Fungal effects on plant–plant interactions contribute to grassland plant abundances: evidence from the field

Plant–fungal interactions can have strong effects on plant abundances, both through direct effects on plant performance and indirect effects on competition and facilitation. Most evidence linking fungi to plant abundances derives from direct fungal effects on initial growth, with little evidence linking fungal effects on plant–plant interactions in intact communities to plant abundances for any plant life‐history stage. We transplanted 4320 individuals belonging to 18 plant species into plots where we removed neighbouring vegetation and suppressed fungi using fungicide in a factorial design. We monitored plant survival and growth for 3 years, using these data to test whether fungi had net effects on how plant–plant interactions affected different plant life‐history components (initial survival/growth, adult survival/growth). We then tested whether these indirect fungal effects or direct fungal effects on plant performance best explained plant commonness (frequency of occurrence) and local density (per cent cover). Finally, we measured differences in root‐associated fungi following fungal suppression and associated these differences with fungal effects on plant performance. Overall, fungi increased competitive effects on survival (i.e. lower survival with fungi intact), but reduced competitive effects on growth of adult plants (i.e. higher growth when fungi intact). Among the focal species, these indirect fungal effects increased survival for more common species relative to rarer species. However, indirect fungal effects on adult growth benefitted rarer species more than common species. Local plant densities were unassociated with indirect fungal effects, but were negatively associated with direct fungal effects on survival and adult growth. This suggests that fungi limit local dominance, thereby indirectly increasing the establishment of common species and the growth of rare species. Synthesis. Using a variety of plant species and suppressing both fungi and neighbours, we show that fungi have net indirect effects, through plant–plant interactions, within intact plant communities. Variation among species in both direct and indirect fungal effects contributed to plant abundances, yet fungal effects did not consistently benefit either common or rare species. However, regardless of commonness, fungi directly reduced growth and survival for species with high local densities, consistent with plant–soil feedbacks limiting species dominance.