Influence of tree water content in the zero-flow maximum temperature difference: A theoretical and experimental approach.

The hydraulic performance of woody species plays an important role for productivity and survival of trees. Sap flux measurements are the most commonly used techniques for the assessment of transpiration and hydraulic response to changing environmental conditions. While multiple heat-tracer style sensor types exist, the most broadly used are Granier-style thermal dissipation sensors. There is mounting evidence that variation in wood moisture content influences the nocturnal maximum temperature (T_max ) baseline of thermal dissipation probes. This wood moisture content variation can be assessed by multiple field measurement techniques. Traditionally, changes in stem water content have been measured by high accuracy, micron scale electronic dendrometers. New techniques leveraging on capacitance sensing technologies commonly used for collecting soil moisture data have also recently been applied at large scales to observe wood moisture content in situ. We pair three years of dendrometer and capacitance sensor measurements of wood moisture content with sap flux observations made using traditional thermal dissipation probes in a mixed forest in northern Lower Michigan. Our study aims to (i) evaluate the coherence between the two measures of wood moisture and to (ii) assess the influences of changes in wood water content on the T_max baseline at long term (seasonal) and short term (interstorm period) time scales. We also present results of a dehydration experiment which pairs measurements of moisture content from Quercus spp wood segments with TDP measured T_max. We demonstrate the species-specific influence of wood moisture content on the T_max baseline and suggest its applicability as a correction factor for long-term sap flux datasets.