ELECTRICAL POTENTIAL ON A TREE - A MEASUR EMENT OF THE SAP FLOW

Potential differences were measured and recorded at the surface of a c hesnut-tree with two parallel one meter long electric dipoles during a few months of 1992. During the month of August, characterized by a qu iet and sunny weather, a daily variation is present, with an amplitude of some ten millivolts, similar on the two dipoles. We propose that t his daily signal is related to the variation of the flux of sap throug h the trunk, in turn due to the daily variation of evapotranspiration on the leaves of the tree. This regular variation is no longer present on the November recordings, after the fall of the leaves, as expected if the working mechanism is electrokinetics. A few applications are d iscussed: mapping of the sap flow with an array of electrodes, action on the sap circulation through the application of an electromotive for ce, interpretation of the spurious magnetotelluric data collected in f orests.