Carbon budget of the Canadian forest product sector

Environmental Science & Policy 2 (1999) 25±41

Carbon budget of the Canadian forest product sector M.J. Apps a, *, W.A. Kurz b, S.J. Beukema b, J.S. Bhatti a a

Natural Resources Canada, Canadian Forest Service, Northern Forestry Centre, 5320-122 Street Edmonton, Alberta, Canada T6H 3S5 b ESSA Technologies Ltd., 3rd Floor, 1765 West 8th Avenue, Vancouver, BC, Canada V6J 5C6

Abstract Although many factors in¯uencing the forest C cycle are beyond direct human control, decisions made in forestry and the forest product sector (FPS) can either mitigate or aggravate the net C balance of terrestrial ecosystems. The Canadian Budget Model of the Forest Product Sector (CBM-FPS) described here, was designed to work with a national scale model of forest ecosystem dynamics (the Carbon Budget Model of the Canadian Forest Sector, CBM-CFS). The CBM-FPS accounts for harvested forest biomass C from the time that it enters the manufacturing process until it is released into the atmosphere. It also accounts for the use and production of energy by the FPS, and emission of CO2 during FPS processing. The CBM-FPS accounting framework uses the characteristics of di€erent forest product types to estimate changes in the storage of C in forest products; it tracks C from the transportation of the harvested raw material through various processing steps in sawmills or pulp mills, to its ®nal destination (product, pulp, land®ll, atmosphere or recycled). Because not all harvested biomass C is released into the atmosphere in the year it is harvested, the model tracks C retained in various short- and long-lived products, and in land®lls. Model results are in general agreement with available data from 1920±1989. Average changes in net C stocks in the FPS, estimated as the di€erence between harvest C input to the FPS and total losses from the forest product sector is estimated to be 23.5 Tg C yr ÿ 1 for the 1985±1989 period. The total FPS pool size at the end of this period is estimated to be 837 Tg C, of which only a fraction (32%) is retained in Canada. The total FPS C stock is small compared to that in the forest ecosystems from which they derive (estimated to contain 86 Pg C in 1989). Nevertheless, the changes in these C stocks contribute signi®cantly to a reduction of the total net atmospheric exchange of the total forest sector (ecosystem and product sector) for that period. # 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Carbon balance; Forestry; Model; Canada; Bioenergy; Emissions; Land®ll

1. Introduction Over the next 50 years, carbon (C), along with other greenhouse gas (GHG) emissions resulting from anthropogenic activities, are projected to lead to important changes in the global climatic system (Manabe and Wetherald, 1986; Schneider et al., 1992; IPCC, 1995). Increases in global mean surface temperatures of 1.5±4.58C, a rise in sea level between 13 to 94 cm, changes in global precipitation and global evapotranspiration of 3±15 and 5±10%, respectively and average * Corresponding author. Tel.: +1-403-435-7305; fax: +1-403-4357359; e-mail: [email protected]

decreases in summer soil moisture are expected to have widespread impacts on the human habitat (Woodwell et al., 1998). The primary anthropogenic sources of GHGs, of which CO2 is the most important, are associated with land-use change, fossil fuel combustion and industrial activities such as cement production (IPCC, 1992; IGBP, 1998). The most important contribution of land-use change to the global GHG balance is deforestation, which presently accounts for ca. 20% of the net annual anthropogenic CO2 emissions to the atmosphere (Houghton et al., 1992; Houghton, 1996; Woodwell et al., 1998). Land-use practices, such as forestry operations, also contribute to the net exchange of GHG with the atmosphere, but the contribution can be either positive or negative.

1462-9011/99 $ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 2 - 9 0 1 1 ( 9 9 ) 0 0 0 0 6 - 4

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M.J. Apps et al. / Environmental Science & Policy 2 (1999) 25±41

It is generally assumed that under sustainable forest management practices, in which logging withdrawals do not exceed net forest biomass increment, the net exchange of CO2 with the atmosphere is negligible (Houghton et al., 1992). Apps and Price (1996) pointed out, however, that changes in vegetation biomass alone do not provide a complete indicator of the net in¯uence of forest sector activities. Changes in C storage in other components of the total system (speci®cally changes in C stocks of litter, soil, forest products and fossil fuel deposits that may be a€ected by forest sector energy-use) must also be taken into account (see Fig. 1 in Apps and Price, 1996). Forestry practices, including logging, clearly also in¯uence forest ecosystem C stocks, but the resultant e€ect on the global C cycle includes more than change in forest biomass. To account for the full e€ect of forestry activities on the net exchange of C between managed forest ecosystems and the atmosphere, changes in all C pools in¯uenced by forestry activities need to be estimated (Apps et al., 1997; IGBP, 1998). Although many studies have been reported that examine factors a€ecting the C balance of forest ecosystems (Prentice et al., 1992; Neilson et al., 1992; Parton et al., 1993; Nabuurs et al., 1997; Peng et al., 1998), most exclude the dynamics of FPS C stocks. The results of such partial-system studies may lead to incorrect conclusions about the role of the forest sector in the terrestrial C budget (IGBP, 1998). Guidelines for estimating national GHG inventories for land-use change and forestry have assumed that wood products are oxidized immediately upon harvest and do not account for net accumulations of FPS C stocks (IPCC, 1997). The Kyoto protocol (UNFCCC, 1997) has generated considerable interest in revising these assumptions to better account for anthropogenic in¯uences on terrestrial C budgets (IPCC, 1997; IGBP, 1998; Schlamadinger and Marland, 1998). The relative magnitude of global estimates of C stocks in forest ecosystems and wood products might suggest that wood products make an insigni®cant contribution to the C budgets of forest ecosystems and o€er limited potential for C sequestration. Such conclusions, however, depend strongly on the accounting system used and the scope of the system considered (Matthews et al., 1996; Apps et al., 1997; IGBP, 1998). The amount of C stored in the FPS depends not only on the management of the forests but also on the ¯ow of C from harvest to ®nal disposition. The role of wood products in sequestering atmospheric C has been demonstrated in a number of country-scale studies (Harmon et al., 1996; Pussinen et al., 1997; Cannel and Dewar, 1995; Skog and Nicholson, 1999) and at the scale of regions or individual stands (Nabuurs, 1996; Marland and Schlamadinger, 1997; Winjum et al., 1998). Most of these studies have concluded that

