Mike Apps

Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems

Knowledge of carbon exchange between the atmosphere, land and the oceans is important, given that the terrestrial and marine environments are currently absorbing about half of the carbon dioxide that is emitted by fossil-fuel combustion. This carbon uptake is therefore limiting the extent of atmospheric and climatic change, but its long-term nature remains uncertain. Here we provide an overview of the current state of knowledge of global and regional patterns of carbon exchange by terrestrial ecosystems. Atmospheric carbon dioxide and oxygen data confirm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s, but became a net carbon sink in the 1990s. This recent sink can be largely attributed to northern extratropical areas, and is roughly split between North America and Eurasia. Tropical land areas, however, were approximately in balance with respect to carbon exchange, implying a carbon sink that offset emissions due to tropical deforestation. The evolution of the terrestrial carbon sink is largely the result of changes in land use over time, such as regrowth on abandoned agricultural land and fire prevention, in addition to responses to environmental changes, such as longer growing seasons, and fertilization by carbon dioxide and nitrogen. Nevertheless, there remain considerable uncertainties as to the magnitude of the sink in different regions and the contribution of different processes.

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.

Towards a standard methodology for greenhouse gas balances of bioenergy systems in comparison with fossil energy systems

In this paper, which was prepared as part of IEA Bioenergy Task XV (“Greenhouse Gas Balances of Bioenergy Systems”), we outline a standard methodology for comparing the greenhouse gas balances of bioenergy systems with those of fossil energy systems. Emphasis is on a careful definition of system boundaries. The following issues are dealt with in detail: time interval analysed and changes of carbon stocks; reference energy systems; energy inputs required to produce, process and transport fuels; mass and energy losses along the entire fuel chain; energy embodied in facility infrastructure; distribution systems; cogeneration systems; by-products; waste wood and other biomass waste for energy; reference land use; and other environmental issues. For each of these areas recommendations are given on how analyses of greenhouse gas balances should be performed. In some cases we also point out alternative ways of doing the greenhouse gas accounting. Finally, the paper gives some recommendations on how bioenergy systems should be optimized from a greenhouse-gas-emissions point of view.

Resilience and vulnerability of northern regions to social and environmental change

Abstract The arctic tundra and boreal forest were once considered the last frontiers on earth because of their vast expanses remote from agricultural land-use change and industrial development. These regions are now, however, experiencing environmental and social changes that are as rapid as those occurring anywhere on earth. This paper summarizes the role of northern regions in the global system and provides a blueprint for assessing the factors that govern their sensitivity to social and environmental change.

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.

Influence of nutrients, disturbances and site conditions on carbon stocks along a boreal forest transect in central Canada

The interacting influence of disturbances and nutrient dynamics on aboveground biomass, forest floor, and mineral soil C stocks was assessed as part of the Boreal Forest Transect Case Study in central Canada. This transect covers a range of forested biomes–-from transitional grasslands (aspen parkland) in the south, through boreal forests, and into the forested subarctic woodland in the north. The dominant forest vegetation species are aspen, jack pine and spruce. Disturbances influence biomass C stocks in boreal forests by determining its age-class structure, altering nutrient dynamics, and changing the total nutrient reserves of the stand. Nitrogen is generally the limiting nutrient in these systems, and N availability determines biomass C stocks by affecting the forest dynamics (growth rates and site carrying capacity) throughout the life cycle of a forest stand. At a given site, total and available soil N are determined both by biotic factors (such as vegetation type and associated detritus pools) and abiotic factors (such as N deposition, soil texture, and drainage). Increasing clay content, lower temperatures and reduced aeration are expected to lead to reduced N mineralization and, ultimately, lower N availability and reduced forest productivity. Forest floor and mineral soil C stocks vary with changing balances between complex sets of organic carbon inputs and outputs. The changes in forest floor and mineral soil C pools at a given site, however, are strongly related to the historical changes in biomass at that site. Changes in N availability alter the processes regulating both inputs and outputs of carbon to soil stocks. N availability in turn is shaped by past disturbance history, litter fall rate, site characteristics and climatic factors. Thus, understanding the life-cycle dynamics of C and N as determined by age-class structure (disturbances) is essential for quantifying past changes in forest level C stocks and for projecting their future change.

Project-based greenhouse-gas accounting: guiding principles with a focus on baselines and additionality

Project-based Greenhouse Gas Accounting : guiding principles with a focus on baselines and additionality

The Carbon Budget of the Canadian Forest Sector: Phase I

An assessment of the contribution of Canadian forest ecosystems and forestry activities to the global carbon budget has been undertaken. The first phase of this study consisted of the development of a computer modeling framework and the use of pub lished information to establish the sectors current role as a net source or a net sink of atmospheric carbon. The framework includes age-dependent carbon sequestration by living forest biomass, net detrital litter fall of carbon to the forest floor, subsequent accumulation and decomposition release in three soil compartments, retention of carbon in manufactured products derived from harvested forest biomass, and burning of forest biomass for energy. There is explicit representation of the role of ecosystem disturbances, such as fire, insect-induced stand mortality, and harvesting (clear-cutting, clear- cutting and slash burning, and partial cutting), as they affect carbon releases and transfers to the forest floor and to the forest product sector. Regrowth of biomass and changes in soil decomposition processes following disturbance are also simulated within the model. In the first phase of the work, national and provin cial data bases were used to provide the first compre hensive estimates of the net carbon exchange between Canadian forest ecosystems and the atmosphere for the reference year 1986.

