Land use and land-use change directly affect the exchange of greenhouse gases between terrestrial ecosystems and the atmosphere. Changes such as the clearing of forests for use in agriculture or as settlements are associated with clear changes in land cover and carbon stocks. Much of the world's land area continues to be managed for food and wood production, human habitation, recreation, and ecosystem preservation without a change in land use. Management of these land uses affects sources and sinks of CO2, CH4, and N2O. Furthermore, the resulting agricultural and wood products contain carbon. The carbon stocks held in these products are eventually released back to the atmosphere, after the products have served their use. Biomass carbon stocks are also used to produce energy that serves as a substitute for, and as complement to, fossil fuels.
Different factors and mechanisms drive land use and land cover transformation. In many cases, climate, technology, and economics appear to be determinants of land-use change at different spatial and temporal scales. At the same time, land conversion seem to be an adaptive feedback mechanism that farmers use to smooth the impact of climate variability, especially in extremely dry and humid periods (e.g., Viglizzo et al., 1995). Land-use change is often associated with a change in land cover and an associated change in carbon stocks. For example, as Figure 1-4 shows, if a forest is cleared, the carbon stocks in aboveground biomass are either removed as products, released by combustion, or decay back to the atmosphere through microbial decomposition. Stocks of carbon in soil will also be affected, although this effect will depend on the subsequent treatment of the land. Following clearing, carbon stocks in aboveground biomass may again increase, depending on the type of land cover associated with the new land use. During the time required for the growth of the new land cover-which can be decades for trees-the aboveground carbon stocks will be smaller than their original value.
Houghton (1991) assessed seven types of land-use change for carbon stock changes: (1) conversion of natural ecosystems to permanent croplands, (2) conversion of natural ecosystems for shifting of cultivation, (3) conversion of natural ecosystems to pasture, (4) abandonment of croplands, (5) abandonment of pastures, (6) harvest of timber, and (7) establishment of tree plantations. We recognize that, depending on the temporal scope of the assessment, classes 6 and 7 may also be considered a land-use practice rather than land-use change.
When forests are cleared for conversion to agriculture or pasture (1,3), a very large proportion of the aboveground biomass may be burned, releasing most of its carbon rapidly into the atmosphere. Some of the wood may be used as wood products; these carbon stocks could thereby be preserved for a longer time. Forest clearing also accelerates the decay of dead wood and litter, as well as below-ground organic carbon (see Figure 1-4). Local climate and soil conditions will determine the rates of decay; in tropical moist regions, most of the remaining biomass decomposes in less than 10 years. Some carbon or charcoal accretes to the soil carbon pool. When wetlands are drained for conversion to agriculture or pasture, soils become exposed to oxygen. Carbon stocks, which are resistant to decay under the anaerobic conditions prevalent in wetland soils, can then be lost by aerobic respiration (Minkkinen and Laine, 1998).
Forest clearing for shifting cultivation (2) releases less carbon than permanent forest clearing because the fallow period allows some forest regrowth. On average, the carbon stocks depend on forest type and the length of fallow, which vary across regions. Some soil organic matter is also oxidized to release carbon during shifting cultivation-but less than during continuous cultivation (Detwiler, 1986). Under some conditions, shifting cultivation can increase carbon stocks in forests and soils, from one cut-regrowth cycle to another. Because shifting cultivation usually has lower average agricultural productivity than permanent cultivation, however, more land would be required to provide the same products. In addition, shorter rotation periods deplete soil carbon more rapidly.
Abandonment of cultivated land and pastures (4,5) may result in recovery of forest at a rate determined by local conditions (Brown and Lugo, 1982; Uhl et al., 1988; Fearnside and Guimar�es, 1996).
Selective logging (6) often releases carbon to the atmosphere through the indirect effect of damaging or destroying up to a third of the original forest biomass, which then decays as litter and waste in the forest (although there are techniques that may reduce these consequences). The harvested wood decays at rates dependent on their end use; for example, fuel wood decays in 1 year, paper in less than a few years, and construction material in decades. The logged forest may then act as a sink for carbon as it grows at a rate determined by the local soil and climate, and it will gradually compensate for the decay of the waste created during harvest. Clear-cutting of forest can also lead to the release of soil carbon, depending on what happens after harvesting. For example, harvesting followed by cultivation or intensive site preparation for planting trees may result in large decreases in soil carbon-up to 30 to 50 percent in the tropics over a period of up to several decades (Fearnside and Barbosa, 1998). Harvesting followed by reforestation, however, in most cases has a limited effect (�10 percent). This effect is particularly prevalent in the tropics, where recovery to original soil carbon contents after reforestation is quite rapid. There are also some cases in which soil carbon increases significantly, probably because of the additions of slash and its decomposition and incorporation into the mineral soil (Detwiler, 1986; Johnson, 1992).
If tree plantations are raised on land that has been specifically cleared (7), initially there would be net carbon emissions from the natural biomass and the soil. The plantations would then begin to fix carbon at rates dependent on site conditions and species grown. To estimate the time scale of carbon uptake in forest plantations, previous work has linked fixation rates to the growth rate over time (Nilsson and Schopfhauser, 1995). Nilsson and Schopfhauser summarize data suggesting the following rates of aboveground carbon accumulation in plantations: 10 t ha-1 yr-1 for coniferous plantations in Australia and New Zealand, 1.5 to 4.5 t ha-1 yr-1 in coniferous temperate plantations of Europe and the United States, 0.9 to 1.2 t ha-1 yr-1 in Canada and the former Soviet Union, and 6.4 to 10.0 t ha-1 yr-1 in tropical Asia, Africa, and Latin America. Even if soil carbon accumulation is considered, these numbers probably represent maximum rates achieved under intensive management that includes the use of fertilizers. However, tree plantations also go through a rotational pattern of harvest, and the long-term estimates of carbon uptake might therefore be much lower than suggested by the foregoing figures.
Changes in land use of the types listed above have led to an estimated net emission of CO2 of 121 Gt C from 1850 to 1990 (Houghton, 1999; Houghton et al., 1999, 2000), as well as an estimated 60 Gt C prior to 1850 (De Fries et al., 1999). Prior to 1950, high- and mid-latitude Northern Hemisphere regions released substantial amounts of carbon from forest clearing and conversion to agricultural use, but this situation has since reversed as many forests presently seem to be in a stage of regeneration and regrowth (Kauppi et al., 1992). The low-latitude tropical belts, on the other hand, have been experiencing high rates of deforestation in recent decades (Houghton, 1994). The wide variation in vegetation carbon density in the low latitudes, however, introduces considerable uncertainty in estimates of carbon stock changes resulting from land-use changes. An estimate of global net emissions of 1.6 � 1.0 Gt C yr-1 from land-use changes from 1980-1989 (Houghton, 1994; Dixon et al., 1994a) was judged to have been on the high side from newer data from the Brazilian Amazon (Schimel et al., 1995). More recent analyses, however, have revised this estimate to even higher figures of 1.7 � 0.8 Gt C yr-1 (Houghton et al., 1999, 2000), 2.0 � 0.8 Gt C yr-1 (Houghton, 1999), and 2.4 Gt C yr-1 (Fearnside, 2000). Most of the carbon emission in the 1980s was from tropical regions (tropical Asia alone accounted for 50 percent of this flux) where deforestation rates averaged about 15 Mha yr-1. Of the major categories of land-use change, the clearing of forests for use as cropland accounted for the largest fraction of CO2 emissions from net land-use change; emissions from conversion to pastures, harvest, and shifting cultivation were lower. These estimates, however, do not include sources and sinks of CO2 caused by land-use management practices not associated with land-use change.
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