Toward biologically meaningful net carbon exchange estimates for tall, dense canopies: multi-level eddy covariance observations and canopy coupling regimes in a mature Douglas-fir forest in Oregon
Abstract
We sought to improve net ecosystem exchange (NEE) estimates for a tall, dense, mature Douglas-Fir forest in the Oregon Coast range located in moderately complex terrain and characterized by weak flows, directional shear, and limited turbulent mixing throughout the diurnal period. We used eddy covariance (EC) observations at two levels and concurrent biological measurements of carbon and water fluxes collected over a period of 6 years (2006-2011) to develop and test a conceptual framework with the goal of i) reducing uncertainty by retaining more measurements for the computation of annual NEE estimates, and ii) producing defendable and biologically meaningful NEE estimates by accounting for the missing sub-canopy respiration due to the weak turbulence. The framework assumes that the scalar exchange between vertical layers can be categorized into discrete canopy coupling regimes, and that advection leads to a systematic loss of scalar from the observational volume that can indirectly be estimated and accounted for as sub-canopy respiration flux when canopy layers are decoupled. The standard deviation of the vertical velocity variance was the most adequate proxy for turbulent mixing strength. It allowed for straight-forward estimation of thresholds used to delineate the exchange regimes and was more sensitive to directional shear and other mechanisms enhancing the mixing. Periods with a decoupled sub-canopy layer dominated and occupied 65 and 88 % of the day- and nighttime periods, respectively. Annual NEE derived from the new framework was estimated as 480 gC m-2 yr-1, which was reduced by 620 gC m-2 yr-1 compared to traditional estimates from single-level EC data filtered using a critical friction velocity. The reduction in NEE was caused by an enhanced ecosystem respiration (ER), while gross ecosystem productivity remained unchanged. Improved ER estimates agreed well with those from independent biological estimates including soil, stem, and foliage respiration within 3 %. Risks and limitations of the new framework are discussed. We conclude that concurrent above- and sub-canopy EC observations are essential to compute meaningful carbon fluxes in tall, dense canopies, which do not lend themselves to standardized processing. The new framework may help including more tall and dense forests in global carbon cycle synthesis and modeling efforts.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B51B0513T
- Keywords:
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- 0315 ATMOSPHERIC COMPOSITION AND STRUCTURE / Biosphere/atmosphere interactions;
- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling;
- 0426 BIOGEOSCIENCES / Biosphere/atmosphere interactions;
- 0428 BIOGEOSCIENCES / Carbon cycling