Evidence for mutualism-parasitism continuum of arbuscular mycorrhizal fungi following defoliation of plains rough fescue (Festuca hallii)

Current ecological theory suggests that mycorrhizal associations exist within a mutualism-parasitism continuum.  There has been little research, however, that examines how this continuum interacts with other ecological pressures to affect host plants.  We conducted a field experiment in a native rough fescue grassland in Alberta, Canada to observe how suppressing arbuscular mycorrhizal (AM) fungi with benomyl affected the response of plains rough fescue (Festuca hallii), a co-dominant species in this system, to competition and defoliation.  Benomyl application effectively suppressed AM fungi, reducing colonization of fescue by 50%.  There was no main effect of AM fungal suppression or interaction with competition treatments, however regrowth of defoliated individuals was significantly altered.  Recovery of defoliated individuals was negatively associated with plant growth potential in control plots, but positively associated in plots where AM fungi were suppressed.  We speculate that while AM fungi were mutualists under low productivity conditions, they became parasitic under high productivity conditions.  The lack of a main effect in this study suggests that mycorrhizal effects may sometimes only be observable when additional, realistic interactions are introduced.  Additionally, main effects in previous studies may have masked the presence of a mutualism-parasitism continuum.