Strong links between teleconnections and canopy CO2 exchange found at a Pacific Northwest old-growth forest

Variability in three Pacific teleconnection patterns are examined to see if net carbon exchange at a low-elevation, old-growth forest is affected by climatic changes associated with these periodicities. Examined are the Pacific Decadal Oscillation (PDO), Pacific/North American Oscillation (PNA) and El Niño-Southern Oscillation (ENSO). We use six years of eddy covariance CO2, H2O and energy fluxes measured at the Wind River AmeriFlux site, Washington, USA and five years of tower-pixel remote sensing data from the Moderate Resolution Imaging Spectroradiometer (MODIS) to address this question. The PDO, PNA and ENSO collectively explained 90% of the variance in annual NEP over the six year period. The forest transitioned from an annual carbon sink (net ecosystem production (NEP) = + 207 g C m-2 year-1) to a source (NEP = - 100 g C m-2 year-1) during two dominant teleconnection patterns between 1999 and 2004. The carbon sink year (1999) occurred during a strong La Niña while the source year (2003) occurred during El Niño. When climate indices are in-phase, i.e. all are negative (cool) or positive (warm), the greatest anomalies in carbon flux and mechanistic variables (light use-, water use-efficiency) were observed. Annual averages were + 0.63 g C m-2 day-1 (- 0.27 g C m-2 day-1) for NEP, 3.1 mg C / g H2O (4.1 mg C / g H2O) for WUE, and 1.7 g C MJ–1 (2.1 g C MJ-1) for LUE during in-phase cool (warm) years.

Teleconnection patterns were connected to variability in the MODIS Enhanced Vegetation Index (EVI) but not to MODIS Fraction of absorbed Photosynthetically Active Radiation (FPAR). This work suggests that any increase in the strength or frequency of ENSO events coinciding with in-phase, low frequency Pacific oscillations (PDO and PNA) could substantially increase or decrease net CO2 uptake in similar Pacific Northwest forests.