Direct and Indirect Human Contributions to Terrestrial Carbon Fluxes A Workshop Summary (2004) / Chapter Skim
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3 The Science Base for Direct and Indirect Human Contribution to Carbon Fluxes
3 The Science Base for Direct and Indirect Human Contribution to Carbon Fluxes
Pages 19-47
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From page 19... ...
Considerable work relevant to fluxes of terrestrial greenhouse gases has been done in many fields, and much of this work is pertinent to assessing direct and indirect human contributions. However, scientists are far from having complete knowledge about these complex phenomena, and current research findings may not be sufficiently developed to meet the needs of the decision-making community, because specific research on direct and indirect effects has not been carried out for many of the critical systems around the world.
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From page 20... ...
For example, Goodale reported that among seven estimates of total forest carbon stocks in the Amazon, the totals ranged from 39 to 93 Pg, and even among those with similar total estimates of carbon stocks, the spatial distribution of biomass varies considerably (Houghton et al., 2001)
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From page 21... ...
According to Houghton, the terrestrial carbon flux due to land use change totaled about 2.2 annually during the 1990s. By far the largest source of carbon was the conversion of forest to cropland in tropical regions, followed by deforestation for pastures and logging.
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From page 22... ...
This can help determine where it is most important to include finely detailed land use information and which data are most important. Estimates of Carbon Stocks and Fluxes from Agricultural Activities Cesar Izaurralde, Joint Global Change Research Institute,6 discussed the effects of agricultural activities on the magnitude and fluxes of soil carbon pools.
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From page 23... ...
after three years of no tillage practice in 138 fields across Saskatchewan. Izaurralde noted that soil inorganic carbon represents 38 percent of the total soil carbon pool (Wilding et al., 2002)
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From page 24... ...
These processes and practices affect carbon pools in above-ground biomass, below-ground biomass, forest floor woody biomass, and soil organic matter. The most common way to define successional or historical state is by the number of years since agricultural abandonment or since the last major disturbance.
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From page 25... ...
With respect to aboveground forest carbon, Potter noted that rangeland in the West is gradually undergoing regrowth due to fire suppression and woody species invasion, leading to a large increase in above-ground carbon storage (Archer, 1995)
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From page 26... ...
Efficacy and longevity of carbon storage practices also depend on mean annual temperature, precipitation, and percent radiation. For example, West presented data from several experiments, which potentially show an increase in soil carbon associated with a climate anomaly in the early 1990s (West, 2003)
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From page 27... ...
Accounting methods, such as CMR curves, can be defined to consider direct human-induced changes in carbon stocks and net greenhouse gas emissions resulting from carbon storage practices. In practice, such a system could be scaled up to estimate direct human-induced effects for the purpose of national inventories by applying information for specific soil or climate classes across the entire country.
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From page 28... ...
. Ojima presented data that showed increasing nitrous oxide emissions over 75 years of high-intensity agriculture but declining carbon sequestration in soil organic matter, leading to a gradual increase in net greenhouse gas emissions.
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From page 29... ...
He asserted that forestry activities are a dominant player in the global carbon cycle because there are large carbon storage pools in terrestrial ecosystems, with the vast majority being stored in trees. While there are good, reliable equations for predicting GPP, the respiratory components are the major uncertainties in the physiological responses of forest carbon stocks.
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From page 30... ...
DeLucia noted that a 41 percent increase in NEP in the year 2050, assuming current land use patterns and current distribution of forests, would offset about 10 percent of the expected fossil fuel emissions. Schlesinger asserted, "The sink is not going to be enough to satisfy all the policy makers' needs if we take global warming seriously." The magnitude of the CO2 stimulation shows notable heterogeneity over time (DeLucia, 2003)
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From page 31... ...
THE SCIENCE BASE 31 variation in respiratory fluxes seems to correspond with El Niño and La Niña cycles, although the regulation of respiration processes remains poorly understood. In a subsequent discussion, Schlesinger pointed out that other drivers, including ozone or drought, may be causing the observed interannual variation in CO2 stimulation.
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From page 32... ...
(2003) Natural Effects on Forest Carbon Dynamics: Demography, Growth, and Fire Nathan Stephenson, U.S.
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From page 33... ...
