Contributions of China ’ s Wood-Based Panels to CO 2 Emission and Removal Implied by the Energy Consumption Standards

Life cycle analysis on wood-based panels in terms of CO2 flux can be used to quantitatively assess the climate change contributions of these materials. In this study, the annual CO2 flux between 1990 and 2015 was calculated through gate-to-gate life cycle analysis of wood-based panels. As implied by the energy consumption standards, China’s wood-based panels used to be carbon sources during the period 1990–2007, with the average contribution to CO2 emissions of 9.20 Mt/year. The implementation of new standards and the development of Cleaner production technologies in China, decreased the energy consumption per panel. China’s wood-based panels acted as a carbon sink between 2008 and 2015, with the average contribution to CO2 removal of 31.71 Mt/year. Plywood produced the largest contributions to the emission and removal of CO2, and was followed by fiberboard and particleboard. China’s wood-based panels, with good prospects and strong demands projected in the future, can potentially contribute to climate change mitigation.


Introduction
The ongoing international concern for climate change has obtained increased attention from developing countries due to their rapidly growing greenhouse gas (GHG) emissions [1].China has committed to a 40%-45% reduction in GHG emissions per unit of gross domestic product (GDP) by 2020 compared with 2005 and has planned to peak in carbon emission reduction by around 2030 [2].The growing concern over the impacts of climate change has emphasized the mitigation potential of forests and forest-derived products, specifically in terms of carbon sequestration [3].The use of wood and wood-based products exerts positive effects on the environment, such as mitigating climate change or reducing waste and other emissions [4].
The wood-based panel industry is an important forest-based one in China.The country is the largest wood-based panel producer worldwide, with a total output of 286.80 million m 3 in 2015 [5].The main panels, namely, plywood, fiberboard, and particleboard account for 87.85% of China's wood-based panel industry.Among wood-based panels, plywood dominates with 57.69% of production and is followed by fiberboard at 23.08% of the total output.The yield of particleboard accounts for 7.08% of the total output [5].Due to the high economic importance of wood-based panels in China, the production has expanded considerably in recent years and is expected to increase with an average annual growth rate of 1.05% from 2015 to 2030 [6].On the one hand, the carbon storage in wood-based panels has increased annually due to an increase in net primary production [7,8].On the other hand, gate-to-gate product manufacturing consumes various energy sources, and almost all energy consumption results in GHG emissions [9].
The major GHG is CO 2 with less contribution from CH 4 and N 2 O [10].The contributions of wood-based panels include the contributions to annual CO 2 emissions and removals [11].Direct emission occurs during production, whereas indirect emission is associated with purchased electricity and product end-of-life.Other emission types are associated with fiber and non-fiber productions, transportation, and product use [12].In compensating for CO 2 emissions, the net removal contributions are obtained on the basis of the estimation of CO 2 stocks [13].
Several studies have focused on environmental impact assessments on wood-based panels.These studies compared the environmental influence of plywood, fiberboard, and particleboard to address issues of global warming, acidification, eutrophication, dust, wastes, and resource consumption [14].Syahirah [15] compared the environmental impacts of panel preparation, shaping, and finishing on the wood-based industry in Malaysia.The growing focus on carbon as an indicator of environmental performance [16] has encouraged studies on carbon flux through the processing stages, carbon stored in products, and carbon footprint for on-site manufacture and cradle-to-product gate processes.Wilson [10,17] and Sakimoto [18] calculated the carbon footprints of production of American particleboard, medium-density fiberboard, and softwood plywood, using the Simapro 7.1 software (Pre'Consultants, Amersfoort, The Netherlands) and assessed the CO 2 fluxes of these materials through the carbon content in the products.Garcia and Freire [19] compared three different tools for assessing environmental impact of particleboard and showed that treating biogenic CO 2 causes the main difference in global warming and greatly benefits global warming.In 2012, Bergman et al. [20] quantified environmental impact using the life cycle assessment method on five wood-based panel products made in North America.
The literature review shows that stored carbon per unit of wood-based panels can offset the carbon emissions from on-site manufacturing and is better than climate-neutral materials with a carbon store equal to their carbon emissions.Some studies have only estimated carbon storage of the national wood-based panel pool and its mitigation potential [21,22].Few studies have comprehensively assessed the contributions of national wood-based panels to CO 2 emissions or removals.Less attention has been attached to GHG contributions to the atmosphere.Current research in China is restricted to carbon footprint or carbon stock calculations per volume of fiberboard and particleboard [23,24].
The climate change contributions should be assessed and the environmental profile of wood-based panels should be improved in China to meet its GHG emission reduction targets.This work focused on the environmental impact of CO 2 emission or removal.CO 2 flux can be used to comprehensively assess the actual effects of products on climate change within their entire life cycle [25].This work derived the annual CO 2 fluxes through the life cycle of wood-based panels to determine their role in climate change between 1990 and 2015.

