Termination of the growing season coincided with
decreases in mean daily air, vegetation and soil temperatures
below approximately 1 – 5 jC. Net daily carbon
uptake and sap flow during fall was also generally lower
than in the spring. This pattern was likely a response to
reduced photosynthetic carbon assimilation due to shorter
days and decreased solar radiation availability in fall
relative to spring. Initial fall freeze events coincided with
short-term reductions in xylem sap flow and temporary
shifts to a net daily release of CO2 by vegetation. These
events ranged from 1 to 5 days and primarily involved
freezing of air and vegetation temperatures rather than
soil. These periods were generally followed by the renewal
of xylem sap flow and a pattern of mean daily net CO2
uptake with the return of favorable temperatures. Seasonally
frozen temperatures, indicated by the persistence (i.e.,
>2 weeks) of air, vegetation and surface soil layer temperatures
below 0 jC, occurred from 4 days to 3.5 weeks
following the end of the growing season. Sap flow and
CO2 flux measurements indicated that vegetation remained
inactive at all sites during the winter months. Temperatures
at the relatively warm KNI and NWT sites remained
near 0 jC with frequent air and vegetation thawing and
freezing events throughout the winter months, while soil
temperatures remained frozen but generally warmer than
4 jC.