Carbon Profile of the Managed Forest Sector in Canada in the 20th

Jul 30, 2014 - USDA Forest Service, Northern Research Station, 271 Mast Road, Durham, New ..... nonresidential construction, furniture, railway ties),...
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Carbon Profile of the Managed Forest Sector in Canada in the 20th Century: Sink or Source? Jiaxin Chen,*,† Stephen J. Colombo,† Michael T. Ter-Mikaelian,† and Linda S. Heath‡ †

Ontario Forest Research Institute, Ministry of Natural Resources and Forestry, 1235 Queen Street East, Sault Ste. Marie, ON P6A 2E5, Canada ‡ USDA Forest Service, Northern Research Station, 271 Mast Road, Durham, New Hampshire 03824, United States S Supporting Information *

ABSTRACT: Canada contains 10% of global forests and has been one of the world’s largest harvested wood products (HWP) producers. Therefore, Canada’s managed forest sector, the managed forest area and HWP, has the potential to significantly increase or reduce atmospheric greenhouse gases. Using the most comprehensive carbon balance analysis to date, this study shows Canada’s managed forest area and resulting HWP were a sink of 7510 and 849 teragrams carbon (TgC), respectively, in the period 1901−2010, exceeding Canada’s fossil fuel-based emissions over this period (7333 TgC). If Canadian HWP were not produced and used for residential construction, and instead more energy intensive materials were used, there would have been an additional 790 TgC fossil fuelbased emissions. Because the forest carbon increases in the 20th century were mainly due to younger growing forests that resulted from disturbances in the 19th century, and future increases in forest carbon stocks appear uncertain, in coming decades most of the mitigation contribution from Canadian forests will likely accrue from wood substitution that reduces fossil fuel-based emissions and stores carbon, so long as those forests are managed sustainably.

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INTRODUCTION Sustainably managed forests are an important part of the global carbon cycle that can contribute to climate change mitigation by sequestering carbon through forest growth and providing fiber to produce harvested wood products (HWP) and bioenergy.1 If used in place of more energy-intensive materials, HWP can reduce fossil fuel-based greenhouse gases (GHGs)2−4 while storing carbon for a long period of time.5,6 Burning wood biomass to produce energy results in reduced fossil fuel consumption and thus reduced fossil fuel-based emissions.3,7,8 However, forest harvesting causes decreases in forest carbon stocks.7,9 Therefore, recent studies7,10−12 have pointed to the need for an integrated approach to evaluate the overall effect of forests on reducing atmospheric GHG by combining long-term forest carbon analyses, life-cycle HWP carbon emissions and removal, and the cumulative effects of using HWP to replace more energy-intensive materials and wood energy in place of fossil fuels. Canada contains 10% of the global forest area13 (397 million hectares, Mha) and has been one of the world’s largest HWP producers.14 Managed forests in Canada were estimated to cover 229 Mha during 1990−2010,13 including forests managed for commercial harvest, as parks and reserves, and as areas where natural disturbances were suppressed. We adopted the above definition of managed forest area and defined Canada’s managed forest sector to include both the managed forest area and HWP originating from the managed forests. Because it is not our intent to define or redefine what is meant by forest management, the © 2014 American Chemical Society

term forest management in this study simply refers to what is occurring on the forest area predefined as managed forests. We further defined a net carbon flow from the atmosphere to the managed forest sector as a carbon sink and a net carbon flow in the opposite direction as a carbon source. Our estimates of carbon sinks and sources in this study simply reflected these definitions. These results alone cannot be used to infer whether the managed forest sector in Canada in the 20th century provided the greatest GHG mitigation contributions because to draw such a conclusion alternative scenarios of forest management and wood uses would need to be generated and evaluated, which was beyond the scope of this study. Forest harvesting has affected less than 0.2% of the forest annually from 1990 to 201013,15 (less than 0.7 Mha per year) and an even smaller fraction prior to 1990.16,17 Deforestation, which is a land use change, has been relatively minor in Canada, with about 1 Mha of forest converted to nonforest land since 1990, resulting in emissions of 4 Tg of carbon (TgC) per year.13 Canadian HWP industries are highly efficient in utilizing harvested wood biomass, either by converting it to HWP or by burning it to produce energy.6 Therefore, the managed forest sector has several means by which it emits or stores carbon, and both natural and industrial processes are important in Received: Revised: Accepted: Published: 9859

