Three years of fully automated and manual measurements of soil CO2 efflux, soil moisture and temperature were used to explore the diel, seasonal and inter-annual patterns of soil efflux in an old-growth (250-year-old, O site) and recently regenerating (14-year-old, Y site) ponderosa pine forest in central Oregon. The data were used in conjunction with empirical models to determine which variables could be used to predict soil efflux in forests of contrasting ages and disturbance histories. Both stands experienced similar meteorological conditions with moderately cold wet winters and hot dry summers. Soil CO2 efflux at both sites showed large inter-annual variability that could be attributed to soil moisture availability in the deeper soil horizons (O site) and the quantity of summer rainfall (Y site). Seasonal patterns of soil CO2 efflux at the O site showed a strong positive correlation between diel mean soil CO2efflux and soil temperature at 64 cm depth whereas diel mean soil efflux at the Y site declined before maximum soil temperature occurred during summer drought. The use of diel mean soil temperature and soil water potential inferred from predawn foliage water potential measurements could account for 80% of the variance of diel mean soil efflux across 3 years at both sites, however, the functional shape of the soil water potential constraint was site-specific. Based on the similarity of the decomposition rates of litter and fine roots between sites, but greater productivity and amount of fine litter detritus available for decomposition at the O site, we would expect higher rates of soil CO2 efflux at the O site. However, annual rates were only higher at the O site in one of the 3 years (597 ± 45 vs. 427 ± 80 g C m−2). Seasonal patterns of soil efflux at both sites showed influences of soil water limitations that were also reflected in patterns of canopy stomatal conductance, suggesting strong linkages between above and below ground processes.
