Author: Santoro M.
Remote sensing observations from space are evolving to operational tools, possibly in complement with measurements taken on the ground. In the forestry sector, the use of satellite data is appealing because these cover large areas within a short period of time providing repeated snapshots of the landscape. One type of sensor that is particularly suited to map and monitor northern forest is a synthetic aperture radar (SAR). Radars send and receive microwaves and are, therefore, unaffected by cloud cover and solar illumination. With an adequate observing strategy, it is possible to extract the best possible information from the data with a relatively small number of images. Such a strategy has been implemented by the Japan Aerospace Exploration Agency (JAXA) for their Advanced Land Observing Satellites (ALOS-1 and ALOS-2). The Phased Array L-band SAR (PALSAR) instrument carried by each of the two satellites provides repeated full coverages of all land masses since 2006 in preferred seasons. For example, the European Nordic countries are imaged every 46 days between late spring and early autumn, when the image contrast over forests is the largest. Maximizing the image contrast means stronger sensitivity of the observations to forest biophysical parameters. As the image contrast also depends on the wavelength at which the sensor operates and L-band is the longest wavelength currently operated in space, the ALOS satellites are currently the remote sensing data source most suited to map and monitor forest variables at landscape to global scales. Being sensitive to forest structure, ALOS observations are therefore closely linked to the carbon content in forests.
With by now more than one decade of observations by ALOS-1 and ALOS-2, it is possible to create time series to identify large-scale changes in the forest landscape. A recently published paper ’Dynamics of the Swedish forest carbon pool between 2010 and 2015 estimated from satellite L-band SAR observations’ investigates the prospect of the ALOS data acquired in 2010 and in 2015 to quantify the dynamics of the carbon pool stored in Swedish forests over five years. This is the first attempt to use nation-wide maps from satellite remote sensing data at high spatial resolution (25 m) and identify sinks and sources of carbon. While estimates of the carbon stored in forests are uncertain at the level of individual pixels, averages and stocks at the landscape, county and national level are reliable. Results indicate that Swedish forests acted as a carbon sink between 2010 and 2015 with a net increase of carbon stocks by 2.5%. These statistics are in line with numbers obtained from data of the National Forest Inventory.
In the Forest Carbon Monitoring project, we plan to implement a similar procedure but develop it further by utilizing both L-band data from ALOS-2 as well as C-band data from the European Space Agency Sentinel-1 satellite. The Sentinel-1 satellite provides C-band observations in 10 m spatial resolution in six-day frequency in Europe. With this procedure we will be able to quantify growing stock volume, above- and below-ground biomass and, thereof, total carbon stored in European forests.
Full citation of the article: Santoro, M., Cartus, O., Fransson, J.E.S., 2022. Dynamics of the Swedish forest carbon pool between 2010 and 2015 estimated from satellite L-band SAR observations. Remote Sensing of Environment 270, 112846. https://doi.org/10.1016/j.rse.2021.112846