FPS C stocks are a€ected signi®cantly by the end use, half-life and decay rate of C in land®lls and long-lived products. In Canada, the work of Kurz et al. (1992) suggests that the FPS could sequester signi®cant C for long periods of time. The present paper examines in more detail the contribution of forest products to the C balance of Canadian forests and forest sector activities. Forest sector C stocks and ¯ows can be divided into two major groups associated with the forest ecosystems themselves and the FPS. Here, the focus is on estimating the latter component, but the recently reported results of Kurz and Apps (1999) (hereafter referred to as K&A) for Canadian forest ecosystem C stocks and ¯ows are used to provide perspective. Forestry operations remove C from forest ecosystems and transfer this C to the FPS. Within the FPS, C is retained in forest products, transferred to land®lls, or returned to the atmosphere by oxidation products through decomposition or combustion. Carbon releases and emissions from the FPS include (i) decomposition and combustion of harvested biomass during various product processing steps (including use of biomass for energy production), (ii) decomposition and combustion of products. Residues and waste in land®lls and (iii) C emissions associated with energy use within the sector. Indirect o€sets of fossil fuel C emissions, through the substitution of wood products for other materials that generate large emissions of C in their production (Marland and Marland, 1992; Matthews, 1996; Schlamadinger and Marland, 1996) are recognized to be very important but beyond the scope of the present paper and are not accounted in the CBM-FPS simulations reported here. The potential to mitigate against human-induced climate change through land-use management options that increase C uptake and storage in biological systems is of great contemporary interest and vigorous debate (IPCC, 1995; IGBP, 1998). For Canada, forests play a central role in these discussions. Many factors controlling the forest sector C cycle, including climate change and other changes in the global environment, are clearly beyond the direct control of national policies. Nevertheless, changes in C stocks in the forest sector are directly (but not exclusively) a€ected by forest management and national forest policy. FPS activities, in particular, may provide potentially important opportunities to mitigate the net exchange of C with the atmosphere at a national scale. There are four basic mechanisms by which mitigation bene®ts may be achieved: (i) improved C storage in forest ecosystems, (ii) longer C storage in forest products, (iii) use of biofuels to displace fossil-fuel use and (iv) use of wood products in place of other products that generate higher GHG emissions for their manufacture.

M.J. Apps et al. / Environmental Science & Policy 2 (1999) 25±41

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Fig. 1. Flow diagram showing the fate of harvested biomass as accounted in the CBM-FPS. Rectangular boxes represent C stocks with di€erent turnover times, ovals and rounded rectangular boxes represent processing steps or intermediate stocks (internally-accounted for diagnostic purposes) and cloud symbols are emissions to the atmosphere. Arrows represent the ¯ow and transfer of C between the di€erent components.

As Canadian forests account for 10% of the world's forested area, the net budget of C exchanges between the Canadian forest sector and the atmosphere can have a signi®cant impact on the global C cycle. Since ca. 1970, Canada's forests have been subjected to increases in natural and anthropogenic disturbances (Kurz et al., 1995). Kurz and Apps (1999) report that these changes in the disturbance regime have resulted in a change in Canada's forest ecosystems; the net C accumulation (about 225 Tg C yr ÿ 1) for the period

1920±1970 appears to have become a small net C loss (about 75 Tg C yr ÿ 1) by 1989. In that work, K&A focus on the changes in forest ecosystem C stocks and although they account for withdrawal of C from the forest by harvest disturbance, they do not account for its fate in the FPS. The data reported in Kurz et al. (1995) indicate that both the area annually harvested and the observed changes in this disturbance rate over time, are small compared to the areas a€ected by natural disturbances (wild ®re and insect-induced

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M.J. Apps et al. / Environmental Science & Policy 2 (1999) 25±41

mortality) and the recent changes in these natural disturbance rates. K&A also point out that not all of the 75 Tg C yr ÿ 1 reported lost from forest ecosystems by 1989 was released to the atmosphere Ð some is retained in the FPS. Earlier work (Kurz et al., 1992) suggests that the net C sequestration in the FPS (net harvest C input minus all losses from accumulated FPS C stocks) is not insigni®cant, yielding a net accumulation of 23 Tg C yr ÿ 1 in the FPS in a nominal reference year (1986). The primary objectives of the present paper are to (1) describe the CBM-FPS model and (2) to examine the historical changes in C stocks, energy use and bioenergy production in the Canadian FPS. The relative importance of these C stocks and changes to the Canadian forest sector (forest ecosystem and products) at the national scale are explored using K&A's recently reported results for Canadian forest ecosystems. The CBM-FPS model is described in detail in Section 2. Particular emphasis is given to the C ¯ows and emissions in the manufacture of pulp and paper products because of their importance in the FPS of Canada (Canadian Council of Forest Ministers, 1993). The model presented here is based on the original forest products module described in Kurz et al. (1992), but has been extensively revised and new data incorporated in its simulations. The reader is, however, referred to the earlier reference (Kurz et al., 1992) for further details that are not repeated here. In the present study, the CBM-FPS is used as a stand-alone model to estimate changes in FPS stocks and emissions over the period 1920±1989, using the historical data for harvest inputs. The FPS analyses presented are limited to the period 1920±1989 by the availability of data from the CBM-CFS analyses reported by K&A, which are used to provide a forest sector perspective (ecosystem plus products). A description of the CBM-CFS, together with changes in the forest ecosystem C stocks for the period 1920±1989, has been given elsewhere (K&A and references therein) and will not be repeated here. Suce it to say that the CBM-CFS tracks all C transfers among C stocks in forest biomass, litter and soils, releases to the atmosphere and withdrawals to the FPS associated with stand-replacing disturbances (®re, insect-induced mortality and harvesting); it accounts for stand-scale forest regrowth of biomass (above and below ground) following disturbance and changes with stand age, and for the decomposition of dead organic matter in litter and soils. 2. The Canadian forest product sector model (CBMFPS) The CBM-FPS tracks freshly harvested biomass C from harvest, through its processing into and transfer

amongst di€erent FPS pools (stocks) until its eventual release into the atmosphere through decomposition or combustion. The model follows the fate of C in the end-use products and accounts for all organic residues generated during the primary and secondary processing of harvested biomass. In the primary processing phases, these residues are in the form of bark, chips and shavings generated in sawmill and solidwood processes as well as organic debris retained in sludge and spent pulp mill liquors associated with pulp and paper processes. Carbon stocks and ¯uxes associated with silvicultural operations are accounted in the forest ecosystem model CBM-CFS (see K&A and references therein for details). Changes in C stocks and direct C emissions in FPS primary and secondary manufacture are accounted in the CBM-FPS, including the use of external energy sources and the production of energy through combustion of biomass residues (bioenergy). Pulp production plays an important part in Canada's forest industry Ð by 1989, C in harvested pulpwood was approximately one third of that of sawlogs destined for solidwood products. For this reason, the CBM-FPS includes a detailed analysis of C ¯ows and emissions in the manufacture of pulp products. Fig. 1 shows the division of the commercial harvest feedstock into solidwood and pulp production streams and the mass-transfers within the FPS (shown by arrows). A small component of the commercial harvest is also allocated to fuelwood. Only the commercial harvest is considered: noncommercial domestic harvest and use of fuelwood is not represented in either CBMCFS or CBM-FPS (the e€ect of such harvest and use is believed to be insigni®cant in Canada today). Recycling of organic C in primary products and the end-fate of all organic residues are explicitly represented by the transfers shown in Fig. 1. The original FPS submodel reported in Kurz et al. (1992) did not separately account for export and import of wood and pulp products. This accounting has been added to the present CBM-FPS and information is now retained on the proportion of harvested C that ends up in forest products exported from Canada. The model also accounts for C that is imported as solidwood products (a small contribution) and as primary or secondary (recycled) feedstock to the FPS as indicated in Fig. 1. Fresh harvest biomass input (in units of Tg C or Tg C yr ÿ 1) to the Canadian FPS may be obtained from the ecosystem model (CBM-CFS) or other scenario model results, or from recorded data (Canadian Forest Service, 1988; Canadian Council of Forest Ministers, 1993; Canadian Pulp and Paper Association, 1994). This ¯exibility permits both historical and projective analyses of the role of the forest sector in the national C budget. It thus facilitates `whatif ' and sensitivity analyses aimed at exploring the consequences of di€erent forest sector options Ð analyses

