5. Synthesis. Overall, this review shows that mycorrhizal networks play a key role in plant communities by facilitating and influencing seedling establishment, by altering plant–plant interactions and by supplying and recycling nutrients.

4. We found 60 cases where seedling species were grown together with larger plants with or without mycorrhizal fungal networks. Mycorrhizal networks promoted seedling growth in 48% of the cases (for 21 seedling species), while negative effects (25%) and no effects (27%) were also common. Seedlings associating with ectomycorrhizal fungi benefitted in the majority of the cases while effects on seedlings associating with arbuscular mycorrhizal fungi were more variable. Thus, the facilitative effects of mycorrhizal fungal networks depend on seedling species identity, mycorrhizal identity, plant species combinations and study system. We present a number of hypothetical scenarios that can explain the results based on cost–benefit relationship of individual members in a network.

3. We address the following questions: (i) are all plant species benefitting from mycorrhizal networks, (ii) is benefit dependent on the size or age of a plant, (iii) is fungal support related to the relative dominance of plants in a community, (iv) are there host dependent barriers and physiological constraints for support and (v) what is the impact of mycorrhizal networks on plant–plant interactions and plant community dynamics? Moreover, using a review of published studies, we test whether mycorrhizal networks facilitate growth of small seedlings that establish between or near larger plants.

2. Many mycorrhizal fungi are not host specific and one fungal individual can colonize and interconnect a considerable number of plants. The existence of these so‐called mycorrhizal networks implies that fungi have the potential to facilitate growth of other plants and distribute resources among plants irrespective of their size, status or identity. In this paper, we explore the significance of mycorrhizal fungal networks for individual plants and for plant communities.

1. Almost all plants are engaged in symbiotic relationships with mycorrhizal fungi. These soil fungi can promote plant growth by supplying limiting nutrients to plant roots in return for plant assimilates.

Is benefit dependent on the size of a plant? Mycorrhizal fungi usually colonize all plant individuals from mycotrophic hosts, irrespective of their size or development. An intriguing question is whether all plant individuals (e.g. small seedlings and larger plants) receive the same amount of benefit from mycorrhizal fungi. To test for general patterns, we performed a literature analysis with studies where seedlings were grown together with larger/adult plants in the presence and absence of mycorrhizal fungi. We made a distinction between studies where seedlings were grown with adult/larger plants from the same species and studies in which seedlings grew together with different plant species. In addition to this, a distinction was made between plants hosting arbuscular mycorrhizal (AM) fungi and ectomycorrhizal (EM) fungi. EM fungi belong to the Basidiomycetes and Ascomycetes and associate with a range of trees, especially those from temperate and tropical forests. AM fungi belong to the Glomeromycota (Schüßler, Schwarzott & Walker 2001) and associate with an estimated 65% of all land plants, including many grasses, herbs and tropical trees (Wang & Qiu 2006; Brundrett 2009). The results of the analysis are shown in the next section. This analysis focuses on experiments where several plants co‐occur in pots, microcosms or in the field. These experiments are ecologically more realistic than those where plants are grown alone in pots with or without mycorrhizal fungi. In pots with multiple plants (e.g. seedlings and adult plants), interactions between plants occur (e.g. competition or facilitation) and the effects of common mycorrhizal networks on plant growth can be tested, thus better simulating conditions usually observed in natural communities.

Is fungal support related to the relative dominance of a plant in the community Many ecosystems are dominated by plants forming mycorrhizal associations (Read 1991). It is still unclear whether plants that dominate a specific plant community also obtain most benefit from mycorrhizal fungi. Studies performed so far gave conflicting results. Hartnett & Wilson (1999), studying tall grass prairie in North America, observed that the dominant C4 grasses obtained most benefit from mycorrhizal fungi. They suggested that mycorrhizal fungi reduced plant diversity in these communities by supporting the dominant plant. Similarly, it is proposed that in some tropical rainforests EM associations encourage dominance of certain tree species (Connell & Lowman 1989). Studies performed with European calcareous grassland provide opposite results. Both Grime et al. (1987) and van der Heijden et al. (1998) show that subordinate plant species benefited most when mycorrhizal fungi were present, while biomass of the dominant grass was not enhanced, or even reduced in presence of mycorrhizal fungi. As a consequence, mycorrhizal fungi enhanced plant diversity in these grassland communities. Note that, in the study by Grime et al. (1987), field roots were used as inoculum to establish a fungal network. It is possible that, beside mycorrhizal fungi, pathogenic fungi were also part of this fungal network. Hence, the negative effects of the fungal network on the growth of the dominant plant could also be due to pathogens. Overall, the studies mentioned above indicate that the ‘status’ and relative dominance of a plant in the community does not determine how much benefit it receives. The results appear to depend on the identity of the dominant plant and its relationship with mycorrhizal fungi. The observations by Grime et al. (1987) and van der Heijden et al. (1998) also imply that there are many factors that determine the dominance of plants in plant communities: in some cases mycorrhizal fungi are important, while in other cases other factors such as growth form, relative growth rate, competitive ability, resistance to stress or disturbance determine plant abundance.

