Models of steady-state and dynamic gas exchange responses to vapor pressure and light in Douglas fir (Pseudotsuga menziesii) saplings.

Three-dimensional empirical models were constructed, depicting the response surface of water use efficiency (WUE) of Pseudotsuga menziesii saplings in relation to different levels of both irradiance and leaf-to-air vapor pressure difference (VPD). The two models developed depict responses of (1) previous season needles during autumn, winter, and early spring and (2) current year needles during the summer. The steady-state stomatal and gas exchange responses to irradiance and VPD suggest that factors determining adaptive stomatal performance in Douglas fir are complex and may differ according to needle age, developmental stage, and season. Stomatal response to light varied seasonally, with the stomata being responsive during the summer and unresponsive during the autumn, winter, and early spring. Previous season needles exhibit higher maximum WUE, but can be less conservative in their total use of water than the more VPD-sensitive maturing needles.Observations of dynamic stomatal responses to step changes in VPD and irradiance were used to propose a simple model depicting a combined stomatal response to sudden changes in both VPD and irradiance similar to those that would occur with the passage of sunflecks in a forest canopy. Step changes in VPD caused transient stomatal movements opposite in direction to that of the final response, while step changes in irradiance resulted in movements only in the direction of the expected final response. On the basis of the model, it was hypothesized that the dynamic response to changes in VPD may serve to enhance the speed of stomatal opening and closing when changes in irradiance are rapid.