M.J. Apps et al. / Environmental Science & Policy 2 (1999) 25±41

Fig. 2. Characteristic retention curves for the four C stock categories represented in the CBM-FPS.

of increasing policy relevance with the establishment of the Kyoto Protocol (UNFCCC, 1997). The C pools shown in Fig. 1 account for product C in di€erent age cohorts in four general classes of forest products, distinguished by their C retention time. The `slow' pool represents lumber and other products that remain in use for long time periods (e.g. structural building products). The `fast' pool contains products used for a shorter period of time (e.g. packaging or lumber used temporarily during construction). The P&P (pulp and paper) pool contains all pulp and paper products. The `land®ll' pool contains all discarded biomass C resulting from various production steps or from the disposal of forest products after accounting for waste combustion. At each time step (usually annual), the CBM-FPS generates new cohorts for each pool in Fig. 1 and ages the other cohorts in each pool using its characteristic retention curves (Kurz et al., 1992). These C retention curves describe how much of the product in that age cohort remains in the pool (Fig. 2). As the pools age, losses of C occur due to decomposition, burning, recycling or transfer of that material to the land®ll pool. It is assumed that material that enters land®ll remains there until it has decomposed. Throughout the model, atmospheric releases of C in di€erent forms (e.g. CO2 and CH4) are separately accounted but this feature of the model has not yet been fully tested and is not reported here. The following paragraphs provide detailed descriptions of the forest product C pools, accounting of energy use and production within Canada's FPS and the estimation of emissions from within the product sector. 2.1. Solidwood product C pools and fuelwood The processing of logs, bolts and other roundwood (the upper left stream in Fig. 1) produces short- and long-lived products. The processing generates residues,

29

some of which decompose during the processing or are recycled, transferred to land®lls, burned as waste and burned to provide energy. The processing of logs and bolts also produces chip residues that provide an important input for the pulpwood stream. The `slow' pool represents C in lumber and forest products that remain in use for long time periods (such as those used in buildings and other structures). The `fast' pool represents non pulp products that remain in use for a shorter period of time (e.g. packaging or lumber scraps) before undergoing disposal (into land®ll or combustion) or recycling into a secondary product. Fuelwood in the CBM-FPS is restricted to commercially harvested wood that is designated for energy production. Additional energy production through combustion of processing residues and discarded forest products is accounted elsewhere in the model (Fig. 1). Historical data on non-commercial (domestic) harvest and its use as ®rewood are not available at a national scale. Such data as do exist (Canadian Council of Forest Ministers, 1993), together with Canada's low population density and easy access to alternative energy supplies from hydroelectric and fossil fuel sources, indicate that such use may be neglected without signi®cant error. For this reason, the fuelwood category in the CBM-FPS does not include unregulated domestic use of ®rewood. Fuelwood is assumed to be completely consumed in the year of harvest and to result in a complete release of C to the atmosphere. Thus in the CBM-FPS model, there is no accounting of C storage in a `fuelwood' pool, no characteristic retention curve and no recycling or secondary transfer of spent fuelwood C. 2.2. Pulp and paper (P&P) C pools The processing of harvested forest biomass destined for pulp and paper production is shown in the second stream at the top of Fig. 1. The P&P pool contains all C in products associated with pulp and paper production. The characteristic retention curve (Fig. 2) for this pool has an initial rapid decline (representing the high turnover of most pulp and paper products) followed by a smaller long-term tail (Kurz et al., 1992). Modi®cations made to the FPS model since Kurz et al. (1992) provide much more detailed accounting of C ¯ows and energy use in the processing of pulpwood. The pulpwood input-stream is ®rst converted into chips, accounting for losses in the debarking process (with the residues burned as hog fuel for within-sector energy use). Chip residues from the solidwood processing stream are used to augment the pulpwood inputstream. As shown in Fig. 1, imports and exports of chips are now also accounted in the primary pulp processing stream (a modi®cation since Kurz et al., 1992).

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M.J. Apps et al. / Environmental Science & Policy 2 (1999) 25±41

During primary processing, the chips are divided into 11 di€erent processing types that include both chemical and mechanical pulp processing and the treatment of recycled materials. These treatments are accounted separately as they have di€erent production eciencies (pulp output/chip input) and di€erent energy requirements. Residues at each stage of processing are burned (as spent liquor for energy or as waste), sent to land®lls, or assumed to decompose immediately. In secondary processing, pulp is converted into ®nal products. The end product C is represented in three end-use categories: market pulp, newsprint pulp and `other' pulp (destined for other paper products such as groundwood, printing, writing, wrapping, tissue, sanitary, linerboard, corrugated, boxboard and building papers). Losses are estimated from the aggregate P&P product C pool that occur through decomposition during production, combustion as waste or for energy and transfers to land®lls. Imports and exports of newly produced pulp and paper products occur before the products are added to the aggregate P&P product pool. In some circumstances, however, insucient product may be produced to account for all the exports that the data suggest for that year. If this happens, the model borrows from the exported pulp and paper products produced in earlier years. 2.3. Land®ll C pool The land®ll pool contains all discarded biomass C resulting from the various production steps and from the disposal of forest products when they have completed their useful life. Its dynamics are unchanged from those given by Kurz et al. (1992). The `land®ll' pool has the same structure as other forest product pools but all losses from it are assumed to go to the atmosphere as combustion or decomposition gases (i.e. there is no recycling of land®ll). Decomposition of land®ll material has both aerobic (CO2) and anaerobic (CH4) components. A particular characteristic of the land®ll retention curve is a signi®cant component having a very long retention time (i.e. this component decomposes at a very low rate) (Micales and Skog, 1997). 2.4. Recycling As shown in Fig. 1, recycling of solidwood products is accounted and is largely unchanged from the representation described in Kurz et al. (1992). Speci®cally, solidwood product recycling is accounted for by taking a proportion (5%) of each age class in the pool and transferring this to the youngest age class of the same pool. This approach does not explicitly account for

