Water balance, transpiration and canopy conductance in two beech stands

Measurements of sap flow, vapour fluxes, throughfall and soil water content were conducted for 19 months in a young beech stand growing at low elevati on, in the Hesse forest. This experiment is part of the Euroflux network, c overing 15 representative European forests. Study of the radial variation o f sap flow within tree trunks, showed a general pattern of sap flux density in relation to the depth below cambium. Among-tree variation of sap flow w as also assessed, in order to determine the contribution of the different c rown classes to the total stand transpiration. Stand sap flow and vapour fl ux, measured with eddy covariance technique, were well correlated, for half hourly as well for daily values, the ratios of the fluxes for both averagi ng periods being 0.77. A strong canopy coupling to the atmosphere was found , omega factor ranging between 0.05 and 0.20 relative to the windspeed. Can opy conductance variation was related to a range of environmental variables : global radiation, vapour pressure deficit, air temperature and soil water deficit. In addition to the effect of radiation and of vapour pressure def icit often found in various other tree species, here beech exhibited a stro ng reduction in canopy conductance when air temperature decreased below 17 degrees C. The model of transpiration was calibrated using data measured in the Hesse forest and applied to another beech stand under mountainous cond itions in the Vosges mountains (east France). Measured and modelled stand t ranspiration were in good agreement. (C) 2000 Published by Elsevier Science B.V. All rights reserved.