Global forest mapping for carbon sink assessment
By Tuomas Häme
Pdf-file of the article, from Positio ICC Special Issue 2011
Natural resources satellites that circulate the globe frequently collect images that can be used for forest monitoring from local to global extents. VTT Technical Research Centre of Finland is developing a satellite image based forest monitoring system that improves the accuracy of forest resources assessment from satellites. The system is developed in the Recover project that is VTT led study with an international consortium.
The world’s total forest area is just over 4 billion hectares, which corresponds to an average of 0.6 ha for every man. The forests store 289 gigatonnes (Gt) of carbon in their biomass alone. However, the global carbon stocks in forest biomass decreased by an estimated 0.5 Gt annually during the period 2005–2010. The main reason for this is considered the reduction of the tropical forest area. In Europe and Asia, the forest area has increased.
Forest is decreasing – or is it?
The most reliable statistical data on global forest extent comes from the Forest Resources Assessment of the Food and Agriculture Organization (FAO) of the United Nations. The assessment is based on questionnaires that are filled in by the UN member countries. The quality and time of information collection vary from country to country.
The information becomes more unreliable when characteristics more complex such as the biomass are concerned. Some researchers think that increased forest growth largely has compensated the carbon reduction by deforestation. On the other hand, particularly the tropical forests are degrading. This means that the ground remains forested but the biomass has permanently decreased. Deforestation and forest degradation are estimated to contribute for more than 15 percent of global greenhouse gas emissions but the figure is uncertain.
Natural resources satellites that circulate the globe frequently collect images that can be used for forest monitoring from local to global extents. An attractive idea is just to make a satellite image map of forests and forget the laborious and expensive forest inventories and questionnaires.
Several global forest and land cover maps have actually been done. The most accurate global mapping so far has been made under a contract by the European Space Agency ESA. This map has 300 meter ground resolution.
However, the comparison of such a map and other coarser maps with each other and ground data has shown that although such maps are applicable to give a general picture on forest distribution they are unreliable as a source of forest inventory data. Even the satellite maps with thirty meter ground resolution can include systematic over or underestimates of forest area, which endangers their use as a source of reliable statistical data.
SilvaSat helps forests to Recover
VTT Technical Research Centre of Finland is developing a satellite image based forest monitoring system that takes into consideration the drawbacks of the present methods. The principal idea is to combine satellite images that have a coarse ground resolution with a sample of very high resolution images. The coarse images can cover the whole area of interest “wall to wall”, up to the whole globe.
The sample of accurate images gives reliable information on the extent of forest area and the degree of degradation. With the help of ground measurements and possible airborne laser scanner data, also forest biomass can be estimated. Combination of “wall to wall” maps with the sampled information makes it possible to compute reliable statistical data on forests everywhere.
Presently, the very high resolution images are collected by commercial satellites on order basis and they are concentrated in urban regions. VTT has suggested launching a specific forest satellite, SilvaSat to collect the very high resolution image sample globally. Such satellite would cost approximately 50 million euro.
VTT is leading an international consortium to apply and further develop the SilvaSat concept and other methods for the monitoring of tropical forests. The project is funded by the 7th Framework Program of the European Commission. The team includes both research organizations and private enterprises. The project, Recover, started in November 2010 and will last three years.
The services that are developed by Recover are particularly meant to support the REDD program – Reduction of Emissions from Deforestation and forest Degradation. The REDD program is an effort to create a financial value for the carbon stored in forests. It offers incentives for developing countries to reduce emissions from forested lands and invest in low-carbon paths to sustainable development. The Recover has its study sites and user partners in Latin America, Africa, and Oceania.
The methods that are developed can be applied globally to improve information on forest resources and their changes and thus to further sustainable forest management.
The author works as a Research Professor in Earth Observation at the VTT Technical Research Centre of Finland. His special expertise area is forest inventory and monitoring from satellite data, contact: Tuomas.Hame[at]vtt.fi
6.8.2011 11:07