Cross validation of open-top chamber and eddy covariance measurements of ecosystem CO sub(2) exchange in a Florida scrub-oak ecosystem

Simultaneous measurements of net ecosystem CO sub(2) exchange (NEE) were made in a Florida scrub-oak ecosystem in August 1997 and then every month between April 2000 to July 2001, using open top chambers (NEE sub(O)) and eddy covariance (NEE sub(E)). This study provided a cross validation of these two different techniques for measuring NEE. Unique characteristics of the comparison were that the measurements were made simultaneously, in the same stand, with large replicated chambers enclosing a representative portion of the ecosystem (75 m super(2), compared to approximately 1-2 ha measured by the eddy covariance system). The value of the comparison was greatest at night, when the microclimate was minimally affected by the chambers. For six of the 12 measurement periods, night NEE sub(O) was not significantly different to night NEE sub(E), and for the other periods the maximum difference was 1.1 mu mol m super(-2)s super(-1), with an average of 0.72 plus or minus 0.09 mu mol m super(-2)s super(-1). The comparison was more difficult during the photoperiod, because of differences between the microclimate inside and outside the chambers. During the photoperiod, air temperature (T sub(air)) and air vapour pressure deficits (VPD) became progressively higher inside the chambers until mid-afternoon. In the morning NEE sub(O) was higher than NEE sub(E) by about 26%, consistent with increased temperature inside the chambers. Over the mid-day period and the afternoon, NEE sub(O) was 8% higher that NEE sub(E), regardless of the large differences in microclimate. This study demonstrates both the uses and difficulties associated with attempting to cross validate NEE measurements made in chambers and using eddy covariance. The exercise was most useful at night when the chamber had a minimal effect on microclimate, and when the measurement of NEE is most difficult.