There is an acceptance that, if used as part of a portfolio of carbon management options, managed sequestration of carbon in terrestrial biomass could have significant contribution to attempts to slow the rise of Ca in the coming decades (Caldiera et al., 2004). The 'coming decades' time frame is significant as models suggest that the future trajectory of Ca rise during the rest of the century will be set in these coming decades. Watson et al. (2000) recognized this potential and concluded that projects that both conserve and increase the size of existing carbon pools should be encouraged. Prospects for significant increases in carbon pools were recently improved by the ratification of the Kyoto Protocol, which provides incentives for Annex 1 (developed countries) to offset domestic carbon emissions by setting up afforestation and reforestation projects in Non-Annex 1 (developing countries). The protocol provides not only incentives for such projects but also protocols for their implementation as part of the Clean Development Mechanism. In Section 2.6 we will assess the potential for carbon sequestration and problems that may arise as a consequence of attempts to manage carbon cycling through the terrestrial biosphere.
It was recently assessed that 7.7% (50 Pg) and 31% (200 Pg) of the carbon stock of terrestrial biomass would be vulnerable to oxidation in the next 20 and 100 years, respectively (Gruber et al., 2004). Land-use changes would account for almost the complete short-term threat, rendering 40 and 100 Pg C vulnerable to oxidation in the next 20 and 100 years, respectively. Of the multitude of potential land-use changes deforestation constitutes the single biggest threat. Given that deforestation over the last 200 years is thought to have constituted 30% of the total anthropogenic increase in Ca over that time frame (Raupach et al., 2004), decreasing the rates of deforestation has considerable potential to slow Ca rise. Indeed it has been calculated that a reduction in deforestation of 3-10% in Non-Annex 1 countries by 2010 would offset 0.053-0.177 Pg C/year, or 1-3.5 % of Annex 1 countries' base year (for Kyoto commitments) emissions (1990) (Watson et al., 2000). Unfortunately deforestation is being driven by social and political factors, not by the needs to protect carbon stocks. These underlying causes will need addressing before deforestation rates can fall. The remaining 10 and 100 Pg C stored in terrestrial biomass is considered vulnerable to oxidation in the coming 20 and 100 years, respectively, due to climate changes and particular effects on species' ability to adapt (Gruber et al., 2004).
Fire suppression is a management strategy that could in theory protect carbon stocks in the coming decades. Schlesinger (1997) calculated that a 20% decrease in fire in tropical savannah and woodlands would protect 1.4 t C/ha/year from oxidation. Many studies have come to similar conclusions. Dixon et al. (1994) concluded that fire management in Russia could lead to long-term carbon storage of 0.6 Pg/year, while Tilman et al. (2000) calculated that fire suppression in the Siberian boreal forests, tropical savannah and woodlands would significantly decrease the rate of increase in Ca by 1.3 Pg C/year. There are, however, several significant problems associated with fire suppression. Tilman et al. (2000) describe how: (i) carbon storage following fire occurs during the period of rapid tree biomass accumulation, e.g. much of the potential for carbon storage in the USA via fire suppression may have already occurred; (ii) some of the increased carbon stores associated with fire suppression represent an accumulation of fuel that might lead to catastrophic, stand-destroying fires; (iii) it is unclear that global climate change might interact with fire frequency to impact carbon storage; and (iv) fire suppression can have negative impacts via its effects on the diversity, composition and functioning of savannah and forested ecosystems.
The greatest potential for increased sequestration of carbon in terrestrial biomass lies in managing forests to increase the carbon stores that living wood and wood products represent. Deriving estimates of the potential for carbon sequestration in terrestrial vegetation by specific management activities at a range of scales has been, and continues to be, the focus of much research.
Nabuurs et al. (2000) concluded that, for the USA, Canada, Australia, Iceland, Japan and the old EU15, alternative forest management practices to improve carbon sequestration were feasible on 10% of the forest area and that in total there was a potential for increased carbon sequestration that averaged 0.6 Pg C/year for the six regions. The potential was variable between regions, with Canada being the highest, and between management activity, with the effectiveness of specific management practices ranging from 0.02 t C ha/year for forest fertilization to 1.2 t C ha/year for a targeted combination of measures in a loblolly pine stand. Nabuurs et al. (2000) also assessed the utility of different management strategies in different regions, and again there was much variability; for example, in the USA, Canada and Australia, fire management had the potential to contribute ~40% of carbon sequestration potential. Restoration of degraded lands accounted for another 40% of the carbon sequestration potential in Australia. Neither strategy was considered useful for carbon sequestration for the old EU15.
