Old-growth CO2 flux measurements reveal high sensitivity to climate anomalies across seasonal, annual and decadal time scales

• Published in: Agricultural and forest meteorology Vol. 161; pp. 1 - 14
• Main Authors: Wharton, S., Falk, M., Bible, K., Schroeder, M., Paw U, K.T.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15-08-2012; Elsevier
• Subjects: 60 APPLIED LIFE SCIENCES; Agricultural and forest climatology and meteorology. Irrigation. Drainage; Agronomy. Soil science and plant productions; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; biometry; canopy; Carbon; Carbon dioxide; carbon markets; carbon sequestration; carbon sinks; Climate; climate models; CO2 flux; eddy covariance; ENSO; ENVIRONMENTAL SCIENCES; Flux; FLUXNET; Forests; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; Interannual variability; Long-term fluxes; mortality; Net ecosystem exchange; Northwest; old-growth forests; PDO; Phase shift; primary productivity; recruitment; Stands; Synecology; Teleconnections; water use efficiency; weather; INE, USA, Pacific Northwest; I, Pacific; CO2 flux; ENSO; Interannual variability; Net ecosystem exchange; Long-term fluxes; PDO; Teleconnections; FLUXNET; Measurement; Teleconnection; Climate; Material flow; Carbon dioxide; Physical environment; Time variation; Gas flux; Seasonal variation; Southern oscillation; flux; Anomaly; Primary forest; Exchange capacity; CO; El Niño; Ecosystem functioning; Gas exchange
• ISSN: 0168-1923; 1873-2240
• DOI: 10.1016/j.agrformet.2012.03.007

► The old-growth forest is a small net sink of carbon over 10 years but exhibits high interannual variability. ► FNEE is closely linked to climate perturbations resulting from major Pacific teleconnection events. ► Nearly 60 years of biometric FANPP data reveal long-term carbon trends associated with the PDO. ► Positive climate years lead to reduced carbon sink activity and increased respiration.
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±40gCm−2yr−1 (a small net carbon sink) while interannual variability was high (∼300gCm−2yr−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.