ecoinvent, 2020). The scraps occurring through the MDF production process are considered to end in open-landfills.

Overview of the methodology used

The methodology used to achieve the objectives of this study builds on material and energy flow calculation, encompassing all stages from plant cultivation to anthropogenic production and products' end of life, to finally evaluate the carbon pools and environmental impacts.

Fig. 2 Calculation steps followed in this study

As shown in Fig. 2, the first step calculates the growth of black locust to determine how much CO2 is captured by photosynthesis, how much biomass is produced in function of time in the different parts of the plant (e.g., belowground, aboveground), and the quantity of residues destined to decay above-and below ground. The carbon in the residues is an input to a SOC model, whose evolution is simulated over the rotation time span. In this way, the yearly content of SOC is estimated (over 105 years), along with the CO2 releases from mineralization or native SOC losses, as detailed in section 2.4. The harvested biomass enters the manufacturing chains and determines the amount of products, associated by-products, and all related flows in the process chain, from harvesting to product manufacturing and end of life. Data from literature have been used to model the material and energy balances for the manufacturing processes.

These foreground flows are then used in three different LCA modeling approaches to respond to different questions as later detailed: static consequential LCA, dynamic LCA using the same consequential modeling of the inventory, and dynamic LCA using the attributional modelling of the background inventory with system expansion in the foreground part. For the dynamic approaches, the material and energy flows have been used as time series with their time occurrence depending on the system's timeline. Both dynamic approaches used temporal predictions for electricity mix (used in the background inventory) based on either the absolute values or the rate of changes.

Black locust growth rate

Black locust is a fast-growing tree species. After seedling planting, it reaches its fastest growth rate at year 20, then the growth rate slows down gradually till the end of the rotation (year 35) [START_REF] Ciccarese | Short rotation forestry and methods for carbon accounting. A case study of black locust (Robinia pseudoacacia L.) plantation in central Italy[END_REF][START_REF] Nicolescu | Black locust (Robinia pseudoacacia L.) as a multi-purpose tree species in Hungary and Romania: a review[END_REF]. The volume of biomass is estimated from the height and diameter at breast height, calculated in function of time [START_REF] Rédei | Growth and Yield of Black Locust (Robinia pseudoacacia L.) Stands in Nyírség Growing Region (North-East Hungary)[END_REF][START_REF] Rédei | Influence of regeneration method on the yield and stem quality of black locust (Robinia pseudoacacia L.) Stands: A case study[END_REF]. The wood volume is converted to biomass dry matter considering a wood density of 785 kg m -3 DM [START_REF] Adamopoulos | Strength properties of juvenile and mature wood in black locust (Robinia pseudoacacia L.)[END_REF]

(SM 2.2).
The whole stand is assumed to grow at the same rate, thus the below-ground biomass is calculated based on a fixed shoot/ root ratio in time, here taken to 1.73 [START_REF] Chen | Arbuscular mycorrhizal symbiosis mitigates oxidative injury in black locust under salt stress through modulating antioxidant defence of the plant[END_REF]. In order to calculate the biomass amount per ha in a specific year, a density varying between 560-2240 trees per ha is considered, according to the growth stage [START_REF] Keresztesi | Breeding and cultivation of black locust, Robinia pseudoacacia, in Hungary[END_REF][START_REF] Nicolescu | Black locust (Robinia pseudoacacia L.) as a multi-purpose tree species in Hungary and Romania: a review[END_REF]. Details and results of simulated annual biomass partition (above-and below ground;

in standing biomass and residues; as woody and non-woody portions) are presented in SM 2.2.

The foliage, pods, and seeds are considered jointly under the umbrella term 'aerial biomass', and are regarded as an annual input to soils from the 6 th year of the rotation [START_REF] Stone | Robinia pseudoacacia[END_REF][START_REF] Warne | Black locust (Robinia pseudoacacia L.). Best management practices in Ontario, Ontario invasive plant council[END_REF]. Similar to the root biomass, the aerial biomass increase in time with the tree growth [START_REF] Ciccarese | Short rotation forestry and methods for carbon accounting. A case study of black locust (Robinia pseudoacacia L.) plantation in central Italy[END_REF]. It was estimated based on a fixed ratio (1.2% of wood biomass, DM).

Simulating SOC on CV-lands for both scenarios, and wood residues decay

The evolution of SOC on CV-lands cultivated with black locust or covered with initial vegetation was simulated with C-TOOL [START_REF] Taghizadeh-Toosi | C-TOOL: A simple model for simulating whole-profile carbon storage in temperate agricultural soils[END_REF]. This model was adopted as one of the few models easily usable (i) with unconventional (non-agricultural) crops, (ii) for perennial systems, and (iii) calibrated for temperate areas [START_REF] Andrade | A review on the interplay between bioeconomy and soil organic carbon stocks maintenance[END_REF]. Among others, it has the advantage that the proportion of C incorporated in the different (virtual) soil pools (each with a different turnover rate affected by clay content and temperature; C-TOOL considers three pools) is easily customizable (e.g., [START_REF] Hansen | Agricultural residues bioenergy potential that sustain soil carbon depends on energy conversion pathways[END_REF]. This ease of customization also applies to the allometric equations used to define the proportion of aboveground residual biomass input to the soil annually. Different applications of the model with perennial crops include [START_REF] Hamelin | Modelling the carbon and nitrogen balances of direct land use changes from energy crops in Denmark: A consequential life cycle inventory[END_REF] and [START_REF] Keel | Large uncertainty in soil carbon modelling related to method of calculation of plant carbon input in agricultural systems[END_REF].

