WINTER AND SPRING THAW AS OBSERVED WITH IMAGING RADAR AT BOREAS

Measurements of the length of the growing season in the boreal regions , during which significant carbon exchange due to metabolic activity o ccurs, may improve current estimates of annual CO2 fluxes at high nort hern latitudes. For coniferous, evergreen forest species, the summer f rost free period bounds the growing season length and period of net ca rbon uptake. Spring soil thaw bounds the period of soil respiration an d decomposition and thus carbon release. The balance of these to excha nges determines whether the boreal region is a net carbon source or si nk. Imaging radar data can potentially be used to monitor these period s of soil and canopy thaw due to the sensitivity of radar to surface f reeze/thaw state. In considering the use of imaging radar, two issues must be addressed. First, the temporal relationship between the time o f freezing and thawing of the forest canopy and soil and the periods o f photosynthetic and respiration activity must be ascertained. Second, the sensitivity of imaging radar to freeze/thaw processes in each of the forest components must be assessed. Of particular interest is the extent to which radar is selectively sensitive to tree and soil thawin g. In 1994, in situ soil, stem and root temperatures, and stem xylem f lux were measured over a complete annual cycle at the Boreal Ecosystem -Atmosphere Study (BOREAS) test sites in Canada. Imaging radar data fr om the European Space Agency Remote Sensing (ERS-1) satellite were als o acquired throughout 1994. The in situ temperature data show clear tr ansitions in soil and stem thawing related to the start of soil respir ation and canopy photosynthesis, respectively. The imaging radar data show clear shifts in backscatter related directly to soil thaw, and po ssibly to canopy thaw, as two independent transitions. These results a re compared to seasonal ecosystem model results for carbon exchange.