C₄ plants have greater water use efficiency than otherwise similar C₃ plants, yet plants with these two photosynthetic pathways commonly coexist suggesting that they occupy different ecological niches. The purpose of this study was to compare water relations and hydraulic traits between two co-occurring, halophytic desert shrubs, Atriplex hymenelytra (C₄) and Isocoma acradenia (C₃), in the Mojave Desert. Based on a pilot study, we hypothesized that Atriplex relied on water in the upper soil layers, while Isocoma had access to groundwater associated with a nearby salt lake, which could potentially move to the upper soil layers via hydraulic redistribution (HR). This process could increase plant water status during the day and reduce the chance of roots and stems from becoming embolized. We hypothesized that, compared to Isocoma, Atriplex would have lower rates of stomatal conductance, lower branch water potentials, higher resistance to embolism formation, would not experience HR and have lower ability to repair xylem embolisms. Measurements between summer 2008 and spring 2009 included repeated, diurnal measurements of stomatal conductance, branch water potentials and percent loss of stem conductance (PLC), as well as monitoring of soil water contents and construction of xylem vulnerability curves using the air seeding method.

Results/Conclusions

Isocoma consistently had higher branch water potentials and higher stomatal conductance during the day than Atriplex. Both species had similar, high rates of nocturnal stomatal conductance. Atriplex was more resistant to embolism formation than Isocoma. Isocoma showed a higher degree of native embolism than Atriplex with evidence for diurnal formation and repair of embolisms occurring at branch water potentials of less than -3 MPa. In 2008-09, no evidence for HR into the surface soil was detected for either species. The finding that neither species experienced HR may have been due to nocturnal transpiration displayed by both species and/or due to the lack of precipitation at our study site during the study period. Our results support the initial hypotheses based on differences in rooting depths between the species and provide further support for previous reports in the literature of differences in hydraulic traits between C₃ and C₄ plants.