© 2004 Heron Publishing—Victoria, Canada
Xylem sap flow and drought stress of Fraxinus excelsior saplings
Andreas Stöhr (1, 2) and Rainer Lösch (1)
1. Abteilung Geobotanik, Heinrich-Heine-Universität, Universitätsstr. 1, 40225 Düsseldorf, Germany / 2. Author to whom correspondence should be addressed (andreas.stoehr@uni-duesseldorf.de) / Received November 6, 2002; accepted April 27, 2003; published online December 15, 2003
Summary
We report on diurnal and seasonal variations in sap flow rate and stem water potential of Fraxinus excelsior L. saplings growing at the edge of a Fraxino—Aceretum forest in western Germany. Because of shallow soil, the trees were
subjected to drought in summer. When soil water availability was not limiting, sap flow rate was related to changes in solar
radiation and vapor pressure deficit. Maximum transpiration rates per leaf area were 3.5–7.4 mmol m–2 s–1, and maximum daily totals were 1.7–3.3 kg m–2 day–1. Under drought conditions, stem water potential dropped to midday minima of –2.6 to –3.5 MPa and sap flow rate was strongly
related to this parameter. After the drought period, reduced apparent (whole-plant) hydraulic conductance was observed, which
was attributed to a continued reduction in stomatal conductance after the drought stress had ceased. A model was developed
that linked sap flow rate directly to climatic variables and stem water potential. Good correlation between measured and simulated
sap flow rates allowed the model to be used for data interpretation.
Keywords:
hydraulic conductance, modeling, transpiration, water relations, water use.