System Dynamic Structure and Functional Unit
The first step in calculating CO 2 flux is to determine the system boundary of wood-based carbon flow, which uses two major system boundaries [26].One is the cradle-to-grave system boundary, which consists of five phases, namely, raw material extraction, product production, packaging and transportation, product use, and waste disposal [27].As illustrated by the flow diagram in Figure 1, the cradle-to-grave analysis applied in the life cycle of harvested wood product (HWP) assesses various environmental impacts over the entire product life cycle, from raw material extraction (the cradle), via production, transportation and use, to waste management (the grave) [28].Wood-based carbon stocks are divided into many sub-fluxes passing through various pools and processes before being released into the atmosphere [29].The other is the gate-to-gate system boundary, which consists of two phases, namely, product manufacture and use [20,30].For our analysis, the gate-to-gate system boundary was used (Figure 2).Forest trimmings and industrial waste wood from sawmills, such as shavings, sawdust, ply trim, veneer, and chips, are sources of raw wood materials.Plywood mostly uses large-diameter timbers as raw materials.Fiberboard and particleboard mostly use small-diameter timbers and wood residues (i.e., harvesting, building, and processing residues) from sawmills and other wood industries as major raw materials.Moreover, bark from logs and sawdust may be used to produce wood-based panels [31].Glueing and hot pressing 6.

Forests
Sawing and sanding  Plywood production involves raw material preparation; peeling, drying, and veneer finishing; gluing and hot processing; sawing and sanding; and packaging.Carbon emission primarily occurs during gluing [32].Fiberboard production includes raw material preparation, pulping, forming, hot processing, and after-treatment.Pulping and forming consume much energy during production [33].Particleboard production comprises raw material preparation, particle preparation, particle drying, particle sizing, slab paving, hot pressing, and after-treatment.The energy is primarily consumed through particle preparation, drying, slab paving, and hot pressing, which account for 87.46% of the total energy consumption [34].The three panels are produced by coordinating various procedures, which also produce pollutants and GHGs.
Using the atmospheric carbon flow as the evaluation objective, IPCC [35] stipulates that direct and indirect CO 2 emissions are positive GHG contributors and that carbon stock and substitution emission reduction are negative GHG contributors.The equation for CO 2 flux is given by: where CF, CE and CS represent the values of CO 2 flux, CO 2 emission, and CO 2 stock, respectively.If the value is above zero, then the product is a net emitter.Otherwise, the product is a net sink.CO 2 emission reduction is stipulated as a negative value [36].The capability of carbon sinking depends on the absolute value.A high absolute value indicates increased contribution to CO 2 removal from the atmosphere [25].
The functional unit describes the quantitative measure of the functions that a product or service provides [37].In this study, the investigated products were wood-based panels.The 2013 Revised Supplementary Methods and Good Practice Guidance Arising from the Kyoto Protocol (The 2013 IPCC Guidelines) [11] stipulates that wood-based panels are reported in cubic meters (m 3 ) solid volume, a functional unit used in previous wood-based panel studies [10,38,39].Therefore, the volumetric unit (m 3 ) was adopted as the reference in the current work to compare the gate-to-gate life cycle CO 2 flux among different panels.

