2 Uncertainties in above- and below-ground carbon stocks
Measurement of both above and below-ground carbon stocks inevitably involves some degree of extrapolation from actual measurements on specific plots to all land areas under consideration. This extrapolation step is often carried out in conjunction with one or more process-based models of the key carbon components using underlying environmental factors as inputs. Uncertainties in the estimates are influenced by: the number and representativeness of the specific plots on which measurements have been carried out; the number of years over which such plots have been studied; the degree of verification of the process models used to assist extrapolation; and the quality of data on environmental factors used by such models.
Above-ground carbon stocks, and in particular standing trees, can usually be measured more cheaply than below-ground carbon. The use of forest-inventory data with appropriate extension factors gives an easy and reliable determination of above-ground biomass. Satellite techniques, supported by "ground truth" studies, can also be used to assess standing biomass with increasing reliability. Soil carbon stocks are less well known but are large and in general change more slowly than above-ground biomass. During the preparation and approval of the IPCC Special Report on Land-use, Land-use Change and Forestry it became clear that there is a range of views among different scientists on the reliability of current estimates for below-ground carbon stock values. This may well be partly due to subjective perceptions, but also reflects the differing amounts of data and heterogeneity of soil systems in different countries. To some extent this difference in perceived reliability of above- and below-ground carbon stocks has affected the policy debate on Articles 3.3 and 3.4 of the Kyoto Protocol.
Changes in land use or management often cause a transition from one dynamic equilibrium situation to another in which various carbon pools change in size. Accurately tracking these changes is complicated by the fact that different carbon pools may re-equilibrate at very different rates. Where several changes occur in sequence, equilibrium may not be established before a further transition is started. Rates of change in below-ground carbon given here (e.g. see summary table above), and in the scientific literature, are generally found to be 1% y-1 or less, whereas comparable rates of change in above-ground carbon in recently afforested areas are typically 5% y-1 or more.
The relative certainty of above- and below-ground carbon stocks in New Zealand reflects the international situation. Our plantation forests are relatively homogeneous and their stemwood content has been assessed and modelled extensively for forest management purposes. There are generally fewer data available for below-ground carbon stock assessment, and differing views on the reliability of estimates of national scale soil carbon stocks based on these.
The values presented here are the best available and uncertainty ranges have been considered carefully. Significant advances have been made in assessing soil carbon stocks in recent years and a consistent and quantifiable pattern of soil carbon changes related to some types of land-cover changes is now emerging. This enables us to say with some confidence that soil carbon is probably decreasing in various land use change situations occurring in New Zealand, the principal exception being restoration of eroded land.
Although uncertainties remain and will need to be addressed, there is no reason why New Zealand should not be able to provide carbon stock assessments for Article 3.4 of comparable or better quality than those of other Annex I countries.
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