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Figure 1. Biomass carbon conversion of harvested wood in three major life stages of harvested wood products (HWP) (Reprinted from ref 6. Copyright 2013 Ontario Ministry of Natural Resources and Forestry). Note: Major life stages of HWP include production, end use, and end-of-life disposal.

carbon stocks of discarded HWP and mill residue in landfills, dumps, and stockpiles, (c) fossil fuel-based GHG emissions from HWP production, (d) methane (CH 4 ) emissions from decomposing wood residue and HWP disposed of in landfills, (e) avoided emissions from using wood residue and collected landfill CH4 in place of fossil fuels to produce energy, and (f) avoided fossil fuel emissions from using HWP to replace energyintensive materials in residential construction. Combining the first four aspects of HWP emissions/removals produces a HWP carbon balance that covers the three major HWP life stages: production, end use, and end-of-life disposal (Figure 1), while the last two components provide an estimate of avoided fossil fuel emissions from wood substitution. Note that the avoided emissions from using mill residue-based energy to substitute for fossil fuels and purchased electricity in HWP manufacturing were included in the calculation of HWP production emissions and, therefore, were a part of the HWP carbon balance. We developed a model for the comprehensive assessments of carbon stocks and emissions for Canadian HWP (HWP-CASE), based on a carbon stock and emission analysis for major types of Canadian HWP that covered the three major life stages.6 We used HWP-CASE to simulate decadal wood carbon stocks and

determining the carbon sink or source of the managed forest sector. Previous studies either did not account for both forest and HWP carbon flows and wood substitution effects in reducing fossil fuel-based GHG emissions,15,18 or they covered a limited time period.19 Studies that focused on the reduced fossil fuel emissions from using HWP to replace other energy-intensive materials and wood energy to substitute for fossil fuels were mostly small-scale case studies or hypothetical analyses.2,3,8,9,20,21 Here we present the first national-level assessment of all main factors affecting the carbon balance of Canada’s managed forest sector for the period 1901−2010. The importance of Canada’s managed forest sector is contrasted with Canada’s fossil fuelbased carbon dioxide (CO2) emissions over the same period.

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METHODS AND MODEL DEVELOPMENT Estimating Wood Products Carbon Balance. We used a production approach to analyze HWP,22,23 which involves attributing the life-cycle carbon stocks and emissions of HWP to the jurisdiction where timber is grown and harvested, regardless of where the HWP are used and disposed of. Using this approach, we estimated (a) carbon stocks of HWP in use, (b) 9860

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since information on the fractions of solid HWP consumption among the major subcategories is lacking. Following Chen et al.,6 the fate of carbon in retired HWP was simulated by assigning HWP to retirement options of recycling, burning with or without energy recovery, composting, or disposal in open dumps or landfills (see Supplementary Figure S1, Supporting Information). When disposed of in open dumps or by composting, sufficient moisture and oxygen allow HWP to decompose aerobically and more completely,24 releasing primarily CO2.25 In landfills, a fraction of a HWP decomposes anaerobically, producing CH4 and CO2 equally; the remaining fraction decomposes extremely slowly or not at all.24 Landfill CH4 emissions were estimated as the mass of carbon in CH4 by considering CH4 generation, collection, and oxidation in landfills (see the Supporting Information, Estimating carbon stock of product in use and in landfills, Estimating carbon stock and methane emission of discarded HWP and mill residue). The estimated CH4 emissions from HWP decomposition in landfills were then converted to CO2eq (CO2 equivalent) using the 25time global warming potential of CH4 over a 100-year time horizon.26 Finally, the CO2eq of CH4 emission was divided by 3.667, the molecular weight ratio of CO2 to carbon (44/12), to convert to a carbon value for integrating CH4 with and comparing it to other components of HWP carbon balance. Significant fossil fuel-based emissions can be avoided by using wood-based energy to replace fossil fuels and by substituting HWP for more energy-intensive construction materials.2,3,20 When used to produce energy, we estimated that each tonne of carbon in wood and CH4 from decomposing HWP in landfills avoided fossil fuel emission of 0.5 and 0.9 tonnes carbon, respectively (see the Supporting Information, Emission reduction from wood substitution, Emission reduction from using wood bioenergy). For HWP used in residential construction in place of nonwood products, we produced an average fossil fuel emission reduction factor of 2.4 tonnes carbon per tonne of carbon in HWP, using references included in Sathre and O’Connor3 that analyzed only HWP used in residential construction where forest carbon was considered (see the Supporting Information, Emission reduction from wood substitution, Emission reduction from using HWP in residential construction). Indirect GHG effects from substituting Canadian HWP for nonwood materials in residential construction, such as market prices and global production of HWP and alternative nonwood materials, were not considered. Carbon stocks and CH4 emissions of retired HWP disposed of in landfills were estimated in HWP-CASE and thus were excluded from the above emission reduction factor to avoid double counting. Timber harvest results in reduced forest carbon stocks.7,9 When using a single emission reduction factor described above, without analyzing forest carbon dynamics of harvest and nonharvest scenarios, the avoided emissions in this study were considered as potential in reducing atmospheric GHG that would only be fully achieved after the carbon stock of the harvested forest regrows to the level it would have had had the forest not been harvested. Because of the temporally dynamic feature of the atmospheric GHG reductions from wood substitution, we separately report them from the decadal and cumulative forest and HWP carbon stock balance. We estimated the avoided fossil fuel emissions for HWP used in residential construction in the period 1951−2010, assuming 64% of HWP used in wood-based house replaced nonwood materials for building concrete- and steel-based houses.2 We did not estimate potential substitution effects for the 46% of

