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.
Highlights
Introduction must be ascertainedSecondly, the sensitivity of spaceborne imaging radar backscatterto freeze/thaw processesmust beIn the boreal forest region and for coniferousforest species assessedI.n particular, it isnecessaryto evaluatewhether backthe summerfrost free period boundsthe growingseasonlength scatter is selectivelysensitive to tree and soil freezing and [Walterand Breck/c,1991, 1986].For both coniferousand de- thawing,or dominated by one of thesecomponents.ciduoustrees,their growthpotential is further limited by their To addressthese questions,in situ soil and stem temperamineral and water uptake
For the broader landscape the tures and stem xylem flux were measured over a complete springsoilthaw boundsthe period of root and soil respiration annual cycle in 1994 at the Boreal Ecosystem-Atmosphere and decompositionE. stimatingthe onsetand the duration of Study (BOREAS) test sites in Canada [Sellerse' l al., 1995]
ERS-1 imagingradar data were collectedfor each overpass of the BOREAS region
Summary
Local Scale 500m diameter for global access,to 783, providing a 168 day repeat for a geodeticportion of the mission.In 1994,ERS-1 wasin a 3 day Towers. The orbit was shiftedto the first geodeticphaseon April 10with an equatorcrossingoptimized for the spaceborneimagingradar(SIR-C/X-SAR) shuttlemis-. Canopyphysiology,biometry Soilfluxes(CO2,CH4, CO) Bole, soil temperature Xylem flux sionwhichflew in April 1994. On April 17 and October 1 the equator crossingswere shifted, still maintaining a 168 day repeat. Data from ERS-1 are transmittedto a number of ground receiving stations, including the National Aeronautics and SpaceAdministrationAlaskaSAR Facility(ASF) at the Universityof Alaska GeophysicaIlnstitutein Fairbanks[Welleret al., 1983; Carsey,1988]. Perature, relative humidity,global short-waveradiation, canopyradiationtemperature,precipitationusinga rain bucketon the tower and an ultrasonicsnowdepth sensoron the ground, air pressure,canopyair temperature,and soil temperatureat.
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