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

Abstract

Carbon dioxide (CO 2) 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 m 3 m −3) and very high (0.35 m 3 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 CO 2 exchange occurred at different times amongst years; a maximum daily CO 2 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 CO 2 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.