Is soil respiration a major contributor to the carbon budget within a Pacific Northwest old-growth forest?

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 μmol m−2 s−1 on a monthly average in summer decreasing to +1 to +2 μmol m−2 s−1 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 m−3) and very high (0.35 m3 m−3) 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 μmol m−2 s−1 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 m−2 day−1 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 m−2 day−1. Annual estimates of soil and understory respiration range from 8.7 to 12.8 Mg C ha−1 year−1 for a period of 5.5 years with an average of 11.1 ± 1.5 Mg C ha−1 year−1. The large observed annual soil efflux is consistent with the presence of large carbon stocks at the Wind River site.