Using eddy covariance measurements for the carbon dioxide flux and hyperbolic relaxed eddy accumulation measurements for the flux of the stable 13C isotope , the separation of assimilation and respiration was investigated. An experiment took place in 2000 at the 'Weidenbrunnen' measuring site of the University of Bayreuth in the hilly region of the Fichtelgebirge, 50° 09' N, 11° 52'E, 765 m a.s.l. over a 19 m high spruce forest. In principle, it is possible to determine from these two fluxes the assimilation and respiration using balance equations and the ratio of the isotopes 13C/12C which differs for respired and atmospheric air. This method is only realistically, if the canopy is well coupled with the air above it. A mixing layer according to Raupach et al. (1996) was frequently found above the forest. It was identified by the examination of wind profile measurements and of scales of coherent structures. Typical scales and the period of ramp structures were analysed using wavelet analysis for the time series of vertical wind velocity and carbon dioxide. Four different exchange conditions were found depending on whether a mixing layer exists (canopy and atmosphere are (a) coupled or (b) not) or not (exchange of momentum and carbon dioxide takes place on (c) similar or (d) different time scales). They will be discussed according to experimental data.