Combining evapo-transpiration measurements with xylem diameter changes

Diameters of tree stems change diurnally due to changes in the water balance of stem tissues. According to cohesion-tension theory measurements of these diameter changes may provide means of obtaining information about sap flow and transpiration [1]. The easiness of the maintenance and the low price of diameter sensors make them advantageous compared with other methods for sap flow masurements. This holds true especially for relatively small size trees, where the surface heating based sapflow monitoring methods are already suffering from inaccuracies [2] but where the stem size is still too small for inserted heating and temperature monitoring elements. We have developed a model for water flow in a Scots pine tree that calculates the diameter change from the measured transpiration rate and the elastic properties of wood. In this study we compare canopy transpiration as derived from the eddy covariance technique to the diameter variations to evaluate the potential of the method. The measurements were carried out at SMEAR II station (Southern Finland, Hyytiälä, 61° 51'N, 24° 17'E) [3]. Eddy covariance measurements represent the whole ecosystem evapo-transpiration whereas the diameter measurements are coupled with transpiration of single trees. However, the measured diameter change and that calculated from the evapo-transpiration rate follow similar pattern. The results of comparisons and problems of scaling the diameter variation to canopy transpiration are discussed.