Wood products are an integral part of the managed forest ecosystem and the forest sector C cycle. They play three roles in the forest sector carbon cycle: (1) a physical pool of carbon, (2) a substitute for more energy-intensive materials and, (3) a raw material to generate energy (Burschel et al., 1993; Nabuurs and Sikkema, 1998; Harmon et al., 1996; Karjalainen, 1996; Matthews et al., 1996; Marland and Schlamadinger, 1997; Apps et al., 1999).
Wood removed from a forest by harvest, whether by thinning or clear-cut, can be viewed as a replacement for the natural mortality that would otherwise occur eventually (albeit at a faster rate). Harvested wood provides renewable raw material for use as fuel, fibre, and building materials; as well as income and employment for rural populations (Glück and Weiss, 1996). Globally, about 3.4 billion m3 of wood are harvested per year, excluding wood that is burned on site (FAO, 1997). Harvest rates are expected to increase at 0.5% per year (Solberg et al., 1996). Of the total harvest, about 1.8 billion m3 is for fuelwood, used mainly in the tropics. The total fuelwood consumption in tropical countries increased from 1.3 to 1.7 billion m3 during the period 1990 to 1995 (FAO, 1997; Nogueira et al., 1998).
If the fossil fuel based energy required to produce and transport forest products is less than that needed for alternative products, then CO2 emissions will be avoided by the use of forest products. Buchanan and Levine (1999) show, for example, that when wood is used for building construction in place of brick, aluminium, steel, and concrete, there can be net savings in CO2 emissions. For construction of small buildings in New Zealand, the carbon substitution effect was larger than the direct carbon storage in wood building products (Buchanan and Levine, 1999). Forest products can also substitute in the marketplace for alternative materials, such as cement, that involve carbon emissions in their manufacture.
A systems approach has been used recently to recognize interdependencies among products and sectors. For example, Adams (1992) and Alig et al. (1997) examined the effects of sequestering C in forests in the USA on the availability of agricultural land, and Sedjo and Sohngen (2000) used a sectoral approach that explicitly recognized interrelations among various wood investment decisions, and between wood investment and C sequestration activities. The systems approach also recognizes the joint product nature of industrial wood and carbon sequestration. In a study in Argentina, for example, Sedjo (1999b) found that timber alone does not generate sufficient returns to justify plantation investment, but the simultaneous sequestration of C can justify investment above some threshold C price. The models do not yet incorporate a potential increase in demand for wood as a fuel to displace fossil fuels.
In the developing world most fuelwood and charcoal use is devoted to satisfying energy needs for cooking (Makundi, 1998). The potential for conservation of fuelwood is significant, both through improved cooking stoves and by substitution with liquefied or gasified biofuels. India, China, and some African countries have large programmes for the distribution of more efficient wood stoves. In India alone 28 million improved stoves have been disseminated (Ravindranath and Hall, 1995). The carbon mitigation costs of improved wood stoves in India range from US$0.10/tC abated (Luo and Hulscher, 1999) to US$12/tC abated (Ravindranath and Somashekar, 1995). A review of case studies in Asia showed an average mitigation cost of US$0.8/tC abated in Thailand to US$1.7/tC in India, through programmes to encourage use of improved wood stoves (Hulscher et al., 1999). The experience with wood stoves shows that when appropriately designed, implemented, and monitored efficient stove programmes can provide substantial benefits to local residents. There are no estimates of the global potential for carbon conservation via this option, however, in India alone it is estimated that 20MtC could be saved annually (Ravindranath and Hall, 1995).
There is also a significant potential for saving fuelwood and charcoal in a large number of small industries. Charcoal making, brick making, pottery making, bakeries, etc. use fuelwood as their primary energy source in many areas. Fuelwood and charcoal consumption in tropical countries is projected to increase from 1.34 billion m3 in 1991 to 1.81 billion m3 in 2010 (FAO, 1993).
Most of the forest harvest in the boreal and temperate zone is for industrial roundwood (i.e., cut logs). About one-half to two thirds of the roundwood finds its way into final products, and the rest is used for energy or ends up as decomposing residues (e.g., Apps et al., 1999). The annual production of roundwood, according to FAO (1997) statistics, corresponds to a harvest flux of about 1.6 billion m3, resulting in about 0.9 billion m3 in final products. This represents a C flux of about 0.3GtC/yr into the product pool.