emissions from manufacturing HWP in Canada from 1951 to 2010 (see the Supporting Information, Conversion from industrial roundwood to harvested wood products). We compiled data of industrial roundwood harvesting (in terms of merchantable volume) in Canada during this period (see Supplementary Table S1, Supporting Information), which was expanded to include bark as well as undersized trees that were also harvested (see the Supporting Information, Conversion from industrial roundwood to harvested wood products, Estimating total harvested wood). The resulting total industrial roundwood volume was converted to carbon using a factor of 0.25, assuming 1 m3 dry wood has a mass of 0.5 tonne and wood is 50% carbon by mass3. Decadal carbon stocks and emissions of HWP produced in Canada between 1901 and 1950 are provided in the Supporting Information (Estimating wood product carbon balance, 1901− 1950). The cumulative carbon stocks and emissions in 1950 were used as the initial conditions for simulating 1951 to 2010 forest carbon stocks and emissions. Major HWP types simulated in HWP-CASE include lumber, structural panels, nonstructural panels, pulp, and paper and paper products. The amount of harvested wood used in producing various types of HWP and used as mill residue to produce energy, and the amount of residue disposed of in landfills or stockpiles was simulated using the fractional shares of industrial roundwood carbon immediately after manufacturing6 (see the Supporting Information, Conversion from industrial roundwood to harvested wood products, Allocating harvested wood carbon to products and residue). Production emissions in HWP-CASE consist of fossil fuel emissions from forest harvesting, wood transportation, HWP manufacturing, and the indirect emissions of purchased electricity consumed by HWP industries, estimated using production emission factors developed by HWP type (see the Supporting Information, Conversion from industrial roundwood to harvested wood products, Production emissions of Canadian harvested wood products). To estimate in-use HWP carbon stocks, solid HWP, e.g., lumber and wood panels, were divided among four end-use categories, single-family house construction, multifamily house construction, residential repair and remodelling (together as residential construction), and other uses (e.g., nonresidential construction, furniture manufacturing), using the end-use matrix we developed for Canadian-made solid HWP (Supplementary Table S5, Supporting Information). Pulp, paper, and paper products were treated as a broad end-use category. Because we used a production approach, end uses of exported Canadian HWP and their carbon stocks and emissions were also included. Carbon retained by in-use Canadian HWP was estimated for each of the above end-use categories using Supplementary Equation S1 (Supporting Information). We used HWP half-lives estimated for the United States for all Canadian HWP (see the Supporting Information, Estimating carbon stock of product in use and in landfills, Estimating carbon stock of HWP in use), including exports from Canada to other countries, because Canada-specific data were lacking and the United States has been the most important consumer of Canadian HWP, with 91 and 86% of Canada-made solid HWP and pulp and paper products consumed in the United States and Canada.6 The “other uses” category accounted for 46% of Canadian solid HWP;6 breaking this broad category into subcategories would improve this carbon balance analysis. However, such a breakdown was not possible 9861