recycling of one product pool into another (i.e. `slow' into `fast' or vice versa). Data to parameterize such a distinction are not available, however and the approximation made in the model was the recommendation of a group of Canadian experts (Kurz et al., 1992). Recycling of pulp and paper products is more complex. Data are available on the amount of recycled materials that are added in each time step during primary processing, but the source of these materials is not known with certainty. In particular, some of the recycled material is imported, while the remainder is derived from pulp and paper pools within Canada. To work around this missing information, in the model it is assumed that all imported recycled material originates from previously exported Canadian paper products. (This is a reasonable approximation since Canada exports signi®cant quanitities of pulp and paper products.) If the imported material is insucient to account for the identi®ed recycling demand, the model then removes C from the Canadian paper product pool. Most of the recycled material is derived from newsprint and other short-lived paper products and it is therefore assumed that recycled material is derived primarily from the youngest product classes. Once enough recycled material is identi®ed, C ¯ows in the subsequent processing follow the paths shown in the lower part of Fig. 1. 2.5. Energy accounting Processing of harvested biomass in the Canadian forest product sector uses a range of di€erent forms and amounts of energy, each of which have di€erent C emissions associated with them. The CBM-FPS accounts for energy used in all processing steps from the time that fresh biomass is harvested and delivered to the processing unit until the ®nal product is produced. It accounts for the transport of raw biomass to the mill but does not account for energy expended in forest ecosystem management (silvicultural activities), transportation of the ®nal product to its place of use, or transportation of recycled materials outside the forest sector (e.g. from municipal newsprint waste collection sites). Five types of energy use, having di€erent C implications, are distinguished in the CBM-FPS: (1) direct fossil fuel combustion, (2) purchased electrical energy, (3) self-generated hydro-electrical energy, (4) energy from fuelwood and woodwaste, including hogfuel and (5) energy from pulp processing residues (spent liquor). CBM-FPS estimates the energy used in each of the 10 pulp-processing types de®ned in Table 1, based on empirical ratios of energy use to product production in each stage of the process. The blend of energy use in each of the ®ve types listed previously depends on the stage in the processing stream (Fig. 1), but is related

M.J. Apps et al. / Environmental Science & Policy 2 (1999) 25±41 Table 1 Types of pulp processes represented in the CBM-FPS model Chemical

Mechanical

Other

Bleached kraft Unbleached kraft Bleached sulphite Unbleached sulphite Semi-chemical Dissolving and special alpha

re®ner thermo-mechanical chemi-mechanical stoneground wood

recycled

to the amount of product produced in that stage. The parameters for these relationships vary across the country and over time, re¯ecting both the state of technology and the relative cost and availability of di€erent energy supplies. Thermal and electrical energy-use data in each of the primary processing types and in the generation of each of the secondary pulp and paper products were synthesized for this study at a national scale for the period 1972±1994 (I. Simonson, personal communication, 1996). The source of these data were the archived records of the Canadian Pulp and Paper Association, who have maintained these data on behalf of its membership (all the major pulp producers in Canada). These data, together with regionally speci®c data de®ning the source of electrical (purchased or self-generated hydro) and thermal (from the combustion of fossil fuels, wood waste and spent liquor) energy were used to generate regionally speci®c energy coecients for each primary and secondary processing type. These coecients were extrapolated to years prior to 1971, using the average for the period 1972±1979. Energy use and production in the processing of solidwood products is handled in the CBM-FPS in a similar manner to the processing of pulp and paper products, although with much less detail. Energy use in the processing is based on coecients that express energy expenditure per ton of product C. The coecients are based on existing data (Luce et al., 1991) and extended over space and time as described above (see also Section 3). Because of the paucity of data, the range of energy technologies and biofuel types, and the inconsistency of reporting by the many diverse small operators, no attempt is made to close the

31

energy budget for solidwood processing. Only two sources of energy (electrical or fossil fuel combustion) are accounted in the CBM-FPS for this sector but the emissions associated with all biomass combustion are included whether for production of energy or solely for waste removal. The CBM-FPS accounts for all biomass combustion but not the use of bioenergy during solidwood-product manufacture. In reality, energy produced as a product or by-product of biomass processing has played an important part in the economics of the FPS. Production of energy through the combustion of hogfuel, spent liquor or wood waste (in all categories shown in Fig. 1) was calculated using the calori®c data shown in Table 2 and modelled estimates of residue production during processing. The CBM-FPS makes independent estimates of bioenergy use (described above) and bioenergy production (using the data in Table 2) in each time step Ð it does not have a built-in constraint that limits the amount of bioenergy used in pulp and paper processing to the amount of energy produced in that process. 2.6. Accounting of emissions In addition to accounting for changes in the C stocks of FPS products and land®ll, the model separately estimates the GHG emissions (CO2, CH4 or CO) associated with each stage of processing and use of harvested biomass. This GHG accounting includes the loss of biomass through combustion or decomposition in product manufacture and use. It also accounts for emissions associated with energy use during processing. The model keeps track of whether these emissions are from biomass (burning as waste, burning for energy or decomposition) or fossil energy (fossil fuel) and the process that generated the emissions (transportation, processing or aging of pools). Emissions associated with fossil fuel use are calculated from the amount of fossil fuel and electrical energy (externally produced) that is consumed during the manufacture of forest products. These emissions are calculated from reported (actual) energy use in the FPS. The emissions associated with bioenergy are based on the model's simulated bioenergy production (described previously), in the absence of reported bioenergy use in the FPS.

Table 2 Bioenergy coecients used in CBM-CFS. Data from: Nielson et al., 1985; I. Simonson, Pulp and Paper Research Institute of Canada, personal communication, 1995 Fuel type

Heating value (GJ Mg wood ÿ 1)

Moisture (%)

Eciency (GJ MgC ÿ 1)

Hogfuel Spent liquor Wood

18±21 14.6 9.8

65 55 33 (wet)

23.4±27.4 16 30

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2.7. Imports and exports Small revisions were made to the earlier version of the forest product sector module (Kurz et al., 1992) to permit separate accounting of imports and exports of forest products. Each of the product C pools in the model now contains three subcategories according to whether the feedstock (1) was harvested in Canada and the products remain in Canada (2) was harvested in Canada but the product was exported out of the country or (3) was harvested outside and the product imported into Canada. The ®rst and third categories, which account for the forest product pools currently in Canada, are required to estimate within Canada (national) accounting of FPS inventories and emissions. The ®rst and second categories permit accounting for the fate of C harvested in Canada regardless of its ®nal destination Ð information that is aimed at assessing the global consequences of decisions within the Canadian forest sector. In the absence of available data, the fate of exported material is assumed to be similar to that of the same categories in Canada: no attempts are made to change production eciencies or other aspects of C dynamics of forest products outside Canada. While this assumption is recognized to be invalid for speci®c instances, the objective of the analyses was to put bounds on the relative importance of imports, exports and domestic product pools. In this context, the aggregate errors arising from the assumption are expected to be small and to not signi®cantly bias the comparative analyses. 3. Input data and simulations As shown in Fig. 1, the CBM-FPS uses annual harvest biomass (converted to C units) as its input driving data. The source of these data can be either historical records or scenarios based on prescriptions or ecosystem model projections. This paper focuses on results obtained using historical and simulated data of forest feedstock for the period 1920±1989. This period was chosen to compliment the existing detailed simulations of the C budget (and simulated FPS feedstock) for the Canadian forest ecosystem (Kurz and Apps, 1999). 3.1. Historical analyses of Canada's FPS The CBM-FPS model was designed to be compatible with the forest ecosystem model (CBM-FPS2) and uses the same spatial unit structure (Kurz et al., 1992). Speci®cally, Canada is represented by 42 spatial units comprised of the intersection of administrative boundaries of provinces and territories with those of the Ecoclimatic Provinces (Ecoregions Working Group, 1989). As described in Kurz et al. (1992), data for log-

