The Social Lives of Trees: Part 2 Mycorrhizal Fungi

By Emma Ewert, graduate student in the Institute’s 15th cohort. Take a look at part one of her series on the social lives of trees!
It is almost impossible to distinguish root systems from the fungi that they are connected to. In fact, the term mycorrhizal means “fungi-root”, and refers to the root system as a whole, including both the tree roots and mycelial hyphae systems (Fungi are most commonly associated with mushrooms, but these visible structures are just the fruiting bodies of a network of thin string-like hyphae that create webs underneath the soil). Within three months of germinating, tree seeds have developed a mycorrhizal network, and while the species of fungi changes throughout its life, a tree will always maintain this partnership. Mycorrhizal fungi have existed for over 400 million years, and evolved along with plants as they moved onto dry ground. All conifers and most broad leafed trees have mycorrhizal partnerships, and it is probable that both  the fungi and the trees need this partnership to survive. Mycorrhizal fungi has three major roles. First, they extend the root system further into the soil, and allow a tree to access 1000 times more soil and water then they could with their roots alone. Secondly, they access and dissolve critical minerals and nutrients such as nitrogen, potassium or phosphorus into compounds that a tree can absorb and use. Finally, they can detect potentially harmful fungi or bacteria, act as an immune system for the tree.

In return for these services, trees release photosynthates through their roots to feed the fungi and bacteria. This transfer is the result of active transport, meaning that trees are deliberately releasing these products rather than losing them through leakage or inefficiency. 20% to 60% of a tree’s total photosynthetic production goes into the soil and its fungal networks. Consider this for a second. Trees use precious energy to photosynthesize light only to give about a half of it away to other organisms. This system contradicts the traditional idea of competition between species in an ecosystem, instead illustrating a remarkable level of interdependence and cooperation. (Insert Diagram Here)
So why do trees willingly release so much of their hard earned carbohydrates into the soil? One part of this answer comes from understanding the way that trees can absorb the minerals they need. Gardeners and farmers are familiar with the fact that plants need nitrogen, phosphate and potassium, the minerals that make up the traditional NPK fertilizers. These minerals are scarce in most soils, and are easily depleted if they are not renewed in some way. In addition, even when they are present, they often exist in compounds that plants are unable to absorb.
The fungal hyphae part of the mycorrhizal system is able to convert these minerals into soluble compounds that a tree can use. Mycorrhizal systems access the minerals in many ways. To use nitrogen as an example, insects and bacteria eat nitrogen-rich compounds and convert them into ammonium and other soluble chemicals that trees can absorb. But some fungi can directly access and convert nitrogen in other ways. Some species release chemicals that break down and dissolve nitrogen, while others enter rocks and “mine” nitrogen or other minerals directly from the rock.
Because hyphae extend the root system so much, they also allow trees to access much more water than they otherwise would. This is especially important in drier areas. In drier areas, trees are much more susceptible when soil has been depleted of fungi and other microbes then the same species in wetter areas. There are probably multiple reasons for this, but the ability of extensive mycorrhizal networks to access so much water is a definite factor.
Furthermore, mycorrhizal networks help detect and repel pathogenic fungi or bacteria. Much like for humans, some bacteria or fungi are beneficial, but many are not. When these pathogens come into contact with the mycorrhizal network, the network uses many different mechanisms to respond. Some fungi and bacteria can recognize pathogens in the soil, and respond directly, while others release compounds that warn the tree of danger. The tree then can change the compounds that it is releasing into the soil, and either repel the pathogen itself, or attract specific bacteria and fungi that can do this job for them.  
The variety of roles that mycorrhizal systems play help explain why a tree can have up to 200 different mycorrhizal fungi species at one time; each fungus has the ability to grow into certain areas and access different sources of water or nutrients, recognize or repel a specific pathogen, or the ability to access and process a different mineral.
While these are the main functions of mycorrhizal networks, many more smaller interactions have also been discovered, and are still being discovered. Trees release a substance tryptophan into the soil, and a specific species of fungi can convert it into a growth hormone. This growth hormone then causes tree roots to grow thicker and extend further into the soil. There are probably many other small interactions like this that occur in the mycorrhizal system, and the rhizome (the area of soil surrounding the mycorrhizal system that houses beneficial bacteria as well), but the system is still being explored, and many of the intricacies have yet to be discovered.
I have a hard time grasping the impact of these incredible discoveries, but the truth is that they are only the tip of the iceberg. The phenomena I have described have been seen and described, but often only in one ecosystem or in one species of tree or plant. So few studies have been done that these interactions are probably much more widespread and much more complex than we know today. One of the interactions that occur as a result of mycorrhizal fungi, is that trees are able to communicate and interact with each other through this network. In the next post, we will delve into this amazing world even deeper.
 
If you are interested in this subject, more information is available from these sources:
H.J. Andrews Experimental Forest: http://andrewsforest.oregonstate.edu/
The Hidden Forest: http://www.indiebound.org/book/9780805014914
The Hidden Half of Nature: http://www.indiebound.org/book/9780393244403