Net Carbon Flux Research Articles

The annual net flux of carbon to the atmosphere from changes in land use 1850-1990*

The annual net flux of carbon to the atmosphere from changes in land use 1850-1990*

The effect of experimental ecosystem warming on CO2 fluxes in a montane meadow

AbstractClimatic change is predicted to alter rates of soil respiration and assimilation of carbon by plants. Net loss of carbon from ecosystems would form a positive feedback enhancing anthropogenic global warming. We tested the effect of increased heat input, one of the most certain impacts of global warming, on net ecosystem carbon exchange in a Rocky Mountain montane meadow. Overhead heaters were used to increase the radiative heat flux into plots spanning a moisture and vegetation gradient. We measured net whole‐ecosystem CO2 fluxes using a closed‐path chamber system, relatively nondisturbing bases, and a simple model to compensate for both slow chamber leaks and the CO2 concentration‐dependence of photosynthetic uptake, in 1993 and 1994. In 1994, we also measured soil respiration separately. The heating treatment altered the timing and magnitude of net carbon fluxes into the dry zone of the plots in 1993 (reducing uptake by ≈100 g carbon m–2), but had an undetectable effect on carbon fluxes into the moist zone. During a strong drought year (1994), heating altered the timing, but did not significantly alter the cumulative magnitude, of net carbon uptake in the dry zone. Soil respiration measurements showed that when differences were detected in dry zone carbon fluxes, they were caused by changes in carbon input from photosynthesis, not by temperature‐driven changes in carbon output from soil respiration. When differences were detected in dry‐zone carbon fluxes, they were caused by changes in carbon input from photosynthesis, not by a temperature‐driven changes in carbon output from soil respiration. Regression analysis suggested that the reduction in carbon inputs from plants was due to a combination of two soil moisture effects: a direct physiological response to decreased soil moisture, and a shift in plant community composition from high‐productivity species to low‐productivity species that are more drought tolerant. These results partially support predictions that warming may cause net carbon losses from some terrestrial ecosystems. They also suggest, however, that changes in soil moisture caused by global warming may be as important in driving ecosystem response as the direct effects of increased soil temperature.

Interannual Variability in Terrestrial Net Primary Production: Exploration of Trends and Controls on Regional to Global Scales

Climate and biophysical regulation of terrestrial plant production and interannual responses to anomalous events were investigated using the NASA Ames model version of CASA (Carnegie–Ames–Stanford Approach) in a transient simulation mode. This ecosystem model has been calibrated for simulations driven by satellite vegetation index data from the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) over the mid-1980s. Relatively large net source fluxes of carbon were estimated from terrestrial vegetation about 6 months to 1 year following El Nino events of 1983 and 1987, whereas the years 1984 and 1988 showed a drop in net primary production (NPP) of 1–2 Pg (1015 g) C from their respective previous years. Zonal discrimination of model results implies that the northern hemisphere low latitudes could account for almost the entire 2 Pg C decrease in global terrestrial NPP predicted from 1983 to 1984. Model estimates further suggest that from 1985 to 1988, the northern middle-latitude zone (between 30° and 60°N) was the principal region driving progressive increases in NPP, mainly by an expanded growing season moving toward the zonal latitude extremes. Comparative regional analysis of model controls on NPP reveals that although Normalized Difference Vegetation Index “greenness” can alone account for 30%–90% of the variation in NPP interannual anomalies, temperature or radiation loading can have a fairly significant 1-year lag effect on annual NPP at middle- to high-latitude zones, whereas rainfall amount and temperature drying effects may carry over with at least a 2-year lag time to influence NPP in semiarid tropical zones.

