Climate indices strongly influence old-growth forest carbon exchange

Abstract

We present a decade and a half (1998–2013) of carbon dioxide fluxes from an old-growth stand in the American Pacific Northwest to identify ecosystem-level responses to Pacific teleconnection patterns, including the El Niño/Southern Oscillation (ENSO). This study provides the longest, continuous record of old-growth eddy flux data to date from one of the longest running Fluxnet stations in the world. From 1998 to 2013, average annual net ecosystem exchange ( F _NEE ) at Wind River AmeriFlux was −32 ± 84 g C m ^−2 yr ^−1 indicating that the late seral forest is on average a small net sink of atmospheric carbon. However, interannual variability is high (>300 g C m ^−2 yr ^−1 ) and shows that the stand switches from net carbon sink to source in response to climate drivers associated with ENSO. The old-growth forest is a much stronger sink during La Niña years (mean F _NEE = −90 g C m ^−2 yr ^−1 ) than during El Niño when the stand turns carbon neutral or into a small net carbon source (mean F _NEE = +17 g C m ^−2 yr ^−1 ). Forest inventory data dating back to the 1930s show a similar correlation with the lower frequency Pacific North American (PNA) and Pacific Decadal Oscillation (PDO) whereby higher aboveground net primary productivity ( F _ANPP ) is associated with cool phases of both the PNA and PDO. These measurements add evidence that carbon exchange in old-growth stands may be more sensitive to climate variability across shorter time scales than once thought.

Keywords

• AmeriFlux
• old-growth forest
• eddy covariance
• net ecosystem exchange
• interannual variability
• Pacific teleconnections