To understand the fluxes of carbon between a specific ecosystem and the atmosphere over a specified period of time and to appreciate its sensitivity to current environmental conditions, disturbance, and climate change, we need to consider the principal component processes within the system that add up to give the overall NEP. The feedbacks among these processes, their speed of response, and their sensitivity to environmental change determine NBP-the future and long-term carbon sequestration potential of the ecosystem.
Figure 1-3 shows measured annual carbon fluxes in a typical boreal, temperate, and tropical forest. NEP is the difference between the gross input of carbon in photosynthesis, GPP, and the sum of the losses of carbon in autotrophic respiration (RA) and RH. The component fluxes accumulated over a year should add up to the annual NEP, which is measured independently as described below (see Section 126.96.36.199). A mass balance of the component fluxes above ground (photosynthesis; foliage, branch, and stem respiration; leaf, branch, and stem litter production; and aboveground NPP) enables an estimate to be made of the amount of carbon internally translocated below ground. A mass balance of the component fluxes below ground (the inputs of litter and translocate from above, root and heterotrophic respiration, fine root turnover, mycorrhizal and root system NPP) enables an approximate estimate to be made of net changes in the pool of soil carbon. There are appreciable errors, however, in measuring all component fluxes (below-ground fluxes in particular), so close agreement between the two estimates of NEP is not to be expected.
The processes of photosynthesis and respiration are functions of several environmental and plant variables, including solar radiation, air and soil temperature and humidity, availability of water and nutrients, atmospheric ozone and other pollutants, leaf area, and foliar nutrition. Climate change therefore affects these processes in several ways. Photosynthesis is likely to be reduced by an increase in cloud cover but increased by enhanced global atmospheric CO2 concentration and, on some sites, by atmospheric nutrient deposition. All respiratory processes are sensitive to temperature, as is the rate of population growth of respiring organs-particularly the fine roots and heterotrophic organisms in the soil. Thus, "soil respiration" is a function of soil temperature (e.g., Boone et al., 1998; Rayment and Jarvis, 2000), which, if increased, leads in the short term to enhanced mineralization of soil organic matter and the release of nutrients-which, in turn, feed back to stimulate photosynthesis, increased leaf area, and tree growth. Evidence is accumulating, however, that in the longer term soil respiration acclimates to the rise in temperature and stabilizes at close to the original rate. In semi-arid and arid regions in particular, the availability of soil water-and thus changes in rainfall patterns-are also of utmost importance for the balance between carbon gains and losses.
|Figure 1-3: Estimated annual total carbon stocks and flows for three representative forest stands in tropical, temperate, and boreal regions. Stocks in bold italics are in tons of carbon per hectare (t C ha-1). Flows are in t C ha-1 yr-1. (a) Tropical rain forest near Manaus, Amazonia, Brazil; (b) temporate deciduous oak-hickory forest, near Oak Ridge, Tennessee, USA; and (c) boreal evergreen black spruce forest, near Prince Albert, Saskatchewan, Canada.|
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