ging operations in each spatial unit were obtained in two di€erent forms: forest area harvested (ha yr ÿ 1) and delivered harvest volume (m3 yr ÿ 1). The ®rst of these data were used in the ecosystem simulations with CBM-CFS (Kurz and Apps, 1999), while the harvest volume data (converted to C units) are used in the historical analyses of the FPS presented here. Additional historical data de®ning the proportions associated with the di€erent processing streams shown in Fig. 1 were compiled from a wide variety of federal (Canadian Forest Service, 1988) and provincial sources as reported in Kurz et al. (1992). The model tracks the fate of harvested material in four categories that correspond to those used by Statistics Canada to report harvest data and these are simulated separately in the CBM-FPS. For simplicity of presentation, however, Fig. 1 combines the Statistics Canada categories `industrial roundwood' (poles, pilings, etc.) and `logs and bolts' (used for lumber, plywood, shingles, shakes and veneers) into a single `solidwood' processing stream. `Pulpwood' is that biomass harvested for the production of pulp, while `fuelwood' is used for boilers and furnaces in industrial or institutional operations. Domestic ®rewood use is not reported in the available statistics at a national scale and is not included in the model as previously explained. The Canadian Pulp and Paper Association (I. Simonson, Pulp and Paper Research Institute of Canada, personal communication, 1995) assembled sector speci®c data from the archived records of its member companies speci®cally for this study. Where necessary, spatial and temporal gaps in the data were augmented by extrapolating available information from adjacent spatial units or by the use of temporal averages. Although the CBM-FPS runs with the same spatial structure as the CBM-CFS (42 spatial units), available FPS data do not permit representation of between province FPS trade. These data limitations required various parameters in the model to be de®ned at di€erent spatial scales. These scales include both national (i.e. the same value is used throughout the country) and provincial (i.e. each province has a di€erent value) scales. 4. Results The recorded changes in the harvest inputs to the FPS over the period 1920±1989 are shown in Fig. 3 together with the simulated estimates from the ecosystem model CBM-CFS. The historical data are those reported by mill operators, while the simulated estimates are based on harvest areas (logging operations). Both data and simulations show a ®vefold increase in harvest at the national scale over the 70-year period. A description and explanation of the spatial dynamics of

M.J. Apps et al. / Environmental Science & Policy 2 (1999) 25±41

Fig. 3. Data and CBM-CFS simulations for harvest feedstock to the Canadian FPS. The data are the actual recorded volumes (Tg C yr ÿ 1) delivered each year to Canadian mills. The CBM-FPS simulations are derived from (independent) data for annual harvest rates (ha yr ÿ 1).

forestry and FPS operations is beyond the scope of this paper but it is noted that the temporal changes shown in Fig. 3 have not been uniformly distributed across the 42 spatial units. There have been signi®cant changes in the allocation of the total harvest stream amongst the di€erent use categories, with a strong shift to sawlog use and a decline in industrial fuelwood since the late 1940s (Fig. 4). This increased input of primary harvest to solidwood product manufacture might suggest a decrease in the relative importance of pulp and paper products and a decrease in the role of bioenergy. Changes in the primary input stream alone, however, tell only part of the story. In reality, there has been a concurrent signi®cant increase in the eciency of the entire FPS, particularly in the use of residues and their transfer from solidwood manufacture to the pulp and paper sector. Thus, as shown by the data in Fig. 5, an increasingly large proportion of the feedstock for pulp and paper production derives from solidwood waste, rather than harvested round wood. The contribution of recycled materials has also increased over the same time period, but still remains (in 1989) a small component of the total pulp and paper feedstock. The changes in bioenergy production and use in the FPS will be discussed in a later section.

33

Fig. 4. Reported changes over time in the allocation of harvested C to the three main wood-processing streams represented in the CBMFPS.

C by 1989 (Fig. 6). Pools with a long turnover time (`slow' pool and land®ll) are, not surprisingly, the largest storage pools, containing 86% of the total. Carbon in exported products derived from Canadian harvests (see Section 2.7) re¯ects the importance of forestry to Canada's economy and balance of trade, accounting for more than two-thirds (69%) of the total pool in 1989. Imports, in contrast, play a minor role, constituting less than 10% of the total pool size in 1989. Increased use of roundwood for solidwood products (Fig. 4) shows up as steady increases in fast and slow lumber production (Fig. 6). Despite the plateau in pulpwood utilization (Fig. 4), the size of the P&P pool has continued to increase (Fig. 6) due to increased saw-chip production and waste wood utilization. 4.2. Energy Energy use varies between di€erent components in the Canadian FPS. Transportation of feedstock from the harvest site to the mill is the largest use of fossil fuel energy (Fig. 7a) in the FPS, accounting for nearly

4.1. Changes in FPS C stocks The total C stock in the three FPS pools (`slow', `fast' and P&P, see Fig. 1) and land®ll has steadily increased over time (Fig. 6). For any given year, these pools account for that year's input (harvest, recycled material and the net of import and export), releases during manufacture that year and the releases from oxidation in products and land®ll of organic material harvested in previous years. The total C in forest products and land®ll is estimated to have reached 836 Tg

Fig. 5. Reported changes in the sources of C entering the pulping process. Woodwaste includes residues (chips) from solidwood processing.

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60% of the sector's total fossil fuel consumption in 1989 (Fig. 7d). Solidwood processing (sawlogs) involves relatively small amounts of externally supplied (fossil fuel and electricity) energy (Fig. 7b). Numerical data for the historic use of bioenergy for solidwood processing at a national scale were not found, although it is known that steam operated sawmills were in use early in the 20th century and waste heat continues to be used for drying lumber in many locations. Although the magnitude of bioenergy use in solidwood processing is unknown, bioenergy contributes signi®cantly to energy budgets in the pulp sector, accounting for an estimated 21% of the total energy use by that industry in 1989 compared to fossil fuel

(35%) and electricity (21%) consumption. The graphs in Fig. 7 (especially panel c) also show that the Canadian FPS has adjusted rapidly to changes in world supplies (and price) of fossil fuel. Thus, in the mid-1970s, variations in fossil fuel use were partly o€set by compensating use of bioenergy in pulp production. Simulated production of bioenergy during the pulping process show the same general trends as the simulated use of that energy (Fig. 8). The present model, however, unrealistically simulates more bioenergy use than is produced. This discrepancy emphasizes inconsistencies in the data used to parameterize bioenergy use and production within the model and the sensi-

Fig. 6. Simulated changes in C stocks in forest products (panels a±c), de®ned by their turnover times, land®ll (panel d) and the total (panel e) over the period 1920±1989. Three categories are shown: C stocks in exported products, C stocks associated with imported products and C stocks derived from Canadian harvests that remain in Canada.

