Background/Question/Methods Previous studies on the interactions between ungulate herbivores and the compensatory responses of scarlet gilia, Ipomopsis aggregata, demonstrated a range of plant fitness responses dependent upon climatic conditions (ranging from under to over-compensation under severe drought to non-drought conditions, respectively). In spite of all we know about fitness compensation in scarlet gilia no one has studied the effects of arbuscular mycorrhizal fungi (AMF) on compensation in scarlet gilia or, for that matter, any other plant species showing patterns of growth compensation following herbivory. In the San Francisco Peaks of Flagstaff, Arizona, two summer long studies were conducted across a natural population of scarlet gilia to assess the compensatory (fitness) response of mycorrhizal removal treatments under drought and non- drought conditions.

Results/Conclusions In 2007, under drought conditions browsed plants with AMF equally compensated when compared to uneaten controls.  However, when AMF were removed browsed plants overcompensated compared to uneaten controls.  These results suggest that AMF were competitors for limited resources (e.g., carbon and water) under the added stress of herbivory during a drought year.  In 2008, under non-drought conditions plants were roughly 2-4 times as productive in terms of fruit production as in the previous year under drought conditions.  A fully factorial experiment of natural herbivory, fungicide and water treatments in 2008 showed that water alone had no significant influence on plant fitness whether they were browsed or not, while fungicide treatments significantly increased the fitness of unbrowsed plants and marginally increased the fitness of browsed plants.  The combinatorial effects of water and fungicide significantly increased fruit production of both browsed and unbrowsed plants over all other treatments further indicating that AMF function as competitors even under more favorable conditions.  Nonetheless, the magnitude of response between drought and non-drought years in compensation following ungulate herbivory is on the order of a 40% increase in fitness following AMF removal in a drought year versus an 16% increase in a non-drought year.  The uneaten controls showed a 40% and 28% increase in fitness following AMF removal in drought and non-drought years, respectively.  Thus, AMF appear to be greater competitors under the more stressful conditions of herbivory and drought.