As my classmates and I stood in front of the chosen site for the community garden we would be creating, I wondered what I was getting myself into. My college campus was constantly undergoing construction of new buildings and renovations, and for many years this vacant lot had been used as a dumping ground for garbage, debris, lumber, and bricks. I crouched down to examine the soil we would be working with, and found it as hard as the concrete sidewalk nearby. There were not even many weeds growing out of the nutrient-deficient ground. This was where we were supposed to grow tomatoes, kale, and squash? This unattractive lot was supposed to become a thriving garden where kids and adults could come together to work and learn and play? My Agroecology course that I anticipated to be a fun and easy part of my quarter was turning out to be much more challenging.

However, my professor knew exactly what to do to transform this place into the beautiful urban farm it would eventually grow into. He inoculated the ground with mushrooms for the winter. He explained that the thin, root-like strands, known as hyphae, would spread through this barren soil and transport water and nutrients throughout the soil and revive it with bacteria and insects. The transformation was unbelievable. Several months after the inoculation of the fungi, the lot began to look like a place where we could dig and plant.

Months later, while installing a drip irrigation system alongside my new friends in the garden, we took a break to snack on some tomatoes and chat about our days. I realized that it was those mushrooms my professor had inoculated into the soil that created the strong foundation for our community.

I often think back to the amazing power of fungi that I observed in that garden. Now, living in the rugged, mountainous, forested ecosystem of the North Cascades, I wonder again how fungi work in this soil and create a foundation for this new community I am in. It was this time that I spent in the garden in California that sparked my interest in studying the hidden processes found beneath the soil and discovering the invaluable role of mycorrhizal relationships in the forest community of the North Cascades.

Understanding the Basics

To understand the process of mycorrhizal relationships, which are the associations between fungi and plants, it is necessary to first learn about fungi in general. Fungi are some of the most ancient organisms that presently exist. They fall under the classification of Eukaryotes and are neither plants nor animals, though they are believed to be more closely related to animals than to plants. Like animals, fungi do not photosynthesize, but rather absorb their food to gain energy. However, like plants, fungi do not move about, and they reproduce through spores, a characteristic shared with more primitive plants like mosses, ferns, and liverworts.

Fungi come in many forms, colors, sizes, and textures. Some are macroscopic and can be observed with the naked eye, while others require a microscope to see. Some appear directly on plants or growing out of the soil, while others are hidden deep beneath the ground or inside of living organisms. There are over 1.5 million different species in the kingdom of fungi, many of which scientists have yet to discover. Something that all fungi share in common though is that they are essential to the health of the ecosystem.

Fungal Relationships

Fungi are generally categorized according to their phylogenic roots, or their evolutionary paths. Another way to organize fungi is by the roles that they play in their communities, which includes their associations with other organisms. Fungal relationships dictate how the fungi obtain their nutrients. A few categories of fungal relationships include saprophytes, parasites, endophytes, lichens, and mycorrhizae. In addition to describing how fungi get their nutrients, these categories also indicate the roles that fungi play in their habitats. While this paper will mostly focus on mycorrhizal relationships, I will first describe a few of the other associations that can occur between fungi and plants, including some personal anecdotes to help the reader picture these processes.

The Artist’s Conch

For the last year, one of my next door neighbors has been an Artist’s Conch. When I step out of my house, looking across the path is a Douglas Fir tree. The tree has thick bark, and its diameter would take at least three adults to wrap their arms around it all the way. However, when looking up towards the top of the tree, it becomes clear that it is no longer living, but it is actually a snag. The top of the tree has been cut down, or perhaps it has snapped off in a storm. There are no branches or needles on it, it is simply a 40 foot tall dead tree.