The simulation was conducted with a one-year time step, for a timespan of only the monthly average temperature is required in terms of meteorological data input (Fig. s1). The required soil characteristics include the soil clay content and the C/N ratio. The former was retrieved from the same database used for identifying CV-lands (GSOC; Food and Agriculture Organization of the United Nations, 2020), the latter, not included in GSOC, was set to 11 based on relevant literature [START_REF] Clivot | Modeling soil organic carbon evolution in long-term arable experiments with AMG model[END_REF][START_REF] Launay | Estimating the carbon storage potential and greenhouse gas emissions of French arable cropland using high-resolution modeling[END_REF]. The average initial SOC content for the CV-lands was evaluated as 42350 kg SOC ha -1 (Shen et al., 2022).

Finally, one other key input required by the model is the annual C input from residual above-and belowground biomass. The aboveground resides includes aerial biomass (foliage, seeds, and pods), which fall down annually (except for the first five years), and stumps, which are crushed and left on the ground as residues during thinning operations and when black locust is logged (year 10 th , 20 th , 35 th of the rotation, respectively). Belowground residues consist of decaying roots and rhizodeposits. Decomposition rate coefficients of 0.12, 0.008, and 0.003 months -1 were set for aerial, aboveground-and belowground wood biomass, respectively [START_REF] Taghizadeh-Toosi | C-TOOL: A simple model for simulating whole-profile carbon storage in temperate agricultural soils[END_REF]. To represent the slower decay of woody above-and belowground residues, the simulation was conducted for an additional 100 years after the last rotation (the C not emitted after 100 years is considered as permanently stored).

For the initial vegetation, consisting of four different land types, the SOC of each land type was simulated individually, then aggregated together based on their respective shares in 1 ha CV-land (C input in Table s19). A decomposition rate coefficient of 0.12 month -1 was used for the initial vegetation' residual biomass inputs to soil (above-and below-ground), as used for the foliage and as used in e.g., [START_REF] Hamelin | Modelling the carbon and nitrogen balances of direct land use changes from energy crops in Denmark: A consequential life cycle inventory[END_REF] and Taghizadeh-Toosi et al. ( 2014)

(both on the basis of [START_REF] Petersen | CN-SIM -A model for the turnover of soil organic matter. I. Long-term carbon and radiocarbon development[END_REF].

2.5 Environmental impact assessment through consequential LCA

Goal, boundary, and functional unit

Consequential LCA is used to evaluate the environmental consequences of (i) cultivating black locust instead of the initial vegetation on the identified CV-lands in France and (ii) using the biomass in anthropogenic products while replacing conventional (fossil-based)

products. This implies that multifunctionality was modeled with system expansion (Fig. 3) and that the life cycle inventories for marginal suppliers were considered. The functional unit is defined as 'using 1 ha of CV-land in France for cultivating perennial crops over one full rotation, and using the harvestable biomass within the bioeconomy'. For the scenarios considered herein, this implies 35 years. The LCA framework follows the ISO 14040 and 14044 standards (ISO, 2006a[START_REF] Iso | Environmental management: life cycle assessment; requirements and guidelines[END_REF]. The system boundary considered for the consequential LCA is shown in Fig. 3, both for the BL (here BLc) and REF (here REFc) scenarios. System BLc involves a variety of substitutions. The vegetable oil (rapeseed oil) and feed (ryegrass silage, rapeseed cake), no longer reaching markets because CV-lands are now used for black locust have to be compensated for (Saez de [START_REF] Saez De Bikuña | A comparison of land use change accounting methods: seeking common grounds for key modeling choices in biofuel assessments[END_REF]. This may happen through two main reactions, namely expansion of nature for agriculture (deforestation) or intensification of agricultural land [START_REF] Tonini | Environmental implications of the use of agroindustrial residues for biorefineries: application of a deterministic model for indirect land-use changes[END_REF], both considered herein under the umbrella term of indirect land use changes (iLUC). According to market reports and literature review, the bio-based CLT and MDF will replace light-clay bricks and polyvinyl-chloride boards (herein referenced as plastic boards), respectively [START_REF] Onyeaju | Comparison of the Thermal Properties of Asbestos and Polyvinylchloride (PVC) Ceiling Sheets[END_REF][START_REF] Takano | A multidisciplinary approach to sustainable building material selection: A case study in a Finnish context[END_REF]. Incineration, at all stages where it happens, is considered with heat recovery, which implies that this heat will not need to be generated the way it would have otherwise been. As a result, corresponding heat generation is avoided. Finally, the use of CLT at the expense of bricks (replaced material) was considered to lead to a 52% reduction of the electricity consumed in the erection and demolition phases, because of the lighter mass [START_REF] Corradini | Product environmental footprint of a crosslaminated timber system: a case study in Italy[END_REF]Guo et al., 2017).

Life cycle inventory

Life cycle inventories for foreground processes (Fig. 1) were established on the basis of literature, models, and a variety of estimation proxies when necessary. These are comprehensively detailed in the SM (Tables s3-s18). The displaced feed service (for the silage ryegrass and rapeseed cake of the initial vegetation) and its substitution by corresponding marginal carbohydrates, protein and lipid ingredients, along with the associated land use changes were modelled according to the methodology described in [START_REF] Tonini | Environmental implications of the use of agroindustrial residues for biorefineries: application of a deterministic model for indirect land-use changes[END_REF].

Landfilling processes (open-landfilling and landfilling of MDF at the end-of-life) were modelled according to the model provided in IPCC ( 2019 with the consequential model.

Life cycle impact assessment and interpretation

The environmental impact assessment was performed with the Environmental Footprint method (EF) recommended by European Commision (Fazio et al., 2018a). Moreover, because all CO2 releases and adsorption are accounted, biogenic CO2 was considered with a global warming potential of 1 kg of CO2-eq per kg biogenic CO2 instead of the value of 0 used in (Fazio et al., 2018a).

