Sap flow dynamics of a beech tree during the solar eclipse of 11 August 1999

During the solar eclipse of 11 August 1999, measurements of sap flow at branch and stem level were made in a 75-year-old beech tree which was 27 m high growing near Ghent, Belgium. The sky was relatively free from clouds during the eclipse which reached its maximum at 10.26 h GMT (=10.36 h True Solar Time). Measurements also included short-wave radiation, air temperature and vapour pressure deficit. The daily variation of the sap flows and of short-wave radiation and vapour pressure deficit were compared to a reference based on mean diurnal cycles. During the eclipse short-wave radiation, air temperature and vapour pressure deficit showed a maximum reduction between the pre-eclipse maximum and the following minimum of respectively 439 W/m ², 3.0 °C and 0.27 kPa. The reduction of the solar energy resulted in the formation of a temporary temperature inversion above the canopy. At the time of maximum eclipse the sap flow rates at branch level and at stem level were reduced by 92 and 76%, respectively, indicating non-stationary flow conditions in the beech tree. Due to the eclipse, the tree reduced its water loss from transpiration by 32.0 kg of water, being about 25% of the total amount which is transpired by the beech tree during a sunny day in August. Regression analysis indicated that both short-wave radiation and vapour pressure deficit changed in a nearly linear fashion during the eclipse. Sap flow rates were given by an exponential equation, allowing the description of sap flow dynamics in terms of time lag and rate of change.