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Figure 2. Carbon balance of Canada’s managed forest sector, 1901−2010 (teragrams of carbon, TgC). (a) Decadal carbon emissions and removals from Canada’s managed forests, harvested wood products, and decadal fossil fuel-based emissions in Canada since 1901. (b) Cumulative carbon emissions and removal of Canada’s managed forest sector. Negative and positive values indicate sinks and sources of atmospheric carbon, respectively. Note: Fossil fuel emissions from manufacturing harvested wood products in Canada were deducted from the national fossil fuel emissions to avoid double counting; mitigation potential through wood substitution was not included.

managed forest area for the period 1990−2010 from previous studies.13,15 The carbon balance analysis for Canada’s total forest area estimated by Chen et al.17 and Kurz and Apps16 were prorated based on area to estimate carbon stock changes for the 229 Mha managed forest area for the periods 1901−1920 and 1921−1989, respectively. To do so, we assumed that rates of forest carbon stock change were similar for the managed forest area and unmanaged forest area in historical periods.17 This assumption was considered valid because the historical harvest rate in Canada was low, affecting only 0.2−0.3% of the managed forest area annually during the period 1901−1989,13,15−17 and before 1950, tree planting and other silviculture practices in Canada were limited.27−29

Canadian-made solid HWP that was consumed by other end uses,6 because of the lack of data, such as the annual fractions of solid HWP consumed by the subcategories in this broad “other uses” category (e.g., nonresidential construction, furniture, railway ties), fractions of solid HWP in these subcategories that have been used to substitute for nonwood materials, and the life-cycle assessment of HWP and nonwood materials in these end-uses. HWP substitution effects in reducing fossil fuel-based emissions prior to 1950 were not estimated because we assumed timber harvest was significantly less in Canada between 1901 and 1950,17 while the conversion rate from timber to solid HWP was considerably lower6 and the use of nonwood construction materials for wood frame house construction was limited in North America. Estimating Forest Carbon Balance. We evaluated carbon stocks in Canada’s current managed forest area retrospectively to 1901 based on the 229 Mha of managed forest area in 1990− 2010,13 which allowed us to evaluate forest carbon for a stable forest area. We obtained the carbon balance of Canada’s

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RESULTS Carbon Balance of Canada’s Managed Forest Sector, 1901−2010. Between 1901 and 2010, Canada’s managed forest sector was estimated to be an overall carbon sink of 8359 TgC (Figure 2b), exceeding the 7333 TgC of Canadian fossil fuel 9862

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emissions accumulated over the same period30 (fossil fuel emissions were converted to carbon). In particular, Canada’s 229 Mha of managed forest area sequestered an additional 7510 TgC (excluding HWP carbon balance), with an average of 0.3 tonnes carbon per hectare per year. Temporal variations in carbon sink/ source behavior are evident as shown in Figure 2a, with forests estimated to be a source during three decades (1901−1910, 1981−1990, and 2001−2010) but otherwise were a carbon sink. Over a 50-year period beginning in 1921, decadal additions to forest carbon ranged from 1250 to 1565 TgC. In contrast, since 1980, Canada’s managed forests fluctuated from being a small sink to a small source of atmospheric carbon, with the carbon stock declining 60.6 TgC between 1981 and 2010 (Figure 2a). In addition to the 7510 TgC increase in forest carbon stock from 1901 to 2010, Canada’s HWP increased in carbon balance by 849 TgC (Figures 2 and 3), which is the sum of the carbon

Table 1. Cumulative Carbon Stocks of Harvested Wood Products (HWP) in Use for Those Produced by Canadian HWP Industries, 1901−2010 (Teragrams of Carbon)a−d solid HWPs

