Abstract: Arbuscular mycorrhizal (AM) fungi are well documented to alleviate physiological stress in plants. While AM fungal benefits under low-water conditions are well studied, AM fungal benefits under high-water conditions are far less understood. Previously, direct experimentation on AM fungal effects on plant performance has largely focused on agricultural crop species, frequently using categorical ambient and drought condition treatments rather than leveraging gradients appropriate for detecting non-linear responses. Thus, there is little understanding about how AM fungi may mediate native, terrestrial plant responses across gradients of water availability. Here, we test the effects of AM fungi on a common, ruderal plants species (Solidago altissima) across a wide range of water availability in a greenhouse experiment. AM fungi improved plant performance at moderate and high levels of water availability, but surprisingly AM fungi did not improve plant performance in the lowest water availability treatment groups. Most importantly, without AM fungi, S. altissima was unable to take advantage of increasing water availability, indicating that even for a ruderal plant species, mycorrhizal associations may be a critical component to plastic responses in plant performance under climate-driven changes in water availability.

Key words: arbuscular mycorrhizal fungi, plant biomass, water availability, non-commercial plants