CO 2 Emission and Energy Consumption Standard
This work calculated the CO 2 emissions from the production process in accordance with energy consumption standards.The overall energy consumption standards of wood-based panel production are China's forestry industry standards issued by State Forestry Administration of the People's Republic of China.These standards are recommended standards and define the relevant terms of on-site manufacture of wood-based panels and the indexes of energy consumption for producing 1 m 3 of panels.q 1 represents the specific value of the indexes and is given in kgce/m 3 .Through the investigation of typical wood-based panel enterprises in China, the energy consumption of different types of enterprises was obtained.Surveyed enterprises accounted for more than 20% of the entire industry, and more than 50% of the total output was investigated.The energy consumptions per unit production of enterprises with an annual production of 50,000 m 3 of plywood and 0.2 million m 3 of particleboard were taken as the basic values [40][41][42], on which the different levels of indexes were based.The index values of different grades can represent the actual energy consumption of Chinese wood-based panel enterprises.The amounts of energy consumption in the entire production system were summed up and converted into standard coal [32][33][34].Energy consumptions are divided into three levels in accordance with the standards, and the specific meaning of each level is as follows.
The third and qualified grades represent the limit values of energy consumption per unit product and are mandatory indicators.The purpose of these grades is to eliminate the backward production capacity of 20%-30%.The second and good grades represent the threshold values of energy consumption per unit product and are mandatory indicators.They are admittance values for production of per unit value of energy consumption for new construction, reconstruction, and expansion of enterprises.The first and excellent grades represent the advanced values of energy consumption per unit product and are recommended indicators.The numerical values indicated by the indexes are in line with the international advanced levels or the leading domestic levels for production of wood-based panels and are the goals of enterprises for on-site manufacture.Energy consumption standards are the principles that enterprises should follow in producing wood-based panels.The enterprises should calculate the energy consumption of the actual production process and match them to the grade in the standards.The comprehensive energy consumption per unit output of an enterprise in actual production refers to the ratio of total energy consumption to qualified output in the same statistical period.
Tables 1 and 2 show the different indexes according to the energy consumption standards between 1990 and 2015.The different levels of indexes were used to judge the actual production level of wood-based panel industry and service as a reference for the government to supervise the consumption of energy for production.The quantitative management of resource consumption by the state may urge enterprises to examine the energy consumption in each production process in accordance with the energy consumption levels and to reduce waste of resources.The backward production capacity that fails to meet the third and qualified grades will be eliminated [43].The implementation of the total energy consumption standards of the wood-based panel industry in China occurs in two stages.The first stage includes LY/T 1529-1999 Total Energy Consumption in Plywood Production (LY/T 1529-1999), LY/T 1451-1999 Comprehensive Energy Consumption for Hard Fiberboard Production on the Wet Process (LY/T 1451-1999), and LY/T 1530-1999 Total Energy Consumption in Particleboard Production (LY/T 1530-1999), which were released in 1989 and implemented in 1990 [32][33][34].
In the first stage, the four major producers of Chinese wood-based panels are Linyi, Shandong Province; Pizhou, Jiangsu Province; Jiashan, Zhejian Province; and Wen'an, Hebei Province.Among these producers, three are located in the northern provinces (The Technical Requirements of Cleaner Production for the Wood-based Panel Industry in China defines the southern and northern provinces as the area with heating facilities and the area without heating facilities, respectively.)(i.e., Linyi, Pizhou, and Wen'an), and the total output and market share of wood-based panels of Shandong, Jiangsu, and Hebei provinces account for more than 80% of the national share.This work selected the northern provinces as regional indicators.
Table 1.Indexes of energy consumption for producing 1 m 3 of panels in the first stage (kgce/m 3 ).
In terms of first-level indexes, the average energy consumptions of plywood, fiberboard, and particleboard were 510, 750, and 375 kgce/m 3 , respectively (Table 1) The  [40][41][42].Replacing the previous energy consumption standards with the current ones includes the implementation of Excellent, Good, and Qualified index levels.The north and south areas are also unified in the same index.The reduction in energy consumptions for wood-based panel production is attributed to the application of cleaner production technologies.China's enterprises have been urged to adopt cleaner production technologies through the improvement of technologies and utilization of clean materials [44].Thus, the calculations based on the "first grade" (Table 1) and "excellent grade" (Table 2) met the requirements for cleaner production in China's wood-based panel industry.According to the statistics, the energy consumption of China's advanced enterprises is 163-182 kgce/m 3 , and that of most small-and medium-scale enterprises is 250 kgce/m 3 [45].In addition, in the research of 20 representative enterprises, the energy consumption per unit of particleboard is 113.03 kgce/m 3 [46].The values of actual practice conform to the first-level indexes.The results of related foreign literature showed that the energy consumptions for plywood production in the United States and Canada are 158.46 and 80.15 kgce/m 3 [47] and that the domestic practical conditions of China are close to advanced levels.Therefore, the index selection conforms to the current situation of the industry and the actual situation.
Table 2. Indexes of energy consumption for producing 1 m 3 of panels in the second stage (kgce/m 3 ).

