Sensitivity of water relations and photosynthesis to summer precipitation pulses for Artemisia tridentata and Purshia tridentata

For much of the western USA, precipitation occurs in pulses, the nature of which determine soil water potential and plant physiological performance. This research utilized three experiments to examine the sensitivity of photosynthesis and water relations for two widespread Great Basin Desert shrub species, Artemisia tridentata (which has both deep and shallow roots) and Purshia tridentata (which reportedly has only deep roots), to (1) variation in pulse magnitude size, (2) the kinetics of responses to pulses, and (3) the relationship between pulse-size and antecedent soil water content. At the study site in the southwestern Great Basin Desert, USA, summer rainfall exhibits a greater frequency of larger-sized events, and longer inter-pulse intervals, compared to annual patterns. Compared to pre-watering values, stem water potential initially increased by about 2.00 MPa for A. tridentata and 1.00 MPa for P. tridentata following watering to simulate an 11.5 mm rainfall pulse. For the same water addition, stomatal conductance increased by 0.3 mol m(-2) s(-1) and photosynthetic CO2 assimilation increased 8-fold for A. tridentata and 6-fold for P. tridentata. Water potential and photosynthetic gas exchange were maximal for both species 2-3 days following a pulse addition. In comparison to P. tridentata, the increase in photosynthesis for A. tridentata was more pronounced for plants treated incrementally with several small pulses compared to plants treated with one pulse of an equivalent total volume. The results indicate that both species can respond to a range of summer rainfall pulse magnitudes within about 2 days, with A. tridentata generally exhibiting larger responses in comparison to the co-dominant shrub species P. tridentata, which at this study site does indeed have shallow roots. In a future climate, the timing and magnitude of summer rainfall pulses will determine the extent to which these two species undergo changes in water status and photosynthetic carbon uptake, with implications for their fitness.