SOIL-MOISTURE VARIATION AND PLANT WATER-STRESS AT THE HARTHEIM SCOTS PINE PLANTATION

During two measurement campaigns in 1992 (the Hartheim Experiment Hart X- and an additional experiment in autumn), measurements of soil moist ure were carried out in a Pinus sylvestris is stand at Hartheim on the Oberrhein. Several methods were used to determine soil water status. They were compared in terms of suitability for estimating stand evapot ranspiration (ET) via soil water depletion. Measurements of tree water potential suggested that conductance of the trees was affected by soi l water depletion during the period of the HartX campaign in spring 19 92. We interpret the observations to indicate a lesser influence of so il water availability on tree transpiration during the autumn experime nt. Eddy correlation and xylem sapflow measurements provided reference ET values with which to compare the stand ET calculations based on so il moisture measurements. Profile measurements of soil moisture showed that particularly in springtime when the lower soil layers are satura ted with water, the water transport from depths below the major rootin g zone is a very important factor affecting evaluation of stand ET. De creases in soil water storage may be determined best with permanently installed soil moisture sensors such as used in tensiometric or TDR me asurements that provide high resolution of changes over time.