Wood-Based Panels
Indexes Values In the second stage, the average energy consumptions for on-site manufacturing of plywood, fiberboard, and particleboard in terms of excellent-grade indexes were 200, 320, and 120 kgce/m 3 , respectively.

IPCC Methodology
The 2006 IPCC Guidelines for National Greenhouse Gas Inventories (The 2006 IPCC Guidelines) identifies three feasible approaches to estimate and report national carbon stock changes of HWPs, namely, the stock change approach, the atmospheric flow approach, and the production approach.Following the 17th session of the Conference of the Parties, carbon accounting of HWP is confined to products in use, where the wood was derived from domestic harvest [48].The principle is the same as the production approache (PA), a universal approach during the second commitment period of the Kyoto Protocol [49].The current study calculated the gate-to-gate CO 2 flux in which the system boundary disregarded trade and excluded carbon in imported wood-based panels [11].The PA was used to calculate only the carbon stock changes.
The first step is to estimate the share of domestically produced wood-based panels.Following the 2013 IPCC Guidelines, the domestic consumption of industrial roundwood was assumed equal to the feedstock used for manufacturing the wood-based panels.The fraction of industrial roundwood from domestic forests was computed on the basis of the production, import, and export of industrial roundwood in accordance with Equation (2): where f IRW (i) is the share of industrial roundwood used in the domestic wood-based panel production with respect to the total consumption of industrial roundwood in year i; IRW P (i), IRW I M (i) and IRW EX (i) represent the carbon content in the produced, imported, and exported industrial roundwood in year i, respectively.The present work assumed that all domestic wood harvests were from sustainably managed forests; therefore, the calculation of carbon stocks did not differ among different afforestation activities.Equation (3) was used to calculate the annual fraction of wood-based panels entering the accounting framework from domestic harvest as follows: where HWP(i) is the number of wood-based panels produced from domestic harvest in year (i), P is the total number of wood-based panels produced in year i.
The annual inflow and outflow for wood-based panels were estimated by applying default conversion factors and a first-order decay function, with the constant annual default decay factors proposed by the 2013 IPCC Guidelines.Changes in carbon stocks in year i were estimated in accordance with the following equations: where C(i) is the carbon stock of wood-based panels at the beginning of year i; C(1900) = 0; k is the first-order decay rate equal to ln HL , where HL is the half-life of the wood-based panels; in f low(i) is the inflow of wood-based panels to the carbon pool in year i; ∆ C(i) is the carbon stock change of each wood-based panel during year i.

Selection of Parameters and Data Sources
The basic density of the wood-based panels in this work was adopted from the national standard GB/T 1933-2009, the Method for Determination of the Density of Wood.The moisture content was adopted from the national standard GB/T 1931-2009, the Method for Determination of the Moisture Content of Wood.The carbon fraction was adopted from the published literature data in China [50].The half-life values and first-order decay rate were based on the default data provided by the 2013 IPCC Guidelines.Table 3 shows the conversion parameters used in the calculation of carbon stocks in wood-based panels.The carbon stock changes were converted to CO 2 by 3.67 [10].Note: various carbon conversion factors of wood-based panels in China are adopted from published studies [11,50].
The data of the total production, import, and export of industrial roundwood and total production of wood-based panels between 1990 and 2015 were from the China Forestry Statistical Yearbook.The stock of wood-based panel carbon pool was cumulative.Calculating the carbon stock required values from the previous year.The 2006 IPCC Guidelines recommends that calculation of the carbon stock of HWP starts from 1900 and assumes that the value prior to 1900 is zero [35].The current production of industrial roundwood and wood-based panels, as well as the trade data recorded in the China Forestry Statistical Yearbook can be traced back to 1949.Data prior to 1949 can be reverse-calculated using 1949 data as a benchmark in Equation ( 6) [51].
where u is the continuous rate of change in industrial roundwood consumption for China at default data of 0.0217; V i is the annual production, import, or export of industrial roundwood and various wood-based panels in year i tracing back to 1900; V i is the annual production, import, or export of industrial roundwood and various wood-based panels in the benchmark year; i is the year; i is the benchmark year.