M.J. Apps et al. / Environmental Science & Policy 2 (1999) 25±41

35

Fig. 7. Energy use by processing sector and type of energy used. Transportation (panel a) includes the transportation of all types of harvest, including fuelwood, from the harvest site to the processing plant. Energy used in processing solidwood products (panel b) and pulp production (panel c) does not include the energy used in the initial transportation of the harvested wood (panel a). Data for bioenergy production within the solidwood processing sector is not available.

tivity of the model to simulated energy use. Fig. 9 shows the energy use per ton of pulp and, indirectly, the simulated changes in the eciency of pulp production. The results, when compared to the limited available data on actual eciencies (Canadian Pulp and Paper Association, 1994) show similar trends Ð increased use of bioenergy and decreased dependency on fossil fuel Ð but di€er in the actual numbers suggested by Fig. 9. At this time, it is not clear whether the di€erences are due to incomplete and imprecise data or incorrect parameterization in the CBM-FPS. 4.3. Emissions Bioemissions (those that arise from combustion or decomposition of previously harvested biomass) vary greatly amongst the di€erent production streams

(Fig. 10). Solidwood processing (Fig. 10a) produces bioemissions only during the combustion of waste wood. (Note that whether or not energy is produced, these emissions are fully accounted in the CBM-FPS even though the production and use of the energy from biomass combustion is not well known.) The increasing use of sawmill residues for pulp and other products after ca. 1960 is re¯ected in the decreasing emissions from wastewood combustion after that time. Emissions from the use of industrial fuelwood (Fig. 10b) were high early in the 20th century and reached a peak during the Second World War in accordance with energy supply and demand in that period. Increasingly ready access to natural gas and other inexpensive energy sources led to a signi®cant post-war decline in the use of industrial fuelwood

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and emissions of the Canadian FPS over the period 1920±1989 and a comparison of these changes with those in the forests of Canada. 5.1. Historical changes in FPS C stocks

Fig. 8. Bioenergy produced and used in the model in the pulp sector. Bioenergy is the sum of that derived from combustion of hogfuel and spent liquor.

(Fig. 10b) Ð there was, however, a small resurgence of fuelwood use in the mid-1980s. Processing of pulpwood generates bioemissions (Fig. 10b) from all three release mechanisms: burning for energy, burning as waste (without energy production) and decomposition in chip piles and land®lls. In the overall pulp sector, increases in emissions from burned-for-energy use is nearly a mirror image of the decreased waste-only combustion of residues (Fig. 10d). This trend re¯ects the increased eciency within the sector after the Second World War. Over the 70-year analysis period, bioemissions from the entire FPS associated with waste reduction only (i.e. no energy production) have declined while those from burned-forenergy have increased. It also appears that much of the bioenergy from fuelwood, at least within the FPS, has been replaced over time by energy production through burning of residues from that sector, making more ecient use of the harvest biomass resource. The rising emissions from decomposition (Fig. 10d) are the result of increasing forest products and land®ll C pools (Fig. 6).

The historical changes in C stocks estimated for the period 1920±1989 use recorded data as inputs to the simulation model. In this respect the model is not substantially di€erent from a number of other models (Harmon et al., 1996; Nabuurs, 1996; Pussinen et al., 1997; Skog and Nicholson, 1999). The results of the analyses suggest that large amounts of C are being stored in forest product pools and land®ll. These FPS C stocks are geographically dispersed, however, and the numerical estimates given here have an inherent imprecision for this reason. In particular, the life cycle characteristics of exported products (characteristic carbon retention curve and recycling fates) are treated as if the products remained in Canada. The model, at present, makes no allowance for variations (whether in response to policies or environmental conditions) in the life-cycle characteristics of exported products in di€erent locations. On the other hand, speci®c data on such variations at large scales are not available and the analyses presented here are likely of the right magnitude. They are certainly adequate for scenario analyses and the ranking of the FPS C-stock implications under di€erent management and policy scenarios. Net C accumulation by the Canadian FPS in 1989 was 45% of the annual harvest Ð considerably higher than the 25% reported by Harmon et al. (1996) for the Paci®c Northwest of the USA. It is uncertain whether this variance is due to real di€erences in the FPS system dynamics or to di€erences in model parameters used. One factor contributing to the apparent disagreement is the di€erence in the degree to which the FPS pools have attained steady-state conditions (where

5. Discussion The CBM-FPS described in this paper represents the ®rst attempt to account for the C pools and ¯uxes in the entire Canadian FPS. The model includes all the main compartments and processes within the FPS that a€ect C stocks and ¯uxes, including product manufacture, energy use, energy production, emissions and the dynamics of the C storage pools within the sector. It also accounts for imports and exports of forest products and has been linked to a national-scale model of forest ecosystem dynamics (CBM-CFS) to provide estimates of total forest sector C stocks and changes in these stocks. The analysis presented here focuses on documenting the historical changes in the C stocks

Fig. 9. Energy-use eciency in the Canadian pulp and paper sector. Energy used in processing pulp per ton of pulp produced is shown for each of the general energy categories accounted in the CBMFPS.

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inputs just balance losses). The amount of harvested wood in Canada has continued to increase substantially over the period of simulation (Fig. 3), thereby continually driving the FPS system away from steadystate. In contrast in the Paci®c Northwest, harvest levels have remained relatively constant. The higher rate of harvest, together with negligible use as fuelwood (Fig. 5), has resulted in higher rates of C accumulation in Canadian forest product sector. In CBM-FPS, land®ll decomposition rates vary with the age of the components of the land®ll (Fig. 2) while Harmon et al. (1996) use a constant rate. This di€erence in parameterization is re¯ected in the di€erent ranking of C stocks in long-term structures (`slow') and land®ll. While the Canadian analysis suggests that land®lls are the primary deposition site (accounting for nearly 50% of the C retained by the FPS), the Paci®c Northwest analysis, in contrast, suggests that long term structural pools hold most (74%) of the FPS C

37

and land®ll pools are ranked second. An FPS C-stocks analysis performed at the US national scale by Skog and Nicholson (1999) found, however, that land®lls are more e€ective C sinks than structural materials, giving support to the Canadian ®ndings reported here. Although all products release C back to the atmosphere through oxidation, pools that delay this release act as temporary storage pools, reducing the proportion of C leaving the terrestrial system (IGBP, 1998). For a given production rate, the amount of C stored in wood products is mainly determined by the life span of these products, captured in the CBM-FPS model by the carbon retention curves. Thus, the slow turnover pool stores C for longer periods of time than the faster turnover pools. To delay the emissions of C back to atmosphere from shorter life-span pools, the product's turnover time may be extended or the endfate of the product changed. Increased recycling of forest products e€ectively acts to increase the residence of

Fig. 10. Changes over time in bioemissions (oxidation of previously harvested forest biomass) from di€erent components of the FPS. The emissions from the entire sector (panel d) include atmospheric releases from FPS C stocks (including land®lls); the other panels show the emissions associated with processing in the di€erent components of the FPS shown in Fig. 1.