For interpretation, a contribution analysis was performed for each impact category, where processes with negative and positive contributions were analyzed separately in two groups. In each group, the processes contributing to at least 80% of the impact were selected for further in-depth analysis, i.e., identifying their sub-processes and substances contributing to the impact (Fazio et al., 2018b). The LCA was facilitated with the SimaPro software, version 9.1.

Dynamic LCA for evaluating climate change impact

Given the long lifetime of the studied system, a time-dependent method is necessary to address the temporal dimension of the climate change impact. Dynamic LCA with the global mean temperature change (GMTC) as an indicator was applied herein. The method and computational tool used were presented in previous works (Tiruta [START_REF] Barna | A climate goal -based , multicriteria method for system evaluation in life cycle assessment[END_REF]CCI-tool, 2021).

The GMTC calculation used the recommended method of IPCC, for the emission timing over the whole life cycle of scenarios [START_REF] Stocker | Climate change 2013 the physical science basis: Working Group I contribution to the fifth assessment report of the intergovernmental panel on climate change[END_REF], for all GHGs included in the ecoinvent v3.5 database (Table s21), and can be summarized by the following main equations.

The atmospheric burden of substance s, Bs (kg), is calculated as the convolution product (symbol * ) between the temporal emissions of the substance, gs (kg.year -1 ), and the concentration -impulse response function of that substance, IRFs :

B (t) = g * IRF = ∫ g t ′ IRF t -t ′ dt ′ (1)
where t and t' are two time scales. The radiative forcing RF is calculated by:

RF (t) = A (t) B (t) = A (t)(g * IRF ) (1)
where As (W.m -2 .kg -1 ) is the radiative efficiency (considered as constant). IRF and A are specific for each GHG [START_REF] Stocker | Climate change 2013 the physical science basis: Working Group I contribution to the fifth assessment report of the intergovernmental panel on climate change[END_REF]. The global mean temperature change generated by the forcer s is defined as the convolution product between its RF and the temperature impulse response function IRFT:

GMTC (t) = RF * IRFT (3)
IRFT is independent of the type of GHG. The mean temperature change at a given time t, GMTC (K), is obtained by aggregating values for all the concerned forcers: Two perspectives were adopted for the dynamic modeling, here referred to 'consequential' and 'attributional', despite not fully embracing all features attributed to these two types of modeling approaches [START_REF] Brandao | Consequential Life Cycle Assessment: What, How, and Why? Encycl[END_REF][START_REF] Schaubroeck | Attributional & consequential life cycle assessment: Definitions, conceptual characteristics and modelling restrictions[END_REF]. These two perspectives imply two main differences. One is the system boundary, as illustrated in Fig. 3 for the consequential perspective and Fig. 5 for the attributional perspective. Both perspectives compare the black locust system to a reference situation. In the consequential perspective, the system expansion reactions (associated with both the products generated in the BL system and the consequences of the foregone use of CV-land) are placed in the BL system (or here BLc).

GMTC(t) = ∑ GMTC (t) (4
Even the foregone use of CV-land (emissions associated with growing ryegrass, rapeseed, and heather) could have been placed in this system for an entirely relative perspective, it was left as a reference (here REFc) for comparison purposes. Two realities are compared in what is here referred to as "attributional perspective". In one (BLa), black locust is grown on CV-land and used to produce heat, CLT, MDF (the latter two being in use for a number of years and disposed of at their end-of-life, the same way as in the BLc system), while vegetable oil is supplied by palm oil and feed by marginal feed ingredients. In the other system (REFa), CV-land is used as CV-land, but to make it equivalent, the alternatives to products generated in the BLa system (heat, bricks, plastic boards) are considered here. Strictly speaking, these two system boundaries are equivalent (and would typically be both labelled as "consequential"). The major difference between the two perspectives lies in the type of background data being used. The consequential perspective considered marginal data, e.g., those foreseen to react to a demand change in the long-term (concretely translating in the use of the so-called consequential background processes), while the attributional perspective used average background data (cut-off processes).

Moreover, for the foreground data requiring heat and electricity, the attributional perspective considered the mix composition predictions made for France for different periods (2020-2030, 2030-2040, 2040-2050), based on [START_REF] Kassara | Trajectories of evolution of the electricity mix 2020-2060: Additional analyses[END_REF] for electricity and (ADEME, 2018) for heat. The heat and electricity mixes after 2050 were considered the same as in 2050 (Table s22- 24). For the consequential perspective, the marginal French electricity mix of Ecoinvent v3.5 was considered (2050; based on predictions from (European Commission, 2021)). For foreground heat, natural gas is considered.

Sensitivity and uncertainty analysis

To explore the robustness of the obtained results and identify the hotspots, sensitivity and uncertainty analysis were conducted for both scenario BLc and REFc. The method and results are described in SM 5 and 6, for the sensitivity and uncertainty of the systems, respectively.

3 Results and discussion

Black locust growth and decay

Based on simulated height and diameter (Fig. s5), the biomass amount on 1 ha CV-land is shown in Fig. 6a and 27% of the DM in above-and belowground wood residues is still undegraded after 35 years.

This falls to 1.5% and 2.8% after 100 years of decomposition.

SOC simulation results

The simulation results with C-TOOL (Fig. 6b) shows that SOC increased at the end of the rotation with black locust while slightly decreased with the initial vegetation. The black locust induces a SOC increase periodically with each rotation, reaching 92040 kg ha -1 at the end of the third rotation (year 2126). The greatest increase is observed during the first rotation with 73650 kg ha -1 . The SOC increase is less important for the subsequent rotations, being only 15540 and 2840 kg ha -1 for the second and third rotation respectively, since the SOC gradually reaches saturation in soils [START_REF] Petersen | An approach to include soil carbon changes in life cycle assessments[END_REF]. Declining SOC stocks are observed following every biomass removal event (harvest, thinning) due to lesser input into the soil.