The annual net flux of carbon to the atmosphere from changes in land use 1850–1990

Rates of land-use change, including clearing for agriculture and harvest of wood, were reconstructed from statistical and historic documents for 9 world regions and used, along with the per ha changes in vegetation and soil that result from land management, to calculate the annual flux of carbon between land and atmosphere. Between 1850 and 1990, changes in land use are calculated to have added 124 PgC to the atmosphere, about half as much as released from combustion of fossil fuels over this period. About 108 PgC are estimated to have been transferred from forests to the atmosphere as a result of human activity, 2/3 from tropical forests and 1/3 from temperate zone and boreal forests. Another 16 PgC were lost from non-forests, largely as a result of cultivation of mid-latitude grassland soils. About 800 × 106 ha of forest were cleared for agricultural purposes, and approximately 2000 × 106 ha were harvested. Conversion of forests to agricultural lands released 105 PgC; harvest of wood released about 20 PgC. These estimates of release include the accumulations of carbon in wood products (17 PgC) and woody debris (4 PgC), the losses of carbon from oxidation of wood products, woody debris, and soil organic matter (373 PgC in total), and the accumulations of carbon in forests recovering from harvest and in the fallows of shifting cultivation (249 PgC). Over the decade of the 1980s the annual net flux of carbon from changes in land use averaged about 2.0 PgC yr-1, higher than the 1.6 PgC yr-1 estimated previously. Almost all of this flux was from tropical regions, where rates of deforestation averaged approximately 15 × 106 ha yr-1. Outside the tropics, regrowth of forests logged in earlier years largely balanced the losses of carbon from oxidation of wood products.

Roots exert a strong influence on the temperature sensitivityof soil respiration

The temperature sensitivity of soil respiration will largely determine the effects of a warmer world on net carbon flux from soils to the atmosphere. CO2 flux from soils to the atmosphere is estimated to be 50–70 petagrams of carbon per year and makes up 20–38% of annual inputs of carbon (in the form of CO2) to the atmosphere from terrestrial and marine sources1,2. Here we show that, for a mixed temperate forest, respiration by roots plus oxidation of rhizosphere carbon, which together produce a large portion of total effluxed soil CO2, is more temperature-sensitive than the respiration of bulk soil. We determine that the Q10 value (the coefficient for the exponential relationship between soil respiration and temperature, multiplied by ten) is 4.6 for autotrophic root respiration plus rhizosphere decomposition, 2.5 for respiration by soil lacking roots and 3.5 for respiration by bulk soil. If plants in a higher-CO2 atmosphere increase their allocation of photosynthate to roots3,4,5,6 these findings suggest that soil respiration should be more sensitive to elevated temperatures, thus limiting carbon sequestration by soils.

Analysis of carbon and water fluxes from the NOPEX boreal forest: comparison of measurements with FOREST-BGC simulations

The ecosystem process model, FOREST-BGC, was applied on a stand in the NOPEX region in central Sweden. It was compared with measured data of net ecosystem carbon flux ( F n) and transpiration ( E Q) on a daily basis. Using the parameterized model, yearly budgets of carbon and water were constructed. F n was obtained from eddy correlation measurements on a tower at heights of 35 and 100 m. E Q was obtained from sap flow measurements using a heat balance method. The model predictions were generally good, considering the relatively low requirements for input parameters. The explained variability of E Q was high (95%), particularly relative to the presence of large water deficit conditions on the site. The explained variability of F n was lower: it was 50% and 66% when compared to the measurements at 35 and 100 m, respectively. These results reflect the large spatial variability of F n and the quantitative differences of measured F n at the two heights over a patchy forest consisting of small stands of different age, density and pine/spruce composition (the validation was made prior to a detailed footprint analysis). The model performed differently for various periods during a year, which demonstrates the value of long-term measurements for model validations. The simulated yearly net carbon ecosystem uptake for the 50-year-old stand with a high leaf area index was 1.99 t ha −1, with a range of 0.55–2.04 t ha −1 for leaf area index of 3–6 observed at the NOPEX site. The model analysis of controls for mass fluxes showed that soil water shortage was the main limiting factor on the NOPEX site in the year studied. The comparative model run for the northern BOREAS site in central Canada indicated that a high atmospheric drought and plant resistance to water flow frequently limited fluxes there. A more maritime climate of NOPEX site permits a larger gross production; however, larger respiration and decomposition rates reduce the quantitative differences of net ecosystem carbon uptake relative to the BOREAS site with a continental climate.

