Stomatal acclimation influences water and carbon fluxes of a beech canopy in northern Germany

The response of beech leaves to leaf-to-air vapour pressure deficit (VPD) is not constant throughout the growing season. In situ measurements of leaf gas exchange showed that this intra-annual variation of stomatal sensitivity to VPD cannot be described adequately by a simple annual course of phenology alone. Reduction in stomata] aperture in dry air was negatively correlated with the sum of photosynthetic photon flux density (PPFD) of the previous three weeks, but was positively correlated with the mean VPD of the previous month. These acclimation effects contribute significantly to explaining variances in stomatal sensitivity across the growing season. The observed mid-term reaction to PPFD is in agreement with recent findings regarding the blue light perception of stomatal guard cells. A mathematical formulation representing these mid-term variations in stomatal sensitivity was incorporated into an ecosystem flux model describing the carbon and water exchange between forests and the atmosphere. Model results were in good agreement with independent eddy covariance measurements above the beech canopy. The model, therefore, is useful for investigating the importance of stomatal acclimation for ecosystem carbon and water balances. At an annual scale, acclimation appears to influence the fluxes moderately (less than 10%). The model suggests, however, that stomatal acclimation optimises the seasonal course of carbon gain and water loss of beech forests significantly with respect to the varying availabilities of water and energy.