Terrestrial ecosystems provide an active mechanism (photosynthesis) for biological removal of CO2 from the atmosphere. They act as reservoirs of photosynthetically-fixed C by storing it in various forms in plant tissues, in dead organic material, and in soils. Terrestrial ecosystems also provide a flow of harvestable products that not only contain carbon but also compete in the market place with fossil fuels, and with other materials for construction (such as cement), and for other purposes (such as plastics) that also have implications for the global carbon cycle (see Box 4.1).
Box 4.1. Stocks and Flows
The global carbon cycle consists of the various stocks of carbon in the earth system and the flows of carbon between these stocks. It is discussed at length in IPCC WG I (Prentice et al., 2001) and IPCC Special Report on LULUCF (IPCC, 2000a) and is illustrated in Figure 4.2.
A consequence of the conservation of mass is that the net of all of the flows (measured as a rate variable in units such as tC/yr) into and out of a given reservoir or stock (measured in units such as tC) during a period of time must equal the change in the stock (tC) in that period. Conversely, a change in stock of a reservoir during a given period must exactly equal the integrated net difference in C flows into and out of that reservoir during that period. Elsewhere in this text the word pool is sometimes used to represent the various reservoirs of carbon in the global carbon cycle. The word sink is used to indicate the net positive flow of carbon into a terrestrial carbon pool.
The maximum rate of net ecosystem carbon uptake cannot occur at the same time as the maximum ecosystem carbon stock (see Figure 4.3). An ecosystem depleted of carbon by past events may have much higher rates of carbon accumulation than a comparable one in which carbon stocks have been maintained. Ecosystems eventually approach some maximum carbon stock a carrying capacity at which time the flows into the carbon pool are balanced by flows out of the carbon pool. Because C sink and C stock in ecosystems cannot be maximized simultaneously, mitigation activities aimed at enhancing the sink and maintaining the biological carbon stock coincide only partially (IGBP, 1998).
Similarly, the maximum rate of C substitution cannot occur at the same time as maximum C conservation. High rates of carbon substitution, through use of forest products or biofuels, generally require high productivity and efficient manufacture and use of derived products.
Carbon taken up by the biosphere may also accumulate in offsite pools as products or in landfills but it continues to oxidize at rates that depend on the conditions of those pools. It is the net of many flows that defines the changes in carbon stocks of off-site pools as well as of on-site pools. Carbon accumulation in off-site pools is an often overlooked, but a potentially important, form of sequestration.
Human activities have changed terrestrial carbon pools. The largest changes occurred with the conversion of natural ecosystems to arable lands. Such disruptions typically result in a large reduction of vegetation biomass and a loss of about 30% of the C in the surface 1 metre of soil (Davidson and Ackermann, 1993; Anderson, 1995; Houghton, 1995a; Kolchugina et al., 1995). Globally, conversion to arable agriculture has resulted in soil C losses of about 50GtC (Harrison et al., 1993; Scharpenseel and Becker-Heidmann, 1994; Houghton, 1995a; Cole et al., 1996; Paustian et al., 2000), and total emissions of C from land use change, including that from biomass loss, have amounted to about 122 ± 40GtC (Houghton, 1995b; Schimel, 1995). Most of the soil C losses occur within a few years or decades of conversion, so that in temperate zones, where there is little expansion of agricultural lands now, losses of C have largely abated (Cole et al., 1993; Anderson, 1995; Janzen et al., 1998; Larionova et al., 1998). Tropical areas, however, remain an important source of CO2 because of widespread clearing of new lands and reduced duration of fallow periods in shifting agriculture systems (Paustian et al., 1997b; Scholes and van Breemen, 1997; Woomer et al., 1997; Mosier, 1998).
The competition for land varies among countries and within a country. Land-use and forestry policies for C management may be most successful when climate mitigation is considered alongside other needs for land, including agriculture, forestry, agroforestry, biodiversity, soil and water conservation, and recreation. Forest fires, for example, are controlled, in many parts of the world, not as a measure for carbon mitigation, but simply because fire threatens areas of human settlement and the habitats of living organisms.
Similarly, biodiversity and landscape considerations have motivated protection of old-growth stands in temperate, boreal, and tropical rain forests from commercial logging. In many cases such decisions have prevented C release into the atmosphere, even though C mitigation was not the initial intent (Harmon et al., 1990). The impact of harvest restrictions on C pool in old-growth forests may be affected by leakage. If one ecosystem is protected but timber demand remains constant, logging may simply be shifted to another, similar ecosystem elsewhere, perhaps to a country where conservation priorities are lower.
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