Coordination of sap flux and water storage at various heights and depths inside stems in two tree species of differing hydraulic strategies

Trees and forests are large reservoirs of terrestrial water and return ~50% of the water from precipitation back to the atmosphere. Thus, understanding how trees store and transport water is critical information for modelers and land managers. It has long been known that trees store large amounts of water. Recently, multiple labeling studies have shown that the duration of this stored water can be on the time scale of weeks to even months, but the functional significance of this long-term stored water is unclear. As a first step to understanding the role of stored water, we measured tree sap flux at three or four heights, and at three sapwood depths, in two species with contrasting hydraulic strategies (ring-porous Quercus falcata and the conifer Pinus taeda L.). This work was performed in a forest in the Piedmont region of North Carolina, USA. Our objective was to determine whether the relationship between sap flux and stored water differed at various heights and sapwood depths. We found that the timing of sap flux was synchronized at all heights and depths, but the magnitude was not. In both species, sap flux was up to 2X greater at mid-tree heights compared to sap flux in lower and upper heights. Sap flux in oak was greater at mid-depth in the sapwood and much greater than sap flux at the shallow depth. In contrast, sap flux in pine increased with depth in the xylem. However, after significant rain events, sap flux was equal at all depths for any given height in both species. This lasted for three to four days after rain events. The large differences in sap flux that we observed across heights of trees and depths of xylem suggest that there is a reservoir of stored water at mid-heights in our studied trees, and this supports/allows for greater sap flux at multiple xylem-depths at this height. While the number of studies showing the importance of capacitance (release of stored water from tissues) in tree water relations is increasing, few studies have studied this phenomenon at multiple heights and depths in trees. The next steps for plant hydraulics scientists and ecohydrologists is to determine exactly which tissues store water, and how long it resides there.