decade

single family house

multifamily house

residential repair and remodelling

other uses

paper

total

1 2 3 4 5 6 7 8 9 10 11

6.2 12.1 17.9 28.7 46.0 61.9 91.4 131.4 175.4 220.9 256.0

0.8 1.6 2.4 3.7 5.8 8.0 12.1 17.2 22.3 27.0 30.3

0.6 1.6 2.8 5.1 9.1 13.6 24.9 43.4 67.1 95.5 118.4

2.5 5.0 7.5 12.0 20.0 25.6 43.1 71.9 111.3 164.8 211.2

2.7 5.3 8.0 11.0 16.5 17.4 29.7 41.5 59.9 84.0 127.3

12.8 25.7 38.5 60.5 97.4 126.5 201.2 305.4 436.0 592.2 743.2

a

Values are cumulative carbon stocks of HWP by end-use category at the end of a decade. bCarbon stocks of each end-use category were assumed to increase linearly from zero in 1900 (Intergovernmental Panel on Climate Change24) to the values in 1930 (Apps et al.18) (decades 1−3). cCarbon stocks of solid HWP used for construction and other uses and carbon stocks of paper and paper products from 1930 to 1950 were estimated from Apps et al.18 (decades 3−5). d Values for 1951 to 2010 were the simulation results of the present study based on the carbon stocks in 1950 and forest harvest volume from 1951 to 2010 (decades 5−11).

Figure 3. Carbon balance of Canada’s harvested wood products (HWP) (teragrams of carbon, TgC), 1901−2010. Note: Mitigation potential through wood substitution was not included.

stock increase of in-use Canadian HWP (743 TgC) (Table 1) and discarded mill residue and retired HWP (782 TgC) (Figure 3), minus HWP production emission (300 TgC) (Supplementary Table S7, Supporting Information) and landfill CH4 emissions (376 TgC, converted from CO2eq) accumulated over the period (Figure 3). In-use HWP carbon stock (743 TgC) included 405 and 211 TgC of solid HWP used in residential construction and all other uses, respectively, and 127 TgC contained by in-use paper and paper products (Table 1). The carbon dynamics of discarded mill residue and retired HWP is very important to HWP life-cycle carbon balance.5,6 Figure 4 illustrates the carbon balance of discarded mill residue and retired Canadian HWP for the period 1901 to 2010 (406 TgC), calculated as the sum of 363, 349, and 69 TgC stored in industrial landfills, municipal landfills, and open dumps and stockpiles, respectively, minus 163 TgC of industrial landfill CH4 emissions in the period 1901−2010 and 213 TgC of CH4 emission from municipal landfills accumulated from 1941− 2010 (converted to carbon from CO2eq). Note that the 100-year global warming potential of CH4 was used when estimating landfill CH4 emissions. The use of solid HWP in residential construction in the period 1951−2010 was estimated to reduce 790 TgC of fossil fuel emissions (Table 2). We also estimated an additional 4 TgC emission reduction provided by using collected landfill CH4 to

Figure 4. Carbon balance of discarded Canadian harvested wood products and mill residue (teragrams of carbon, TgC), 1901−2010.

produce energy from 1981 to 2010. The use of mill residue-based energy was estimated to reduce 279 TgC of HWP production emission for the period 1951−2010 (Table 2), which was included in calculating HWP production emissions and thus should not be interpreted as additional contributions. A sensitivity analysis identified important modeling parameters/variables in HWP-CASE that were then used in an uncertainty analysis (see the Supporting Information, Uncertainty analysis for the carbon balance of Canada’s harvested wood products, 1951−2010, Sensitivity analysis). Uncertainty was assessed by selecting the most influential parameters affecting the carbon balance of Canadian HWP and their contribution in reducing fossil emissions through material substitution (Supplementary Table S8, Supporting Information) 9863

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Table 2. Decadal Emission Reduction from Using Harvested Wood Products (HWP) To Replace Other Energy-Intensive Materials in Residential Construction and from Using Wood Residue in Place of Fossil Fuels To Produce Energy, 1951-2010 (Teragrams of Carbon, TgC) HWP used in construction emission reductiona wood biomass used to produce energy emission reductionb

1951−1960

1961−1970

1971−1980

1981−1990

1991−2000

2001−2010

total

31.2 48.0 25.2 13.6

57.9 89.1 47.9 25.8

84.5 130.0 74.2 40.0

103.6 159.4 104.3 56.4

121.2 186.4 128.3 70.0

114.8 176.6 138.9 77.5

513.2 789.5 518.8 283.3

a

Calculated using the emission reduction factor of 2.4 tonne of carbon per tonne of carbon contained in solid HWP used in construction in each decade, based on an assumption that 64.1% of the HWP used in residential construction replaced more energy-intensive materials.2 bCalculated using the emission reduction factor of 0.5 tonne of carbon per tonne of carbon in wood biomass burned to produce energy, including the cumulative emission reduction of 3.7 TgC from using collected landfill methane to generate energy in the last four decades.