Socialism or capitalism in soil? It is tempting to compare mycorrhizal networks with socialist systems, where all individuals have equal opportunities and where wealth and power are distributed more evenly. On the contrary, a capitalist mycorrhizal network would be privately controlled for profit by the plant or plants establishing the network. Our analysis provides both examples of ‘socialist’ and ‘capitalist’ tendencies of mycorrhizal networks. In several cases small seedlings obtained more benefit (in terms of biomass gain) compared with the larger plants that established the mycorrhizal networks (e.g. Eissenstat & Newman 1990; van der Heijden 2004), pointing to socialist tendencies. However, in all studies performed so far, the actual investments (in terms of carbon/energy input into the network) by small and large plants were not determined and there is no empirical evidence that resources are preferentially allocated to small seedlings. It is not unlikely that in many cases the larger plants facilitated establishment of the small seedlings by (i) providing improved mycorrhizal inoculum potential and (ii) reducing the carbon cost of establishing a functioning mycorrhizal network around the seedlings’ roots. However, there are also several examples which show that small seedlings receive proportionally the same or even less benefit from networks as larger plants (Table 1). In terms of total biomass gains then, the larger plants thus benefit more from mycorrhizal networks, pointing to capitalist tendencies. Mycorrhizal networks can also be viewed as part of ‘superorganisms’ (sensuClements 1936), with the fungal species in the network being redundant physical extensions of the roots that translocate nutrients freely between plants. However, each fungal species has its own niche and mycorrhizal fungi differ in many ways including growth rate (Olsson, Jakobsen & Wallander 2002), soil type preference, resistance to stress and disturbance (Oehl et al. 2003), ability to acquire nutrients (Jakobsen, Smith & Smith 2002), ability to solubilise nutrients from organic matter and plant host range (Molina, Massicotte & Trappe 1992). Moreover, mycorrhizal fungi have evolved mechanisms for recognizing and preventing fusion of non self tissue. For instance, in AM fungi, hyphal fusions have only been observed between individuals of the same genotype while fusion between individuals of different genotypes, species or families do not occur (Giovannetti, Azzonlini & Citernesi 1999; but see Croll et al. 2009). Soils are, thus, colonized by several independent networks simultaneously competing for nutrients and roots. Moreover, in many cases mycorrhizal plants acquire nutrients not directly from the soil in competition with other plants, but from their fungal networks. A fungus may compete with other fungi for soil nutrients, and then deliver those nutrients to the various plants it colonizes. The distribution of the nutrients to plants in a network are then a function of variations in compatibility between a fungal individual and its colonized plant hosts, and variation in carbon flow to the fungus among the plants.

Carbon and mineral nutrient transfer through mycorrhizal networks One important consequence of mycorrhizal networks is that nutrients, carbon and water can be transferred from one plant to another. The significance of interplant carbon & nutrient transfer has been widely debated (see reviews by Simard, Durall & Jones 2002 & Selosse et al. 2006). Selosse et al. (2006) estimated that up to 40% of plant nitrogen in receiver plants can be derived from donor plants (e.g. nitrogen fixing plants) and be transferred through mycorrhizal networks. In most situations this proportion is probably much lower, as nitrogen is usually limiting plant productivity, making it unlikely that plants give it away for ‘free’. Moreover, nitrogen fixation by nitrogen fixing plants is energetically expensive, implying that direct transfer from a nitrogen fixer to a non‐nitrogen fixer is probably low. The significance of interplant carbon transfer has been unequivocally shown in mycoheterotrophic plants which parasitize on mycorrhizal networks from which they obtain carbon and nutrients (see above). In addition, evidence for carbon movement between green plants comes from Simard et al. (1997) and Lerat et al. (2002). There is debate about the ecological significance of C transfer between plants via mycorrhizal networks (Robinson & Fitter 1999). Graves et al. (1997) and Wu, Nara & Hogetsu (2001) have shown that C fixed by one plant and transferred to another remains in the root system, and presumably the hyphae, of the second plant. However, C must move out of the root system in mycoheterotrophs and the studies by Simard and Lerat suggest this can happen in green plants as well. What is not clear in these studies is the source of the carbon translocated from the fungi. It is likely that some of the C atoms are part of amino acids such as glutamine or glutamate that are transferred as N sources to the plants from the fungi. How these amino acids influence the energy budget of these plants, especially those that cannot fix their own carbon, remains to be determined.

