Redistribution of water within the aboveground part of trees

Abstract

Recent progress in sap flow measurement techniques demonstrates that, together with transpiration, hydraulic redistribution (HR) is determinant for the study of tree water relations. Without HR trees would not transpire at the observed rates in many conditions. Results from direct sap flow measurements across the world illustrate the widespread nature of HR through roots in the soil profile (belowground). Information about redistribution of water within the aboveground parts of the tree is scarcer but it may take place when the driving forces significantly differ within the tree compartments. In this work we illustrate, through sap flow records in different xylem layers measured by multi-point heat field deformation (HFD) sensors, the occurrence of aboveground hydraulic redistribution in trees for different species and sites across Europe and Canary Islands, with temperate climate varying from continental to mediterranean. Similarly as in roots HR can be characterized by reverse flows in some plant parts when evaporative demands are low (examples are shown in stem of mature scots pine or in leaf of palm Phoenix canariensis). Contrary, HR in aboveground part of trees can occur during sharp increase or decrease of driving forces and can be characterized by asynchronous patterns of flow variation in the conducting xylem of tree organs (examples are demonstrated in branches of oak trees Quercus suber L. and Quercus pubescens and in trunk of dragon tree Dracaena draco.