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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.

Summary1. Arbuscular mycorrhizal (AM) symbiosis has been shown to enhance some plant traits to which pollinators are known to respond. Moreover, in gynodioecious species pollinators prefer hermaphrodite flowers over female ones, but the role of fungal symbiosis in sex-specific pollinator attraction is unknown. 2. We examined how plant gender dimorphism and AM symbiosis affect floral visitors and reproductive output in the gynodioecious plant Geranium sylvaticum. Floral visitors were monitored in a common garden experiment using mycorrhizal plants inoculated with either Glomus claroideum or Glomus hoi and in non-mycorrhizal condition. 3. We hypothesized that because of the larger flower display and pollen presence in hermaphrodites, (i) hermaphrodites receive more floral visitors than females. However, as females produce higher numbers of seeds than hermaphrodites, we predicted that (ii) effective pollinators visit both sexes equally and that (iii) AM symbiosis, although affecting floral characteristics and insect behaviour in general, should not have sex-specific effects on insects that transport pollen. 4. Our results confirm the hypotheses as hermaphrodites received 1AE5 times as many floral visitors than females. However, the most important pollinator group, the bumblebees, did not prefer either sex, which may be important for the maintenance of both sexes in gynodioecious populations. We further corroborated that AM symbiosis, although affecting flower size and the amount of pollen, did not affect bumblebee behaviour, whereas other hymenopterans showed a deterrence for G. hoi inoculated plants. 5. This work reports the impact of AM symbiosis on the behaviour of floral visitors in a sexually dimorphic plant species. Positive interactions between plants, floral visitors and AM have been documented previously. However, potential negative tritrophic interactions between plants, floral visitors and AM may also occur. Our results suggest that plant sex and the mycorrhizal status of the plant also influence floral visitor groups that are unlikely to carry out pollination. This may benefit the plants if these floral visitors carry fungal diseases and consume pollen and nectar without effectively vectoring pollen.

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Mycorrhizal symbiosis involves reciprocal transfer of carbon and nutrients between shoots on the one hand and roots colonized by symbiotic fungi on the other. Mycorrhizas may improve the mineral nutrient acquisition rates, but simultaneously increase the belowground demand for carbon. Mycorrhizal plants will have a selective advantage over non‐mycorrhizal ones if they are more cost‐efficient in terms of carbon cost per unit of acquired mineral nutrient. However, we demonstrate here in a simple model system that this is not a necessary condition. Mycorrhizas may evolve even when they are less cost‐efficient, provided that photosynthesis and/or growth are strongly nutrient‐limited. This result implies a unique hypothesis for the evolution of mycorrhizal associations which may be inherently cost‐inefficient as compared to plant roots. Such symbioses may have evolved when the superior nutrient acquisition rate of fungi combines with the relatively high photosynthetic nutrient use efficiency of the host plant. Consequently, provided that mycorrhizas are really cost‐inefficient, the selective advantage of mycorrhizal plants will disappear when an increase in the nutrient acquisition rate is not associated with a sufficiently high nutrient use efficiency of photosynthesis, as at high soil nutrient levels or due to a loss of leaf area, shading or low temperatures.