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C in these products, especially the products with a shorter life span. The model results indicate a substantial increase in Canadian use of recycled material in pulp production since 1970. Land®lls appear to have accumulated large pools of C. Over the 70-year analysis period, a total of 1940 Tg C was harvested from Canadian forest ecosystems and an additional 52 Tg C imported as products. In 1989, an estimated 837 Tg C, or 43% of the cumulative harvested C was stored in forest product pools and land®lls in Canada and abroad. Land®lls contained 461 Tg C or 55% of the retained C. The model results are sensitive to the decomposition rate of products in land®lls and these rates are not well established. However, while Aragno (1988) reported that 35±40% of organic matter in a land®ll could be converted into methane and carbon dioxide under ideal conditions, the decomposition within actual land®lls would be negligible (Micales and Skog, 1997; Skog and Nicholson, 1999), suggesting that the storage in land®lls may be higher than simulated in the present study. Further evidence for very low decomposition rates in actual land®lls is provided by reports of recovered newspapers that are still legible after being in land®ll for more than 20 years (Rathje and Murphy, 1992). Direct measurements of land®ll C storage and the attribution of its origin is, however, likely to remain impractical and improved models of decomposition processes in land®ll need to be developed. Despite the sources of numerical error, it is clear that land®lls may provide an important long-term storage for C, especially under anaerobic conditions. Price et al. (1998) found that land®ll repositories of residues played an important role in the net C budget of an operational forest enterprise. Micales and Skog (1997) studied the decomposition of forest products in land®lls and reported that US land®lls may serve as large carbon sinks, e€ectively preventing large quantities of C from being released back into the atmosphere.

Fig. 11. Comparison of the total pulp production simulated by the CBM-FPS model with observed data (national-scale data for pulp production prior to 1937 are not available).

Fig. 12. Comparison of the pulp produced from recycled material in the CBM-FPS (national-scale data prior to 1972 are not available).

Testing of the CBM-FPS simulation model is problematic, as there are few independent experimental observations and no independent model estimates at the national scale. There are, however, a number of limited comparisons between data and simulations that can be made to test key steps within the model. Fig. 11 compares the CBM-FPS estimates of Canadian pulp production with actual data reported by Canada's pulp and paper industry (I. Simonson, Pulp and Paper Research Institute of Canada, personal communication, 1995). The data and simulations are in close agreement, especially from 1950 to the early 1980s. The residual errors in the simulated estimates are likely associated with unaccounted details of changes in the eciency of pulp production within each of the 10 processing types (Table 1). Recycled materials are handled di€erently amongst the 10 pulp processing types. Nevertheless, simulated and actual pulp production from recycled feedstock also match reasonably well over the post-1972 period for which data were available (Fig. 12). This match is remarkably good given that changes in the relatively small, recycled component are dwarfed by the increases in other feedstock to the pulp process (Fig. 5). Several other internal checks with independent data were also performed. Thus the CBM-FPS estimates of the production of newsprint and other pulp products (internal diagnostic pools, see Fig. 1) agree with available data well Ð within a few percent on average over the 17-year period (1972±1989) for which data exists (I. Simonson, Canadian Pulp and Paper Association, unpublished data, 1995). Market pulp simulations tend to be somewhat higher (22%) than the available data. It should be noted, however, that the data are not easily obtained and may be a signi®cant underestimation of reality. Similarly, the model appears to simulate the proportion of pulp produced by mechanical versus chemical processes acceptably (after mid-1950 the simulations were consistently within 20% of the available data).

M.J. Apps et al. / Environmental Science & Policy 2 (1999) 25±41

5.2. FPS emissions and energy use Most of FPS energy use (090%) in Canada is within the pulp and paper sector (Fig. 7). There has been an overall increase in C entering the pulping process since 1970 and a marked increase in the use of wastewood for bioenergy production by the pulp and paper sector. The increased production of energy from forest waste material has reduced the C stocks in forest products (including land®ll) from what it would have been. Nevertheless, Schlamadinger and Marland (1996) argue that the net e€ect of bioenergy production and use is a reduction of C emissions to the atmosphere through the replacement of fossil fuel use. Such o€sets, however, do not show explicitly in forest sector accounting in the IPCC Guidelines for national GHG inventories, where fossil fuel emissions are accounted separately from land-use change and forestry (IPCC, 1997). They are not included in the present accounting of the FPS C budget presented here. Moreover, estimation of the bioenergy o€set in C-stock terms requires a larger systems approach that includes alternative energy systems (such as proposed by Schlamadinger et al., 1997) and is beyond the scope of the present paper. In Canada, most of the FPS C emissions during manufacture are from the pulping process. The emissions from fuelwood, sawlog processing and fossil fuel have decreased over time, but further reductions require changes in energy production and consumption within the FPS. The largest potential for such reductions is likely through the increased use of forest waste products (including recycled materials) for energy production. At present, the model calculates energy use and production independently from one another and these estimates do not agree with each other (Fig. 8). In addition, in the absence of historical data, the energy use coecients in the model are assumed to be static from 1921 to 1971. There are reasons to believe, however, that energy use eciency has changed considerably over this period and that as a result the emissions associated with energy use (Fig. 7) prior to 1971 are likely underestimated in the present study. The recently reported ®ndings of K&A indicate that Canadian forest ecosystems may have been a net source of GHG to the atmosphere after ca. 1980. Averaged over the period 1985±1989, K&A's results indicate that Canadian forest ecosystems lost 69 Tg C yr ÿ 1, due to an increase in disturbances in the period 1970±1989, in which harvesting plays only a small role. The present work shows that not all this loss in forest ecosystem C went to the atmosphere; an estimated 25 Tg C yr ÿ 1 accumulated in forest product pools resulting in a lower net release to the atmosphere of about 44 Tg C yr ÿ 1 in this period.

