By : Chantalle Simmons, Christopher Kreuzweiser, Cindy Graves
There is a group of fungi which grow associated with plant roots in a symbiotic relationship. These are called mycorrhizae fungus root. A large volume of the soil is penetrated by fine, highly branched fungal hyphae which are "extensions" of the tree's own root system. As the fungal hyphae are very absorptive, and more efficient than the plant's roots themselves, they take up mineral nutrients from the soil and then pass some of these minerals to the plant. In return, the fungi receive sugars and other nutrients from the plant's photosynthetic processes. Hence the symbiotic relationship.
The most common species of mycorrhizae are known as endomycorrhizae ( endon = within ). The fungus grows in the root's outer layer and sends the highly branched hyphae inside the cell walls.
Ectomycorrhizae ( ecktos = outside ) is a separate form of mycorrhizae, in which a sheath develops around the outside of the root and extend hyphae between cells of the outer layers of the roots.
Mycorrhizae also contain nitrogen fixing bacteria which fix atmospheric nitrogen. Nitrogen, in a usable form is one of the plant's most important requirements. However, plants are defendant upon the activity of soil microorganisms which in corporate nitrogen through this form of nitrogen fixation. After, nitrogen fixation has occurred usable nitrogen compounds, N02 and NH3 are available for both the micro-organism and the plants.
This, after the nitrogen fixing bacteria in the mycorrhizae harvest nitrogen from the air, the nitrogen is used both by the fungus and the host tree.
The most common trees in this type of relationship are woody plants. In particular : Pine, Fir, Spruce, Larch, Douglas Fir and Hemlock. These trees depend on these mycorrhizae forming fungi for nutrient uptake. It is believed that this occurrence occurred as far back as 400 million years ago, traceable back to the early fossils of the rooting systems of plants.
Mycorrhizal fungi also produce hormones which encourage the production of new root tips of the tree and therefore increase the tree's useful life span. In addition, a mycorrhizal infection is beneficial to plant in situation where nutrients are deficient or the plant faces strong competition from other organisms. This results in an increased surface area available for mineral and water uptake.
Hence, it is evident that the mycorrhizal relationship with certain trees is beneficial to both organisms. The fungi gain valuable sugars and nutrients from the host and the tree experiences enhanced growth for a variety of reasons if the fungus is present.
Mycorrhizae have a link with the Northern Flying Squirrel. The squirrel is common in northern Canada, Alaska and North-Western United States of America. This squirrel is nocturnal and its diet consists mainly of fungus, hence it is a "mycophagist". Mycorrhizal hypogeous fungi - those which fruit below the ground, form the principle diet of the squirrel.
Exactly what is the main link between the hypogeous ‚ mycorrhizal fungi and the flying squirrel??
Let us examine this relationship:
The squirrels nest and reproduce in the tree canopy. At night, they descend to the ground and feed on hypogeous sporocarps. These sporocarps contain nutrients which are beneficial to the small animals, such as the squirrels which eat them. But they also contain water, nitrogen fixing bacteria, fungal spores and yeast.
After the sporocarps are consumed by the squirrels, pieces of them move to the stomach where the fungal tissue is digested. They then move to the small intestine where they are absorbed, and finally into the cecum where they are concentrated and mixed. The sporocarps are later released as pellets, after spending more than a month after ingestion. These pellets are excreted through the rectum and still contain the fungal spores, nitrogen fixing bacteria and yeast.
Each pellet contains four important components to the forest :
These pellets may be dispersed throughout the forest in various ways allowing for the reproduction of the mycorrhizae fungi depending on where the pellets fall. If they land near a rootlet of a tree, the root might become host to a new colony of mycorrhizal fungus as spores germinate. Hence, the Flying Squirrel is an important component within the forest system.
We must remember how fundamentally connected these organisms are in order to maintain the delicately balanced ecosystem of the forests. If we do not ensure the survival of one species, another will surely perish. Thus fungi, seemingly insignificant organisms, have a vital role in the huge cycle of life.
Arms, Karen and Camp, Pamela. "BIOLOGY : Third Edition" Saunders College Publishing, Toronto, 1987.
L. Johnson. "BIOLOGY" . Wm. C. Brown Company Publishers, Iowa,1983.
Maser , C . The Redesigned Forest, R. & E. Miles. San Pedro, 1988.
Alternate Version
done in 1997
In looking at the ecological issues surrounding forestry and its industries, it is easy to lose sight of the forest for the trees. While trying to develop plans for sustainable land use and logging both government officials and industries have focused solely on maintaining a regular crop of trees rather than focusing on sustaining a functioning ecosystem that preserves its biodiversity and the many links between species that are vital within an ecosystem. In many cases, the public has been lulled into believing that by replanting trees in logged areas and growing back crops of commercially valuable trees a complete forest with inter- species connections and wildlife shall return.