Potential contributions of vertically migrating Rhizosolenia to nutrient cycling and new production in the open ocean

We present a numerical model of nutrient uptake and photosynthesis during migrations of the marine diatom Rhizosolenia that was developed to estimate fluxes of carbon and nitrogen due to these migrations in the open ocean. The predicted specific rate of increase of Rhizosolenia was 0.11–0.15 day 1 , whereas the total time for one migration cycle ranged between 3 and 5 days. Using published estimates of Rhizosolenia abundance, we estimate that new primary production due to Rhizosolenia migrations ranges between 0.018 and 0.033 mmol N m −2 day −1 . These values represent up to 17% of new production due to turbulent diffusive fluxes of nitrate into the euphotic zone and are of the same order of magnitude as new production due to nitrogen fixation in tropical oceans. Large-scale contributions of Rhizosolenia to oceanic new production are limited by their relatively low standing crop. Variations in the formulation of losses with depth greatly affected gross and net fluxes of carbon and nitrogen. Better characterization of losses of Rhizosolenia and improved estimates of its abundance will help determine more accurately the contributions of Rhizosolenia to global biogeochemical cycles.

Moss and soil contributions to the annual net carbon flux of a maturing boreal forest

We used input and decomposition data from 14C studies of soils to determine rates of vertical accumulation of moss combined with carbon storage inventories on a sequence of burns to model how carbon accumulates in soils and moss after a stand‐killing fire. We used soil drainage—moss associations and soil drainage maps of the old black spruce (OBS) site at the BOREAS northern study area (NSA) to areally weight the contributions of each moderately well drained, feathermoss areas; poorly drained sphagnum—feathermoss areas; and very poorly drained brown moss areas to the carbon storage and flux at the OBS NSA site. On this very old (117 years) complex of black spruce, sphagnum bog veneer, and fen systems we conclude that these systems are likely sequestering 0.01–0.03 kg C m−2 yr−1 at OBS‐NSA today. Soil drainage in boreal forests near Thompson, Manitoba, controls carbon storage and flux by controlling moss input and decomposition rates and by controlling through fire the amount and quality of carbon left after burning. On poorly drained soils rich in sphagnum moss, net accumulation and long‐term storage of carbon is higher than on better drained soils colonized by feathermosses. The carbon flux of these contrasting ecosystems is best characterized by soil drainage class and stand age, where stands recently burned are net sources of CO2, and maturing stands become increasingly stronger sinks of atmospheric CO2. This approach to measuring carbon storage and flux presents a method of scaling to larger areas using soil drainage, moss cover, and stand age information.

Influence of methodology and assumptions on reported national carbon flux inventories: An illustration from the Canadian forest sector

The Intergovernmental Panel on Climate Change (IPCC) has developed guidelines to standardize the international reporting of greenhouse gas emissions and removals by signatory nations of the UN Framework Convention on Climate Change. With regard to forest sector carbon fluxes, the IPCC guidelines require only that those fluxes directly associated with human activities (i.e., harvesting and land-use change) be reported. In Canada, the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS2) has been used to assess carbon fluxes from the entire forest sector. This model accounts for carbon fluxes associated with both anthropogenic and natural disturbances, such as wild fires and insects. We combined model results for the period 1985 to 1989 with additional data to compile seven different national carbon flux inventories for the forest sector. These inventories incorporate different system components under a variety of seemingly plausible assumptions, some of which are encouraged refinements to the default flux inventory described in the IPCC guidelines. The resulting estimated net carbon fluxes varied from a net removal of 185,000 kt carbon per year of the inventory period to a netemission of 89,000 kt carbon per year. Following the default procedures in the IPCC guidelines, while using the best available national data, produced an inventory with a net removal of atmospheric carbon. Adding the effect of natural disturbances to that inventory reversed the sign of the net flux resulting in a substantial emission. Including the carbon fluxes associated with root biomass in the first inventory increased the magnitude of the estimated net removal. The variability of these results emphasizes the need for a systems approach in constructing a flux inventory. We argue that the choice of which fluxes to include in the inventory should be based on the importance of these fluxes to the overall carbon budget and not on the perceived ease with which flux estimates can be obtained. The results of this analysis also illustrate two specific points. Even those Canadian forests which are most free from direct human interactions—forests in which no commercial harvesting occurs—are not in equilibrium, and their contribution to national carbon fluxes should be included in the reported flux inventory. Moreover, those forest areas that are subject to direct management are still substantially impacted by natural disturbances. The critical effect of inventory methodology and assumptions on inventory results has important ramifications for efforts to “monitor” and “verify” programs aimed at mitigating global carbon emissions.