and obtaining their high/low boundary values from published literature (Supplementary Table S9, Supporting Information). Boundary values were used to estimate the potential ranges of the carbon balance of Canada’s HWP and avoided emission from wood substitution for the period 1951 to 2010. Based on the estimated carbon balance of 193 TgC in 1950, the carbon balance of Canadian HWP in 2010 was between 429 and 1354 TgC, with 849 TgC estimated using average parameter values. The total avoided emissions from using landfill CH4 to generate energy, and from using HWP in residential construction, ranged from 136 to 1258 TgC, with an average estimate of 794 TgC.

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DISCUSSION

While not included in our analysis, Canada’s unmanaged forest area was also a medium to large carbon sink during the period 1901−1980 because we estimated managed forest carbon stocks as a proportion of the carbon sink in the total forest area reported by others.16,17 From 1981 to 2010, the unmanaged forest area likely fluctuated from being a small source when natural disturbance was severe to an otherwise small sink, as estimated in previous studies15−17 for managed and unmanaged forests in Canada. Fire frequency in Canada’s forest was high in the late 19th century,31 and fires, some from natural causes but mostly humaninduced, burned large areas of forest.27 Timber harvesting, as well as land use change that converted some forestlands to agricultural lands by European settlers, were other main factors driving Canada’s forests to a reduced forest land area and a younger forest age structure at the end of the 19th century,27 resulting in a source of 30 TgC per year between 1895 and 1910.17 The fire frequency in Canadian forests decreased in the 20th century, largely due to climate,31,32 and since about 1960 has been further reduced by improved fire suppression.27,32 From 1921 to 1980, the low disturbance rates, combined with forest regrowth in areas disturbed in the late 19th century as well as low rates of land use change from forest to nonforest, allowed Canada’s forests to sequester a significant amount of carbon.17 Therefore, although the large forest carbon sink in the period 1921−1980 occurred on the managed forest area, it was mainly due to natural growth of younger forests. From 1981 to 2010, the total carbon stock on this area had been relatively stable. Looking ahead, future increases in Canadian forest carbon stocks appear uncertain. Forest fires result in large direct forest carbon emissions, and insect outbreaks reduce growth rates and kill trees over time, transferring live tree carbon to dead organic carbon pools that release carbon through decomposition. As illustrated in Figure 5, timber harvest in Canada remained at a relatively low and stable level in the period 1990−2010, which tended to have low and

Figure 5. Area of natural forest disturbance and forest harvesting in Canada (million hectares per year, Mha yr−1) in relation to forest carbon balance (teragrams of carbon per year, TgC yr−1), 1990−2010 (produced on the basis of Stinson et al.15 and Natural Resources Canada13).

stable impacts on forest carbon stocks.33 In comparison, natural forest disturbance varied significantly among years, and severe natural disturbances, in addition to forest harvest, often drove Canada’s forests to be a carbon source.15 Based on previous studies,16,17 similar conclusions can be drawn for 1901−1990. In the 21st century, it is anticipated that climate change will cause forests in Canada to be increasingly subject to natural disturbances, potentially leading to large carbon releases and making Canada’s forests a large carbon source.15,33,34 Although forest harvesting in Canada has remained at a relatively low level, without harvesting Canada’s forests might have been an even larger carbon sink over the 20th century. The greatest harvesting-induced forest carbon reductions would have occurred in the tolerant hardwood forests of eastern Canada and the coastal coniferous rainforests of British Columbia, where forests of long-lived species in the latter forest type are associated with very long natural disturbance cycles.35 In such forests, harvesting old-growth stands produces a large carbon stock reduction.36,37 In contrast, in the boreal forest that accounts for 77% of Canada’s forest area, natural fire has been the dominant driver of forest carbon change, especially in central and western Canada.38,39 In such landscapes, if fire suppression reduces area burned, then some harvesting of shorter-lived boreal species may result in a landscape with more forest carbon stocks, compared to a fire-prone natural landscape without fire suppression.9 The introduction of modern forest fire suppression in Canada in the 1960s may be responsible for reducing the area that otherwise would have burned,40,41 during the same period in which 9864