A few inches above eye level, there is a big shelf fungus attached to the bark. With several shades of brown on top, and white on the bottom, the fungus almost resembles a clam shell. It is the size of my head, perhaps even bigger. I have had the pleasure of greeting my Artist’s Conch every day for a year, observing it in the rain, snow, and sun. My eyes automatically travel to it every single time I go in or out of my house. It has changed very little throughout the year. Despite a lack of visible change, I know that it is slowly absorbing its nutrients from the snag, decomposing it and transforming it into rich soil. I hypothesize that if I one day come back to visit my old neighborhood, the fungus will be much bigger, and the snag will be further along in its journey to being recycled into the soil to welcome new plants into the neighborhood.

The Decomposers

The Artist’s Conch described falls into the role of a decomposer, also known as a saprophytic fungus. These fungi obtain their nutrients by feeding on dead, organic matter. They release enzymes and acids that break down molecules of dead matter into simpler molecules. Saprophytes are essentially recycling agents, as they recycle carbon, hydrogen, nitrogen, phosphorus, and minerals from dead organisms into nutrients that can be used by living organisms. Saprophytic fungi are either primary, secondary, or tertiary decomposers, depending on the stage of the decomposition process to which they contribute. Some will appear immediately after the organism dies, while others will contribute to the decomposition process further along in the plant’s post-death stages. Saprophytes are vital to the forest community as they return important nutrients into the soil.

The next three categories of fungi partake in symbiosis, which is defined as an association between at least two living organisms. The three categories are parasites, endophytes, and mycorrhizae.

Parasitic Fungi

Parasitic fungi obtain their nutrients from other living organisms. In this type of relationship, the fungus benefits while its host suffers. Most of these fungi are microscopic, although their effects are easily seen through spots or lesions on leaves or shoots. Necrotrophs are parasitic fungi that quickly invade and kill a plant host, and then act as saprotrophs, feeding on the dead matter. Biotrophs invade a living plant and take a long time to kill it. While parasitic fungi are harmful to their host organisms, they can be seen as a way of creating a healthier ecosystem as they can clear out the weakest plants. Recent scientific findings have indicated that factors such as the changing climate can contribute to some plants being more vulnerable to parasitic fungi, which can lead to some devastating effects. It is also interesting to note that parasitic fungi can also invade other fungi.

Endophytes

Endophytic fungi form relationships with plants by spreading their mycelia, or root system, between plant cell walls. These associations are either commensalistic or mutualistic, either being harmless to the plant or offering benefits. The fungus always benefits in this situation, as it obtains its nutrients from the plant. In many endophytes, the plant benefits through an enhanced ability to absorb nutrients as well as assistance in warding off parasites, infections, and predation. An example is an endophyte in certain grasses that releases mycotoxins which are toxic to livestock, but harmless to the grass. This allows the grass to be protected from being eaten. Endophytes can also release alkaloids inside of fir needles and other plant parts that are harmful to insects, thus protecting the plant. This is an especially helpful adaptation to long-living trees, as insects have short life spans and evolve quicker than trees that lives for hundreds of years. Since these trees cannot adapt to the insects as quickly as the insects evolve to harm the tree, endophytes can step in to protect the trees by evolving at a more comparable rate as the insects.

Lichens

Lichens are the combination of a fungus, known as the mycobiont, and a photosynthetic partner, which can be either algae or cyanobacteria. The fungus provides the structure for the organism while its partner provides the nutrients through photosynthesis. In the case of lichens, the fungus can also exist independently of the algae. However, when it is combined with the algae, it acts as a separate organism. They can inhabit countless surfaces and climates. Some lichens are extremely sensitive to pollution, allowing scientists to use them as indicators of air pollution.

Saprophytes, parasites, endophytes, and lichens are a few examples of associations that fungi have with plants. This is just an introduction to the vast variety in roles that fungi play in the natural world. The rest of this paper will highlight the process of mycorrhizal relationships, the topic that first sparked my interest in fungi in the sunny Californian garden.