CO 2 Emissions
The annual CO 2 emissions in the production of the wood-based panel industry were derived and are shown in Figure 3.

CO2 Emissions
The annual CO2 emissions in the production of the wood-based panel industry were derived and are shown in Figure 3.The numbers of standard coal were converted to CO2 with a conversion coefficient of 2.54 t CO2/tce [52].On-site CO2 emissions for manufacturing plywood, fiberboard, and particleboard were 1.30, 1.91, and 0.95 t CO2/m 3 , respectively, during the period 1990-2007.From the production and trade data in the China Forestry Statistical Yearbook, the average annual CO2 emitted to the atmosphere was found to be 7.80 Mt in the first stage.
In the second stage, China's wood-based panel industry experienced a new round of rapid growth, with an average annual yield of 171.96 million m 3 .On-site manufacturing of 1 m 3 of plywood, fiberboard, and particleboard produced 0.51, 0.81, and 0.30 t CO2, respectively.The average annual CO2 emitted to the atmosphere was 21.84 Mt CO2, which is 1.8 times of the first stage, due to the rapid growth of yields and low energy consumptions.
Total CO2 emission was found to grow in each stage.The annual CO2 emissions of plywood manufacturing were the highest at approximately 17.58 Mt/year and were followed by those of fiberboard (15.78 Mt/year).Particleboard had the lowest annual CO2 emission contributions due to the low yields and CO2 emission per unit.

CO2 Stock Changes
Forests sequester atmospheric CO2 to produce wood, and carbon stored in wood products is emitted as CO2 and CH4 when burned [53].As shown in Figure 4, China's annual CO2 stock changes of wood-based panels continuously increased from 1990 to 2015.Using established national data and the PA to carbon storage, the net increase in CO2 stored in 1990 was calculated to be 1.88 Mt whereas The numbers of standard coal were converted to CO 2 with a conversion coefficient of 2.54 t CO 2 /tce [52].On-site CO 2 emissions for manufacturing plywood, fiberboard, and particleboard were 1.30, 1.91, and 0.95 t CO 2 /m 3 , respectively, during the period 1990-2007.From the production and trade data in the China Forestry Statistical Yearbook, the average annual CO 2 emitted to the atmosphere was found to be 7.80 Mt in the first stage.
In the second stage, China's wood-based panel industry experienced a new round of rapid growth, with an average annual yield of 171.96 million m 3 .On-site manufacturing of 1 m 3 of plywood, fiberboard, and particleboard produced 0.51, 0.81, and 0.30 t CO 2 , respectively.The average annual CO 2 emitted to the atmosphere was 21.84 Mt CO 2 , which is 1.8 times of the first stage, due to the rapid growth of yields and low energy consumptions.
Total CO 2 emission was found to grow in each stage.The annual CO 2 emissions of plywood manufacturing were the highest at approximately 17.58 Mt/year and were followed by those of fiberboard (15.78 Mt/year).Particleboard had the lowest annual CO 2 emission contributions due to the low yields and CO 2 emission per unit.

