Substitution Effects On Emission Reduction And Time To Carbon Sequestration Parity Of Harvested Wood Products In China

Climate change is recognized as the primary environmental problem faced by humankind today.Forestry carbon sequestration is listed as one of the key ways to achieve carbon neutrality.The potential of forestry greenhouse gas（GHG）mitigation is determined by forest management behavior.Harvested wood products（HWP）can inversely affect forest management and the forest carbon sink.On the one hand,HWP can retain carbon for various periods of time depending on end uses.On the other hand,using HWP instead of more GHG-intensive materials and employing wood bioenergy as a substitute for fossil fuels result in reduced fossil fuel emissions.How to dynamically manage the carbon regulation function of the national forestry system and how to scientifically assess the substitution effect and time to carbon sequestration parity of HWP are important academic and practical questions relevant to the real-world problem of accurate assessment of forestry carbon in China.Nonetheless,the existing system of assessment of forestry ecosystem carbon has certain limitations in terms of its mechanism and methodology.On the one hand,there are deficiencies in the carbon cycle.Substituting wood for fossil fuels and HWP for fossil fuel-intensive products affords permanent and cumulative reductions in atmospheric carbon.The current system of forestry carbon assessment takes into account only carbon dynamics between forest carbon sequestration and forest product carbon storage without considering the HWP substitution effect as well as suffers from the common misunderstanding of the“carbon neutral”hypothesis,which ignores the time needed to achieve a net emission reduction.On the other hand,there are deficiencies in the current carbon evaluation model and evaluation indicators.First,the logic of the HWP displacement factor（DF）needs to be revised.The DF is based on the analysis of the carbon footprint in a product life cycle and has not solved the problem of regional representation in evolution and application.Emissions from the production of different HWP and alternative nonwood materials are often different,and energy mixes of these industrial sectors vary by region or country;therefore,using a single average DF to estimate substitution benefits may not be accurate.Second,the metrics employed to determine the time of achievement of net mitigation benefits lack completeness and accuracy and fail to take into account the appropriate baseline scenarios that include the no-harvest baseline scenario that may offer a better carbon emission mitigation option for wood products than harvesting.The main research tasks are as follows:First,in this study,we determine the correlation mechanism of“forest HWP”carbon dynamics over the entire life cycle.On the basis of“carbon debt repayment”theory,we clarify the theoretical logic and mathematical principles of the existing system of forestry carbon assessment and incorporate the HWP substitution effect into the“forest HWP”carbon cycle,which solves the problem of the separation of carbon circulation caused by independent studies on a forest and HWP.Scientifically assessing the dynamics of carbon flow caused by the use of HWP and accurately defining the dynamic life cycle inventory of biological carbon are preconditions for achieving the overall goal of this study.Second,in this project,we revise and expand the DF of the HWP substitution.To estimate the wood substitution effects in a specific sector at the national level more accurately,we apply the weighted method in this work.Based on national HWP market segments,the overall DF for all HWP consumed in China is estimated by means of HWP consumption fractions of each HWP end use subcategory in total HWP consumption in China as weighting factors.The weighted DF represents an innovation in the assessment of HWP substitution effects and solves the key methodology problem of this study.Third,in this work,we determine offset time to carbon neutrality of HWP.The dynamic life cycle assessment method will be utilized to compute the temporal distribution of biological carbon emissions and storage and to determine“time to carbon sequestration parity”of different emission reduction targets.On the basis of assumptions of the existing metrics,we consider appropriate baseline scenarios that include the no-harvest baseline scenario.Different metrics will affect the implementation of the forestry carbon emission reduction.To accurately quantify the net impact of biochar on climate change,it is necessary to scientifically set up baseline scenarios and determine offset time to carbon neutrality.Fourth,in this study,we establish a model for the evaluation of the integrated mitigation contribution of the forestry sector depending on time:Dynamic forest products carbon substitution assessment model（DFCS model）.After identifying the correlation mechanism of“forest HWP”carbon dynamics over the whole life cycle,after revising the DF of HWP substitution,and after determining offset time to carbon neutrality of HWP,we construct the dynamic carbon substitution evaluation model.The framework of the model includes three aspects:a model hypothesis,model framework,and model mathematical structure.We focus on material flow of the key consumption market according to the percentage of each HWP end use subcategory.We sort out the complete carbon dynamic inventory of each subsystem and fill the data gap reflecting the national level of HWP substitution in China.The model accurately measures net GHG emission reduction capacity and time to carbon sequestration parity of HWP.Fifth,in this work,we measure and compare the key factors affecting the time to carbon sequestration parity of HWP to test the importance of the factors involved in the evaluation of emission mitigation and to assess effects of the model assumptions by means of empirical data.We compare the time to carbon sequestration parity and cumulative carbon emission reduction for HWP within a target time range for different forest rotation periods,different HWP end use subcategories,and different end-of-life disposal techniques;these data are intended to verify the realization of the overall goal of this study.The main conclusions are as follows:（1）National-level DFs for wood substitution in China.The weighted average DF for HWP substitution in the construction sector was estimated to be 3.48 t C/t C,and the DF for HWP substitution in furniture manufacturing was estimated to be 1.36 t C/t C.After we combined HWP substitution analyses of the construction and furniture manufacturing,the overall HWP DF was estimated to be 2.90 t C/t C.That is,for each t C contained in the HWP used to substitute for nonwood materials in the construction and furniture manufacturing sectors in China,GHG emissions could be reduced by 2.90 t C on average.When wood-based pellets replaced coal in heat and power generation,the life cycle GHG emissions significantly diminished.