The Chanterelle

Last fall, as the Vine Maples were turning red and yellow around me and the weather began to turn colder, I was invited to go mushroom hunting at Bacon Creek. My friends could not wait to go, describing to me how many mushrooms they find there year after year. I pulled on my rain pants, rain boots, and raincoat, and headed to this famous bountiful land of mushrooms. We parked on the side of the road, and began to head straight up the hill.

We climbed over fallen trees, dodging branches, but keeping our eyes on the ground to search for mushrooms. What we found, was indeed a bounty. There were Chanterelle mushrooms everywhere. They were easy to spot, with their brightly colored yellow to orange fruiting bodies. Their sizes vary from thumb sized, to weighing two pounds. We identified them by their apricot-like smell. We felt their gills beneath the cap, feeling the soft, thin, ridges extending part of the way down the stem. The caps resembled warped frisbees slightly turned upwards like a funnel.

These Chanterelles spread their hyphae throughout the soil, connecting the trees together and enriching the soil. These golden, delicious organisms are integral to the forest’s survival. Though the hyphae are as thin as hair strands, they transport water and nutrients to the Douglas Firs and Hemlocks around them. Without these mushrooms, the forest community would cease to thrive. Eating my Chanterelle dinner that evening, I was happy to know that the mycelia remain in the soil to keep the forest standing, and that the fruiting bodies would be back next year.

Mycorrhizal Relationships

Mycorrhizal relationships are mutualistic, meaning both the fungi and the plant benefit. The word “mycorrhizae” means “fungus-root.” The process was discovered in 1885 by a German botanist named Albert Bernhard Frank. It is believed to have evolved millions of years ago when terrestrial plants first spread across Earth. There are over 6,000 identified species of fungi which form mycorrhizal relationships with over 240,000 plant species. They occur in all habitats, from deserts, to lowland tropical rainforests, to subalpine temperate forests. Without them, most plants would not survive and the landscape would look much different.

Structure

To understand the process of mycorrhizal relationships it is important to first understand the structure of the parties involved. The individual strands that connect plants and fungi to one another are called hyphae. Mycelium is the name for a mass of hyphae, which is essentially the vegetative part of the fungus. Mycelium can spread out to amazing distances. It is believed that the largest organism in the world was a mycelial mat in Eastern Oregon that was estimated to be 2,400 acres and 2,200 years old. Some mycorrhizal relationships include fungi that have fruiting bodies, while some do not. Truffles are a type of underground fruiting body involved in mycorrhizae. Ectomycorrhizae are fungi that form sheaths around the outside of host plant roots, while endomycorrhizae enter the root cells of host plants.

Roles in the Community

Mycorrhizal fungi play several roles in the ecosystem. They accelerate plant growth by transporting nutrients both between fungi and plants, as well as between different plants. They also protect plants from disease by inoculating the roots with antibiotics. Mycorrhizae allow for plants to grow in seemingly impossible conditions, such as highly acidic soil. A more recent finding showed that mycorrhizae can also send signals to plants to warn them of an herbivore attack. Mycorrhizae are often the first organisms to appear in a disturbed area and can help restore an ecosystem to a thriving community.

Nutrient Uptake

It is believed that as many as 95% of all plants have mycorrhizal associations. Plants that are not mycorrhizal tend to grow in open habitats where there is less competition for nutrients. Mycorrhizae essentially create networks that transport nutrients, water, sugars, and minerals. They are vital to plant growth as they increase their ability to absorb what they need. The fungi are able to break down phosphates, metal ions, and sugars into a form that the plant roots can absorb.

Plant to Plant Connections

Mycorrhizae link together plant communities by transferring nutrients from one plant to another. This allows for ecosystems to be more resilient. A study by Simard compared the flow of nutrients through the mycelia of three species of trees: a Douglas Fir, a Paper Birch, and a Western Red Cedar. The Douglas Fir and the Paper Birch shared the same ectomycorrhizal fungus, while the Western Red Cedar had an endomycorrhizal fungus. The researchers covered the Douglas Fir to simulate shade. What they found, was that the mycorrhizae responded by transporting sugars from the Paper Birch to the Fir. Over 9% of the net carbon compounds that the fir was receiving was from the roots of the birch. Even more astounding, was that the amount of sugars transferred was directly proportional to the amount of shading created by the researchers.