CO 2 Stock Changes
Forests sequester atmospheric CO 2 to produce wood, and carbon stored in wood products is emitted as CO 2 and CH 4 when burned [53].As shown in Figure 4, China's annual CO 2 stock changes of wood-based panels continuously increased from 1990 to 2015.Using established national data and the PA to carbon storage, the net increase in CO 2 stored in 1990 was calculated to be 1.88 Mt whereas it was calculated to be 125.49Mt in 2015.The annual average increments of plywood, fiberboard, and particleboard were 19.06, 15.70, and 4.98 Mt CO 2 , respectively.Plywood had the highest annual output and thus had the most significant changes in CO 2 pool.The carbon stock of China's HWP products rapidly increased after 2003 mainly due to the rapid development of China's forestry that gained an important position in the sustainable development of China's national economy.After 2001, the production of China's wood-based panels increased at a rapid speed, thereby increasing the annual stock.Wood-based panel contributions varied from approximately 1.54 Mt CO2 to 27.74 Mt CO2/year between 1990 and 2007.The largest emitter was plywood (i.e., 4.61 Mt CO2/year), followed by fiberboard (i.e., 4.37 Mt CO2/year) and particleboard (i.e., 0.22 Mt CO2 CO2/year).In this stage, woodbased panels were carbon sources and CO2 emissions of on-site manufacture exceeded the CO2 stored in the products.The annual CO2 emission contributions to the atmosphere were 9.20 Mt.With the increase in outputs, China's wood-based panels produced large amounts of CO2 to the atmosphere.

Annual CO2 Fluxes and Contributions
The implementation of new standards led to the decrease in energy consumption per panel in The carbon stock of China's HWP products rapidly increased after 2003 mainly due to the rapid development of China's forestry that gained an important position in the sustainable development of China's national economy.After 2001, the production of China's wood-based panels increased at a rapid speed, thereby increasing the annual stock.The carbon stock of China's HWP products rapidly increased after 2003 mainly due to the rapid development of China's forestry that gained an important position in the sustainable development of China's national economy.After 2001, the production of China's wood-based panels increased at a rapid speed, thereby increasing the annual stock.Wood-based panel contributions varied from approximately 1.54 Mt CO2 to 27.74 Mt CO2/year between 1990 and 2007.The largest emitter was plywood (i.e., 4.61 Mt CO2/year), followed by fiberboard (i.e., 4.37 Mt CO2/year) and particleboard (i.e., 0.22 Mt CO2 CO2/year).In this stage, woodbased panels were carbon sources and CO2 emissions of on-site manufacture exceeded the CO2 stored in the products.The annual CO2 emission contributions to the atmosphere were 9.20 Mt.With the increase in outputs, China's wood-based panels produced large amounts of CO2 to the atmosphere.Wood-based panel contributions varied from approximately 1.54 Mt CO 2 to 27.74 Mt CO 2 /year between 1990 and 2007.The largest emitter was plywood (i.e., 4.61 Mt CO 2 /year), followed by fiberboard (i.e., 4.37 Mt CO 2 /year) and particleboard (i.e., 0.22 Mt CO 2 CO 2 /year).In this stage, wood-based panels were carbon sources and CO 2 emissions of on-site manufacture exceeded the CO 2 stored in the products.The annual CO 2 emission contributions to the atmosphere were 9.20 Mt.With the increase in outputs, China's wood-based panels produced large amounts of CO 2 to the atmosphere.

Annual CO2 Fluxes and Contributions
The implementation of new standards led to the decrease in energy consumption per panel in the second stage.On the contrary, an increasing trend of the production of wood-based panels with an annual growth rate of approximately 25% was observed.In 2008-2015, wood-based panels acted as a carbon sink.CO 2 storage in wood products offset the manufactured CO 2 released to the atmosphere [20].The average annual CO 2 removal contributions were 31.71Mt/year, with the contributions of plywood, fiberboard, and particleboard at 15.17, 9.59, and 6.95 Mt CO 2 /year, respectively.Considering the same trend with the first stage, plywood played the most important role in removing CO 2 .The greater the output, the greater the contributions to CO 2 removal.
The CO 2 contributions between the two stages were the exact opposite.The difference between CO 2 storage and emissions varied from positive to negative; therefore, the wood-based panel pool in China changed from a carbon source to a carbon sink.This finding was mainly due to the fact that energy consumption per panel for on-site manufacture decreased and panel production significantly increased.

