According to the guidelines provided by the Intergovernmental Panel on Climate Change (IPCC), long-term averages should be used as weather data when calculating the carbon balance of forests. When the interannual variation caused by weather is excluded, uncertainty estimates for soil carbon stock change become unrealistically small.
"Our results show that the uncertainty estimates of greenhouse gas inventories depend on the calculation method and on how the input data for the model, such as weather and litterfall data, have been averaged," says Aleksi Lehtonen, researcher at the Natural Resources Institute Finland (Luke).
Weather conditions strongly affect the litter production by vegetation and the decomposition of organic matter, in particular, and thus soil carbon stock changes. Upland soil carbon sink accounts for approximately 20% of the forest carbon sinks.
In the study published in the Canadian Journal of Forest Research, the role of year-to-year variation in weather conditions was examined by increasing the estimated uncertainty of litter input originating from tree needles, foliage and fine roots by a 5% random error. This seemed to have little effect on the total uncertainty of the litter input, but the uncertainty of the upland soil carbon sink estimate multiplied.
The reliability of carbon sink estimates is important
Reliable uncertainty estimates are needed for the prioritisation of research relating to the development of greenhouse gas inventories and for the assessment of the importance of emission sources and carbon sinks.
"Information provided by greenhouse gas inventories is used to support climate policy, for example. Therefore, it is important that, in addition to the size of the carbon sink, we also know what kind of uncertainties pertain to the figures reported," Lehtonen says.
The results of the study also stress the need for long-term monitoring of the ecosystem using permanent sample plots so as to be able to determine interannual variation and take it into account in calculations.
Finland's GHG inventory is one of the highest quality inventories in Europe
The member states of the European Union report their GHG emissions and removals to the Secretariat of the United Nations Framework Convention on Climate Change (UNFCCC) and the European Commission annually. Finland's GHG inventory is regarded as one of the highest quality inventories in the European Union.
In Finland, upland soil carbon stock changes are quantified using the Yasso07 soil model, in which variations in weather conditions can be taken into account. This model, combined with input data derived from forest inventory, is also used by many other countries in the EU.
The Natural Resources Institute Finland is responsible for reporting emissions and removals resulting from agriculture, land use and forestry.
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