39

Accounting of emissions (and o€sets) from energy use is carried out in an entirely di€erent section of the IPCC Guidelines (IPCC, 1997) than those associated with forestry and land-use change. Moreover, estimating these for Canada is complicated by the diverse nature of the energy-supply system and geographical variances that apply. A rough estimate using the data of Fig. 7d and data from Schlamadinger et al. (1997), suggests that the fossil fuel emissions alone could be as high as 14 Tg C yr ÿ 1 in 1989. Electrical energy use in the FPS yield lower C emissions both because electrical use is less than 30% of fossil fuel use (Fig. 7d) and because both hydro and nuclear sources supply much of Canada's electrical demand. 6. Conclusions The C stocks in the entire Canadian forest sector (excluding peat deposits) in 1989 is estimated to be 86.6 Pg C, of which 71.3 Pg C is found in the dead organic matter of litter and soils, 14.5 Pg in living biomass and only 0.8 Pg C in FPS stocks. Although the FPS C stocks contain less than 1% of the total forest sector C, they grew by nearly 25 Tg C yr ÿ 1 in 1989. The FPS C stocks thus play a signi®cant role in the net forest sector exchange with the atmosphere and o€set more than one-third of the net C release from Canada's forest ecosystems reported by K&A. The net C emissions from energy use in the FPS have not been calculated in detail in the present study but preliminary estimates (>14 Tg C yr ÿ 1) suggest that if accounted to the FPS, they would signi®cantly reduce the net sink attributed to the sector. O€setting such fossil emissions, however, are the avoided emissions associated with the use of non-wood products whose manufacture generates higher emissions (Schlamadinger and Marland, 1996; Matthews, 1996). The results obtained in the present study indicate that the C stocks of the Canadian FPS are still in a very unsaturated state and still accumulate C more quickly than they lose C through decomposition from the accumulated pool. Adding to this non-equilibrium condition is the steady increase in harvest inputs. The steady increase in Canadian FPS C stocks is due both to decreases in bioemissions from within the sector (Fig. 10) and to the steady increase in harvest inputs (Fig. 3). As Houghton (1996) pointed out, however, FPS C stocks can remain an atmospheric CO2 sink only as long as harvest inputs continue Ð once these cease, FPS stocks can only act as sources. There are a number of ways by which the e€ective FPS atmospheric CO2 sink could be enhanced and extended. Obvious mechanisms include continued improvements to production eciencies, increased product lifetimes and the increased use of bioenergy.

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Increased harvest rates would also increase C sequestration by the forest product sector, but the changes in forest ecosystem pools must also be taken into account. Hendrickson (1990) pointed out that, all other factors being equal, the ratio of turnover times in FPS C stocks relative to turnover times in dead organic matter is the critical factor: the longer C can be held in FPS pools, the greater the bene®t. Carbon stored in land®lls with a negligible decomposition rate remains there for a long time serving as a signi®cant C sink without being a€ected by global climate change (Micales and Skog, 1997). Dewar (1990) arrived at a similar conclusion and extended the turnover time concept to forest ecosystem rotation lengths; harvesting long-lived stands for short-lived products gives the least C bene®t and below a threshold value may lead to negative net C budgets. These studies (and others such as Marland and Schlamadinger, 1997) are generally focussed on stand-level assessments and scaling these up to a national level for a forest landscape as complex as Canada's is non-trivial. Work is presently underway to examine various scenarios with a linked CBM-CFS and CBM-FPS system approach. This linked system can be used to simulate the probable outcomes of various policy options at di€erent spatial and temporal scales and thereby to assist in making better informed resource management decisions.

Acknowledgements We wish to give special acknowledgement to Dr. Ivor Simonson and the Canadian Pulp and Paper Association for their generous provision of summary statistics from their member companies in the Canadian pulp and paper industry. We also thank Dave Luck and Statistics Canada for other industry data and the many representatives of Federal and Provincial Agencies, Universities and Industry Organizations who have provided data and intellectual assistance in the model development over the past decade. The assistance of Tamara Lekstrum, and Dr. David Halliwell in compiling data and performing simulations for this project is greatly appreciated. The authors are grateful to Dr. Celina Campbell and several anonymous reviewers for their constructive comments on an earlier draft of this manuscript. This research has been supported in part by the Energy from the Forest (ENFOR) program managed by the Department of Natural Resources Canada, Canadian Forest Service on behalf of the federal Panel on Energy Research and Development (PERD).

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Rathje, W., Murphy, C., 1992. Five major myths about garbage and why they are wrong. Smithsonian 23, 113±122. Schlamadinger, B., Apps, M.J., Bohlin, F., Gustavsson, L., Jungmeier, G., Marland, G., Pingoud, K., Savolainen, I., 1997. Towards a standard methodology for greenhouse gas balances of bioenergy systems in comparison with fossil energy systems. Biomass and Bioenergy 13, 359±375. Schlamadinger, B., Marland, G., 1998. The Kyoto Protocol: provisions and unresolved issues relevant to land-use change and forestry. Environmental Science and Policy 1, 313±328. Schlamadinger, B., Marland, G., 1996. The role of forest and bioenergy strategies in the global carbon cycle. Biomass and Bioenergy 10, 275±300. Schneider, S.H., Mearns, L., Gleick, P.H., 1992. Climate change scenarios for impact assessment. In: Peters, R.L., Lovejoy, T.W. (Eds.), Global Warming and Biological Diversity. Yale University Press, New Haven, CT, pp. 38±55. Skog, K.E., Nicholson, G.A., 1999. Carbon cycling through wood products: the role of wood and paper products in carbon sequestration. Forest Products Journal, in press. UNFCCC, 1997. Kyoto Protocol to the United Nations Framework Convention on Climate Change. Document FCCC/CP (1997.7/ Add.1. Http://wwww.unfccc.de. Winjum, J., Brown, S., Schlamadinger, B., 1998. Forest harvests and wood products: sources and sinks of atmospheric carbon dioxide. Forest Science 44, 272±284. Woodwell, G.M., Mackenzie, F.T., Houghton, R.A., Apps, M., Gorham, E., Davidson, E., 1998. Biotic feedbacks in the warming of the Earth. Climatic Change 40, 495±518. Nielson, R.W., Dobie, J. and Wright, D.M., 1985. Conversion factors of the forest products industry in western Canada. Special publication no. SP-24R. Forintek Canada Corp. Vancouver, BC, Canada, 92 pp. All authors have been working on di€erent aspects of carbon budgets in forest ecosystems for a number of years but have very di€erent and complimenting backgrounds: Mike Apps (physics, pharmacy and forestry), Werner Kurz (wood science and forest ecology), Sarah Beukema (mathematics, zoology and biology) and Jagtar Bhatti (forest soils and nutrient cycling). Drs Mike Apps and Jagtar Bhatti are research scientists with the Canadian Forest Service at the Northern Forestry Centre. Dr Apps received his PhD in Physics (University of Bristol) in 1972, joined the Canadian Forest Service in 1980 to examine trace uranium-series radionuclides in forest vegetation and since 1986 has been developing models of forest dynamics. He is an Adjunct Professor in Renewable Resources at the University of Alberta. Dr Bhatti has been with the Canadian Forest Service since obtaining his PhD in Forest Soils (Florida) in 1994. His main interests are nutrient and carbon cycling and since joining Dr Apps has focused on the role of disturbances in these cycles. Dr Werner Kurz and Ms Sarah Beukema are systems ecologists with ESSA Technologies Ltd. In 1989, Dr Kurz obtained a PhD in forest ecology from the University of British Columbia (where he remains an Adjunct Professor) and joined ESSA where he leads ESSA's forest modelling team. Sarah Beukema has BA degrees in Mathematics and Biology and earned an MSc. in Zoology and Applied Mathematics (University of British Columbia) before joining ESSA's forest modelling team.