Conclusions and outlook In this review, we have shown that mycorrhizal fungal networks play a key role in natural ecosystems. Mycorrhizal fungi can facilitate seedling establishment and plant growth by acquiring limiting nutrients. Many plants benefit from fungal support, but there are also a considerable number of cases where there is no, or even a negative effect of mycorrhizal fungi. The results presented in our analysis clearly reflect this as we observed significant growth stimulation of small seedlings by mycorrhizal fungal networks in only 48% of cases, while negative effects occurred in 25% of the cases. Mycorrhizal fungal networks have the ability to support co‐occurring plants of different sizes, but effects are highly variable. Thus, mycorrhizal networks do have some similarities to socialist systems in that small plants can benefit from networks that are supported by bigger plants in the community. However, whether this is actually occurring is highly context dependent, and varies with study system (e.g. plant species identity, fungal identity, nutrient availability). Beside effects on plant growth, we identified a number of other important ecological functions of mycorrhizal networks in soil. These include recycling of nutrients, prevention of nutrient losses, contribution to soil structure, food for other organisms, and mycorrhizal fungal networks acting as hyphal highways for bacterial dispersion. For a better understanding of the impact of mycorrhizal fungal networks on seedling growth and ecosystem functioning, several key questions need to be answered. First, cost–benefit relationships of individual plants connected to mycorrhizal networks are still poorly understood. It is unclear which plants invest, and how this is related to the amount of benefit received. The fact that most (if not all) physiological studies are performed with single plants, grown in highly simplified study systems, without hyphal interconnections to other plants, does not contribute to a better understanding of process occurring in mycorrhizal networks. The use of dual labelling (with 13C; 14C, 15N 33P) as performed by some investigators is important. Second, we have shown here that mycorrhizal networks are important for seedling establishment in several cases. However, in order to draw more precise conclusions, additional studies are required (e.g. studies with EM fungal networks showed positive effects in 75% of the cases. However, this conclusion is based on seven independent studies). Furthermore, as far as we know, there are no studies which tested whether hyphal networks formed by plants with ericoid or orchid mycorrhizas promote seedling establishment of nearby plants. Third, the contribution of mycorrhizal networks to nutrient uptake by plants and nutrient cycling in natural ecosystems is still poorly understood and mainly based on experiments performed in the laboratory (but see Hobbie & Hobbie 2006). Fourth, there are many other factors that facilitate seedling establishment and plant growth as discussed in this issue of the Journal of Ecology. The relevance of mycorrhizal networks compared with these other factors is often poorly understood. Fifth, most studies have been performed with mycorrhizal fungi that can be easily cultured. However, molecular techniques have shown that in the case of AM fungi, 60% of environmental sequences do not match with AM fungi that have been brought into culture (van der Heijden, Bardgett & van Straalen 2008). Hence, it will be extremely important to cultivate these fungi and assess their ecological relevance. Sixth, the spatial distribution and movement of nutrients in mycorrhizal networks is still poorly understood. Finally, our climate is changing and periods of drought or heavy rainfall are expected to increase in many countries. It is important to understand how these changes influence the stability of mycorrhizal networks and their ability in facilitating plant growth.

Acknowledgements We would like to thank Erik Lilleskov and Mari Moora for discussion and providing data. Toby Kiers and David Read commented on a very early version of this paper. We thank the editor and the two referees for helpful and constructive comments. Financial support was provided to M.G.A.V.D.H by the Swiss Federal Government and the Swiss National Science Foundation award 31003A_125428 and to T.R.H by the National Science Foundation award DEB‐0614381, the Mianus River Gorge Preserve, and the USDA Forest Service.

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