In order to increase carbon storage in a forest, one might reasonably expect a greater response by decreasing the death rate than by increasing the growth rate. Whereas a 10 percent increase in growth at a constant death rate results in a 10 percent increase in equilibrium forest stand mass, if the growth rate remains constant and the death rate decreases by 10 percent, one might expect a 50 percent increase in stand biomass.
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From page 34... ...
on forest carbon sequestration at the biome level. Stephenson asserted that a network of forest gauging stations, checked annually, is needed to understand the causes of tree mortality and measure growth and reproduction for living trees.
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From page 35... ...
is equal to the three effects - natural, indirect human-induced effects, and direct humaninduced effects - direct effects could be calculated by: Direct human-induced effects = C - natural effects minus indirect humaninduced effects. For inventory reporting, at least from a policy viewpoint, it may not be necessary to sort out every other effect individually.
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From page 36... ...
Additional workshop discussions on separating direct and indirect effects are summarized in Chapter 4. Implications for Indirect and Natural Effects on National and International Greenhouse Gas Inventories Christopher Field, of the Carnegie Institution, addressed indirect and natural effects for national and international greenhouse gas inventories.
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From page 37... ...
Field then described the limited relevance of past carbon cycle research to the problem of assigning credit for carbon management. He presented a sample project to separate indirect effects from direct effects for ammonia fertilization in a forest, assessing carbon gains that are corrected for indirect effects at the landscape scale and losses due to harvesting, disturbance, and leakage.
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From page 38... ...
Over a decade or so, the constraints from control plots could be improved with additional information from manipulative experiments and bottom-up models, and in the long term, bottom-up and topdown models ought to provide precise, flexible constraints. Research Needed to Enable Partitioning of Direct and Indirect Effects Jim Randerson, University of California at Irvine, discussed the research needed to enable partitioning of direct and indirect carbon sinks.
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From page 39... ...
He said that two-thirds of the CO2 anomalies observed between 1997 and 2001 were caused by fire, although climate effects strongly influence the ability of humans to use fire as a method for land clearing. Randerson suggested that the primary climate factor regulating future carbon fluxes from terrestrial ecosystems is drought stress allowing fire use, rather than temperature stimulating increased microbial respiration.
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From page 40... ...
. The following are the site specific datasets that Birdsey classified as essential for separating direct and indirect effects: · species/biome response curves (e.g., growth response to CO2 and nitrogen, carbon responses to management activities and land use by age class)
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From page 41... ...
He also noted that scientists have a limited ability to partition some direct and indirect effects at this scale but not all. Birdsey speculated that in 10 years, the grid and temporal increments will come down to a grid scale of 1 to 5 kilometers in increments of 10 years or a grid scale of 5 to 50 kilometers in increments of 1 year due to the increased availability of datasets and improved computer processing capability.
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From page 42... ...
Fire and other disturbances can cause rapid temporal changes to ecosystem structure and carbon fluxes on the timescale of hours, thereby initiating a process of succession that can take years to centuries until full recovery. Most importantly, because most relevant biological processes are nonlinear, models based on average values of important underlying heterogeneities are likely to be highly inaccurate for long-term predictions.
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From page 43... ...
forest carbon measurement, focusing primarily on "bottom-up approaches." Heath said examining a number of smaller areas and summing them up for a total is an example of a bottom-up approach, which requires an explicit estimate for each area. For example, if the area and the carbon per area are known, they can be multiplied
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From page 44... ...
. A set of empirical or fundamental process models convert the inventory data to estimate forest carbon pools, and a forest sector model projects the estimates through 2050.
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From page 45... ...
In contrast, indirect effects in Alaskan forests may be more important since these areas are less affected by humans and grow slowly. Heath emphasized that resources are limited, and it is costly and complicated to measure carbon sequestration accurately, without trying to separate direct and indirect effects.
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From page 46... ...
In estimating carbon fluxes, Brown highlighted a concern about researchers' limited understanding of the forest carbon stocks where deforestation is occurring, based on little spatial representation of land use change. Some biomass maps are available that could, in theory, be combined with remote sensing data to produce a much better estimate of land use changes.
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From page 47... ...
Brown also stated that more research to identify historical land use patterns and quantify forest degradation and fragmentation are needed to better understand changes in tropical forest carbon stocks. Most of the work on indirect effects in tropical forests has utilized modeling (e.g., CO2 effect studies by Tian et al., 1998; McKane et al., 1995; Potter et al., 1998)
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