2.6 Statistical analysis
All data analyses were completed using JMP Pro 12 (JMP, 2012). Sap flux density across years and zones was determined using a paired t-test. The zone was defined as a riparian buffer, mid-hillslope, and upland-hillslope. The t-test was selected, and the significance level was set to α < 0.05 to determine which year or zone was statistically different from each other. The power model was used to determine the nonlinear relationship between daily VPD and sap flux density. The explanatory and response variables in the bivariate plots were log-transformed to produce the power model, y =axb. The effects test was used to determine if the evaporative demand changed across the zones and if there was an interaction between VPD and the zones. VPD was log transformed to reflect the power function of the relationship between VPD and species sap flux density. P values < 0.05 indicate that the independent variables and/or their interaction were significantly related to sap flux density.