Unlike the black locust system, the SOC in the initial vegetation system decreased from 42350 to 40680 kg ha -1 during the cultivation (105 years). The main difference between black locust and initial vegetation is the C input. According to the literature data for vegetation yield and to the simulation results for black locust growth, the aboveground C inputs from initial vegetation is higher than that from black locust aerial biomass (foliage, seeds and pods) (1.27 and 0.91 t ha -1 year -1 respectively). The higher C input implies that a greater quantity of C is humified; as a result, the SOC of the initial vegetation is higher than black locust until year 2031. But at years 10, 20, and 35 of the rotation, there are extra C inputs from logged stems, especially from the belowground root portion (below-ground: 4.65, 19.23, 65.24 t C ha -1 from the first-, second thinning, and harvesting respectively), making the SOC of the black locust system being higher than the one of the initial vegetation. Meanwhile, compared to the initial vegetation residues, the gradual decomposition of black locust residues keep the C in the soils for a longer time, leading to the SOC of black locust exceeding the SOC of the initial vegetation system by year 2041 (second thinning). SOC evolution for black locust and initial vegetation during the cultivation period.

Harvestable biomass: stem, branch and bark.

Considering the 105 years studied herein, an annual SOC increase of 880 kg ha -1 is observed for the black locust system, corresponding to an increase rate of 21‰ per year, which can be compared to the 4‰ target reported by [START_REF] Minasny | Soil carbon 4 per mille[END_REF] to offset the global annual GHG emissions. Looking at each rotation in isolation, this yearly increase rate is 50‰ during the first rotation and 30‰ during the first-two rotations, indicating that short-term cultivation might be more efficient. [START_REF] Ciccarese | Short rotation forestry and methods for carbon accounting. A case study of black locust (Robinia pseudoacacia L.) plantation in central Italy[END_REF] simulated one rotation of black locust with the Yasso model, and observed an annual SOC increase of 110 kg ha -1 year -1 before the final harvestwhich is half of our result (220 kg ha -1 year -1 for the first rotation). On the other hand, [START_REF] Wang | Carbon sequestration of mature black locust standson the loess plateau, China[END_REF] reported, based on field measurements, an average annual SOC increase of 930 kg ha -1 (over 25 years). They measured this increase to 2200 kg ha -1 annually from year 5-10 but only 290 kg ha -1 from year 10-25. Both studies also highlighted the contribution of dead wood incorporated in soils.

It should be noticed that the high SOC at the end of harvesting does not last for a long time. Fig. s2 shows that following the last stem harvest, the SOC decreases gradually since there is no extra C input anymore. After 51 years (2177) and under the conservative assumption that the soil is left bare, all SOC gained with the black locust cultivation is lost, the SOC stock being equivalent to the initial level of 2022.

Environmental impact results from the static LCA

The LCA results are shown in Fig. 7 (climate impact; global warming potential for a 100-y time horizon, GWP100) and Fig. 8 (all other impacts); the negative contributions represent avoided processes or emissions while positive contributions represent the processes induced by implementing the studied scenario. The difference between these represents the net impact of a given category and scenario. Results are expressed per functional unit.

Climate change

Both scenarios lead to a negative characterized impact for climate change, but scenario BLc leads to a larger magnitude than scenario REFc (-2202 vs -51 t CO2-eq). In scenario BLc (Fig. 1a), the tree cultivation and the avoided plastic boards processes shape the net negative score, respectively contributing with -1283 and -2509 t CO2-eq. The material substitution contributes the most to negative restuls, which is also observed in a recent LCA study on woody products [START_REF] Cordier | Dynamic LCA of the increased use of wood in buildings and its consequences: Integration of CO2 sequestration and material substitutions[END_REF]. The avoided plastic board process is an aggregation of several activities. About 61% of its impact is due to petrochemical plastic manufacture, while 24% stems from incineration at the end-of-life (Fig. s4). The negative score of tree cultivation reflects the net 1305 t CO2 captured by black locust in one rotation, offsetting the positive GHG emission due to the iLUC in the same process (21 t CO2-eq).

On the other hand, emissions during the biomass logging (combining harvesting, thinning and pruning processes), and the disposal of MDF products are the main positive contributors, with similar magnitudes. For logging , the GHG emission comes from the residue decomposition, with 68% due to CO2 and 26% due to CH4. For the product disposal, since 45%

of CLT is recycled to produce MDF at the end of life, the disposal of MDF plays a more critical role (MDF: 545 vs CLT: 139 t CO2-eq emitted). The important contribution of MDF disposal to climate change is due to the CO2 emitted during the incineration (mainly comes from the woody part). Processes with contributions lower than 1% of the total impact (in absolute) were attributed to categories others (+) or (-) (Process contributions were computed as follows: e.g., process A has a contribution of 1 and process B of -1, then the contribution of process A is 1/(abs(1)+abs(-1))=50%; details in SM). FU: functional unit.

For scenario REFc (Fig. 7b), natural grasslands and woody moorlands are shown as net contributors to the climate change impact. This reflects the emissions of CH4 and N2O due to the on-site decomposition of residues. Intensive grasslands and rapeseed lands contribute to the total negative score with 68% and 32% respectively, which in both cases is essentially due to the avoided marginal vegetable oil (from rapeseed oil production) and marginal feed (from rapeseed meal and silage production). Based on the results of section 3.2 (increase of 92040 kg SOC ha -1 in 105 y), planting black locust increases, in average over the course of one rotation, the CV-lands SOC by an amount equivalent to 112 t CO2-eq, while maintaining the initial vegetation leads to SOC decreases corresponding to 2 t CO2-eq. Compared to the net results of the two scenarios, the effect of the C change in soils is relatively small, representing 5% and 4%

of the net scores in scenario BLc and REFc, respectively.

