Influence of clipping and soil water status on vesicular-arbuscular mycorrhizae of two semi-arid tussock grasses

Summary Using two tussock grasses, Agropyron desertorum, a grazing-tolerant species, and A. spicatum, a grazing-sensitive species, we measured the responses of vesicular-arbuscular mycorhizae (VAM) to clipping and soil moisture status. The clipping was carried out over a 4-year period (1982–1985). For the soil moisture × clipping studies, a rainout shelter was used to create drought stress, and irrigation was used to create high moisture conditions over a 2-year period (1984 – 1985). No consistent patterns in VAM infection frequency and VAM fungal spore counts were observed, either between species or among treatments. Increased moisture tended to increase total root and total VAM length, but there were no consistent moisture × clipping effects. Nor did the phenology of the fungus follow consistent patterns: there were similar lengths of root containing vesicles and arbuscules in 1984, and predominantly vesicles in 1985. The greatest effect was a large difference in VAM activity between years regardless of treatment, despite our attempts to use extreme environmental conditions and a rigorous sampling design. The VAM of both plant species were highly correlated. We postulate that VAM are highly plastic and that they respond to environmental pressures that we do not yet understand.