Figure 4.4: Indicative figure displaying historical changes in land use in three world regions. The presented values should not be taken as absolute, because the historical evidence is often only anecdotal (Mather, 1990; Kauppi et al., 1992; Palo and Uusivuori, 1999; Farrell et al., 2000).
The temperate zone is the most populated zone of the world, while the boreal zone is quite sparsely populated. For thousands of years forest area has diminished, particularly in the temperate zone, as forests were cleared for agriculture and pasture. Clearing of the European Mediterranean region began ca 5000 years ago; in Central Europe and in China deforestation occurred in early Medieval times; in parts of Russia and Mongolia forest clearing occurred in late Medieval times; and in North America clearing occurred mainly in the 19th century (Mather, 1990, see Figure 4.4). Since the mid 20th century the net forest area of the temperate zone has no longer decreased but has instead increased (Kauppi et al., 1992). The inner parts of the boreal zone in Siberia, Alaska, and Canada have not been subject to significant land-use management. The opportunities present to store carbon in terrestrial ecosystems in the boreal and temperate zones are thus very much determined by historical land-use change and the associated losses of carbon (Kurz and Apps, 1999).
Understanding the historic and current net sink of C in the temperate and boreal zones is important to assessing the potential of present and future management options. In general, estimates of C flows have been based on a variety of methods and data, resulting in a wide range of reported values for C flows per region. The confidence level in each separate value is therefore low. For example, for European forests the estimates of the present C sink vary from almost 0 to 0.5 GtC/yr (Nabuurs et al., 1997; Martin et al., 1998; Valentini et al., 2000; Schulze, 2000). For Canada, early estimates, based on a static assessment, indicated a net sink of 0.08GtC/yr for the mid-1970s (Kurz et al., 1992); whereas subsequent analyses, accounting for changes in forest disturbances over time (see section 4.2.3), indicated that Canadian forests became a small net source of C (0.068GtC/yr) by the early 1990s (Kurz and Apps, 1999). Estimates of carbon accumulation in woody biomass for the USA also show a large uncertainty. While the average rate for the USA C sink ranges from 0.020 to 0.098GtC/yr for the 1980s and 1990s (Birdsey and Heath, 1995; Turner et al., 1995; Houghton et al., 1999), atmospheric inversion models applied to the North American continent suggest a sink of 1.7 ± 0.5GtC/yr, largely south of 51ºN (Fan et al., 1998), but with very low levels of confidence (Bolin et al., 2000).
In the less intensively managed forests of Russia and Canada, changes in mortality associated with natural disturbances appear to dominate over management influences (see Section 4.2.4). In European Russia, managed forest ecosystems were estimated to be a sink of 0.051GtC/yr between 1983 and 1992, but the less actively managed Siberian forest was a net source of 0.0810.12GtC/yr (Shepashenko et al., 1998). The available estimates for Siberia differ even more than for the other regions mentioned above, and their confidence level may be low (Schulze et al., 1999).
Recent FAO statistics on 55 countries in the temperate and boreal zones indicate a general increase in the forest carbon stock (trees only) of 0.88GtC/yr (UN-ECE/FAO, 2000). Changes in forest management and changes in the environment have contributed to this trend. In Europe, the trend is consistent with the observation of increased growth in individual stands noted by Spiecker et al. (1996). The FAO statistics indicate that between the 1980s and 1990s both net annual increment and timber fellings increased, but that the rate of change was lower for fellings than for growth, resulting in a substantial increase in the carbon sink from the 1980s to the 1990s (Kuusela, 1994; Kauppi et al., 1992; Sedjo, 1992; Dixon et al., 1994; UN-ECE/FAO, 2000). The carbon sink in live woody vegetation was on the order of 10% of the fossil fuel CO2 emissions in the USA and in western Europe, and higher in the 1990s than in the 1980s (c.f. Kauppi et al., 1992).
These relatively high sequestration rates are not a result of active policies
aimed at climate mitigation, but less rather appear to be related to general
trends in land use and land-use change. In the USA, Schimel et al. (2000)
and Houghton et al. (1999) estimate that the observed sink is a result
mainly of changes in land use and land management, rather than a response to
changes in the environment. The latest observations, based on forest inventory
data (UN-ECE/FAO, 2000), are reflected in the Special Report on LULUCF (IPCC,
2000a). The IPCC (2000a) estimates that the total global terrestrial biopheric
sink in the 1990s amounted to 0.7GtC/yr, despite a source from land-use change
in the tropics of 0.9GtC/yr
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