Other environmental impacts

All impact results except climate change are presented in Fig. 8. Among all fifteen environmental impacts of Fig. 8, there are four and twelve with a net positive score for scenario BLc and REFc, respectively ( Similar to climate change, avoided conventional products is the main contributor to the negative impact in scenario BLc. This particularly applies for plastic boards, especially due to the suspension polymerization operation (Fig. s6). However, the avoided use of plastic boards does not consistently translates with negative impacts. Without their incineration at the end-oflife, extra heat and electricity production are induced in the BLc system. Hence, avoiding the disposal of plastic boards generates burdens in ionizing radiation, freshwater eutrophication, and land use, representing 13%, 81%, and 46% of the positive scores, respectively. It also explained why in these three impacts, scenario BLc is worse than scenario REFc. In addition, in land use impact, rapeseed and silage ryegrass avoid palm oil and animal feed and the related necessary land, which reinforce this result.

In the scenario BLc, positive impact contributions come from multiple source across the different impacts, but agricultural management and products manufacturing tend to be two key contributors. The major agricultural management activities are tree logging processes (first-and second thinning, pruning, and harvesting). The diesel used in machines for logging and skidding trees to open-dry is the primary contributor, with some exceptions (e.g. PM10 from dust being the main contributor for respiratory inorganics). Since there is no fertilizer and pesticide use, the influence of black locust cultivation is negligible except in land use. Nevertheless, the agricultural activities during the cultivation (like the chemical application and diesel used for agricultural machines) are the main impact sources in scenario REFc, as observed in the LCA of common crops [START_REF] Silva | Do wood-based panels made with agro-industrial residues provide environmentally benign alternatives? An LCA case study of sugarcane bagasse addition to particle board manufacturing[END_REF].

As widely admitted in woody board production [START_REF] Piekarski | Life cycle assessment of medium-density fiberboard (MDF) manufacturing process in Brazil[END_REF][START_REF] Yuan | Do green wooden composites using lignin-based binder have environmentally benign alternatives? A preliminary LCA case study in China[END_REF], the resin (urea-formaldehyde resin here) used as additive is the main positive score contributor (47%) in the manufacture chain, especially in mineral resource use. Furthermore, free formaldehyde emitted from the cure of urea-formaldehyde resin is another potential pitfall, causing more than half (56%) of positive score in cancer human health impact. Therefore, increasing the use of cellulosic materials in the fiberboard production for avoiding the resin use can be an efficient pathway to reduce the environmental impacts [START_REF] Puettmann | Cradle to gate life cycle assessment of north American cellulosic fiberboard production[END_REF].

Fig. 8 LCA results for BLc (left bar) and REFc (right bar) scenarios for all impact categories of the EF method, with the exception of climate change.

3.4 Dynamic LCA results

Dynamic LCA in consequential modelling

GMTC results for the whole 269 years period considered (timeline in Fig. 4) are presented in Fig. 9. In the result analysis, distinction is made between the GHG (and related GMTC) stemming from the biomass (growth, decay, utilization, and end of life), noted as "bio", and the other GHGs coming from anthropogenic processes referenced as "non-bio" (despite stemming from biogenic processes like e.g., iLUC). The GMTC indicator has the property of additivity, thus the 'total' GMTC is the sum of GMTC from bio and non-bio contributors.

Concerning the REFc scenario, the GMTC (Total REFc; red curve) is positive during the cultivation period (up to 2126) except the first five years due to the avoided products. During the cultivation, 76% of captured CO2 is released in a short time from the residue decomposition.

Moreover, 3% of the C emited by anaerobic decoposition of residues is as CH4, in the first ten years (2022)(2023)(2024)(2025)(2026)(2027)(2028)(2029)(2030)(2031). The contribution of CH4 represents up to 25% of REFc's bio GMTC, but this contribution decreases sharply (due to the short life time of CH4 in athmosphere), shortly after the last cultivation period, leading to the decrease of bio GMTC from 2131 onwards.

Meanwhile, non-bio GMTC decreases continously because the main contributor is the avoided CO2 from the replaced feedstock, a long-live GHG, thus with long term effect. The short lifespan of biogenic CH4 and the relatively long-term CO2 emissions from non-bio sources lead the non-bio GHG to dominate the trend of scenario REFc after the year 2150. For the BLc scenario, in the first years of cultivation, the CO2 captured is low and SOC mineralization is relatively high (Fig. 6b), making the bio GMTC slightly positive (Fig. 9, Bio BLc). The BLc GMTC becomes negative in year 2028 (Fig. 9, Total BLc), reflecting, among others, the carbon captured in both biomass and as SOC as the black locust stand grows. The CH4 contribution to bio GMTC represents a maximum of 9%, CO2 being the largest contributor.

The increase of bio GMTC (Bio BLc) after the last rotation is explained by the end of CO2 capture, and the bio GHG emissions from the end of life incineration of MDF and CLT (the stored biogenic C is released back to the atmosphere).

The GMTC of non-bio sources (Non-bio BLc) is positive at the beginning, due to the emissions from tree plantation and iLUC. After the second thinning (year 2041), non-bio GMTC becomes negative, and between 2042 and 2047, closely follow the GMTC of bio BLc.

However, from 2048 onwards, bio and non-bio GMTC drastically change paths, with non-bio GMTC importantly decreasing due to the plastic boards and bricks replacement by the biobased products (MDF and CLT), involving manufacturing, use, and end-of-life processes. The net GMTC is negative just 5 years after the start of the first rotation, revealing that replacing the scenario REFc by BLc can mitigate global warming from the 5 th year onwards.

