Defoliation changes mycorrhizal benefit and competitive interactions between seedlings and adult plants

1. Competition is considered to be one of the main biotic factors shaping plant communities. Experiments on the role of mycorrhizal symbiosis in plant competition have reported conflicting results. 2. We studied the effect of arbuscular mycorrhizal symbiosis on below-ground interactions between seedlings and established adult plants in a system where plants compete for below-ground resources. In this glasshouse experiment, seedlings of Gnaphalium norvegicum were grown with and without an arbuscular mycorrhizal fungus, in the presence and absence of a large conspecific adult plant. The ability of adult plants to support the fungus was modified by defoliating 0%, 50% or 75% of its leaf area. 3. We found that mycorrhiza increased below-ground competitive intensity. The mycorrhizal benefit to the seedlings was low in the vicinity of non-defoliated adult plants, but increased with increasing defoliation intensity of the adult plant. This was mirrored by reductions in mycorrhizal benefit that adult plants gained at the highest level of defoliation. 4. These results emphasize the importance of below-ground competition during seedling establishment and show that competition for mycorrhiza-mediated resources may be an important factor underlying seedling establishment in nutrient-poor systems. Defoliation of neighbours can increase the beneficial effect of mycorrhizae to seedlings establishing in the vicinity of larger plants, suggesting that grazing or mowing may improve seedling establishment by decreasing below-ground competition.