Xylem sap velocity in relation to weather and air pollution

Abstract

Measurements of xylem sap velocities have been used to assess the influence of atmospheric conditions and air pollutants on the transpiration of full-grown trees. For a period of four months sap velocities of oak, beech and Douglas fir were measured on an hourly basis using the heat-pulse technique. Simultaneously, concentrations of S02, NO, N02 and 03 were measured along with air temperature, light intensity, air hurnidity, precipitation, wind speed and wind direction. To analyse these time series, a technique is introduced which can handle time-dependent relations: the stochastic response model. This statistical model is a submodel of the dass of structural models and is estimated by means of the KaIman filter. The influence of weather conditions on heat-pulse velocities (HPV) is prevailing: 80% of the variance is explained by the single variable vapour pressure deficit. No influence of gaseous pollutants on HPV could be assessed. Possible explanations are discussed.