The Effect of Local Atmospheric Circulations on Daytime Carbon Dioxide Flux Measurements over a Pinus elliottii Canopy

The daytime net ecosystem exchange of CO2 (NEE) was measured in an even-aged slash pine plantation in northern Florida from 1999 to 2001 using the eddy covariance technique. In August 2000, two clear-cuts were formed approximately 1 km west of the study site. A statistical approach was used to determine whether the clear-cuts induced changes in CO2 concentration, wind direction, horizontal and vertical wind speeds, and temperature, as measured by instruments above the plantation canopy and, in turn, whether any such changes affected daytime NEE. The NEE estimates were first transformed so that mean responses to incident radiation and vapor pressure deficit were removed using empirically derived functions for each 30-min period. Prior to the clear-cuts, there were significant interactive effects of CO2 concentration and some wind statistics on NEE at the tower when wind was flowing from the direction of the future clear-cuts. Even in this relatively homogenous forest, with flat topography, the CO2 source strength differed with wind direction prior to the clear-cuts. After the clear-cuts, additional twoand three-way interactive effects became significant during flows from the direction of the clear-cuts. There was also a 16.6% reduction in the integrated measure of daytime NEE over 487 days after the clear-cuts. The results herein suggest that the development of local circulations over the clear-cuts contributed to low-frequency effects on the NEE.