Surface soil layer temperatures
were also constrained near 0 jC for 1– 7 weeks following
initial thawing of air and vegetation temperatures until the
overlying snow pack was depleted. Site soil temperature
measurements and nearby snow depth and snow water
equivalent measurements showed that the initiation of
seasonal snow pack depletion coincided with a rapid rise
in surface soil layer temperatures, indicating an influx of
water into previously frozen upper soil layers from snowmelt.
Initiation of the growing season corresponded closely
with the timing of snowmelt and the associated increase in
surface soil layer temperatures to within F 1 jC. These
results are consistent with previous observations of boreal
and subalpine evergreen coniferous forests, that the seasonal
shift of net CO2 exchange in the spring coincides with the
influx of snowmelt water into upper soil layers (Monson et
al., 2002; Jarvis & Linder, 2000). Mean daily vegetation and
air temperatures were predominantly above 1 jC during this
period, while warmer temperatures generally corresponded
with enhanced xylem sap flow and net daily CO2 uptake by
vegetation.