Water Relations of Xeric Grasses in the Field: Interactions of Mycorrhizas and Competition

SUMMARY The effects of vesicular‐arbuscular mycorrhizal inoculum and competition from non‐mycorrhizal annual plants were tested on Agropyron smithii Rydb. and Agropyron dasystachyum (Hook.) Scribn. growing in an inoculum‐poor soil. Mycorrhizal A. smithii decreased stomatal resistance ( r s ) and increased leaf water potential compared to the non‐inoculated plants, but only during the driest portion of the growing season. The presence of annuals caused increased r s of A. smithii , but only of non‐inoculated plants. Mycorrhizal plants had the same r s with or without annuals, indicating that mycorrhizas may alleviate the detrimental effects of competition on r s of A. smithii. By contrast, neither inoculation nor annuals significantly affected water relations of A. dasystachyum. Mycorrhizas also did not increase N and P concentrations or percentage cover of either grass, but both had delayed phenology when mycorrhizal. This research was done during a wet year when the effects of mycorrhizas may be subtle. Long‐term observations are necessary to determine the importance of mycorrhizas under field conditions.