Understanding the forest carbon cycle is important for ecologists, land managers, and policy makers concerned with climate change.  Release of CO₂ from the forest floor by soil respiration is a significant contributor to the forest carbon budget, and inter-annual variability in soil respiration can have a large effect on ecosystem respiration.  Studies on daily and seasonal soil respiration patterns and response to disturbances are available, but multi-year studies are rare. Our study examined how biophysical controls influence diurnal and annual trends in soil respiration over four years (2005-2008) at a temperate Douglas-fir and Western Hemlock old-growth forest within the Wind River Experimental Forest and T.T. Munger Research Natural Area, in Washington State. Soil respiration was measured continuously by eight clear auto-chambers arranged in a 15 meter radius circular plot during the growing season.  Soil moisture, soil temperature, and meteorological variables were also measured.  
Results/Conclusions

Soil respiration measurements from the auto-chambers varied as much as four-fold, showing some local variability.  However, measurements among chambers were highly correlated (typically, R=0.7-0.9, p<.01), and displayed the same diurnal and annual trends in soil respiration across chambers, which indicated little or no chamber x environment interaction.  Diurnal trends in soil respiration were not related to soil temperature or soil moisture, and we observed a hysteresis effect of soil respiration with respect to soil temperature.  Inter-annual variability in soil respiration over the growing season was strongly negatively related to vapor pressure deficit, which is an indicator of canopy photosynthesis, and not related to soil temperature or soil moisture.  During the four years of observation, mean growing season soil respiration rate steadily declined, and did not reflect inter-annual variability in precipitation or temperature.  Overall, soil respiration in this old-growth forest appears to be driven by photosynthesis.  Future work should compare biophysical controls on soil respiration within and among various forest ecosystems, such as sub-alpine coniferous forest and deciduous forest.