Werner A. Kurz

A Large and Persistent Carbon Sink in the World’s Forests

Net average global annual uptake of atmospheric carbon dioxide by forests was 1.1 petagrams of carbon, roughly one-sixth of fossil fuel emissions. The terrestrial carbon sink has been large in recent decades, but its size and location remain uncertain. Using forest inventory data and long-term ecosystem carbon studies, we estimate a total forest sink of 2.4 ± 0.4 petagrams of carbon per year (Pg C year–1) globally for 1990 to 2007. We also estimate a source of 1.3 ± 0.7 Pg C year–1 from tropical land-use change, consisting of a gross tropical deforestation emission of 2.9 ± 0.5 Pg C year–1 partially compensated by a carbon sink in tropical forest regrowth of 1.6 ± 0.5 Pg C year–1. Together, the fluxes comprise a net global forest sink of 1.1 ± 0.8 Pg C year–1, with tropical estimates having the largest uncertainties. Our total forest sink estimate is equivalent in magnitude to the terrestrial sink deduced from fossil fuel emissions and land-use change sources minus ocean and atmospheric sinks.

Mountain pine beetle and forest carbon feedback to climate change

The mountain pine beetle (Dendroctonus ponderosae Hopkins, Coleoptera: Curculionidae, Scolytinae) is a native insect of the pine forests of western North America, and its populations periodically erupt into large-scale outbreaks. During outbreaks, the resulting widespread tree mortality reduces forest carbon uptake and increases future emissions from the decay of killed trees. The impacts of insects on forest carbon dynamics, however, are generally ignored in large-scale modelling analyses. The current outbreak in British Columbia, Canada, is an order of magnitude larger in area and severity than all previous recorded outbreaks. Here we estimate that the cumulative impact of the beetle outbreak in the affected region during 2000–2020 will be 270 megatonnes (Mt) carbon (or 36 g carbon m-2 yr-1 on average over 374,000 km2 of forest). This impact converted the forest from a small net carbon sink to a large net carbon source both during and immediately after the outbreak. In the worst year, the impacts resulting from the beetle outbreak in British Columbia were equivalent to ∼75% of the average annual direct forest fire emissions from all of Canada during 1959–1999. The resulting reduction in net primary production was of similar magnitude to increases observed during the 1980s and 1990s as a result of global change. Climate change has contributed to the unprecedented extent and severity of this outbreak. Insect outbreaks such as this represent an important mechanism by which climate change may undermine the ability of northern forests to take up and store atmospheric carbon, and such impacts should be accounted for in large-scale modelling analyses.

Forest carbon sinks in the northern hemisphere

There is general agreement that terrestrial systems in the Northern Hemisphere provide a significant sink for atmospheric CO2; however, estimates of the magnitude and distribution of this sink vary greatly. National forest inventories provide strong, measurement-based constraints on the magnitude of net forest carbon uptake. We brought together forest sector C budgets for Canada, the United States, Europe, Russia, and China that were derived from forest inventory information, allometric relationships, and supplementary data sets and models. Together, these suggest that northern forests and woodlands provided a total sink for 0.6–0.7 Pg of C per year (1 Pg = 1015 g) during the early 1990s, consisting of 0.21 Pg C/yr in living biomass, 0.08 Pg C/yr in forest products, 0.15 Pg C/yr in dead wood, and 0.13 Pg C/yr in the forest floor and soil organic matter. Estimates of changes in soil C pools have improved but remain the least certain terms of the budgets. Over 80% of the estimated sink occurred in one-third of the forest area, in temperate regions affected by fire suppression, agricultural abandonment, and plantation forestry. Growth in boreal regions was offset by fire and other disturbances that vary considerably from year to year. Comparison with atmospheric inversions suggests significant land C sinks may occur outside the forest sector.

A 70-YEAR RETROSPECTIVE ANALYSIS OF CARBON FLUXES IN THE CANADIAN FOREST SECTOR

The Carbon Budget Model of the Canadian Forest Sector (CBM–CFS2) is a framework for the dynamic accounting of carbon pools and fluxes in Canada’s forest ecosystems and the forest product sector. The model structure, assumptions, and supporting databases are described. The model has been applied to estimate net ecosystem carbon fluxes for Canada’s 404 Mha forest area for the period 1920–1989. Changes in disturbance regimes have affected the forest age class structure and increased the average forest age during the period 1920–1979. The resulting changes in dead organic matter and biomass carbon during this period were estimated with the model. In the last decade of the analysis, large increases in disturbances, primarily fire and insect damage, have resulted in a reduction in ecosystem carbon storage. The estimates of biomass pool sizes obtained are consistent with those of other studies, while dead organic matter carbon pool estimates remain somewhat uncertain. Sensitivity analysis of several sources of u...

