The traditional hypothesis that old-growth forests are carbon neutral is under debate as recent studies
show evidence of net carbon sequestration. Here, we present a decade (1998–2008) of carbon dioxide,
water and energy fluxes from an old-growth stand in the American Pacific Northwest to identify
ecosystem-level responses to climate variability, including teleconnection patterns. This study provides
the longest, continuous record of old-growth eddy flux data to date. From 1998 to 2008, average annual net ecosystem exchange (FNEE) was −49 ± 40 g C m−2 yr−1 (a small net carbon sink) while interannual variability was high (∼300 g C m−2 yr−1) and indicated that the stand is able to switch from net carbon sink to source in response to climate forcing. Seasonal and annual FNEE variability was strongly linked to climate anomalies associated with major teleconnections and the subsequent responses of driving mechanisms (e.g., water use efficiency, light use efficiency, canopy conductance) to local weather (e.g., cloudiness). Biometric measurements of aboveground net primary productivity (FANPP) provided a ∼60 year record of growth, recruitment, and mortality responses to a longer range of climatic conditions, including shifts in the Pacific Decadal Oscillation (PDO). A negative trend in FANPP generally matched the warm PDO phase shift starting in 1977. As climate models predict future warming in the Pacific Northwest, our results suggest that any perturbations towards a warmer, drier state, such as would occur during positive climate phases, may have significant impacts on regional terrestrial carbon budgets through increasing respiration without subsequent, offsetting increases in carbon assimilation in these old-growth forests.
