Finnish Forest Research Institute  Metla

Press release 17.1.2006

Accuracy of assessments on emissions of greenhouse gases in the forest sector must be improved

Forests are key factors in the global carbon flux and carbon sequestration in forests may mitigate global climate change. The UN Framework Convention on Climate Change (FCCC) and the Kyoto Protocol require participating countries to report on changes in forest carbon stocks. The forest sector is required to improve accuracy of greenhouse gas (GHG) inventories. In particular, assessment methods for estimating soil carbon stock changes should be developed. The Finnish Forest Research Institute (Metla), European Forest Institute (EFI), and the Technical Research Centre of Finland (VTT) collaborated during 2003-2005 to identify the most likely causes of uncertainty in assessing the forest carbon sinks.

A carbon sink binds carbon dioxide from the atmosphere slowing down the global warming process. Information gained from national forest inventories regarding average tree growth and annual removals can be used to estimate forest carbon sinks. Important as an error source causing uncertainty in the carbon sink represented by vegetation is the annual variation in forest tree growth and removals. The level of uncertainty could be reduced if more accurate growth estimates were annually updated and data about removals, including details such as the exact place and soil type, were available.

Estimating soil carbon stocks is generally considered very unreliable and the methods available are based on modelling, because the currently available forest inventories do not support the monitoring of soil carbon stock. Among the factors detrimental to the reliability of the carbon sink estimates is the state of knowledge on the initial soil carbon. Another group of uncertainties is the actual system parameters used in the soil model. To calculate an estimate of soil carbon stocks, data about litter input is needed; this can be calculated on the basis of biomass stocks and removals. In addition to the uncertainties due to litter inputs, also the fact that the decomposition rate of litter depends on temperature makes it more difficult to estimate soil sinks.

There are two essential challenges in forest carbon inventories. Firstly, the inter-annual changes in carbon amounts are small compared to the existing carbon stocks. Secondly, there is strong inter-annual variation within forest ecosystems and only a limited amount of data on this is available. To distinguish even the smallest of changes is a challenging task.

The estimates of carbon sinks represented by the forest sector contain significant uncertainties, when compared with emission values calculated for other sectors. Hence, to reduce the uncertainty of the Finnish GHG inventory we must be able to reduce especially the uncertainty concerning soil carbon sink assessments. This Metla, EFI and VTT joint project, Uncertainty assessment of forest carbon balance was funded by the Ministry of Environment and the Ministry of Agriculture and Forestry.

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