Risk of natural disturbances makes future contribution of Canada's forests to the global carbon cycle highly uncertain

A large carbon sink in northern land surfaces inferred from global carbon cycle inversion models led to concerns during Kyoto Protocol negotiations that countries might be able to avoid efforts to reduce fossil fuel emissions by claiming large sinks in their managed forests. The greenhouse gas balance of Canadas managed forest is strongly affected by naturally occurring fire with high interannual variability in the area burned and by cyclical insect outbreaks. Taking these stochastic future disturbances into account, we used the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) to project that the managed forests of Canada could be a source of between 30 and 245 Mt CO2e yr−1 during the first Kyoto Protocol commitment period (2008–2012). The recent transition from sink to source is the result of large insect outbreaks. The wide range in the predicted greenhouse gas balance (215 Mt CO2e yr−1) is equivalent to nearly 30% of Canadas emissions in 2005. The increasing impact of natural disturbances, the two major insect outbreaks, and the Kyoto Protocol accounting rules all contributed to Canadas decision not to elect forest management. In Canada, future efforts to influence the carbon balance through forest management could be overwhelmed by natural disturbances. Similar circumstances may arise elsewhere if global change increases natural disturbance rates. Future climate mitigation agreements that do not account for and protect against the impacts of natural disturbances, for example, by accounting for forest management benefits relative to baselines, will fail to encourage changes in forest management aimed at mitigating climate change.

Boreal Forests and Tundra

The circumpolar boreal biomes coverca. 2 109 ha of the northern hemisphere and containca. 800 Pg C in biomass, detritus, soil, and peat C pools. Current estimates indicate that the biomes are presently a net C sink of 0.54 Pg C yr−1. Biomass, detritus and soil of forest ecosystems (includingca. 419 Pg peat) containca. 709 Pg C and sequester an estimated 0.7 Pg C yr−1. Tundra and polar regions store 60–100 Pg C and may recently have become a net source of 0.17 Pg C yr−1. Forest product C pools, including landfill C derived from forest biomass, store less than 3 Pg C but increase by 0.06 Pg C yr−1. The mechanisms responsible for the present boreal forest net sink are believed to be continuing responses to past changes in the environment, notably recovery from the little ice-age, changes in forest disturbance regimes, and in some regions, nutrient inputs from air pollution. Even in the absence of climate change, the C sink strength will likely be reduced and the biome could switch to a C source. The transient response of terrestrial C storage to climate change over the next century will likely be accompanied by large C exchanges with the atmosphere, although the long-term (equilibrium) changes in terrestrial C storage in future vegetation complexes remains uncertain. This transient response results from the interaction of many (often non-linear) processes whose impacts on future C cycles remain poorly quantified. Only a small part of the boreal biome is directly affected by forest management and options for mitigating climate change impacts on C storage are therefore limited but the potential for accelerating the atmospheric C release are high.

An inventory-based analysis of Canada's managed forest carbon dynamics, 1990 to 2008.

Canadas 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 Canadas 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 Canadas 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, Canadas 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].

Could increased boreal forest ecosystem productivity offset carbon losses from increased disturbances

To understand how boreal forest carbon (C) dynamics might respond to anticipated climatic changes, we must consider two important processes. First, projected climatic changes are expected to increase the frequency of fire and other natural disturbances that would change the forest age-class structure and reduce forest C stocks at the landscape level. Second, global change may result in increased net primary production (NPP). Could higher NPP offset anticipated C losses resulting from increased disturbances? We used the Carbon Budget Model of the Canadian Forest Sector to simulate rate changes in disturbance, growth and decomposition on a hypothetical boreal forest landscape and to explore the impacts of these changes on landscape-level forest C budgets. We found that significant increases in net ecosystem production (NEP) would be required to balance C losses from increased natural disturbance rates. Moreover, increases in NEP would have to be sustained over several decades and be widespread across the landscape. Increased NEP can only be realized when NPP is enhanced relative to heterotrophic respiration. This study indicates that boreal forest C stocks may decline as a result of climate change because it would be difficult for enhanced growth to offset C losses resulting from anticipated increases in disturbances.

TELSA: the Tool for Exploratory Landscape Scenario Analyses.

A motorcycle has a fuel tank disposed near the front end of a motor cycle body frame and positioned substantially upwardly of the body frame, a riders seat disposed rearwardly of the fuel tank, and an engine disposed downwardly of the body frame. A receptacle for storing a crash helment, for example, is disposed rearwardly of the fuel tank, forwardly of the riders seat, and upwardly of the engine. The receptacle is openable and closably by at least a portion of the riders seat which is movable back and forth or pivotally movable about an end thereof.

Carbon budget of the Canadian forest product sector

Although many factors influencing 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 diAerent 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 final destination (product, pulp, landfill, 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 landfills. Model results are in general agreement with available data from 1920‐1989. Average changes in net C stocks in the FPS, estimated as the diAerence between harvest C input to the FPS and total losses from the forest product sector is estimated to be 23.5 Tg C yr ˇ1 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 significantly 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.