The spatial variability of energy and carbon dioxide fluxes at the floor of a deciduous forest

Fluxes of carbon dioxide, water and sensible heat were measured using three different eddy covariance systems above the forest floor of a closed decid uous forest (leaf area index approximate to 6). The primary objective was t o examine the representativeness of a single eddy covariance system in esti mating soil respiration for time scales ranging from one-half hour to more than one week. Experiments were conducted in which the eddy covariance sens ors were in one of three configurations: i) collocated, ii) separated horiz ontally or iii) separated vertically. A measure of the variation between th e three systems (CV',related to the coefficient of variation) for half-hour carbon dioxide fluxes was 0.14 (collocated systems), 0.34 (vertically sepa rated systems at 1, 2 and 4 m above the surface), and 0.57 (systems horizon tally separated by 30 m). A similar variation was found for other scalar fl uxes (sensible and latent heat). Variability between systems decreased as t he number of half-hour sampling periods used to obtain mean fluxes was incr eased. After forty-eight hours (means from ninety-six half-hour samples), C V' values for carbon dioxide fluxes were 0.07, 0.09 and 0.16 in the colloca ted, vertically separated and horizontally separated experiments, respectiv ely. The time dependence of variability has implications on the appropriate ness of using short-term measurements in modelling validation studies. Ther e are also implications concerning the appropriate number of half-hour samp les necessary to obtain reliable causal relationships between flux data and environmental parameters. Based on the longer-term measurements, we also d iscuss the representativeness of a single eddy covariance system in long-te rm monitoring of soil respiration and evaporation beneath forest canopies u sing the eddy covariance method.