Responses of High Latitude Ecosystems to Global Change: Potential Consequences for the Climate System

Terrestrial ecosystems of high latitudes occupy approximately one-fourth of the Earths vegetated surface. Substantial climatic warming has occurred in many high latitude areas during the latter half of the 20 th Century (Serreze et al. 2000), and evidence continues to mount that this warming has been affecting the structure and function of terrestrial ecosystems in this region (Stow et al. 2004; Hinzman et al. 2005). It is important to understand these changes because they may have consequences for the functioning of the climate system, particularly in the way that (a) radiatively active gases are exchanged with the atmosphere, (b) water and energy are exchanged with the atmosphere, and (c) fresh water is delivered to the Arctic Ocean (Chapin et al. 2000a; McGuire et al. 2003). The exchange of water and energy has implications for regional climate that may influence global climate, while the exchange of radiatively active gases and the delivery of fresh water to the Arctic Ocean are processes that could directly influence climate at the global scale. Over the past decade the IGBP-GCTE high latitude transects have become important foci for research on responses of high latitude terrestrial regions to global which has been augmented by carbon storage studies along a transect in Finland; one in Canada, the Boreal Forest Transect Case Study (BFTCS); and one in Alaska. The high latitude transects generally span substantial temperature gradients (mean annual temperature of 5° to –15°C) both within and among transects (McGuire et al. 2002). Temperature along each transect co-varies with precipitation and photosynthetically active radiation. Disturbance regimes including fire and insects are also variable among the high latitude transects. For example, fire is essentially non-existent in much of Scandinavia, but burns annually an average of approximately 1% of the boreal forest along the EST (McGuire et al. 2002; Fig. 24.2). Similarly, land-use and land-cover change also varies among the high latitude transects (Kurz and Apps 1999; McGuire et al. 2002, 2004). Each of the transects provides a different perspective into the responses of high latitude ecosystems to global change. In this chapter we first summarize how climate, disturbance regimes, and land cover in high latitudes have changed during the last several decades. We then summarize the results of ecological research along these transects that have contributed towards a richer understanding of high latitude terrestrial responses to these changes. We conclude with a discussion of challenges and opportunities for integration. …

Historic carbon budgets of Ontario’s forest ecosystems

Carbon (C) budgets of Ontario’s forest ecosystems for the period 1920–1990 were calculated using the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS2). Results show that total forest biomass C in Ontario increased from 1.83 Pg (10 15 g) to 2.56 Pg between 1920 and 1970, then decreased to 1.70 Pg by 1990. Carbon in soil and forest floor dead organic matter (DOM) increased from 8.30 to 11.00 Pg between 1920 and 1985 but decreased to 10.95 Pg by 1990. Ontario’s forest ecosystems acted as a C sink sequestering 41–74 Tg (10 12 g) C per year from 1920 to 1975, but became a C source releasing 7–32 Tg C per year (5-year average) after 1975. Disturbances (fire, insects and harvesting) enhanced both direct and indirect C emissions, and also affected average forest age and C sequestration. Net primary production (NPP), net ecosystem production (NEP), and net biome production (NBP) were affected by both disturbances and average forest age. Forests in the boreal (BO, 62.66 M ha), cool temperate (CT, 7.77 M ha) and moderate temperate (MT, 0.20 M ha) regions had different C dynamics. However, boreal forests dominated Ontario’s forest C budget because of the large area and associated C stock. Detailed C budgets for 1990 were also analyzed. The average forest ages in 1990 were 36.2 years for BO, 43.4 years for CT, and 92.1 years for MT regions, respectively. The total C stock of Ontario’s forest ecosystems (excluding peatlands) was estimated to be 12.65 Pg, including 1.70 Pg in living biomass and 10.95 Pg in DOM and soil. Average C density was 179 Mg ha � 1 (10 6 g) (24 Mg ha � 1 for biomass and 155 Mg ha � 1 for DOM and soil). The total net C balance (excluding harvest removal) was � 31.8 Tg. NPP, NEP and NBP were 267.6, � 28.2 and � 40.6 Tg per year, respectively. The young age (36.2) of Ontario’s boreal forests indicates a great potential for C sequestration and storage. Roughly 1 Pg C could be sequestered with a 10-year increase in forest age. A less severe disturbance regime and/or higher NPP would convert Ontario’s forest ecosystems back to a C sink. # 2002 Elsevier Science B.V. All rights reserved.