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Hidden Partners: Mycorrhizal Fungi and Plants

Ectomycorrhizal Fungi and the Wider Environmental Community

The effects and roles of ectomycorrhizal fungi in the wider ecological community have only been given serious consideration within the last two decades and is still poorly understood.

It appears that mycorrhizal fungi have evolved different strategies on how to inhabit and compete within their environment, known as "foraging strategies." Some types and species appear to have evolved towards optimal host searching strategies, while other groups (such as the ectomycorrhizae) have evolved towards increased competition with other mycorrhizal fungi for soil nutrients. Mycorrhizae have also evolved and adapted to different soil conditions such as pH and soil composition. All of these factors and more have allowed for an incredibly rich and diverse number of mycorrhizal species and ecological pathways. Given that mycorrhizal associations are involved with more than 90% of all land plant species, the effect that mycorrhizal fungi have on their community is thus quite profound.

As fungi produce the carbon containing enzymes needed to degrade the organic material they encounter, they use the energy supplied from their host plant and release carbon into the surrounding substrate, thereby altering its chemical make-up. The chemical make-up of the soil can also be altered by dead decomposing fungi, which release large amounts of nitrogen from their cell walls.

Because ectomycorrhizae typically colonize tree species, the amount of carbon available to them is much greater than other types of mycorrhizal fungi which typically colonize herbaceous plants. This is theorized to be one reason for the ectomycorrhizal formation of large mycelial groupings. The fungus can spend less time and energy foraging for another host plant, and instead spend more energy supplying the current host plant with nutrients, thus making the situation more productive for the tree. The fungus will also benefit because a healthy tree is able to produce more carbohydrates than the fungus itself can use.

Plants also play an active role in helping their mycorrhizae compete for nutrients. For example, ectomycorrhizal plants usually have high levels of secondary compounds in their leaves such as lignin. Due to the high levels of these secondary compounds, the availability of inorganic nitrogen (the nitrogen source absorbed by most other mycorrhizal fungi) is very low in the surrounding soil, while levels of organic nitrogen (which ectomycorrhizae and Ericoid mycorrhizae can utilize) are high. Thus the plant helps its own mycorrhizal fungus colony to out-compete other mycorrhizae, which will in turn help the ectomycorrhizal plant to out compete non-ectomycorrhizal plants.

However, not all landscapes are dominated by a single species and most land plants are colonized by non-ectomycorrhizal fungi. How can this be? Complex landscapes such as forests and grass lands with hundreds of plant species can exist because different types of mycorrhizal fungi utilize different forms of nitrogen and phosphorus (organic and inorganic). A single plant can also be colonized by several species of fungi, thus broadening its potential nutrient base. In this way, plants are able to compete yet coexist with each other.

Page 6: Mycorrhizal Fungi and the Future
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