According to the SAR (IPCC, 1996), the current global stock of C in forest products is about 4.2GtC and the net sink is 0.026GtC/yr. Other sources suggest a stock of 10-20GtC (Sampson et al., 1993; Brown et al., 1996b) and a global sink of 0.139GtC/yr (Winjum et al., 1998). There is a large uncertainty in the estimates. Even if the high end of the range is correct, the C sink in wood products appears small compared to the current rate of C sequestration in boreal and temperate forest ecosystems. Whether the physical pool of carbon in wood products in use acts as a sink depends on the relative rates of input and output from the product pool, i.e., the difference between the production of new products and the decay of the C stock in existing products (Apps et al., 1999).
Options to increase physical sequestration of carbon in wood products include:
Several studies have been carried out on the impacts of these measures on the amount of carbon sequestered in wood products. These studies generally conclude that the sink potential is quite small at the national or global level (Karjalainen, 1996; Nabuurs, 1996; Marland and Schlamadinger, 1997).
Use of wood as a fuel reduces CO2 emissions from fossil fuels (Hall et al., 1991; Brown et al., 1996a; Nabuurs, 1996; Marland and Schlamadinger, 1997). Where the costs of growing biofuels on agricultural lands are higher than the costs of using fossil fuel, some form of incentive may be required to generate significant shifts to biofuels (Sedjo, 1997). The use of abandoned forest products for energy rather than disposal as waste can provide additional opportunities for displacing use of fossil fuels (Apps et al, 1999). Chapters 3 and 6 provide further discussion of the use of bioenergy within the energy sector.
Micales and Skog (1997) estimate that of the total amount of carbon-based products disposed of in the USA in 1993, as either paper or wood products, 28TgC (out of a total domestic harvest of approximately 123TgC/yr) will remain stored in landfills. Heath et al. (1996) and Karjalainen et al. (1994) emphasize the increasing role of landfills as a store of C. Production of methane through anaerobic decomposition deserves to be considered when evaluating the mitigation potential.
While C sequestration in wood products can reach saturation, the C benefits of materials substitution can be sustained. Assuming a material substitution effect of 0.28tC/m3 of final wood product (Burschel et al., 1993), and a flux corresponding to a roundwood volume of 0.9 billion m3 annually, the substitution impact of industrial wood products may be as large as 0.25GtC/yr. Although this estimate is highly uncertain, it is possible that for wood products the substitution impact is larger than the sequestration impact. This substitution is additional to the sinks in wood products mentioned before.
Globally, wetlands contain large reserves of organic carbon - about 300 to 600GtC (Gorham, 1991; Eswaren et al., 1993; Scharpenseel, 1993; Kauppi et al., 1997). A major portion of this carbon is found in peat-forming wetlands (peatlands), often associated with forests, in both northern (302Mha, 397GtC) and tropical (50Mha, 144GtC) biomes (Zoltai and Martikainen, 1996). Over the long term, peatlands gradually accumulate additional carbon, because decomposition is suppressed under flooded conditions (Harden et al., 1992; Mitsch and Wu, 1995; Rabenhorst, 1995; Zoltai and Martikainen, 1996; Kasimir-Klemedtsson et al., 1997). The beneficial effect of this carbon accumulation, however, is at least partially offset by release of methane, which is also a GHG (Gorham, 1995).
There are few opportunities to augment the accumulation of carbon in wetlands by improved management. Drainage of forested peatlands, largely concentrated in boreal regions, can enhance tree growth significantly, but the net ecosystem carbon changes are less clear some studies report large net gains while others indicate large net losses of carbon to the atmosphere (see review by Zoltai and Martikainen, 1996). A more important mitigation measure, from the perspective of atmospheric CO2, is the preservation of the vast carbon reserves already present (van Noordwijk et al., 1997) in peatlands. Drainage of wetlands for agricultural or other uses results in rapid depletion of stored C (Kasimir-Klemedtsson et al., 1997).
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