Mycorrhizal network

Mycorrhizal networks (also known as common mycorrhizal networks or CMN) are underground hyphal networks created by mycorrhizal fungi that connect individual plants together and transfer water, carbon, nitrogen, and other nutrients and minerals.

Nutrient exchanges and communication between a mycorrhizal fungus and plants.

The formation of these networks is context dependent, and can be influenced by factors such as soil fertility, resource availability, host or myco-symbiont genotype, disturbance and seasonal variation[1] (due to enrichment of nitrogen in soil affect michozial communities or effect of human activities of human affecting nitrogen cycle).

By analogy to the many roles intermediated 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.[2][3].

Substances transferred

Several studies have demonstrated that mycorrhizal networks can transport carbon,[4][5] phosphorus,[6] nitrogen,[7][8] water,[1][9] defense compounds,[10] and allelochemicals [11][12] from plant to plant. The flux of nutrients and water through hyphal networks has been proposed to be driven by a source–sink model,[1] where plants growing under conditions of relatively high resource availability (e.g., high-light or high-nitrogen environments) transfer carbon or nutrients to plants located in less favorable conditions. A common example is the transfer of carbon from plants with leaves located in high-light conditions in the forest canopy, to plants located in the shaded understory where light availability limits photosynthesis.

Types

There are two main types of mycorrhizal networks: arbuscular mycorrhizal networks and ectomycorrhizal networks.

  • Arbuscular mycorrhizal networks are formed between plants that associate with glomeromycetes. Arbuscular mycorrhizal associations (also called endomycorrhizas) predominate among land plants, and are formed with 150–200 known fungal species, although true fungal diversity may be much higher.[13] It has generally been assumed that this association has low host specificity. However, recent studies have demonstrated preferences of some host plants for some glomeromycete species [14][15]
  • Ectomycorrhizal networks are formed between plants that associate with ectomycorrhizal fungi and proliferate by way of ectomycorrhizal extramatrical mycelium. In contrast to glomeromycetes, ectomycorrhizal fungal are a highly diverse and polyphyletic group consisting of 10,000 fungal species.[16] These associations tend to be more specific, and predominate in temperate and boreal forests.[13]

Benefits for plants

Several positive effects of mycorrhizal networks on plants have been reported. These include increased establishment success, higher growth rate and survivorship of seedlings;[17] improved inoculum availability for mycorrhizal infection;[18] transfer of water, carbon, nitrogen and other limiting resources increasing the probability for colonization in less favorable conditions.[19] These benefits have also been identified as the primary drivers of positive interactions and feedbacks between plants and mycorrhizal fungi that influence plant species abundance [20]

Mycoheterotrophic and mixotrophic plants
Monotropastrum humile—an example of a myco-heterotrophic plant that gains all of its energy through mycorrhizal networks

Myco-heterotrophic plants are plants that are unable to photosynthesize and instead rely on carbon transfer from mycorrhizal networks as their main source of energy. This group of plants includes about 400 species. Some families that include mycotrophic species are: Ericaceae, Orchidaceae, Monotropaceae, and Gentianaceae. In addition, mixotrophic plants also benefit from energy transfer via hyphal networks. These plants have fully developed leaves but usually live in very nutrient and light limited environments that restrict their ability to photosynthesize.[21]

Importance at the forest community level

Connection to mycorrhizal networks creates positive feedbacks between adult trees and seedlings of the same species and can disproportionally increase the abundance of a single species, potentially resulting in monodominance.[5][17] Monodominance occurs when a single tree species accounts for the majority of individuals in a forest stand.[22] McGuire (2007), working with the monodominant tree Dicymbe corymbosa in Guyana demonstrated that seedlings with access to mycorrhizal networks had higher survival, number of leaves, and height than seedlings isolated from the ectomycorrhizal networks.[17]

Introduction

The importance of mycorrhizal networks facilitation is no surprise. Mycorrhizal networks help regulate plant survival, growth, and defense. Understanding the network structure, function and performance levels are essential when studying plant ecosystems. Increasing knowledge on seed establishment, carbon transfer and the effects of climate change will drive new methods for conservation management practices for ecosystems.