Temporal Variations in the Carbon Budget of Forest Ecosystems in Spain

Temperate and boreal forests of the Northern Hemisphere have recently been identified as important carbon sinks. Accurate calculation of forest carbon budget and appraisal of the temporal variations of forest net carbon fluxes are important topics to elucidate the “missing sink” question and to follow up the changing carbon dynamics in forests. In this article, recent carbon budgets of the forests of a region in northern Spain have been calculated using data from forest inventories and a carbon cycle model for the human perturbations to the forests. Two methods are used, one that requires data from two forest inventories (growing stocks), and another that uses data from only one inventory (forest growth). The methods are complementary and give a picture of the temporal evolution of the forest net carbon flux. Forests in northern Spain are important carbon sinks, accumulating, on average, 2.91 ± 0.47 Tg C/yr (1.46 Mg C·ha−1·yr−1) from 1972/1973 to 1986/1988 (these figures have been derived using data from two forest inventories). The big errors associated with the net fluxes of carbon make it difficult to draw out firm conclusions about the temporal dynamics of carbon budgets, but it can be concluded that the net carbon flux from the forests in the north of Spain has experienced drastic changes through the period 1968/1973 until 1982/1988: the sink strength has diminished in this period from 8.4 ± 4.4 to 4.9 ± 3.6 Tg C/yr (each figure derived from data from one inventory), as a result of the increase of emissions due to human perturbations of the forests (harvest and forest fires), which have not been compensated by an equivalent forest growth. Northern forests represent the largest proportion of the forest carbon sink in Spain. This is a consequence of the high productivity and areal coverage of the fast-growing tree species that dominate the landscape of the region. Growing stock (timber volume) in the forests of the region has increased by almost a third in ≈15 yr. Temporal evolution of tree densities (stem diameter distributions) and timber volumes during those years indicate maturation of the forests on average. The growth in timber volume is shown to be due mainly to the increase of mean tree heights in the same diameter classes and not to a shift in the stem diameter distribution toward bigger diameter classes. Reasons for this growth of timber volume are discussed, and although climatic and CO2 fertilization effects may play a role, it is concluded that increase in the growing stock is also compatible with “normal” stand evolution.

TEMPORAL VARIATIONS IN THE CARBON BUDGET OF FOREST ECOSYSTEMS IN SPAIN

Temperate and boreal forests of the Northern Hemisphere have recently been identified as important carbon sinks. Accurate calculation of forest carbon budget and appraisal of the temporal variations of forest net carbon fluxes are important topics to elucidate the “missing sink” question and to follow up the changing carbon dynamics in forests. In this article, recent carbon budgets of the forests of a region in northern Spain have been calculated using data from forest inventories and a carbon cycle model for the human perturbations to the forests. Two methods are used, one that requires data from two forest inventories (growing stocks), and another that uses data from only one inventory (forest growth). The methods are complementary and give a picture of the temporal evolution of the forest net carbon flux. Forests in northern Spain are important carbon sinks, accumulating, on average, 2.91 ± 0.47 Tg C/yr (1.46 Mg C·ha−1·yr−1) from 1972/1973 to 1986/1988 (these figures have been derived using data from two forest inventories). The big errors associated with the net fluxes of carbon make it difficult to draw out firm conclusions about the temporal dynamics of carbon budgets, but it can be concluded that the net carbon flux from the forests in the north of Spain has experienced drastic changes through the period 1968/1973 until 1982/1988: the sink strength has diminished in this period from 8.4 ± 4.4 to 4.9 ± 3.6 Tg C/yr (each figure derived from data from one inventory), as a result of the increase of emissions due to human perturbations of the forests (harvest and forest fires), which have not been compensated by an equivalent forest growth. Northern forests represent the largest proportion of the forest carbon sink in Spain. This is a consequence of the high productivity and areal coverage of the fast-growing tree species that dominate the landscape of the region. Growing stock (timber volume) in the forests of the region has increased by almost a third in ≈15 yr. Temporal evolution of tree densities (stem diameter distributions) and timber volumes during those years indicate maturation of the forests on average. The growth in timber volume is shown to be due mainly to the increase of mean tree heights in the same diameter classes and not to a shift in the stem diameter distribution toward bigger diameter classes. Reasons for this growth of timber volume are discussed, and although climatic and CO2 fertilization effects may play a role, it is concluded that increase in the growing stock is also compatible with “normal” stand evolution.

