Xylem diameter changes as an indicator of stand-level evapo-transpiration

Xylem diameters of living trees change diurnally. According to the cohesion-tension theory these changes result from changes in sapwood water content. A model based on the assumptions of that theory was used to predict diameter changes from the water vapour exchange in a boreal Scots pine forest. The total evapo-transpiration was measured with the eddy-covariance technique and the contribution of the understorey with open chambers together with the monitoring of the soil water content by the Time-Domain Reflectometry (TDR) method. Comparisons between measured and calculated xylem diameter changes are presented and discussed. The results reveal that diameter changes follow quite well the water vapour exchange in the cases when there was no rain.