The potential of forests and the forest sector to mitigate greenhouse gas (GHG) emissions is widely recognized, but challenging to quantify at a national scale. Forests and their carbon (C) sequestration potential are affected by management practices, where wood harvesting transfers C out of the forest into products, and subsequent regrowth allows further C sequestration. Here we determine the mitigation potential of the 2.3 × 106 km2 of Canada's managed forests from 2015 to 2050 using the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3), a harvested wood products (HWP) model that estimates emissions based on product half-life decay times, and an account of emission substitution benefits from the use of wood products and bioenergy. We examine several mitigation scenarios with different assumptions about forest management activity levels relative to a base case scenario, including improved growth from silvicultural activities, increased harvest and residue management for bioenergy, and reduced harvest for conservation. We combine forest management options with two mitigation scenarios for harvested wood product use involving an increase in either long-lived products or bioenergy uses. Results demonstrate large differences among alternative scenarios, and we identify potential mitigation scenarios with increasing benefits to the atmosphere for many decades into the future, as well as scenarios with no net benefit over many decades. The greatest mitigation impact was achieved through a mix of strategies that varied across the country and had cumulative mitigation of 254 Tg CO2e in 2030, and 1180 Tg CO2e in 2050. There was a trade-off between short-term and long-term goals, in that maximizing short-term emissions reduction could reduce the forest sector's ability to contribute to longer-term objectives. We conclude that (i) national-scale forest sector mitigation options need to be assessed rigorously from a systems perspective to avoid the development of policies that deliver no net benefits to the atmosphere, (ii) a mix of strategies implemented across the country achieves the greatest mitigation impact, and (iii) because of the time delays in achieving carbon benefits for many forest-based mitigation activities, future contributions of the forest sector to climate mitigation can be maximized if implemented soon.

Quantitative assessment of Canada’s boreal forest mitigation potential is not yet possible, though the range of mitigation activities is known, requirements for sound analyses of options are increasingly understood, and there is emerging recognition that biogeophysical effects need greater attention. Use of a systems perspective highlights trade-offs between activities aimed at increasing carbon storage in the ecosystem, increasing carbon storage in harvested wood products (HWPs), or increasing the substitution benefits of using wood in place of fossil fuels or more emissions-intensive products. A systems perspective also suggests that erroneous conclusions about mitigation potential could result if analyses assume that HWP carbon is emitted at harvest, or bioenergy is carbon neutral. The greatest short-run boreal mitigation benefit generally would be achieved by avoiding greenhouse gas emissions; but over the longer run, there could be significant potential in activities that increase carbon removals. Mitigation activities could maximize landscape carbon uptake or maximize landscape carbon density, but not both simultaneously. The difference between the two is the rate at which HWPs are produced to meet society’s demands, and mitigation activities could seek to delay or reduce HWP emissions and increase substitution benefits. Use of forest biomass for bioenergy could also contribute though the point in time at which this produces a net mitigation benefit relative to a fossil fuel alternative will be situation-specific. Key knowledge gaps exist in understanding boreal mitigation strategies that are robust to climate change and how mitigation could be integrated with adaptation to climate change.

Canada's forests play an important role in the global carbon (C) cycle because of their large and dynamic C stocks. Detailed monitoring of C exchange between forests and the atmosphere and improved understanding of the processes that affect the net ecosystem exchange of C are needed to improve our understanding of the terrestrial C budget. We estimated the C budget of Canada's 2.3 × 106 km2 managed forests from 1990 to 2008 using an empirical modelling approach driven by detailed forestry datasets. We estimated that average net primary production (NPP) during this period was 809 ± 5 Tg C yr−1 (352 g C m−2 yr−1) and net ecosystem production (NEP) was 71 ± 9 Tg C yr−1 (31 g C m−2 yr−1). Harvesting transferred 45 ± 4 Tg C yr−1 out of the ecosystem and 45 ± 4 Tg C yr−1 within the ecosystem (from living biomass to dead organic matter pools). Fires released 23 ± 16 Tg C yr−1 directly to the atmosphere, and fires, insects and other natural disturbances transferred 52 ± 41 Tg C yr−1 from biomass to dead organic matter pools, from where C will gradually be released through decomposition. Net biome production (NBP) was only 2 ± 20 Tg C yr−1 (1 g C m−2 yr−1); the low C sequestration ratio (NBP/NPP=0.3%) is attributed to the high average age of Canada's managed forests and the impact of natural disturbances. Although net losses of ecosystem C occurred during several years due to large fires and widespread bark beetle outbreak, Canada's managed forests were a sink for atmospheric CO2 in all years, with an uptake of 50 ± 18 Tg C yr−1 [net ecosystem exchange (NEE) of CO2=−22 g C m−2 yr−1].

