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The Unseen Ecology of Dirt: The Story of Mycorrhizae
By: Scot Pipkin, Director of Education and Engagement
“There’s a world going on underground.” —Tom Waits
If you spend time at Santa Barbara Botanic Garden, you will hear many people talk about the “web of life.” It’s an elegant and instructive metaphor illustrating the strong but sometimes invisible connections between organisms. Disturb one strand and others will be affected. Lose many strands and the structure ceases to function.
As I write this article, the Garden is celebrating Bird Month, as a perfect example of the web of life. Not only do birds rely on plants for nesting, roosting, and direct food sources (fruits and seeds), they also rely heavily on the insects that are associated with plants. If we take the example of the Order Lepidopteran (butterflies and moths), we find that they represent up to 70% of a songbird’s diet (Piel, Tallamy, Narango, 2021). Most butterflies and moths rely on a limited set of larval host plants to survive. We can just look at the example of monarch butterflies (Danaus plexippus) and milkweed (Asclepias spp.) to get a sense of how the relationships function in the web of life. If we zoom more closely into this metaphorical web, we might notice something fascinating about the strands themselves. Like yarn or rope, these strands, which look like single, solid entities at our scale, actually consist of smaller fibers woven together, intertwined to create strength.
On any scale, the web of life metaphor reminds me to be aware of my connection to others, including those of the non-human (i.e., plant, animal, mineral) world. I have a friend who works as a hydrologist. They are quick to clarify that their interest and specialty are surface water, not groundwater hydrology. “Once it goes underground,” they say, “I pretend it doesn’t exist anymore. What happens down there is too complicated.” In many ways, I have felt a similar sense about ecology. The things that happen above ground, on terra firma, are within my realm of comprehension. Once we start asking questions about what’s happening beneath our feet, all bets are off. Don’t even get me started on marine or aquatic ecology. As I’ve gotten older and my curiosity more refined, the dark, often dusty (around here, anyway) world of soil ecology has remained mysterious; however, it has become less intimidating. Any sense of fear I’ve had has been replaced with allure.
Establishing the Roots
Standing under the shade of an oak tree in California, say, a coast live oak (Quercus agrifolia), blue oak (Quercus douglasii), valley oak (Quercus lobata), or canyon live oak (Quercus chrysolepis), one might feel the urge to dig down — beneath the layer of decaying leaves, twigs, and bark and into the earth. Across the threshold of duff and piercing the truly subterranean world, it might not take too long to encounter mighty roots mirroring a mighty canopy. Careful inspection of these woody threads would reveal that, like the metaphorical strands in the web of life, these structures are more than what they seem. In fact, these roots will begin to look a lot like the strands of the web of life previously described: intertwined lengths of living tissue exchanging energy and building resiliency. It’s one of the great and ancient symbiotic relationships in nature. It is an amazing partnership between plant and fungus.
Oaks (Quercus spp.) are not the only plants that benefit from this relationship with fungus. In fact, it is estimated that up to 90% of plants globally are involved in these relationships, where fungus in Class Mycorrhizae (etymologically speaking, “myco” refers to fungus and “rhizome” refers to root) works with plant roots. The ubiquitousness of mycorrhizal relationships with plants hints at their significance. In fact, research demonstrates that the presence of fungus in a plant’s roots greatly increases the plant’s ability to uptake water; absorb essential nutrients such as carbon, nitrogen, and phosphorus (Simard, 2018); fight off pathogens (Duchesne, 1994; Linderman, 1994); and even reduce a plant’s susceptibility to herbivory (Koziol et al., 2018).
A closer study of mycorrhizal symbiosis is revealing fascinating insights into soil ecology. Researcher Suzanne Simard has spent decades studying such relationships in the temperate rainforests of western Canada, and what she has learned is awesome in the most literal sense. Based on Simard’s experiments, it is becoming clear that the mycorrhizal relationship with plants isn’t as simplistic as sharing resources. Mycorrhizal networks enable nutrient sharing between trees of multiple species and even facilitate communication within the forest. It appears that through the mycorrhizal network, trees can democratically allocate resources to each other and promote the growth of seedlings. Within the network, hubs of activity center around particular trees, often within a certain age class. These “Mother Trees” help dictate the distribution of resources within the network (Simard, 2018). Through these partnerships between Mother Trees and fungus, there is an ability to recognize both kin and individual neighbors (ibid). The health of the forest as we experience it is mediated in large part by what is happening below ground. A special shoutout goes to old growth habitats. Those are the realms of Mother Trees.