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likely also resulted in underestimated avoided emissions, since some of these HWP were used in place of more energy-intensive materials.44,45 National scale studies such as described here, with improved analysis of forest and HWP carbon balance and wood substitution effects, could result in better understanding of the contributions of forest sector to climate change mitigation, if complemented by comparisons to alternative forest management and wood use scenarios. This in turn may lead to improved policies and practices in managing these aspects of global carbon cycling.

harvesting of Canadian forests increased. If equal areas of forest (of similar age and species) were harvested as would have burned, a shift from natural to anthropogenic disturbance could have reduced forest carbon emissions to the atmosphere, in effect trading-off natural emissions from combustion in forest fires with removal of forest carbon by harvesting.42 The objective of tying the area available for harvesting to long-term reductions in area burned is considered a trade-off to achieve ecological objectives in disturbance prone forests.9,31,43 Such landscape objectives, although established to achieve ecological and timber supply goals, can contribute to reduction of atmospheric GHG if the net difference in forest carbon stocks between burned and harvested forests over time, plus the carbon balance of HWP, produce a net removal of carbon from the atmosphere. The carbon stock of HWP, unlike that of forests, is largely immune to accidental emissions,5,11,23 but the HWP carbon stock can decline if the carbon output from HWP in use and in landfills exceeds the carbon input,4 which can happen if forest harvest declines significantly. In comparison, the avoided fossil fuel emissions from material substitution are permanent and cumulative,3,7,20 although there will be a delay to fully achieve the mitigation contribution until the forest regrows.8 Because the maximum atmospheric GHG reductions from wood substitution are dependent on forest regrowth, the uncertainty in forest carbon stocks influences the reliability of the substitution effects. The decline in the combination of the carbon balance of Canada’s managed forest sector and fossil fuel CO2 emissions since 1970 (Figure 2b) was a result of large increases in fossil fuel combustion. The increased life-cycle carbon balance of Canadian HWP from 1901 to 2010 was equivalent to removing 12% of Canada’s total fossil fuel emissions in the period 1901−2010. Once fully achieved after the harvested forest regrows, the mitigation contribution from using HWP in residential construction in the period 1951−2010 will equal 11% of Canada’s total fossil fuel emission in the period 1901−2010. Since these results were obtained largely without policies, incentives, or objectives for reducing emissions from HWP production or use, or for forest management practices to maximize forest carbon, it is likely that the increase in the carbon balance of the Canadian managed forest sector and the avoided fossil fuel emissions from HWP substitution reflected the lower end of their potential. Canada’s HWP industries have been highly successful in achieving high utilization efficiency of wood removed from the forest, with current utilization of more than 90% of biomass in HWP or for bioenergy.6 Areas where improved emissions reduction can be achieved include: increased HWP use in construction to replace more emissions intensive materials; recycling and reuse of retired HWP before eventually burning them for energy or placing them in landfills; increased capture of CH4 at landfills; greater use of energy from methane combustion to replace fossil fuel energy; and the use of more intensive silviculture, where ecologically appropriate, to promote rapid forest regeneration, and increase forest growth rates.42 One area where the increase of HWP carbon balance may be greater than reported here is the carbon stocks of in-use HWP, estimated using the equation that exponentially decreased HWP in use. Using this method, 28% of HWP used in residential construction was estimated as retired after 40 years, which is probably higher than what actually has occurred.43 Overestimating the early retirement of HWP results in not only underestimated carbon stock of HWP in use, but also overestimated landfill CH4 emissions. Ignoring the 46% HWP used for all “other uses”, as well as HWP produced prior to 1950,

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ASSOCIATED CONTENT

* Supporting Information S

Additional details on (a) estimating the life-cycle carbon stocks and emissions of Canadian HWP in the period 1901−2010 and (b) uncertainty analysis for the carbon balance of Canadian HWP. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (705) 946-7486; Fax: (705) 946-2030; e-mail: jiaxin. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Lisa Buse and Trudy Vaittinen, both from Ontario Forest Research Institute, Ontario Ministry of Natural Resources and Forestry: Lisa reviewed and commented on an early version of this manuscript, and Trudy helped to produce the artwork.

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REFERENCES

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