Sap flow and daily electric potential variations in a tree trunk

Electric potential has been monitored since December 2003 in the roots and at two circumferences and one vertical profile in a standing poplar (Populus nigra). Electric potential is sampled using 6 mm diameter stainless steel rods, inserted 5 mm deep in the sapwood and is referenced to an unpolarisable lead/lead chloride electrode installed 80 cm deep in the soil. Diurnal variations are observed with seasonal differences. During winter, diurnal variations depend on the measurement point, with variable amplitudes and sometimes anti-correlations between electrodes. By contrast, a stable and coherent organisation is established in the spring, with larger amplitudes, and lasts during summer. Dedicated experiments have been performed to rule out a direct effect of temperature on the electrodes, and thus, demonstrate a genuine electrical source in the tree. Daily electrical variations have been reported previously, and have been interpreted as electrokinetic effects associated with sap flow. However, a comparison of the electrical signals with a direct measurement of the sap flow by a continuous heat flow method, shows that the electrical variation, although clearly correlated to sap flow, is not simply proportional to it. In a living system, electrokinetic effects, in addition to thermoelectrical effects, are probably modified significantly by additional electrochemical effects, such as membrane diffusion potentials, ion active transport by proteins or action potentials. Electric potential variations in trees may thus, reveal physical mechanisms in living systems not accessible by other methods. A better understanding of the electrical response of trees associated with sap flow may improve the knowledge of transfer processes between the soil and the atmosphere. This is important for the understanding of adaptive response of trees, the modelling of water and carbon balance in relation to climate change, and the quantification of the contribution of trees to the migration, retention and dispersion of contaminants.