Forest Carbon Storage

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Forests exert a disproportionately large influence on the global carbon cycle. The Earth's forests store over 1,200 Pg of carbon (1 Pg [petagram] equals 1 x 1,015 g.), which is approximately half of all organic carbon in the terrestrial biosphere. More than 80 percent of this forest carbon resides in boreal and tropical forests, with the remainder in temperate forests. Terrestrial vegetation, especially in forests, interacts strongly with the atmosphere, with impacts on carbon dioxide concentrations and fluxes. Every year, the terrestrial biosphere removes approximately 120 Pg of carbon from the atmosphere through photosynthesis, while releasing an almost identical amount of carbon back to the atmosphere as carbon dioxide through plant and soil respiration. By comparison, humans release approximately 6 Pg of carbon to the atmosphere as carbon dioxide through fossil fuel combustion. Forests also modify weather patterns and local- to regional-hydrologic cycles, and higher rates of plant productivity in forests translate to larger export of products, including biomass feedstock for paper, dimension lumber, and biofuels.

Globally, two to five times more carbon is stored in forest soils than aboveground forest biomass. Despite the large above and belowground differences in carbon storage, only 60 Pg carbon out of the 120 Pg carbon fixed annually by terrestrial vegetation through photosynthesis is allocated belowground for the construction and maintenance of root systems and mycorrhizae, the belowground symbiotic fungal partners of plants that allow plants to increase their exploration of soil for nutrients. Notably, this below-ground carbon flux is the Earth's third largest biologically mediated carbon flux after oceanic photosynthesis and terrestrial photosynthesis.

From an evolutionary perspective, this flow of carbon represents the currency with which chloroplasts in leaves (photosynthetic endosymbionts) acquire nutrients, water, and structural support from mycor-rhizae. Most of the carbon allocated belowground is rapidly released back to the atmosphere, with up to 80 percent released within one year of entering soil. When only net primary production is considered (that is, gross photosynthesis less plant respiration) the relative proportion of net primary production in forests that is allocated to belowground may be 20-50 percent of what is allocated to aboveground net primary production, in contrast to grassland or tundra ecosystems where the opposite pattern is found. The carbon that remains belowground takes the form of newly-formed soil organic matter or coarse roots, the two main long-lived stocks of organic carbon in soil. The roots of large trees may be extensive, for resource acquisition and to support large aboveg-round biomass.

Climate change, along with elevated carbon dioxide, increased nutrient deposition rates, invasive species, and land use change are likely having large, often site-specific (and sometimes opposing) effects on forest carbon budgets. Together, these changes on a global scale have the potential to drastically alter how

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forests interact with the atmosphere. There is great uncertainty regarding how the balance of photosynthesis and respiration will change in response to these global change variables, because many effects are opposing. Because so much organic carbon is stored belowground, accurate estimates of how belowground carbon allocation and belowground process rates in forests will respond to climate change are needed to understand how climate change will alter root growth, carbon inputs belowground, and soil organic matter formation. These, in turn, will influence biotic and abiotic properties of soil such as bulk density, cation exchange capacity, and water-holding capacity, which in turn affect plant growth.

climatic effects on forests

Increasing temperature, nutrient deposition rates, and atmospheric carbon dioxide may stimulate forest production, but these effects may saturate, or be canceled by, increased water limitations to forest productivity due to warmer or drier conditions or by increases in atmospheric plant toxins such as ozone. Warmer temperatures may drive higher respiratory process rates, which may increase carbon losses from forests. However, substrate limitations to microbial decomposition rates in soils or to biochemical processes in plants (acclimation) may limit losses of ecosystem carbon to the atmosphere. Uncertainty increases as ecosystems adapt or become modified by these global change variables.

For example, plant stress may increase with climate change, resulting in greater susceptibility to pests and pathogens. A change in nutrient availability or a warmer climate may, therefore, alter species composition by favoring the recruitment and survival of more tolerant plant species over species previously occupying sites under a cooler climate or nutrient-poor conditions. Climate change may also alter disturbance regimes. For example, in a warmer and drier world, it is anticipated that certain plant species will experience more frequent periods of drought stress and susceptibility to pest and pathogen attacks, resulting in higher mortality and fuel loads. Higher fuel loads and a changing climate will interact to increase intensity and frequency of major fire disturbances.

In already warm and dry ecosystems, warming and drying, therefore, may cause a large transfer of carbon from soils and vegetation to the atmosphere, and these changes would result in a positive feedback on global warming by accelerating the increase in atmospheric carbon dioxide concentrations. Conversely, in currently cool and wet climates, forest productivity may increase as growing season length and nutrient availability increase with warming, potentially resulting in an increase in carbon storage. This would have the opposite negative feedback effect on future warming through a net reduction in the rate at which atmospheric carbon dioxide concentrations are increasing.

Superimposed on the uncertainty of how global change variables will impact forests and forest carbon budgets is the uncertainty concerning ever-evolving patterns of human land use, especially the conversion of forest to nonforest, decisions over abandonment that result in the opposite process, as well as the magnitude and direction of changes in the amounts and distribution of total annual precipitation. The global sum of these changes is receiving tremendous scientific attention, but uncertainty of predicted outcomes from these effects and their interactions remains high.

SEE ALSo: Afforestation; Carbon Sequestration; Carbon Sinks; Deforestation.

BIBLIoGRAPHY. J.T. Houghton, et al., eds., Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge University Press, 2001).

Christian P. Giardina Institute of Pacific Islands Forestry USDA Forest Service

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