Stomatal Sensitivity to Vapor Pressure Deficit

Stomatal sensitivity is proportionally related to the magnitude of G_s at low D (D = 1 kPa) when soil moisture is not limiting (Granier et al., 1996), and it can be derived as:

where G_Sref is the intercept (i.e., the value of G_s at D = 1 kPa in a log-linear relationship), and -m represents the slope of the regression fit representing stomatal sensitivity to D (i.e., dG_s/dln D). By analyzing data from a variety of sources, including both porometric and sap flux derived G_s, -m was demonstrated to be approximately 0.6 (Oren et al., 1998). In our study, the -m and G_Sref for E. urophylla in the wet and dry seasons was calculated to characterize the response of G_s to drought.

A boundary line analysis of the relationship between D and G_s was performed for the dry and wet seasons. The datasets of G_s for each tree were binned by radiation (nine levels from 0 to 1600 μmol m^-2 s^-1). The data at night (Q₀ = 0) were excluded because plant physiological response at night is much more complicated than that in the daytime (Oren et al., 2001). The relationship between the mean lnD and G_s of each subset was linearly fitted, and the intercept and slope corresponded to the G_Sref (G_s at D = 1 kPa) and sensitivity in response to D (dG_s/dlnD, mmol m^-2 s^-1 kPa^-1), respectively (Oren et al., 1998). Then, the relationship between G_Sref and -dlnd/dG_s for the two seasons was fitted.

In order to determine the radiation regulation of stomatal conductance under different water conditions, the G_Sref under different light conditions was normalized by the value of the maximum Q₀ of each tree in both seasons, and the relationship between the G_Sref and mean Q₀ at that level for all of the trees was fitted with an exponential function expressed as:

Where a refers to the maximum dependent variable, i.e., the max G_Sref (G_Sref-max).