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

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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).
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