Water Transport

It has been discussed how mycorrhizae benefit plants through the exchange of nutrients and sugars between the fungi and their host plants. Another benefit is the fungal ability to access water in harsh conditions and provide water to their host plants. Because mycelial networks can spread out so far and deep throughout the soil, they can access water much easier than plant roots would be able to. This is often seen in desert ecosystems, as many cacti are mycorrhizal and depend upon those relationships for survival.

Disease Resistance

Another benefit is the ability of the mycorrhizal fungi to protect the plants from pathogens. There are several ways that this can occur. Many of the fungal species involved in these associations inoculate their host plant’s roots with antibiotics to protect them. They can also encourage the growth of other microorganisms in the plant that decrease pathogens. Mycorrhizae can also protect the plant roots by surrounding the roots with a thick sheath which certain pathogens cannot physically break through.

Warning Signals

A recent study by Babikova and colleagues showed yet another benefit that fungi can provide to their host plants. They found that mycorrhizal fungi can send warning signals from one plant to another to alarm them of an herbivore attack. Often, when a sap-sucking herbivore, such as an aphid, infests a plant, the plant releases VOCs, or Volatile Organic Compounds. VOCs act as a repellant to the aphid’s predator insects. The study examined effects of the mycelial networks between aphid-infested plants and aphid-free plants. What they found was that the aphid-infested plant shared its VOCs with the aphid-free plant through a fungus, making it appear as if the plant had already been attacked, so it was left alone by herbivores. This study indicated another form of protection that plants can receive from the fungi they associate with.

Factors that Increase Mycorrhizae

A study by Kuhnert and colleagues resulted in the surprising finding that the biomass of fungi is actually highest in the winter. The researchers measured the amount of hyphae during the summer and the winter in vegetated and unvegetated soils in an alpine habitat. The insulating properties of snow cover, which keep the soil at a consistent temperature of 0°C, allows for more fungal activity, and less microbial activity within the soil. Plant matter fallen during the fall season allows the fungal matter to decompose during the winter months. Additionally, the mycelium holds on to the dissolved nitrogen from the fallen plant matter, and then releases it into the soil in the early spring for more plants to grow. Although in the winter months it may seem that the soil is just as inactive as the plants above ground, in reality, mycorrhizae are very active in the cold winter months.

Climate Change Impacts on Mycorrhizae

Observations of phenological changes, or changes in seasonal life cycles of plants and animals, show a correlation between climate change and change in life cycles. There have also been some noted changes in mycorrhizal activity that can be correlated with changing climate. Mycorrhizal fungi depend on their host plants and respond to activity in their hosts. In the United Kingdom, there have been astounding findings in regards to the fruiting season of mushrooms in the last 60 years. In the 1950s, the average fruiting period for 315 different species of fungi was 33.2 days. Between 2000 and 2010, the average period for the same species was 74.8 days, which is over twice the previous number. Researchers predict that these longer seasons of fruiting will alter the plant communities of certain ecosystems through increased decomposition rates, which could alter the soil.

Conclusion

As an undergraduate student, I first witnessed the powerful capacity of mycorrhizae to build a thriving garden community. Now, living in the North Cascades ecosystem, I have lived alongside fungi and observed the ways they shape the forest community. Mycorrhizae remind me of the fact that we all have some kind of role that we play in our world. Though those roles may be hidden beneath the soil, the forest would not stand without the existence of the hyphal networks. Fungi are some of the most ancient organisms in the natural world, and each mushroom I find has a story, a connection to another organism, and an important job to do.

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