The potential for carbon sequestration in Non-Annex 1 countries is especially relevant, as Article 3 of the Kyoto Protocol makes provision for Annex 1 countries to offset domestic CO2 emissions by implementing afforestation and reforestation projects in Non-Annex 1 countries. In specific NonAnnex 1 countries considerable potential for carbon sequestration in forests has been assessed. Sathaye and Ravindranath (1998) suggest that in 10 tropical and temperate countries in Asia 300 million hectares may be available for diverse forest management strategies and that afforestation or plantation projects could increase carbon stocks by 70-100 t C/ha in many places. Specifically commercial timber forestry in Indonesia and India could increase carbon stocks by 165 and 120 t C/ha, respectively.
In 2001 a special report of the IPCC was prepared to provide a state-of-the-art assessment of the global potential for carbon sequestration strategies relevant to land use, land-use change and forestry (Watson et al., 2000). This assessment concluded that global carbon sequestration in terrestrial ecosystems of 1.2-1.5 Pg C/year was possible by 2010. Carbon sequestration in above- and belowground biomass as a consequence of afforestation and/or reforestation projects accounted for 0.2-0.6 Pg C/year; of this estimate, improved management practices within a given land use resulted in an additional 0.57 Pg C/year and land-use change of 0.44 Pg C/year. By 2050 a cumulative increase in carbon stocks in terrestrial ecosystems of up to 100 Pg C was assessed as possible - a sequestration equivalent to ~10-20% of projected fossil fuel emissions during that period. A recent study by Cannell (2003) concluded that increasing terrestrial carbon sinks and biomass energy substitution could sequester 0.2-5 Pg C/year, with 5 Pg C/year considered the theoretical maximum achievable and 0.2 Pg C/year the most conservative actually achievable estimate.
Problems associated with managed carbon sequestration
The expansion of forestlands and decreases in rates of deforestation are considered win-win options, in that they not only increase carbon sequestration but should result in a number of key ecological improvements, including preservation of biodiversity, protection of top soils and watersheds (Schulze et al., 2002). However, there is concern that particular management options may ultimately prove to be less carbon-sound than others; indeed there is the danger that some may not result in a net increase in carbon sequestration. Most, if not all, these concerns have been central to the drafting of the protocols for the inclusion of biological sinks into the Kyoto Protocol. Sanz et al. (2004) describe both the 'loophole' argument, where biological sinks are seen as a way of doing little to reduce industrial emissions, and the 'floodgates' argument, whereby scientific and technical limitations to assessing carbon sequestration could allow for inflated claims of carbon sequestration. Fundamental to the success of carbon sequestration projects is ensuring the 'permanence' of carbon sinks, the 'addition-ality' of carbon sequestration over and above what would have occurred naturally and the protection against 'leakage', whereby a project to increase carbon storage in one location increases carbon release in another, while at the same time removing apparent 'perverse incentives' to clear off old-growth forests and replace them with a fast-growing tree species. This latter concern arises from the lack of protection towards old-growth forests in the Kyoto Protocol (Schulze et al., 2003).
A final concern is the need for complete carbon accounting over the complete lifetime of a project as well as knowledge of the ultimate use of the carbon pools, not just increases in aboveground biomass. The dangers of a narrow focus on wood production rather than complete carbon accounting are clearly illustrated in a recent study by Deckmyn et al. (2004), who modelled carbon sequestration for two different afforestation projects, initiated on agricultural land. They demonstrated that while NPP and wood production of a short-rotation poplar coppice (SRC) was much higher than that for an oak-beech forest (OBF), complete accounting after 150 years revealed that carbon stored in all carbon pools in the OBF was double that of the SRC. However, even though wood produced in the OBF was used for much longer-lived wood products, that from the poplar plantation substituted for fossil fuels, which ultimately resulted in a fourfold higher mitigation potential for the SRC of ~26 t CO2 ha/ year. Complete carbon flux accounting over the life of a project also requires appreciation of the carbon costs of all components of the project. It has been argued that the carbon costs of applications of water or fertilizer on forests to increase productivity could offset the increase in carbon sequestration that they yield, due to the carbon costs of production and application (Schlesinger, 1997). Care must also be taken that application of fertilizer or irrigation does not trigger oxidation of soil carbon stores by microbes whose activity is also water- or nitrogen-limited.
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