Sapflow in the Sugar Maple in the Leafless State

Abstract

Summary Techniques have been developed to study sap pressure changes, concentration changes and exudation rates in single trees ( Acer saccharum ). Our data are consistent with the idea that gas bubbles may expel sap by lateral displacement of liquid rather than by mechanical fibre compression of conduits as implied by Sauter (1974). Pressures typically become supra atmospheric by day and subatmospheric by night in step with meterological changes. Phenomena such as localised unfreezing of the branches through solar radiation can complicate the interpretation of data. Twigs may thaw while the trunks remain frozen and sometimes exudation may be demonstrated even though the air temperature is sub zero. Though trees appear to behave as single hydraulic units, if the lateral conductivity of sap is poor, they may be more accurately envisaged as a bundle of similar replicates. Solute concentration of sap increased upwards in main trunks. The low solute concentration in the roots seems to explain the insignificance of root pressure under field conditions. The fluctuating pressures in normal intact maple trunk probably serve to distribute solutes to buds in preparation for the growing season. Reduction in soil water balance, root impermeability and the effect of transpiration probably combine to prevent exudation in summertime. Techniques which might increase the efficiency of tapping are discussed.