The Intergovernmental Panel on Climate Change recommends that countries that use advanced (Tier 3) models to meet their international reporting obligations on forest greenhouse gas emissions and removals evaluate model predictions against independent field data. Unfortunately, estimates of total ecosystem C stocks and stock changes are scarce and consequently the recommended evaluations are rarely completed. The Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) is the core model of Canada's National Forest Carbon Monitoring, Accounting, and Reporting System that implements an Intergovernmental Panel on Climate Change Tier 3 approach. It accounts for biomass, dead organic matter, and soil C pools as affected by natural and anthropogenic disturbances. We used data from a recent study of total ecosystem C stocks for black spruce (Picea mariana (Mill.) BSP) and balsam fir (Abies balsamea (L.) Mill.) boreal forest chronosequences of different disturbance origins in Newfoundland, Canada, to evaluate C stock and stock change predictions from the CBM-CFS3. Results indicated that the accuracy of the CBM-CFS3 is high for landscape-scale estimation of C stocks. Comparison of estimates stratified by lead species or disturbance type indicated that model accuracy could be improved at finer scales by increasing specific model parameters such as the snag fall rate and woody debris decay rates relative to default parameters.

The scientific community, forest managers, environmental organizations, carbon-offset trading systems and policy-makers require tools to account for forest carbon stocks and carbon stock changes. In this paper we describe updates to the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) implemented over the past years. This model of carbon-dynamics implements a Tier 3 approach of the Intergovernmental Panel on Climate Change (IPCC) Good Practice Guidance for reporting on carbon stocks and carbon stock changes resulting from Land Use, Land-use Change and Forestry (LULUCF). The CBM-CFS3 is a generic modelling framework that can be applied at the stand, landscape and national levels. The model provides a spatially referenced, hierarchical system for integrating datasets originating from different forest inventory and monitoring programs and includes a structure that allows for tracking of land areas by different land-use and land-use change classes. Ecosystem pools in CBM-CFS3 can be easily mapped to IPCC-defined pools and validated against field measurements. The model uses sophisticated algorithms for converting volume to biomass and explicitly simulates individual annual disturbance events (natural and anthropogenic). Several important scientific updates have been made to improve the representation of ecosystem structure and processes from previous versions of CBM-CFS. These include: (1) an expanded representation of dead organic matter and soil carbon, particularly standing dead trees, and a new algorithm for initializing these pools prior to simulation, (2) a change in the input data requirement for simulating growth from biomass to readily available merchantable volume curves, and new algorithms for converting volume to biomass, (3) improved prediction of belowground biomass, and (4) improved parameters for soil organic matter decay, fire, insect disturbances, and forest management. In addition, an operational-scale version of CBM-CFS3 is freely available and includes tools to import data in standard formats, including the output of several timber supply models that are commonly used in Canada. Although developed for Canadian forests, the flexible nature of the model has enabled it to be adapted for use in several other countries.

Forest carbon switch The forests of British Columbia are suffering a severe infestation with the mountain pine beetle ( Dendroctonus ponderosae ). Climate change is thought to have contributed to the severity of this outbreak by allowing it to expand its range into formerly inhospitable areas. An analysis of the likely impact of the outbreak during the period 2000 to 2020 suggests that it will convert the forest from a small net carbon (C) sink to a large net C source. This change — and similar effects caused by other insect pests and forest fires — could put North American forest carbon sinks at risk, and should be taken into account when modelling the impact of climate change on carbon cycling.