Such community-level implications aren’t limited to the ancient cedar and hemlock forests (Family Pinaceae) of the Pacific Northwest where Simard conducts her research. Here in California, researchers observed the transfer of nitrogen from gray pine (Pinus sabiniana) to nearby blue oaks (Southworth, 2013), suggesting that similar dynamics may be at play in the oak woodlands of California as they are in the forests of Canada. In fact, these mycorrhizal networks benefit a number of species in the plant community, including buckbrush (Ceanothus cuneatus), and other annual plants (He et al., 2006). Additional studies in the foothills of the Sierra Nevada reveal that dozens of mycorrhizal fungi can be associated with blue oaks and those communities can differ in significant ways from nearby interior live oak (Quercus wislizeni) associations (Morris et al., 2008). This study was only examining the diversity of ectomycorrhizae, or mycorrhizae whose hyphae (fungal equivalent to roots) do not penetrate the cell walls of the host roots. This class of mycorrhizae is particularly noteworthy because it includes chanterelles (Cantharellus spp.); agarics (Amanita spp.), including Amanita muscaria, one of the infamous “magic mushrooms”; and other common mushrooms of forests and woodlands. Mycorrhizae, therefore, are responsible for the health of our plant communities and a significant food source for humans the world over.
Biodiversity Loss and Recovery
Given the importance of mycorrhizae to plant communities, it stands to reason that we should be prioritizing the conservation of soil integrity in addition to saving the plants we care about. Conversely, we can also know that by saving native plants, even individuals in some cases, we are protecting the networks that support a healthy landscape on a broader scale. There is evidence that suggests soil disturbance ranging from tilling to grazing to development can significantly impact mycorrhizal fungi. In one study in coastal sage scrub communities in California, it appears that conversion to nonnative grasses has tilted the soil microbial balance to favor mycorrhizae that provide more benefit to the introduced plants.
This represents a possible competitive advantage for the invasive plants as they might be able to utilize their mycorrhizal advantage to outcompete native species (Brooks, et al., 2022).
The interconnected web of life is only one aspect of why biodiversity is so critical to ecological function on our planet. Living systems influence abiotic (nonliving) components of our world profoundly. In the steep terrain of Southern California, mycorrhizae work with plant roots to stem the pull of gravity and stabilize soils. Mycorrhizae help create soil aggregates through biochemical and biological processes (Rillig and Mummey, 2006). When living strands are broken through disturbance, soil is more easily eroded, leading to slumping, incised channels, and sometimes catastrophic slope failures. More than wildfire and flooding, human activities are a primary source of soil disturbance on our planet. The scale at which we convert native habitats for mining, forestry, agriculture, development, and transit is unparalleled. As a result, mycorrhizal communities are being negatively impacted by human activities across the globe (Xavier and Germida, 1999).
Your Home Garden Can Help
By now, it should be evident that mycorrhizal fungi are essential for supporting healthy habitats. In the face of massive disturbance and destructive practices, it might feel intimidating to think about what we as individuals can do to promote soil biodiversity and plant health. Fortunately, we have the potential to make real impacts in our local communities.
For those who have access to land, whether it be a yard or community site, what could be more beautiful than facilitating healthy soil interactions that could beget even more biodiversity? If you are a homeowner or caretaker of a property with land, consider this: for supporting mycorrhizal communities, any plant is better than bare soil or impervious concrete, a California-native plant is even better, and a locally adapted native plant is best. There is limited but potentially significant evidence that local genes might play a role in colonization by local mycorrhizal fungi (Van Geel, et al. 2021). Though rare, some local nurseries sell plants whose genetics they can confidently say come from nearby populations. But it all starts with native plants.
While it is possible to purchase mycorrhizal inoculants for installation in your own yard, it might not be as simple as acquiring a product and inoculating the earth. Fungi might be a little bit fickle, or at least adapted to more specific conditions. Some inoculants may be effective with some plants or soil conditions but not colonize as vigorously in others. Manzanitas (Arctostaphylos spp.), for instance, have relationships with an entire group of mycorrhizal fungi (known as ericoid mycorrhizae), and some plants — like manzanita — benefit most from multiple mycorrhizal associations at once. Mycorrhizal communities may also change over time as a landscape or habitat matures. Inoculation can work for some species and might not be a bad idea in a landscape that has seen significant recent disturbance, but it may not be a necessary step toward healthy soil. Spores have been spreading for a billion years and fungi have found a way to colonize the various corners of this planet. There is hope that your small patch of soil could be the landing spot for a microscopic hitchhiker looking for a plant root to share energy with. Try to make that home welcoming.
In an age when climate change and threats to the web of life loom large over our environmental psyche, it’s easy to fall into a paralyzed sense of despair. This is a normal response. The problems are so big and we as individuals have seemingly so little recourse, it’s easy to disassociate from the topic altogether. But I say, despair not! We have to fight for the things we love and what could be more beautiful than helping the next generation of Mother Trees get established?
