Site-Scale Ecosystem Carbon Balance Significance of Space-Based Radar
Observations of Terrestrial Ecosystem FreezeThaw Dynamics
Steve Frolking1, Kyle McDonald2, John Kimball3, JoBea Way2,
Reiner Zimmermann4, and Steve Running3
Recent results from the BOREAS program indicate that mature boreal forest stands have a near-zero
annual carbon flux, i.e., carbon uptake through photosynthesis nearly balances carbon release
through respiration. Simulation and observation indicate significant interannual variability in carbon
balance at a site. Much of this variability may be due to changes in growing season length, for which
an important control is the timing of spring snowmelt and ecosystem thaw. An important step in
assessing year-to-year changes in the boreal net carbon flux is to determine a method for accurately
monitoring the interannual variation in growing season length. We report on initial analysis of space-
borne active microwave observations, coupled with in-situ temperature and snow depth observations
and ecosystem model simulations. We analyzed NASA Scatterometer (NSCAT) backscatter cross
sections from 9/96 through 6/97 for three sites in the BOREAS Study Area in central Canada. The
scatterometer operated at a frequency of 13.995 GHz (Ku band, 2.1-cm wavelength). The NSCAT
data had 25-km spatial resolution and twice-daily temporal coverage. Our objective is to assess the
utility of using radar-derived ecosystem freezethaw state to determine growing season length.
At the northern site the NSCAT signal showed a strong seasonality, with winter backscatter cross
sections of 8 dB, versus 12 dB in early summer and fall. The southern BOREAS sites had less
seasonality (10 dB in winter, 11 dB in early summer and fall). At all three sites there was a strong
shift in backscatter during spring snow melt (46 dB). At the southern site with the greatest fraction
of deciduous cover, the NSCAT backscatter rose steadily from 11 to 9.2 dB during the spring
leafout (June 1997); the more coniferous sites showed little rise during this period. All sites showed
shifts in backscatter of 1 to 2 dB that were coincident with snowfall, periods of extreme cold weather
(Tmax < 25C), or brief midwinter thaws. Previously developed radar freezethaw transition detec-
tion algorithms based on shifts in backscatter from mean summer or winter values gave reasonable
results for the northern site but were not successful at the two southern sites.
NSCAT's strong spring snowmelt signal (at the beginning of snowmelt) coincided with the end of
the ecosystem's steady winter respiration period; net carbon uptake by the forest ecosystem began
when the snow had completed melting about 30 days later. The end of the growing season occurred
in late September 1996 in both model simulations, when diurnal minimum air temperatures were
consistently below 0C. The NSCAT backscatter cross section dropped by about 1 dB at this time,
but this was very near the beginning of the record. Shifts in NSCAT backscatter during the winter did
not correspond to major shifts in NEE as the soils were cold and the ground snow covered all winter
long.
1
EOS Institute, University of New Hampshire, Durham, New Hampshire 03824, USA
2
NASA-Jet Propulsion Laboratory, Pasadena, California 91009, USA
3
School of Forestry, University of Montana, Missoula, Montana 59812, USA
4
Bitek, University of Bayreuth, D-95540, Bayreuth, Germany
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