Methodological and Data Constraints
The underlying assumptions presented in this work can be improved through in-depth analyses such as (1) considering the substitution of emission reduction to decrease the life cycle of GHG emissions, (2) studying the climate change mitigation potential of wood-based panels through the cradle-to-grave system boundary, and (3) achieving acute and specific data and methods.Some methodological and data constraints are discussed below.
Analysis based on default data and methods presents a methodological weakness, because of the absence of available data.The accuracy and quality of results in this work depended on the data and method quality.Using the default data and methods provided by IPCC and previous publications introduced uncertainties.This work calculated the CO 2 stock changes based on the default half-lives that were distinguished among main wood-based panels.A total of 25 years were used as half-lives in the current work whereas some studies estimated them to be shorter than 25 years [50,51].This work might have overestimated the CO 2 stock changes.
The 2013 IPCC Guidelines describes three methods based on the level of detail and accuracy of the available data: (1) instantaneous oxidation (Tier 1), (2) first-order decay (Tier 2), and (3) country-specific methods (Tier 3).The country-specific method can be used if a large number of detailed data and methodologies are available.Under the Tier 3 method, accurate country-specific information is applied to improve the accuracy of the estimates.If consistent, transparent, and verifiable parameters (i.e., service life information and conversation factors of products) are applied to country-specific methods, then the estimates of carbon removal contributions can increase in accuracy and reliability.
The current national environmental standard for the wood-based panel industry stipulates that imported wooden raw materials should come from sustainable forests and that national wooden raw materials should comply with the forestry laws and regulations in China [54].The carbon in wood-based panels is allocated to particular forest activities because different estimates of contributions depend on the wood origin.The harvest from deforestation is regarded as instantaneous oxidation.This work disregarded deforestation activities because of the lack of data.Thus, the CO 2 contributions were overestimated.Forest activities should be differentiated in the future.

Potential of China's Wood-Based Panels in Reducing CO 2 Emissions of On-Site Manufacture
The wood-processing industry standard in China has a low requirement in energy consumption indexes [55].The energy consumption per unit output of China's wood-based panel is 4.8 times that of the world [56].As presented in Table 4, CO 2 emissions during the wood-based panel production were higher in China than those in the United States.The gate-to-gate life cycle CO 2 fluxes of China's wood-based panels were lower than those of the United States.China's wood-based panel industry has a high CO 2 emission reduction potential in on-site manufacture.However, the decrease in energy consumption indexes for wood-based panel production indicates that China's wood-based panels can potentially reduce CO 2 emissions of on-site manufacture.China is committed to reducing or avoiding pollutant emissions during production, service, and use.The implementation of cleaner production technologies reduces or eliminates pollution from the environment by improving the technologies and using clean materials.For more than 10 years, the index values of total energy consumption standards have declined.The reduction in energy consumption during production shows that China's wood-based panels present strong GHG reduction capacity and energy-saving potential.

Ways to Enhance CO 2 Removal Contributions of China's Wood-Based Panels
Climate change can be mitigated through the use of wood products that reduce carbon emissions and remove carbon [58,59].Global carbon emissions can be mitigated using two methods: (1) directly reducing carbon emissions and (2) indirectly increasing carbon storage [60].
Cleaner production technologies must be carried out to reduce energy and resource consumption in production and thus reduce CO 2 emissions [14].Energy consumption occupies a large proportion in the cost of wood-based panel products; the energy costs of plywood, fiberboard, and particleboard are 25%, 30%, and 30%, respectively [46,61,62].
Carbon mitigation or slowing carbon emissions over time can also be achieved using various CO 2 stock approaches, such as extending the life span of HWP, recycling and reusing disposed HWPs, and improving the production efficiency of HWPs.These approaches cannot directly mitigate carbon emissions or indirectly increase carbon storage.However, they can provide a significant time lag between carbon sequestration from the atmosphere and carbon emissions back into the atmosphere [60].