Tips for Getting Started
Before prepping any ground or buying a tree from a nursery, it’s important to reconnect with nature. Fall back in love with the earth that sustains us and remind yourself that YOU are a strand in the web of life — reliant on other connections and capable of supporting a great load. Go outside and spark your curiosity. Walk around barefoot and connect with the earth. Carefully flip a decaying log and see what life lies just underneath the surface. Mycorrhizal fungi are an incredible, miraculous component of the world we inhabit, but theirs is just one story unfolding around us at all times. By cultivating your curiosity and establishing a connection to the natural world, you will be positioning yourself to be a better advocate for and steward of the web of life.
Once you have reconnected with the natural world, there are actionable steps you can take to improve soil health and local ecology. Using natural mulch and home composting are great ways to contribute to soil biology and help create a good environment for organisms like mycorrhizal fungi to colonize. Bottomline, soils are home to an array of microbes, herbivores, hunters, decomposers, and other organisms that contribute to our local biodiversity. Planting native plants helps ensure that mycorrhizal networks can get established and local ecology has the opportunity to recolonize our lives.
Even if you don’t have access to a patch of earth to tend, there are significant steps you can take to advocate for native biodiversity and our local webs of life. I am consistently inspired by a friend who is a dedicated advocate for nature in our communities. They attend public meetings, provide public comment on development plans, serve on one of the local public tree advisory commissions, among other things. In turn, I’ve become activated to attend meetings, write comment letters, and use my voice to advocate for nature in our communities. What I’ve realized in the process is these are real policies with longstanding implications and precious few people are speaking up. Your voice can change the direction of local policy for decades and that should feel empowering. In fact, the Garden can be another “friend” encouraging you to get involved by signing petitions, writing letters, and taking more action to support native plants and habitats.
So, go outside, commune with the earth, contribute to her well-being, plant native plants, and use your voice to let others know that we are all connected in this vast web. Like the spores of a chanterelle, your actions will multiply and intertwine with others to create something astonishing. O
Citations
Duchesne, L. C. (1994). Role of ectomycorrhizal fungi in biocontrol. Mycorrhizae and plant health (pp. 27-46). The American Phytopathological Society.
Piel, G., Tallamy, D. W., & Narango, D. L. (2021). Lepidoptera Host Records Accurately Predict Tree Use by Foraging Birds. Northeastern Naturalist, 28(4), 527-540. https://doi.org/10.1656/045.028.0410
Linderman, R. G. (1994). Role of VAM fungi in biocontrol. Mycorrhizae and plant health (pp. 1-26). The American Phytopathological Society.
Morris, M. H., Smith, M. E., Rizzo, D. M., Rejmánek, M., & Bledsoe, C. S. (2008). Contrasting ectomycorrhizal fungal communities on the roots of co-occurring oaks (Quercus spp.) in a California woodland. The New Phytologist, 178(1), 167-176. https://doi.org/10.1111/j. 1469-8137.2007.02348.x
Pickett, B., Irvine, I. C., Arogyaswamy, K., Maltz, M. R., Shulman, H., & Aronson, E. L. (2022). Identifying and Remediating Soil Microbial Legacy Effects of Invasive Grasses for Restoring California Coastal Sage Scrub Ecosystems. Diversity, 14(12), 1095. https://doi.org/10.3390/d14121095
Rillig, M. C., & Mummey, D. L. (2006). Mycorrhizas and soil structure. The New Phytologist, 171(1), 41-53. https://doi.org/10.1111/j.1469-8137.2006.01750.x
Simard, S. W. (2018). Mycorrhizal Networks Facilitate Tree Communication, Learning, and Memory. In F. Baluska, M. Gagliano & G. Witzany (eds.), Memory and Learning in Plants (pp. 191-213). https://boomwachtersgroningen.nl/wp-content/uploads/2019/04/ Simard2018_Chapter_MycorrhizalNetworksFacilitateT-1.pdf
Southworth, D. (2013). Oaks and mycorrhizal fungi. Oak: Ecology, types and management (pp. 207-218). https://tinyurl.com/bdfybv28
Van Geel, M., Aavik, T., Ceulemans, T., Träger, S., Mergeay, J., Peeters, G., van Acker, K., Zobel, M., Koorem, K., & Honnay, O. (2021). The role of genetic diversity and arbuscular mycorrhizal fungal diversity in population recovery of the semi-natural grassland plant species Succisa pratensis. BMC ecology https://doi.org/10.1186/ s12862-021-01928-0