（2）Substitution effects on emission reductions and time to carbon sequestration parity of HWP in China.{1}Time to carbon sequestration parity of plywood.When the emission reduction effect of HWP substitution was not considered,the carbon sequestration parity of plywood was achieved within 18 years.Under the low-DF scenario（DF=1.36）,the carbon sequestration parity of plywood was achieved within 10 years.Under average-DF（2.9）and high-DF（3.48）scenarios,the carbon sequestration parity of plywood was achieved within 0years.The cumulative carbon emission reductions in 100 years under the four scenarios turned out to be 748.64,2373.64,4213.71 and 4906.72 kg CO₂eq.{2}Time to carbon sequestration parity of fiberboard.When we did not consider the emission reduction effect of HWP substitution,carbon sequestration parity of fiberboard was achieved within 16 years.Under the low-DF scenario,the carbon sequestration parity of plywood was achieved within 2 years.Under the average-and high-DF scenarios,the carbon sequestration parity of plywood was achieved within 0 years.The cumulative carbon emission reductions in 100 years under the four scenarios proved to be 622.53,2247.53,4087.60,and 4780.62 kg CO₂eq.{3}Time to carbon sequestration parity of particleboard.When we did not consider the emission reduction effect of HWP substitution,carbon sequestration parity of particleboard was achieved within 18 years.Under the low-,average-,and high-DF scenarios,the carbon sequestration parity of plywood was achieved within 0 years.The cumulative carbon emission reductions in 100 years under the four scenarios were found to be 472.32,2097.32,3937.39,and 4630.40 kg CO₂eq.{4}Examining empirical results to evaluate the model assumptions.When we did not consider the emission reduction effect of HWP substitution or when we tested the low-DF scenario,plywood yielded the greatest cumulative carbon emission reductions,followed by fiberboard and particleboard.Achieving carbon neutrality for plywood took the longest time,which required time to carbon sequestration parity of more than 10 years.Under the average-DF scenario and high-DF scenario,wood products could manifest immediate net emission reductions.（3）Measurement and comparison of time to carbon sequestration parity of HWP under different scenarios.（1）Carbon sequestration parity for different forest rotation periods.With the extension of the rotation period,time to carbon sequestration parity shortened,which could produce a net emission reduction faster,but the cumulative carbon emission reduction was found to be reduced within 100 years.（2）Carbon sequestration parity for different HWP end use subcategories.Increasing the use of HWP to replace nonwood materials in the construction sector in China,especially in the nonresidential construction sector,which has a high DF,could significantly contribute to GHG mitigation.Consequently,time to carbon sequestration parity shortened,and the cumulative carbon emission reduction was found to increase within 100years.（3）Carbon sequestration parity for different end-of-life disposal techniques.When we analyzed the emission reduction effect of HWP substitution for end-of-life disposal techniques,collecting methane in landfills was found to greatly reduce carbon emissions in the life cycle of HWP,thereby possibly shortening time to carbon sequestration parity and increasing the cumulative carbon emission reduction.The main contributions include the following:{1}The revision and expansion of carbon debt repayment theory.The simple"carbon neutral"hypothesis tends to cause accounting errors and hence is increasingly replaced by the concept of“carbon sequestration parity.”We expanded the application of carbon debt repayment theory in forestry economics in terms of three aspects:sustainable forest management,an optimal rotation period,and life cycle analysis of HWP.{2}Optimization and improvement of the DF model and carbon sequestration parity metric.We applied the weighted method based on national HWP market segments.The weighted DF represents an innovation in the assessment of HWP substitution effects and solves the key methodology problem of our study.We determined the mechanism of“carbon debt repayment”and expanded the operation of carbon sequestration parity for different HWP substitution scenarios.The main policy implications include the following:{1}Optimization of HWP life cycle carbon management.We should extend service life of HWP,increase the proportion of hardwood and other forest products with long service life,and improve HWP life cycle carbon storage.We should strengthen the verification and optimization of GHG emissions of HWP during the life cycle and improve the HWP substitution effect.To reduce emissions,we should also increase energy utilization and recovery and reuse rates of waste forest products.{2}Adjustment of the market structure of the forest product industry.The forestry carbon sink should be included in the optimal rotation period evaluation model to plan an optimal rotation period combination.From a long-term perspective,the proportion of plywood should be increased to enhance the carbon emission reduction potential of HWP.The use of HWP in the construction sector should be increased to improve the national HWP DF and cumulative carbon emission reduction.{3}Establishment of carbon management organizations and technical standards in the forestry industry.We should implement the deployment of forestry carbon sink management organizations,speed up the creation of the technical-standard system,and strengthen forestry carbon science,technology,and policy research.{4}Improvement and structural upgrading of the carbon-trading market.For dealing with climate change,the development of the carbon market will become the institutional choice and a development trend for major countries of the world.China’s carbon market supervision and improvement will be accelerated to encourage and guide industrial transformation and structural upgrades.