van Gorsel, E., Leuning, R., Cleugh, H.A., Keith, H., Kirschbaum, M.U.F., Suni, T. (2008). Estimating net ecosystem exchange in moderately complex topography. Agricultural and Forest Meteorology 148: 1174-1180. 


Abstract. Even moderately complex topography can lead to significant horizontal and vertical advection and a consequent underestimation of nocturnal CO2 effluxes derived from eddy covariance measurements on a single tower. The standard approach to select nighttime eddy flux data uses a threshold in friction velocity to exclude periods when advection is important but this is problematical in situations where turbulence is intermittent. van Gorsel et al. (2007, TellusB, 59B, 397-403) have developed an alternative method that estimates the CO2 flux from the maximum of the eddy flux plus change in storage term in the period after sunset when stable stratification develops. During this time the advection terms do not contribute significantly to the mass balance of the air layer below the eddy flux instruments at Tumbarumba flux station (SE Australia). Advection dominates only later in the night, following the development of large horizontal and vertical gradients of CO2. As net nighttime and daytime fluxes are often of similar magnitude but opposite in sign, underestimation of respiration can lead to large errors in annual ecosystem carbon budgets.


Nocturnal respiration rates obtained by using the new approach are systematically higher compared to previously published results that used a u*-threshold filter and are in excellent agreement with independent chamber measurements. Revised Net Ecosystem Exchange (NEE) calculations are therefore significantly reduced compared to previously published ones (with a residual of 5.6 +/- 0.5 tC ha-1 year-1) and compare well with results from an ecosystem model (-0.2 +/- 0.7 tC ha-1 year-1).


Due to a drought period in 2003 and a subsequent insect attack NEE at Tumbarumba has been highly variable. Within 5 years the forest changed from being a carbon sink of -7.5 tC ha-1 year-1 to a source of +2.5 tC ha-1 year-1. This high variability demonstrates the importance of long-term high-resolution time series when interpreting ecosystem data. It

further confirms the importance of using multiple measurement techniques and modelling to improve confidence in the mutually constrained NEE values.


Keywords: advection, gravity flows, respiration, eddy flux, cold-air drainage, chamber, forest growth model


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