Desert dogma revisited: Coupling of stomatal conductance and photosynthesis in the desert shrub, Larrea tridentata

The success of the desert shrub Larrea tridentata (creosotebush) has been largely attributed to temperature acclimation and stomatal control of photosynthesis (A) under drought stress. However, there is a paucity of field data on these relationships. To address this void, we conducted a joint field and modelling study that encompassed a diverse set of environmental conditions. At a Larrea-dominated site in southern New Mexico we manipulated soil moisture during the growing season over a 2-year period and measured plant pre-dawn water potential (Ψ_pd), stomatal conductance (g) and A of individual shrubs. We used these data to develop a semi-mechanistic photosynthesis model (A-Season) that explicitly couples internal CO₂ (C_i) and g. Vapour pressure deficit (VPD) and Ψ_pd affect instantaneous g in a manner that is consistent with a biophysical model of stomatal regulation of leaf water potential. C_i is modelled as a function of g, derived from a simplification of a typical A-C_i curve. After incorporating the effects of growing temperature on stomatal behaviour, the model was able to capture the large diurnal fluctuations in A, g and C_i and the observed hysteresis in g versus C_i dynamics. Our field data and application of the A-Season model suggest that dogma attributed to Larrea's success is supported with regard to stomatal responses to VPD and Ψ_pd, but not for mechanisms of temperature acclimation and CO₂ demand.