The mitigation effectiveness increases with the cultivation duration, with a peak around year 2136 (i.e. 10 years after the end of the last rotation), with a temperature reduction of -3.94E-09 K by ha of black locust cultivation. A minor part of the net negative GMTC is due to the bio flows, and reflect the higher CO2 absorption of black locust compared to the initial vegetation.

However, it is the non-bio GHG flows of BLc, and hence the avoided materials, that essentially determine the net GMTC result (Fig. 9).

Dynamic LCA in attributional modeling

The GMTC results for the attributional perspective ( average background processes with system expansion) are presented in Fig. 10, for REFa (top) and BLc (bottom) scenarios. The impact of each system is referenced as "net" REFa and BLc, obtained by the aggregation of all system's contributors. In this modeling approach, only the CO2 capture processes result in negative GMTC, all other processes generate impacts, hence positive GMTC.

In REFa system, the bio REFa is the same as the bio REFc, and will accordingly not be further detailed. In the first decades, the impacts of agricultural management and later activities (Non-bio REFa), involving oil mill operation and ryegrass silage, are negligible compared to the GMTC from biogenic GHG flows. In REFa system, the biggest contributor is the plastic board (production, use, and end-of-life incineration), dominating the net REFa after 2043. CO2 and CH4, emitted as pulse in the years of MDF manufacture and disposal, are the primary contributors to GMTC, while conventional brick and heat production contribute little to the net GMTC result. The REFa system, despite its slightly negative net GMTC during the first years, is, as for REFc, shown as inadequate for global warming mitigation.

The net GMTC of the BLa system is negative in short and mid term with an overshot in the second halph of the century. Albeit very few studies exist combining carbon flows of the whole life cycle of the plant with dynamic GMTC evaluation, this kind of behavior with negative and positive domains was observed for other wood-based building materials [START_REF] Negishi | Evaluating climate change pathways through a building's lifecycle based on Dynamic Life Cycle Assessment[END_REF]. The bio contributor of BLa scenario is the same as in consequential modeling and was analyzed in section 3.4.1. The non-bio GHG emissions stem from iLUC, wood logging operation, MDF&CLT production, disposal processes, and all related background processes, and always lead to positive GMTC. The GMTC from non-bio sources increases continuously, with sharp slopes when impulse emissions occur from these processes. At the end of anthropogenic activities, non-bio GMTC remains at a highe level for long term due to the accumulation of CO2 (long lived gas) emitted during the whole time span. Because the CO2 capture vanishes with the harvesting of wood, the net GMTC increases and becomes positive in 2072, then decreases sharply with the tree growth during the second rotation. Following the same trend, after the second rotation, the GMTC becomes positive for a while and continues to increase due to the non-bio contributors and additional processes. Concerning the additional products, induced feed ingredients affect the GMTC more than palm oil, CO2 emission during the agricultural management being the major contributor. However, it should be highlighted that the SOC level simulated after three rotations (ca. 134000 kg ha -1 ) is significantly higher than the average SOC in French lands covered by grass or forest (81000-85000 kg ha -1 , [START_REF] Pellerin | Stocker du carbone dans les sols français. Quel potentiel au regard de l'objectif 4 pour 1000 et à quel coût?[END_REF], and well above the 41000 kg ha -1 observed after 105 years of the initial vegetation system. Nevertheless, SOC deserves more attention and experimental investigation in real conditions in order to validate the simulation results. Such experiments will, in contrast to the simulation approach, require many years.

Combining the simulation results for the SOC, biomass growth, decay, and amount of products allows calculating the total C stock and comparing it to the C from CO2 captured during the whole period (Fig. 11). The difference between stocks and captures represents the biogenic C released into the atmosphere (as CO2 and CH4). The C stock in products is higher than in soil. The net stock represents the negative emissions that the BL scenario could induce (81%, 39%, 15% of total captured C, in the year 2050, 2100, 2290 respectively. Fig. s11 in SM).

Annualizing the CO2 absorption to compare with annual grass, black locust could capture twice the amount captured by the initial vegetation, similar to a vigorous energy grass [START_REF] Hamelin | Modelling the carbon and nitrogen balances of direct land use changes from energy crops in Denmark: A consequential life cycle inventory[END_REF]. Compared to 14 wood species cultivated in France (Albers et al., 2019a), black locust could fix more C than the average of these species over the same period while attaining maturity well before the average maturity time of about 80 years.

Fig. 11 Carbon captured over the cultivation period, and biogenic carbon stocks during the lifetime of the BL scenario.

In the value chain proposed, bio-based products can replace fossil-based products with problematic environmental footprint at the level of their manufacturing and, significantly, of their end-of-life management.

The static LCA highlighted that replacing the initial vegetation with the black locust value chain is beneficial for the environment in 13 out of 16 environmental impacts (Table 1, italic fonts), mainly due to the fossil-based replacement. The use of resins/plastics in both biobased and fossil-based products is critical in many impacts as highlighted by the LCA and confirmed by other studies (e.g., [START_REF] Nakano | Life cycle assessment of wood-based boards produced in Japan and impact of formaldehyde emissions during the use stage[END_REF], Silva et al., 2013, etc.), therefore less or no resin additive is matter of investigation [START_REF] Kouchaki-Penchah | Life cycle assessment of medium-density fiberboard manufacturing process in Islamic Republic of Iran[END_REF][START_REF] Puettmann | Cradle to gate life cycle assessment of north American cellulosic fiberboard production[END_REF]. The high environmental impacts of plastic production could be mitigated by other choices for the board material, as highlighted by the sensitivity analysis (SM section 5). Further, with the development of plastic recycling and replacement, less GHG emissions could be expected from production and incineration [START_REF] Deng | Life cycle assessment of biobased fibre-reinforced polymer composites[END_REF][START_REF] Schwarz | Plastic recycling in a circular economy; determining environmental performance through an LCA matrix model approach[END_REF][START_REF] Ye | Life cycle assessment of polyvinyl chloride production and its recyclability in China[END_REF].