In the last 20-30 years, logging methods and silvaculture have progressed taking into account soil conditions, drainage, the needs of different tree species, wildlife, and public concerns to determine the logging and silvaculture methods to be used on any particular cutblock. Cutblock sizes have decreased and in some cases,they have been shaped to emulate forest fires. Rather than leaving cut areas bare, they are replanted with tree species of potential commercial value and genetic traits conducive to the lumber mill not necessarily those that sustain genetic diversity. Although a great deal of research has gone into farming regular crops of trees, there is little known about the ecosystem from which they come and the full breadth of the interactions that exist with the life around them. An example of just one of the interactions that exist between a tree and its surrounding ecosystem can be found in the conifer and mixed wood forests of Canada and the northern United States in the combination of the northern flying squirrel and mycorrhizae.
The term mycorrhizae relates to types of fungi that evolved into symbiotic associations with plant roots. The fungi draw important nutrients from the soil in "exchange" for simple carbohydrates from the host plants both of which are passed through hyphae that have entered the plant's root hairs. (Janerette 1991) Conifers such as pine, spruce, fir, hemlock and Douglas fir which form the basis of the northern forests depend on mycorrhizae for a large part of their nutrient uptake. (Maser 1988; Harley and Smith 1983). Mycorrhizae enhance their ability to survive in poor soil conditions, promote root growth, prevent the invasion of pathogens and increase the tree's ability to withstand extreme conditions. (Janerette 1991) The fungi, depending on their species, generally have fruiting bodies, sporocarps, below ground with hyphae extending to penetrate the root hairs of the tree to form a more efficient nutrient absorption system than the roots alone could provide. Along with this, the fungi serve as hosts to a nitrogen-fixing bacteria that fixes atmospheric nitrogen using an extract from the fungi for food-needed nitrogen that is passed onto the tree which cannot fix nitrogen for itself.(Maser 1988) The fascinating link here that shows the close linkages within an ecosystem is the connection between the sporocarps and the flying squirrel which is in turn dependent on the tree. The mycorrhizal sporocarps which are found below ground are one of the primary food sources for the northern flying squirrel and other small animals. These fruiting bodies provide their essential nutrients. (Fogel and Trappe 1978; Gronwall and Pehrson 1984; Sanders 1984) The sporocarp even produces a particular odor as it matures to attract these animals to forage for it. In doing so, the fungi encourages its own spread and reproduction. When the northern flying squirrel eats the sporocarp, it takes in as well the nitrogen-fixing bacteria and an essential yeast which along with the spores pass untouched through its digestive system. After a period of time, even months, they are excreted in fecal pellets to form little mycorrhizal units with the necessary nutrients to grow under appropriate conditions to once again form a symbiosis with a root system. The squirrel in turn, is dependant on the tree for its nests and reproduces in the tree tops and comes down from the tree at night to feed.(Maser 1988) Looking at this cycle it is easy to see how each aspect depends on the other. When a tree is removed the squirrel must move and take its mycorrhizal pellets elsewhere and the mycorrhizae cannot survive as it is unable to photosynthesize its own food and is a poor saprophyte (Maser 1988). When trees are planted to restock a cutblock they do not have the mycorrhizae they require limiting their growth. If the trees replanted are not those that match the particular fungi in that block, then they do not necessarily get the nutrients that they need. Hence when looking at logging and reforestation issues, the flying squirrel habitat and continuation of the appropriate mycorrhizae must also be considered. The connections between coniferous trees, the flying squirrel and mycorrhizae are only one aspect of an infinite number of ecological links that exist which we do not yet understand. Not only do we need to seek to learn more about these ecosystems to regenerate them as they are destroyed by society's constant demands for wood and lumber but it is in the best interests of the forest industries to learn more about these interactions for long-term sustainability. By discovering more and more about the workings of these ecosystems we may all be able to profit from these ecosystems without exploiting them, no longer separating the trees from the forest.
Bibliography.
Fogel, R. and Trappe, J.M. 1978 Fungus consumption (mycopphagy) by small animals. Northw. Sci. 52:1©31
Gronwall, O. and Pehrson, A. 1984. Nutrient content in fungi as a primary food of the red squirrel "Sciurus vulgaris L. Oecologia, 64:230-231.
Harley, J.L. and Smith, S.E. 1983. Mycorrhizal Symbioses. Academic Press, New York. NY. 483pp.
Janerette, Carol A. An Introduction to Mycorrhizae. The American Biology Teacher, January 191, vol.53, no. 1 pp13.
Maser, Chris. "The Redesigned Forest ". R & E Milnes, San Pedro: 1988.
Sanders, S.D. 1984 Foraging by Douglas tree squirrels (Tamiascirus douglasii : Rodentia) for conifer seeds and fungi. Ph.D thesis, Univ. of California, Davis. 95pp.