Spatial and temporal variations in net carbon flux during HAPEX-Sahel

Micrometeorological measurements of the surface flux of carbon dioxide were made at a number of spatially separate sites within the HAPEX-Sahel experimental area. Differences in the timing of plant development caused by differences in rainfall (both quantity and frequency) over the experimental area exert a major influence on the absolute magnitude of the surface flux of carbon made simultaneously over the experimental area. Surface fluxes are presented for the three major types of vegetation in the Sahelian landscape, namely, millet, savannah and tiger bush. The atmospheric and surface controls on the surface flux of CO 2 are described for these three land surfaces and the fluxes are compared between sites. The comparison shows that modellers need to take into account the heterogeneity not only of the type of surface vegetation but also the variability in the life cycle stage of the vegetation, when scaling up to larger areas.

TROPICAL DEFORESTATION AND THE GLOBAL CARBON BUDGET

▪ Abstract The CO2 concentration of the atmosphere has increased by almost 30% since 1800. This increase is due largely to two factors: the combustion of fossil fuel and deforestation to create croplands and pastures. Deforestation results in a net flux of carbon to the atmosphere because forests contain 20–50 times more carbon per unit area than agricultural lands. In recent decades, the tropics have been the primary region of deforestation. The annual rate of CO2 released due to tropical deforestation during the early 1990s has been estimated at between 1.2 and 2.3 gigatons C. The range represents uncertainties about both the rates of deforestation and the amounts of carbon stored in different types of tropical forests at the time of cutting. An evaluation of the role of tropical regions in the global carbon budget must include both the carbon flux to the atmosphere due to deforestation and carbon accumulation, if any, in intact forests. In the early 1990s, the release of CO2 from tropical deforestation appears to have been mostly offset by CO2 uptake occurring elsewhere in the tropics, according to an analysis of recent trends in the atmospheric concentrations of O2 and N2. Interannual variations in climate and/or CO2 fertilization may have been responsible for the CO2 uptake in intact forests. These mechanisms are consistent with site-specific measurements of net carbon fluxes between tropical forests and the atmosphere, and with regional and global simulations using process-based biogeochemistry models.

Global net carbon exchange and intra‐annual atmospheric CO2 concentrations predicted by an ecosystem process model and three‐dimensional atmospheric transport model

A generalized terrestrial ecosystem process model, BIOME‐BGC (for BIOME BioGeoChemical Cycles), was used to simulate the global fluxes of CO2 resulting from photosynthesis, autotrophic respiration, and heterotrophic respiration. Daily meteorological data for the year 1987, gridded to 1° by 1°, were used to drive the model simulations. From the maximum value of the normalized difference vegetation index (NDVI) for 1987, the leaf area index for each grid cell was computed. Global NPP was estimated to be 52 Pg C, and global Rh was estimated to be 66 Pg C. Model predictions of the stable carbon isotopic ratio 13C/12C for C3 and C4 vegetation were in accord with values published in the literature, suggesting that our computations of total net photosynthesis, and thus NPP, are more reliable than Rh. For each grid cell, daily Rh was adjusted so that the annual total was equal to annual NPP, and the resulting net carbon fluxes were used as inputs to a three‐dimensional atmospheric transport model (TM2) using wind data from 1987. We compared the spatial and seasonal patterns of NPP with a diagnostic NDVI model, where NPP was derived from biweekly NDVI data and Rh was tuned to fit atmospheric CO2 observations from three northern stations. To an encouraging degree, predictions from the BIOME‐BGC model agreed in phase and amplitude with observed atmospheric CO2 concentrations for 20° to 55°N, the zone in which the most complete data on ecosystem processes and meteorological input data are available. However, in the tropics and high northern latitudes, disagreements between simulated and measured CO2 concentrations indicated areas where the model could be improved. We present here a methodology by which terrestrial ecosystem models can be tested globally, not by comparisons to homogeneous‐plot data, but by seasonal and spatial consistency with a diagnostic NDVI model and atmospheric CO2 observations.