Nature of Cleaner Production Technologies in China's Wood-Based Panel Industry
Cleaner production is based on holistic and preventative approaches; if this technology is implemented society-wide, then societies can be sustainable [63].Cleaner production has been adopted by China as a primary tool to fight against industrial pollution [64].During the 12th Five-Year Plan (2011-2015), the energy consumption per unit of GDP fell by approximately 16% compared with the 1.03 tce in 2010 [65].To achieve national emission reduction targets, cleaner production technologies have been applied in China's wood-based panel industry.The values in Table 5 show the grades for cleaner production technologies in wood-based panel production.
The first grade represents the advanced level of international cleaner production, the second grade represents the advanced level of cleaner production in China, and the third grade represents the basic level of cleaner production in China.
China's energy consumption standards for wood-based panels have been revised in recent years (Tables 1 and 2) and energy consumptions for production of 1 m 3 of wood-based panels have decreased.These amendments conform to the requirements of cleaner production t in China.
Table 5. Indexes of cleaner production technologies for wood-based panel production in China (kgce/m 3 ).

Fiberboard
First grade q 1 ≤ 200 Second grade q 1 ≤ 390 Third grade q 1 ≤ 440 Particleboard First grade q 1 ≤ 120 Second grade q 1 ≤ 150 Third grade q 1 ≤ 200 Another reason for the decrease in energy consumption for production of 1 m 3 of wood-based panels was the large increase in output.The comprehensive energy consumption per unit output of an enterprise in actual production refers to the ratio of total energy consumption to qualified output in the same statistical period.Clean production technologies reduce the total energy consumptions for on-site manufacture.An increase in output means low energy consumption per unit of product.

Conclusions
China is one of the major countries that aim to reduce GHG emissions.As a pillar of the forestry industry, the production and use of wood-based panels in China have a significant impact on the contribution of GHGs.Assessing the life cycle of CO 2 fluxes revealed the role of China's wood-based panel industry in mitigating climate change and quantifying CO 2 emission and removal contributions in different panels.For our analysis, we estimated CO 2 emissions through product production and CO 2 stored during their useful lives.The results showed that China's wood-based panels were carbon sources between 1990 and 2007.As implied by the energy consumption standards for wood-based panel production, energy consumption per panel for on-site manufacture decreased.Panel production in China also increased considerably.The two factors had made China's wood-based panels a carbon sink in 2008-2015.Between the two stages, plywood produced the largest contributions whereas particleboard provided the smallest contributions.The changes in annual CO 2 fluxes showed that the use of wood-based panels met the GHG emission targets in China.Therefore, cleaner production technologies must be carried out to reduce energy consumption and GHG emissions in panel production.China's wood-based panels, with good prospects and strong demands projected in the future, can potentially contribute to climate change mitigation.

Fig 2 .
Fig 2. Gate-to-gate life cycle and process chain of wood-based panels.

Figure 1 .
Figure 1.Wood-based carbon flow through the cradle-to-grave life cycle of harvested wood product (HWP).

Fig
Fig 2. Gate-to-gate life cycle and process chain of wood-based panels.

Figure 2 .
Figure 2. Gate-to-gate life cycle and process chain of wood-based panels.

Figure 3 .
Figure 3. On-site CO2 emissions for the manufacture of China's wood-based panels.

Figure 3 .
Figure 3. On-site CO 2 emissions for the manufacture of China's wood-based panels.

Figure 5
Figure 5 exhibits the gate-to-product gate CO2 fluxes of wood-based panels between 1990 and 2015.

Figure 4 .
Figure 4. CO 2 stock changes of China's wood-based panels.

Figure 5
Figure 5 exhibits the gate-to-product gate CO 2 fluxes of wood-based panels between 1990 and 2015.

Figure 5
Figure 5 exhibits the gate-to-product gate CO2 fluxes of wood-based panels between 1990 and 2015.

Note: the arrangement
is based on two standards: Cleaner Production Standard-Wood-Based panel Industry (Medium-density Fiberboard) and Cleaner Production Standard-Wood-Based panel Industry (Particleboard).
Wood-based carbon flow through the cradle-to-grave life cycle of harvested wood product (HWP).

2 .
Gate-to-gate life cycle and process chain of wood-based panels.

Table 3 .
Various carbon conversion factors of wood-based panels.

Table 4 .
CO 2 fluxes through the gate-to-gate life cycle of wood-based panels in China and the United States (t/m 3 ).
[57]: CO 2 emissions and stocks of wood-based panels in the United States are adopted from Consortium for Research on Renewable Industrial Materials Report[57].The carbon calculation of plywood is divided into two types on the basis of region: PNW (Pacific Northwest) and SE (Southeast).