Carbon and energy exchange between an old-growth forest and the atmosphere

CO2 and energy exchange were measured above a temperate Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) and Western Hemlock (Tsuga heterophylla (Raf.) Sarg.) old-growth forest at the Wind River Canopy Crane Research Facility in the Cascade Mountains in southern Washington State. Continuous measurements were conducted from July 1998 through 2004 using the eddy-covariance technique at 70 meters on a construction crane tower and 3 meters above the forest floor.; The results of long-term measurements showed a high degree of interannual variability in the net ecosystem carbon exchange at the Wind River Canopy Crane site. The observed Pacific Northwest old-growth forest was found to be a large carbon sink under favorable climatic conditions, which include lower than normal spring and early summer temperatures and additional precipitation during the summer. While the first complete year of observation (1999) was such a comparatively large carbon sink (-2 Mg C ha-1 yr -1), the site is on average a moderate sink (-0.5 Mg C ha-1 yr-1). However 2001 was found to be a small source of carbon of 0.5 Mg C ha-1 yr-1.; Maximum observed soil and understory fluxes were 6 mumol m-2 s-1 on a monthly average in summer decreasing to 1-2 mumol 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 5 cm during periods of moderate soil moisture, but moisture effects required a modification of the equation at low and very high soil moisture. Annual estimates of soil and understory respiration range from 8.7 to 11.5 Mg C ha-1 yr-1 for a period of 5.5 years with an average of 10.9 Mg C ha-1 yr-1 . The large observed annual soil efflux is consistent with the presence of large carbon stocks at the Wind River site. Enhanced or suppressed respiration has significant influences on the carbon balance of the entire stand. In years with low overall respiration the stand can act as a strong carbon sink (1999), whereas increased respiratory fluxes in other years can turn the ecosystem into a weak to moderate source (2001).