Mass, heat and momentum exchange between a mature eucalyptus forest and the atmosphere

Abstract

The exchanges of momentum, sensible heat and CO 2 between a mature Eucalyptus forest and the atmosphere were measured between April 17 and 27, 1975, using aerodynamic flux-profile relationships. The accuracy of these estimates of the flux are heavily dependent on the reliability of the measurements of the very small gradients of mean windspeed, temperature and carbon dioxide concentrations above the forest, and on the proper determination of the aerodynamic parameters of the forest; viz., the zero plane displacement d, the roughness length z 0 or a friction coefficient C f. d, z 0 and C f were determined from wind profiles under neutral conditions on the assumption that the logarithmic law is valid for measurements made “close” to the roughness elements. It was further assumed that these parameters are independent of atmospheric stability, and that d may be identified with the height of the mean sink for momentum within the canopy. A common value of d was used in the calculation of the fluxes of momentum, sensible heat and CO 2 from profiles of windspeed, temperature and CO 2. The fluxes of momentum calculated using wind profiles agreed well in all stability classes with the estimates of τ computed using a low-level windspeed and the friction coefficient C f. These latter values of τ also provided estimates of the fluxes of sensible heat and CO 2 which were equally as good as those obtained from aerodynamic flux-profile formulae. In view of the great difficulty in establishing the correct values of d and C f, it is desirable that other studies involving the flux-gradient approach should use an independently measured flux (such as sensible heat) as a tracer for estimating the CO 2 flux. The results of the present study show that the eucalypt forest has rates of CO 2 exchange which are comparable with other natural vegetation types. Typical midday rates of photosynthesis were 1 – 7 · 10 −7 kg CO 2 m −2 s −1 and night-time respiration rates of 0 – 3 · 10 −7 kg CO 2 m −2 s −1.