3.4 Relationship between VPD, sap flux density, and zones
A power function model was used to determine the relationship between the daily growing season (May-October) VPD and sap flux density from the monitored tree species in 2015 and 2016. Daily sap flux density in all species were significantly related to VPD across all zones in both years with the highest r2 values occurring in 2015 when compared to 2016 (Table 3). The nonlinear relationship generally led to increased deviations at higher VPD values.