A Test of Mutual AId in Common Mycorrhizal Networks: Established Vegetation Negates Benefit in Seedlings

By acquiring symbiosis from the mycelium supported by neighboring plants, seedlings become connected to a common mycorrhizal network (CMN). Although the maintenance of a CMN may lead to mutual benefit among the neighboring plants, the benefits may be unequally distributed if the plants differ in their sink strength for the shared resources in the CMN. Hence, seedlings may not obtain any net benefit from a CMN maintained by competitively superior, mature plants. In a controlled greenhouse experiment, we showed that the presence of a mycorrhizal adult plant does not improve the growth of seedlings, although solitary seedlings benefit from mycorrhizae. Seedlings of the perennial, mycorrhizal, subarctic herbs Antennaria dioica, Campanula rotundifolia, Sibbaldia procumbens, and Solidago virgaurea were germinated alone or in the vicinity of an adult established Sibbaldia plant. The seedlings were either left nonmycorrhizal (NM) or were inoculated by spores or by a hyphal network connected to established Sibbaldia plants. Three arbuscular mycorrhizal fungi (AMF) were involved: Glomus claroideum, G. hoi, or an undescribed Glomus sp. isolate. In the absence of an adult Sibbaldia plant, the plant species responded differently to each AMF species, of which G. claroideum was most infective and most improved the growth of solitary seedlings. In NM seedlings grown alternatively with or without a NM adult plant, competition had no or only weakly adverse effects on seedling growth. Interestingly, the situation was the same in seedlings grown with a mycorrhizal adult plant: although heavily infected, seedlings did not show improved growth as compared to NM seedlings. A common mycorrhizal network may well imply some mutual aid for the connected plants, but competitive interactions within the CMN overwhelm any benefits. Corresponding Editor: S. H. Faeth.