Seedling establishment

Seedling establishment research often is focused on forest level communities with similar fungal species. However mycorrhizal networks may shift intra- and interspecific interactions that may alter pre-established plants physiology. Shifting competition can alter the evenness and dominance of the plant community. Discovery of seedling establishment showed seedling preference is near existing plants of con-or heterospecific species and seedling amount is abundant.[23] Many believe the process of new seedlings becoming infected with existing mycorrhizae expedite their establishment within the community. The seedling inherit tremendous benefits from their new formed symbiotic relation with the fungi.[24] The new influx of nutrients and water availability, help the seedling with growth but more importantly help ensure survival when in a stressed state.[25] Mycorrhizal networks aid in regeneration of seedlings when secondary succession occurs, seen in temperate and boreal forests.[23]

Seedling benefits from infecting mycorrhizae
Increased infectivity range of diverse mycorrhizal fungi
Increased carbon inputs from mycorrhizal networks with other plants
Increased area means greater access to nutrients and water
Increased exchange rates of nutrients and water from other plants.
Mature Douglas fir

Several studies have focused on relationships between mycorrhizal networks and plants, specifically their performance and establishment rate. Douglas fir seedlings' growth expanded when planted with hardwood trees compared to unamended soils in the Oregon Mountains. Douglas firs had higher rates of ectomycorrhizal fungal diversity, richness, and photosynthetic rates when planted alongside root systems of mature Douglas firs and Betula papyrifera than compared to those seedlings who exhibited no or little growth when isolated from mature trees. .[26] The Douglas fir was the focus of another study to understand its preference for establishing in an ecosystem. Two shrub species, Arctostaphylos and Adenostoma both had the opportunity to colonize the seedlings with their ectomycorrhizae fungi. Arctostaphylos shrubs colonized Douglas fir seedlings who also had higher survival rates. The mycorrhizae joining the pair had greater net carbon transfer toward the seedling.[27] The researchers were able to minimize environmental factors they encountered in order to avoid swaying readers in opposite directions.

In burned and salvaged forest, Quercus rubrum L. establishment was facilitated when acorns were planted near Q. montana but did not grow when near arbuscular mycorrhizae Acer rubrum L. Seedlings deposited near Q. montana had a greater diversity of ectomycorrhizal fungi, and a more significant net transfer of nitrogen and phosphorus contents demonstrating ectomycorrhizal fungi formation with the seedling helped with their establishment. Results demonstrated with increasing density; mycorrhizal benefits decrease due to an abundance of resources that overwhelmed their system resulting in little growth as seen in Q. rubrum.[28]

Mycorrhizae networks decline with increasing distance from parents, but rate of survival was unaffected. This indicated that seedling survival has a positive relation with decreasing competition as networks move out farther.[29]

One study displayed the effects of ectomycorrhizal networks in plants who face primary succession. In his experiment, Nara transplanted Salix reinii seedlings inoculated with different ectomycorrhizal species. He found that mycorrhizal networks are the connection of ectomycorrhizal fungi colonization and plant establishment. Results showed increased biomass and survival of germinates near the inoculated seedlings compared to inoculated seedlings.[30]

Studies have found that association with mature plant equates with higher survival of the plant and greater diversity and species richness of the mycorrhizal fungi. However, these studies have not considered the threshold status of competing for resources and the benefit for the mycorrhizal networks.

Carbon transfer

Carbon transfer has been demonstrated by experiments using isotopic 14C and following the pathway from ectomycorrhizal conifer seedlings to another using mycorrhizal networks.[31] The experiment showed a bidirectional movement of the 14C within ectomycorrhizae species. Further investigation of bidirectional movement and the net transfer was analyzed using pulse labeling technique with isotopes 13C and 14C in ectomycorrhizal species Douglas fir and Betula payrifera seedlings.[32] Results displayed an overall net balance of carbon transfer between the two until the second year where the Douglas fir received carbon from B. payrifera.[33][34] Detection of the isotopes was found in receiver plant shorts, expressing carbon transfer from fungus to plant tissues.

When ectomycorrhizal fungi receiver end of the plant has limited sunlight availability, there was an increase in carbon transfer, indicating a source-sink gradient of carbon among plants and shade surface area regulates carbon transfer.[35]

Water transfer

Isotopic tracers and fluorescent dyes have been used to establish the water transfer between conspecific or heterospecific plants. The hydraulic lift aids in water transfer from deep-rooted trees to seedlings. Potentially indicating this could be a problem within drought-stressed plants which form these mycorrhizal networks with neighbors. The extent would depend on the severity of drought and tolerance of another plant species.[36]

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See also

References
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