Carbon dioxide (CO2) exchange was measured above the forest floor of a temperate Douglas-fir (Pseudotsuga menziesii (Mirbel)
Franco) and Western Hemlock (Tsuga heterophylla (Raf.) Sarg.) old-growth forest in southern Washington State. Continuous flux
measurements were conducted from July 1998 to December 2003 using the eddy-covariance technique. Maximum observed fluxes
were +6 mmol m2 s1 on a monthly average in summer decreasing to +1 to +2 mmol m2 s1 in winter and early spring. Nighttime
soil and understory exchange was well described by an exponential function of soil temperature at a depth of 50 mm during periods
of moderate soil moisture, but moisture effects required a modification of the equation at low (0.25 m3 m3) and very high
(0.35 m3 m3) soil moisture. Interannual variation was observed in soil respiration and understory carbon exchange and linked to
interannual variability in soil moisture and temperature. Maximum CO2 exchange occurred at different times amongst years; a
maximum daily CO2 flux was measured as early as May in 2000 and as late as July in 2001. Summer understory photosynthesis was
shown to be up to 2 mmol m2 s1 with some interannual variability. Understory net photosynthesis never exceeded net CO2
efflux on a half-hourly basis, implying at no time was all of the soil respiration recycled by understory photosynthesis. Maximum
daily carbon exchange ranged from +5 to +7 g C m2 day1 in the summer months and was greatly reduced (but was still non-zero)
in the wintertime due to lower soil temperatures, with daily values ranging from +0.5 to +1 g C m2 day1. Annual estimates of soil
and understory respiration range from 8.7 to 12.8 Mg C ha1 year1 for a period of 5.5 years with an average of
11.1 1.5 Mg C ha1 year1. The large observed annual soil efflux is consistent with the presence of large carbon stocks at
the Wind River site.