All conducted LCA modeling approaches indicate that the black locust system is generally better than the initial situation, either by measuring the consequence of this replacement as negative kg CO2-eq and negative net GMTC, or by measuring/comparing the actual impact. BLa scenario could be a negative emission strategy in the near-and mid-term (2030-2063) when net GMTC is negative. It is certainly a mitigation solution from 2026-2029, when the net GMTC is lower than that of REFa system but still positive (Fig. 10). The black locust system alone satisfies the neutrality in this decade (2027), but its implementation requires producing feedstock and oil elsewhere, hence attention must be paid to the net GMTC of the whole BLa system. Its increase in the second half of the century could be mitigated with appropriate solutions at the level of all manufacturing and end-of-life technologies, i.e. less fossil-based background processes and promoting wood material recycling instead of incineration. If the systems are scaled-up to the whole CV-lands in France, the REFa system increases the global mean temperature with 0.0020°C and 0.00576C by 2050 and 2100. BLa system decreases the temperature with 0.0006°C in the mid-term, but increases it by 2100 with 0.0002°C, with a peak in between which must be mitigated. Of course, these values are only orders of magnitude, to be relativized to the feasibility of using the whole CV-land potential.

Conclusions

This study investigated the relevance of using black locust as biopump to contribute to global warming mitigation following the CSAAP concept. A value chain was proposed from tree planting on CV-lands in France, to bio-based products manufacturing, use, and end of life, as an alternative to leaving the CV-lands as they are presently. This system runs from the first year of cultivation to the end of three rotations and to the end-of-life of the last product produced from biomass, encompassing 267 years.

Simulation results of biomass growth, decay, and SOC changes showed, throughout three rotations, that the black locust system induces more C stock in soils (considered as negative emission) than the initial vegetation, for an annual average SOC increase of 21‰. This also applies with regards to the C stored in the biomass of both systems. It was found that 15% of the carbon captured by 3 rotations of black locust is still stocked in technosphere after 260 years.

Replacing the initial vegetation with the black locust value chain was shown to mitigate global warming and was highlighted as beneficial to the environment for 13 of the 16 impact categories evaluated through consequential LCA. Hence, improving climate change was shown to result in minor trade-offs with the other environmental impacts. It should be noted that avoided plastic, associated to the secondary product MDF, is key to climate change impact and to the performance of most other impacts.

Considering the climate goals, black locust value chain implementation results in climate cooling effect by net negative emissions, in the near and mid-term, but with a GMTC overshot (BLa) observed from the second half of the century, which must be mitigated. SOC has little contribution to carbon storage. Therefore, this study highlighted the importance of acting on emissions stemming from manufacturing and end-of-life (all background processes), which offset the benefit of C capture to achieve the goal of neutrality.

The dynamic approach used for global warming evaluation clearly shows that the mitigation potential is relative to the time scale. Approaching the end-of-life of the system results in emissions from land use change (if black locust is no more cultivated), and from the end-of-life management of the products, leading to the C stocks depletion and C emissions.

Therefore, if such a strategy is implemented in reality, a continuous assessment of the carbon flows and other GHG is necessary, together with adapted solutions (e.g. recycling instead of landfilling, technologies with no emissions, etc.) to preserve the climate benefit.

Finally, these results highlight the benefit of using biopump for applying the CSAAP concept in mitigating climate change and other environmental impacts, providing the scientific basis for policy makers. However, there are some limitations that deserve further investigations.

The carbon balance at the level of plant cultivation should be confirmed by experiments or specific investigation for each CV-land locally. In the practical applications, the land use cannot be guaranteed for such a long time, and the mono cultivation of one specie is unrealistic.

Moreover, if all the available CV land in France were used for the planting of black locust and if all the wood were used only for the proposed products, the production would far exceed the market. Indeed, with the development of technology and market, there will be more and more bio-based products instead of petrochemical materials, and the marginal products substituted in the consequential LCA will be challenged.

This study provided an integrated and comprehensive framework for proposing and analyzing mitigation solutions for global warming, which could be applied to other cases. Other combinations biopump/bio-based product could be analyzed; it is also possible to propose a panel of combined solutions with different plants and products for a given region. Processes with contributions lower than 1% of the total impact (in absolute) were attributed to categories others (+) or (-) (Process contributions were computed as follows: e.g., process A has a contribution of 1 and process B of -1, then the contribution of process A is 1/(abs(1)+abs(-1))=50%; details in SM). FU: functional unit.

Fig. 8 LCA results for BLc (left bar) and REFc (right bar) scenarios for all impact categories of the EF method, with the exception of climate change impact.

