Stomatal behaviour in a beech canopy : an analysis of Bowen ratio measurements compared with porometer data
On the basis of measurements of stand transpiration and microclimate, the bulk stomatal or bulk leaf conductance (g L ) of a beech forest in northern Germany was calculated for periods in which leaves were fully expanded and the canopy was dry. This conductance depends strongly on light and humidity conditions above the forest. During periods with photosynthetic photon flux densities Q > 1200 μmol m -2 s -1 , g L was reduced from 1500 mmol m -2 s -1 at a vapour pressure deficit D = 0.5 kPa to 500 mmol m -2 s -1 at D = 2 kPa. Light saturation of g L was not reached until Q = 1200 μmol m -2 s -1 at low D, or until even higher Q at higher D. The dependence of g L on Q and D was described mathematically by a non-linear equation that requires two empirical parameters. Values for g L as simulated by this equation provided a satisfactory agreement with independent porometer data collected on single leaves and scaled up to the canopy. A comparison of stomatal and aerodynamic conductances showed a strong coupling between the forest canopy and the atmosphere, indicating that transpiration of the beech forest is controlled mainly by the stomata.