Nonlinear responses of desert shrubs to episodic rainfall events

The physiological responses of desert shrubs to rapidly varying soil and atmospheric water status are equivocal. While much of the wide variations in observed responses in gas exchange and plant water potential can be readily attributed to nonlinear plant-soil-atmosphere couplings, there is a paucity of data to quantify these behaviors in the field. This motivated us to conduct an integrated field and modeling study on Larrea tridentata, the dominant shrub of North American warm deserts. We monitored photosynthesis, stomatal behavior, and water potential of 16 individual shrubs exposed to natural and simulated rainfall during two growing seasons at a field site in southern New Mexico. We use these field data to develop a new photosynthesis (A) model that explicitly links internal CO2 (Ci) and stomatal conductance (g). A threshold response is assumed for instantaneous g such that stomata operate at their maximum g (gmax) for low vapor pressure deficits (VPD); thereafter, g decreases nonlinearly with increasing VPD. Predawn water potential and growing temperature interact with VPD to affect g and gmax. Ci is a linear function of g and the intercept is the minimum achievable Ci, which varies with temperature and light intensity. This model is able to reproduce Larrea's responses to episodic soil and atmospheric water availability, capturing the large diurnal and seasonal fluctuations in A, g, and Ci. We use this model to show that the many conflicting conclusions regarding desert shrub responses to rainfall are explained by the lack of sufficient field data and by several key nonlinear plant-soil-atmosphere couplings.