Fig. 1

 1 Fig. 1 Process flow diagram showing the key activities considered for (a) black locust

  105 years from 2022 to 2126, corresponding to three consecutive rotations of black locust. Meteorological conditions in France were predicted by SICLIMA (DRIAS CERFACS, IPSL, last updated May 2013) for the RCP4.5 climate trajectory (Representative Concentration Pathway; Chen et al., 2021), downscaled by the model CNRM-CERFACS-CM5/CNRM-ALADIN63. In C-TOOL,

Fig. 3

 3 Fig. 3 System diagrams showing the boundaries considered in consequential LCA of business

  ), which proposes a time-dependant decay function, and that C emissions arise as 14% CH4-C, and 86% CO2-C. According to the IPCC (2019) landfilling model, 15% of the products' biogenic C is considered to end as emissions to air. For the static LCA, the overall SOC change simulated with C-TOOL was, for both systems (BLc and REFc), annualized (i.e. distributed equally) over the 35 years of the rotation. All the background inventories were taken from Ecoinvent v3.5 (ecoinvent, 2020),

)

  In scenario BL, trees are cultivated on 1 ha of land for three consecutive rotations, from 2022 to 2126. The same time span is considered for the initial vegetation lasting on the CVlands in scenario REF. The temporal inventory (limited herein to GHGs) was obtained by placing each process on the timeline shown in Fig.4. For long-lasting biogenic GHG emissions (e.g., those stemming from the disposal of CLT and MDF), the longest emission time was set as 100 years because the C released after 100 years can be considered permanently stored(European Commission-Joint Research Centre., 2010). Therefore, the temporal scope spans over the implantation of the first rotation in 2022, up to the end of the emissions, in 2290, from the last MDF disposal, covering 267 years in total. The manufacturing processes were considered to occur in the year of biomass harvesting. The time step for the inventory calculation was set to 1 year, regarded as sufficiently fine for climate change impact calculation.

Fig. 4

 4 Fig. 4 Timeline of processes for dynamic LCA.

Fig. 5

 5 Fig. 5 System diagrams of business as usual (REFa) and black locust (BLa) systems, built on

Fig. 6

 6 Fig. 6 Simulated (a) biomass growth and decay for black locust during one rotation, and (b)

Fig. 7

 7 Fig.7 Climate change impact for (a) BLc and (b) REFc. Processes with contributions lower than

Fig. 9

 9 Fig.9GMTC results for all GHG sources from REFc and BLc scenarios, and the

Fig. 10 .

 10 Fig. 10. GMTC evolution in REFa (above) and BLa (bottom) scenario with time. 1 st , 2 nd , 3 rd

Fig. 1 Fig. 2

 12 Fig. 1 Process flow diagram showing the key activities considered for (a) black locust cultivation, harvesting (green) and residual biomass management (orange), and (b) manufacture and disposal of cross-laminated timber (CLT; yellow) and medium density fiberboard (MDF; blue

Fig. 5

 5 Fig. 5 System diagrams of business as usual (REFa) and black locust (BLa) systems, built on REF and BL scenarios respectively.

Fig. 6

 6 Fig. 6 Simulated (a) biomass growth and decay for black locust during one rotation, and (b) SOC evolution for black locust and initial vegetation during the cultivation period. Harvestable biomass: stem, branch and bark.

Fig. 7

 7 Fig.7 Climate change impact for (a) BLc and (b) REFc.Processes with contributions lower than 1% of the total impact (in absolute) were attributed to categories others (+) or (-) (Process contributions were computed as follows: e.g., process A has a contribution of 1 and process B of -1, then the contribution of process A is 1/(abs(1)+abs(-1))=50%; details in SM). FU: functional unit.

Fig. 9

 9 Fig.9GMTC results for all GHG sources from REFc and BLc scenarios, and the consequence of replacing REFc with BLc (Net GMTC). Total BLc = Bio BLc + Non bio BLc; Total REFc = Bio REFc + Non bio REFc; Net GMTC = Total BLc -Total REFc. 1 st , 2 nd , 3 rd refer to the rotation number.

Fig. 10 .

 10 Fig. 10. GMTC evolution in REFa (above) and BLa (bottom) scenario with time. 1 st , 2 nd , 3 rd refer to the rotation number. Total BLa = Bio BLa + Non bio BLa; Net BLa = Total BLa+ Palm oil + Feed; Total VGa = Bio VGa + Non bio VGa; Net REFa = Total VGa + Heat + Plastic board + Brick.

Fig. 11

 11 Fig. 11 Carbon captured over the cultivation period, and biogenic carbon stocks during the lifetime of the BL scenario.

Table 1

 1

, in red). Comparing scenario BLc to REFc, cultivating black locust instead of the initial vegetation benefits all environmental impacts except ionizing radiation, freshwater eutrophication, and land use.

Table 1 .

 1 Summary of the impact results. Positive impact values on grey background. BLc is better than REFc for the impacts highlited in italic bolds.

	Impact		Unit	BLc	REFc
	Climate change		kg CO2 eq	-2202407	-51061
	Ozone depletion		kg CFC11 eq	-0.034	0.00488
	Ionising radiation, HH kBq U-235 eq	9937	1847
	Photochemical ozone	kg NMVOC eq	-8217	478
	formation, HH
	Respiratory inorganics disease inc.	-0.39	0.030
	Non-cancer	human	CTUh	-0.17	0.26
	health effects
	Cancer human health	CTUh	-0.049	0.0022
	effects
	Acidification terrestrial	mol H+ eq	-4137	4616
	and freshwater
	Eutrophication		kg P eq	407	-11.2
	freshwater
	Eutrophication marine kg N eq	-1318	883

  The biggest GHG contributor is the avoided CO2 emission from the plastic board life cycle. The avoided plastic boards are counted at the time when MDF is produced, namely during first and

	second thinning, harvesting, and CLT recycling (years 2032/2067/2102, 2042/2077/2112,
	2057/2092/2127, 2108/2142/2178 respectively). The sharp decrease around 2057 is due to more
	plastic boards being avoided by more MDF being produced at the final harvest.

Acknowledgments

This work was carried out within the research project Cambioscop (https://cambioscop.cnrs.fr), financed by the French National Research Agency, Programme Investissement d'Avenir (ANR-17-MGPA-0006) and Region Occitanie (18015981). Additional funding was supplied by the Chinese Scholarship Council (201801810082).

Supplementary material

Supplementary document contains : documented LCA inventory, details on modeling approaches, results.