Wood water content influences sap flux estimations under water limited conditions in a deciduous forest in Michigan

Sap flux measurements are the most common individual-scale measurements and are used as a proxy for transpiration through the conservation of mass. While multiple heat-tracer style sensor types exist, the most broadly used are Granier-style thermal dissipation probes (TDP). Beginning in 2014, work at University of Michigan Biological Station pioneered continuously monitoring wood water content using capacitance sensors in mature trees. This unique data set has been used to demonstrate the key role of stem-stored water, or the trees’ capacitance, to buffer transpiration against water stress. Furthermore, increasing evidence has shown diurnal variations in the hydraulic capacitance of stems as a result of changes in water stress under laboratory conditions. These variations may induce inaccuracy in the nocturnal maximum temperature (T_max) baseline of TDP and cause underestimation of sap flux measurements. Therefore, it is critical to study the dynamics between wood water content and sap flux measurements under natural conditions to establish the likely impact of these variations and their influence on estimations of transpiration.

We pair continuous time-series measurements of wood water content with raw sap flux observations made using traditional thermal dissipation probes in a mixed forest in northern lower Michigan. We demonstrate that decreases in wood water content result in increases in the T_max signal of the thermal dissipation probe under water stressed conditions. This behavior is in accordance to the theory of heat conductance and the specific heat capacity of solids with respect to changes in water content. Our results suggest that the diurnal dynamics of wood water content may be an important source of error in sap flux data during drought and other water limited conditions, and should potentially be considered for use as a correction factor when using thermal based sap flux measurement techniques.