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.
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 (
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