The solubility pump of carbon in the subtropical gyre of the North Atlantic

The subduction of carbon is examined using abiotic models of the solubility pump in the subtropical gyre of the North Atlantic. The importance of the seasonal cycle of the mixed layer, and advection of carbon, is examined using sensitivity experiments with a Lagrangian model of the carbon system. The rate of subduction of carbon is found to be strongly influenced by the gradients in mixed-layer thickness over the gyre and, to a lesser extent, modified by the end of winter bias in the properties of subducted fluid. A seasonally-cycling geochemical model of the carbon system is then developed for the North Atlantic. The model is diagnosed to examine the seasonal exchange in carbon between the atmosphere and ocean induced by the seasonal warming and cooling. There is a net annual air-sea flux of carbon into the subtropical gyre of the model due to undersaturation ofpco, with respect to the local equilibrium with the atmosphere. The undersaturation is due to advection of carbon by the circulation. Along the path of the Gulf Stream, northward advection and cooling of the low latitude waters is so rapid that the surface waters are significantly undersaturated in carbon. Due to its long equilibration period, there is a resultant air-sea flux of carbon dioxide over the northern flank and interior of the subtropical gyre. Warm, low carbon water from the tropics is fluxed into the southern flank of the subtropical gyre in the Ekman layer, inducing an oceanic uptake of carbon there. The model experiments suggest that it is necessary to account for advection to close the carbon budget in the observed time-series measurements at Bermuda.

Seasonal variations of CO2 and water vapour exchange rates over a temperate deciduous forest

AbstractLong‐term and direct measurements of CO2 and water vapour exchange are needed over forested ecosystems to determine their net annual fluxes of carbon dioxide and water. Such measurements are also needed to parameterize and test biogeochemical, ecological and hydrological assessment models. Responding to this need, eddy covariance measurements of CO2 and water vapour were made ever a deciduous forest growing near Oak Ridge, TN, between April 1993 and April 1994. Periodic measurements were made of leaf area index, stomatal resistance, soil moisture and pre‐dawn leaf water potential to characterize the gas exchange capacity of the canopy.Four factors had a disproportionate influence on the seasonal variation of CO2 flux densities. These factors were photon flux densities (during the growing season), temperature (during the dormant season), leaf area index and the occurrence of drought The drought period occurred during the peak of the growing season and caused a significant decline in daily and hourly CO2 flux densities, relative to observations over the stand when soil moisture was plentiful.The annual net uptake of carbon was calculated by integrating flux measurements and filling missing and spurious data with the relations obtained between measured CO2 fluxes and environmental forcing variables. The net flux of carbon for the period between April 1993 and April 1994 was ‐525 g C m−2 y−1. This value represents a net flux of carbon from the atmosphere and into the forest. The net annual carbon exchange of this southern temperate broadleaved forest exceeded values measured over a northern temperate forest (which experiences a shorter growing season and has less leaf area) by 200 g C m−2 y−1 (cf. Wofsy et al 1993).The seasonal variation of canopy evaporation (latent heat flux) was controlled mostly by changes in leaf area and net radiation. A strong depression in evaporation rates was not observed during the drought Over a broadleaved forest large vapour pressure deficits promote evaporation and trees in a mixed stand are able to tap a variety of deep and shallow water sources.

Strategies for reducing carbon emissions on the tropical rain forest: The case of the Brazilian Amazon

The Brazilian Amazon deforestation has been responsible, in part, for the increasing concentrations of CO2 in the atmosphere. In 1990, the amount of carbon released to the atmosphere (as CO2) from Brazilian Amazon deforestation was 3.5 % to 4.9 % and 250 % to 360 % of the World and Brazilian annual emission from fossil fuels, respectively. In fact, the forest removal is a major contributor to local, regional and global environmental changes. Today, the deforested lands account for an amount superior to 430,000 Km 2. In addition to increase of the greenhouse effect, the foreseeable consequences of deforestation are : losses of the soil nutrients, surface erosion, decline in water percolation, evapo-transpiration and the annual pluviosity, losses in the biodiversity, etc.. On the other hand, deforestation is halted and replaced with a rational forest management, a reuse of degraded lands for agro-forestry and biomass production for energy and industrial purposes, we can reduce the pressure on forests lands and the net carbon flux will be reversed. In this paper, we argue the possibilities to achieve a sustainable biomass valorisation in the forest and degraded lands in Brazilian Amazon, with a special interest in the job creation, the cost of these strategies and their carbon flow.

