A mycorrhizal network (also known as a common mycorrhizal network or CMN) is an underground network found in forests and other plant communities, created by the hyphae of mycorrhizal fungi joining with plant roots. This network connects individual plants together. Mycorrhizal relationships are most commonly mutualistic, with both partners benefiting, but can be commensal or parasitic, and a single partnership may change between any of the three types of symbiosis at different times. Mycorrhizal networks were discovered in 1997 by Suzanne Simard, professor of forest ecology at the University of British Columbia in Canada. Simard grew up in Canadian forests where her family had made a living as foresters for generations. Her field studies revealed that trees are linked to neighboring trees by an underground network of fungi that resembles the neural networks in the brain. In one study, Simard watched as a Douglas fir that had been injured by insects appeared to send chemical warning signals to a ponderosa pine growing nearby. The pine tree then produced defense enzymes to protect against the insect. The formation and nature of these networks is context-dependent, and can be influenced by factors such as soil fertility, resource availability, host or mycosymbiont genotype, disturbance and seasonal variation. Some plant species, such as buckhorn plantain, a common lawn and agricultural weed, benefit from mycorrhizal relationships in conditions of low soil fertility, but are harmed in higher soil fertility. Both plants and fungi associate with multiple symbiotic partners at once, and both plants and fungi are capable of preferentially allocating resources to one partner over another. Mycorrhizal associations have profoundly impacted the evolution of plant life on Earth ever since the initial adaptation of plant life to land. In evolutionary biology, mycorrhizal symbiosis has prompted inquiries into the possibility that symbiosis, not competition, is the main driver of evolution. Referencing an analogous function served by the World Wide Web in human communities, the many roles that mycorrhizal networks appear to play in woodland have earned them a colloquial nickname: the "Wood Wide Web". Many of the claims made about common mycorrhizal networks, including that they are ubiquitous in forests, that resources are transferred between plants through them, and that they are used to transfer warnings between trees, have been criticised as being not strongly supported by evidence.
Definitions and types As a scientific term, mycorrhizal network has broad meanings and usage. Scientific understandings and thus publications utilize more specific definitions arising from the term common mycorrhizal network (CMN). The keyword "common" requires that two or more individual plants are connected by the same underground fungal network, through which matter of various types and functions may flow. The plants themselves may be individuals of the same or different species. In turn, the fungal network that is composed of threadlike hyphae may be limited to a single type or entail several. The kinds of evidence deemed necessary for supporting scientific conclusions, along with the tendency for disputes to arise, depend in part on the definitions used. There are two main types of mycorrhizal networks. These are determined by the two main categories of fungal growth forms. Arbuscular mycorrhizal networks are those in which fungal hyphae not only enter the plant's roots but also penetrate into the cells themselves. Ectomycorrhizal networks send hyphae into the roots where they thread their way between the plant cells but do not penetrate cell walls. The arbuscular type is the most common among land plants and is regarded as the ancestral type. However, tree species comprising the canopy of temperate and especially boreal forests in the Northern Hemisphere tend to associate with ectomycorrhizal fungi. Plant and fungal partners within a network may enact a variety of symbiotic relationships. Earliest attention was given to mutualistic networks by which the plant and fungal partners both benefit. Commensal and parasitic relationships are also found in mycorrhizal networks. A single partnership may change between any of the three types at different times.
Knowns, unknowns, and controversies The mycorrhizal symbiosis between plants and fungi is fundamental to terrestrial ecosystems, with evolutionary origins before the colonization of land by plants. In the mycorrhizal symbiosis, a plant and a fungus become physically linked to one another and establish an exchange of resources between one another. The plant provides to the fungus up to 30% of the carbon it fixes by photosynthesis, while the fungus provides the plant with nutrients that are limiting in terrestrial environments, such as nitrogen and phosphorus. As this relationship has been better investigated and understood by science, interest has emerged in its potential influence on interactions between different plants, particularly in the possibility that connectivity through the mycorrhizal network may allow plants to positively impact the survival of other plants. Evidence and potential mechanisms for a variety of plant-plant interactions mediated by the mycorrhizal symbiosis have been presented, but their validity and significance is still controversial.
Proposed effects and functions of the mycorrhizal network
Potential nutrient and photosynthate transfer between plants Since multiple plants can be simultaneously colonized by the same fungus, there has been interest in the possibility that inter-plant transfer of nutrients may occur via mycorrhizal networks, with photosynthates moving from a 'donor' plant to a 'recipient' plant. Numerous studies have reported that carbon, nitrogen and phosphorus are transferred between conspecific and heterospecific plants via AM and ECM networks. Other nutrients may also be transferred, as strontium and rubidium, which are calcium and potassium analogs respectively, have also been reported to move via an AM network between conspecific plants. It is possible that in this way, mycorrhizal networks could alter the behavior of receiving plants by inducing physiological or biochemical changes, and there is evidence that these changes have improved nutrition, growth and survival of receiving plants.
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