5.6.1.1. Carbon Pools and Flux

Carbon pools in the world's forests are estimated to be 348 Gt C in vegetation and 478 Gt C in soil (to 1 m) (updated since Dixon et al., 1994b, by Brown, 1998). Bolin and Sukumar (2000) based their numbers on Dixon et al. (1994b). The largest vegetation and soil carbon pools are in tropical forests (60 and 45% of the total, respectively) because of their large extent and relatively high carbon densities. Carbon stocks in forests vary, depending on the type of forest in relation to climate, soil, management, frequency of disturbances, and level of human-caused degradation.

Based on traditional carbon inventories, terrestrial ecosystems were shown to be carbon sources during the 1980s in the SAR (Brown et al., 1996); a high- and mid-latitude forest sink was exceeded by the source from low-latitude forests. However, recent work—using atmospheric measurements and modeling—suggests that terrestrial ecosystems appear to be net sinks for atmospheric carbon, even when losses from land-use change are taken into account (Bolin and Sukumar, 2000). For the 1980s, a net storage increase of 0.2 ± 1.0 Gt C yr^-1 by terrestrial ecosystems (largely in forests) was estimated as the difference between a net emission of 1.7 ± 0.8 Gt C yr^-1 from land-use changes (primarily in the tropics) and global terrestrial uptake of 1.9 ± 1.3 Gt C yr^-1 (Bolin and Sukumar, 2000). In the 1990s, estimated net emissions from land-use change decreased slightly to 1.6 ± 0.8 Gt C yr^-1, and global terrestrial uptake increased to 2.3 ± 1.3 Gt C yr^-1, resulting in a net terrestrial uptake of 0.7 +1.0 Gt C yr^-1 (Bolin and Sukumar, 2000). This terrestrial net sink of carbon arises as the net effect of land-use practices (agricultural abandonment and regrowth, deforestation, and degradation); the indirect effects of human activities (e.g., atmospheric CO₂ fertilization and nutrient deposition); and the effects of changing climate, climatic variation, and disturbances. The relative importance of these different processes is known to vary strongly from region to region.

Regional source and sink relationships also have been inferred by techniques of inverse modeling of observed atmospheric CO₂ gradients and circulation patterns (Ciais et al., 1995; Fan et al., 1998). These estimates are relatively imprecise and are difficult to relate to those that are based on forest inventory data. For example, the study by Fan et al. (1998) suggests that 1.4 Gt C yr^-1 was taken up by terrestrial biota in North America in the 1980s. However, mechanistic models and measurements based on forest inventories do not agree with the magnitude or spatial distribution of this carbon sink (Holland and Brown, 1999; Potter and Klooster, 1999).

Temperate forests are considered to be net carbon sinks at present, with estimates of 1.4-2.0 t C ha-1 yr^-1 (Nabuurs et al., 1997; Brown and Schroeder, 1999; Schulze et al., 1999). These findings are consistent with recent estimates of carbon in woody biomass, based on statistics for 55 temperate and boreal countries; these statistics indicate a general increase in forest biomass from the 1980s to the 1990s (UN-ECE/FAO, 2000c). Changes in forest management (reduction of harvest levels, increased regeneration effort, and administrative set-asides), as well as changes in the environment (N and CO₂ fertililization), appear to have contributed to this trend, but the relative contribution of different factors varies among forest regions and countries (Kauppi et al., 1992; Houghton et al., 1998, 1999; Brown and Schroeder, 1999; Liski et al., 1999; Nadelhoffer et al., 1999).

In boreal forests, carbon budgets vary strongly among different forest types (Apps et al., 1993; Bonan, 1993, Shvidenko and Nilsson, 1994). Although some boreal forest regions currently appear to be net carbon sources (Shepashenko et al., 1998; Kurz and Apps, 1999), others appear to be net sinks, varying between 0.5 and 2.5 t C ha-1 yr^-1 (Shvidenko and Nilsson, 1994; Jarvis et al., 1997). The difference between annual increment and net fellings reported to the FAO (reported in UN-ECE/ FAO, 2000c) does not account for changes in the frequency and severity of disturbances that have a large influence on source and sink relationships in boreal forests (Kasischke et al., 1995; Kurz and Apps, 1999; Bhatti, 2001). For example, detailed analyses of forest inventory data together with observed changes in disturbance over time indicate that Canadian forest ecosystems changed from a modest sink (0.075 Gt C yr^-1) from 1920-1970 to a small net source of 0.050 Gt C yr^-1 in 1994 (Kurz and Apps, 1999). Similarly, in Russia between 1983 and 1992, managed forest ecosystems in the European portion, where disturbances were relatively controlled, were a sink of 0.051 Gt C yr^-1 but a net source of 0.081-0.123 Gt C yr^-1 in the less intensively managed Siberian forests of the east (Shepashenko et al., 1998). Factors that were not included in these analyses that may offset losses of biotic carbon from disturbed forests include nitrogen deposition and CO₂ fertilization (Chen et al., 2000; Schimel et al., 2000), but experimental verification of these influences is not yet possible from inventory data.

In the tropics, forests are still reported to be a net carbon source as a result of land-use change. Although some studies suggest net carbon uptake in some tropical forests (Grace et al., 1995; Phillips et al., 1998), losses associated with high rates of forest clearing and degradation exceed such gains. The magnitude of the net carbon source from the tropics has been reported to be about 0.1 Gt C yr^-1 lower in the first half of the 1990s than in the 1980s (Houghton and Hackler, 1999; Houghton et al., 2000; Houghton, 2001), mostly because of reduced rates of deforestation in the 1990s (FAO, 1996, 1997a).

In summary, carbon stored in forest ecosystems appears to be increasing, and at an increasing rate with about 0.2 Gt C yr^-1 being stored in the 1980s and 0.7 Gt C yr^-1 in the 1990s. Most storage occurs in temperate forests, with a small net source from tropical forests and boreal forests varying depending on the disturbance regime they experience.

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