The role of forest and bioenergy strategies in the global carbon cycle

Forest and bioenergy strategies offer the prospect of reduced CO 2 emissions to the atmosphere. Such strategies can affect the net flux of carbon to the atmosphere through 4 mechanisms: storage of C in the biosphere; storage of C in forest products; use of biofuels to displace fossil-fuel use; use of wood products which often displaces other products that require more fossil fuel for their production. We use the mathematical model GORCAM (Graz/Oak Ridge Carbon Accounting Model) to examine these mechanisms for 16 land-use scenarios. Over long time intervals the amount of C stored in the biosphere and in forest products reaches a steady state and continuing mitigation of C emissions depends on the extent to which fossil fuel use is displaced by the use of bioenergy and wood products. The relative effectiveness of alternative forest and bioenergy strategies and their impact on net C emissions strongly depend, for example, on the productivity of the site, its current usage, and the efficiency with which the harvest is used. When growth rates are high and harvest is used efficiently, the dominant opportunity for net reduction in C emissions is seen to be fossil-fuel displacement. At the growth rates and efficiencies of harvest utilization adopted in many of our base scenarios, the net C balance at the end of 100 years is very similar whether trees are harvested and used for energy and traditional forest products, or reforestation and forest protection strategies are implemented. The C balance on a plantation system that provides a constant output of biomass products can look different than the balance of a single parcel of land.

Terrestrial sources and sinks of carbon inferred from terrestrial data

Two approaches have been used to calculate changes in terrestrial carbon storage with data obtained from terrestrial ecosystems, rather than with atmospheric or oceanographic data. One approach is based on the changes in carbon that result from changes in land use (conversion of forest to agricultural land, abandonment of agricultural land, harvest and regrowth). The other approach uses measurements of forest biomass obtained through forests inventories to determine change directly. These latter studies may also calculate changes in the amount of carbon stored in wood products and soil, but in this respect the two approaches are similar. If a significant fraction of the missing carbon sink is to be found in mid-latitude forests, one would expect direct measurement of biomass to show greater accumulations of carbon than analyses in which calculated accumulations result only from regrowth following previous harvests or abandonment of agricultural land. Data from Canada, the conterminous US, Europe, and the former USSR show this circumstance to be correct. Accumulations of carbon in biomass and soil are 0.8 PgC yr −1 greater than expected from past management practices (land-use change). In the tropics (where forest inventories are rare), the total net flux of carbon from changes in land use (1.6 PgC yr −1 ) is consistent with recent estimates of flux based on atmospheric data, but the geographic distribution of the flux is not the same. Globally, terrestrial ecosystems are calculated to have been a net source of 0.8 ± 0.6 PgC yr −1 during the 1980s. DOI: 10.1034/j.1600-0889.1996.t01-3-00002.x

Forest Management for Mitigation of CO2Emissions: How Much Mitigation and Who Gets the Credits?

Forestry projects can mitigate the net flux of carbon (C) to the atmosphere in four ways: (1) C is stored in forest biomass — trees, litter and soil, (2) C is stored in durable wood product, (3) biomass fuels displace consumption of fossil fuels, and (4) wood products often require less fossil-fuel energy for their production and use than do alternate products that provide the same service. We use a mathematical model of C stocks and flows (GORCAM) to illustrate the inter-relationships among these impacts on the C cycle and the changing C balance over time. The model suggests that sustainable management for the harvest of forest products will yield more net C offset than will forest protection when forest productivity is high, forest products are produced and used efficiently, and longer time periods are considered. Yet it is very difficult to attribute all of the C offsets to the forestry projects. It is, at least in concept, straightforward to measure, verify, and attribute the C stored in the forests and in wood products. It is more challenging to measure the amount of fossil fuel saved directly because of the use of biomass fuels and to give proper attribution to a mitigation project. The amount of fossil fuel saved indirectly because biomass provides materials and services that are used in place of other materials and services may be very difficult to estimate and impossible to allocate to any project. Nonetheless, over the long run, these two aspects of fossil fuel saved may be the largest impacts of forestry projects on the global C cycle.