Winter and spring thaw as observed with imaging radar at BOREAS

Abstract

Measurements of the length of the growing season in the boreal regions, during which significant carbon exchange due to metabolic activity occurs, may improve current estimates of annual CO2 fluxes at high northern latitudes. For coniferous, evergreen forest species, the summer frost free period bounds the growing season length and period of net carbon uptake. Spring soil thaw bounds the period of soil respiration and decomposition and thus carbon release. The balance of these two exchanges determines whether the boreal region is a net carbon source or sink. Imaging radar data can potentially be used to monitor these periods of soil and canopy thaw due to the sensitivity of radar to surface freeze/thaw state. In considering the use of imaging radar, two issues must be addressed. First, the temporal relationship between the time of freezing and thawing of the forest canopy and soil and the periods of 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 thawing. In 1994, in situ soil, stem and root temperatures, and stem xylem flux were measured over a complete annual cycle at the Boreal Ecosystem-Atmosphere Study (BOREAS) test sites in Canada. Imaging radar data from the European Space Agency Remote Sensing (ERS-1) satellite were also acquired throughout 1994. The in situ temperature data show clear transitions in soil and stem thawing related to the start of soil respiration and canopy photosynthesis, respectively. The imaging radar data show clear shifts in backscatter related directly to soil thaw, and possibly to canopy thaw, as two independent transitions. These results are compared to seasonal ecosystem model results for carbon exchange.