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University of Michigan Biological Station

The University of Michigan Biological Station (UMBS) was founded in 1909.

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Changes in the hysteresis between VPD and sap flux velocity as related to local weather in different tree species in northern Michigan Forests

Atmospheric
Climate
Vegetation
Project Abstract: 
Sap flow measurements are commonly used to monitor plant water use and transpiration in response to environmental variables, such as water availability, light, and vapor pressure deficit (VPD) (A.B. Roddy and T.E. Dawson, 2013). Understanding the response of sap flux to these variables is important for modeling how plant water use changes under daily and seasonally varying conditions. VPD, which varies substantially over daily and seasonal timescales, is an important driver of plant transpiration. However, daily increases and decreases in VPD often do not produce the same results in sap velocity, causing hysteresis in the daily relationship between sap velocity and VPD. For a given VPD that occurs in both the morning and the evening, sap velocity is higher in the morning than in the evening, producing a clockwise rotation in the hysteresis curve. In the summer, there is greater hysteresis in the relationship between sap velocity through main stems and VPD than there is in the winter, and the magnitude of hysteresis is positively related to maximum daily VPD (O’Grady et al., 1999, 2008; Zeppel et al., 2004).
Investigator(s): 
Ashley Matheny
Status of Research Project: 
Active
Years Active: 
2018
Research sites: 
UMBS AmeriFlux Tower
Methods: 
Xylem sap flow measuring systems are being increasingly used to quantify whole-plant water use, especially in woody plants (Swanson,1994, Smith and Allen, 1996, Kostner et al., 1998; Wullscheleger et al., 1998). Granier (1985;1987) developed a dual-probe sap flow measuring system, which is now referred as dissipation probe (TDP) method where sap flux density, is calculated by means of an empirical calibration equation with the only variable being the temperature difference between a vertically separated heated needle (downstream/upper probe) and reference needle (upstream/lower probe) inserted radially into the stem 10-15 cm apart. The downstream needle is heated at constant power (0.2 W) with along its length (0.02 m) while the upstream (lower) probe is left unheated to measure the ambient temperature of the wood tissue (Ping LU et al, 2004). Under conditions of thermal equilibrium of the system established between the sensor probe and its surroundings (wood and sap), and for a constant sap flux density, it can be assumed that input of heat by the Joule effect is equal to the quantity of heat dissipated by convection and conduction at the wall of the probe (Granier, 1985; Cabibel and Do, 1991).