Morels and Elms Part Two: Theory

Morels grow in many different areas in a variety of conditions. The Morel Data Collection Project1 has identified several distinct North American morel species in disparate habitats. Some seem to appear most frequently in relationship to certain trees, others sprout after forest fires, and others seem to be stimulated by disturbing the ground where they live.

The morel I am studying has been classified by the MDCP as the Classic North American Yellow Morel. According to the MDCP, this mushroom has been found in all of the above habitats. There is one habitat it seems to have conquered exclusively, however, at least among morels — the area around an elm tree recently killed by Dutch elm disease.

There are two features which distinguish the relationship morels have with elms from those they have with other trees. The first is that while morels appear around live trees such as ash and sycamores, they most often appear under elms after the trees are dead or dying. Second, while the live trees will only produce a mushroom or two, dead elms can produce more than a hundred. There's obviously something different going on here.

The prevailing theory about why morels appear under dead elms is that when the trees die, they release their stored nutrients into the soil, which is used by the underground morel organism to produce mushrooms. This seemed implausible to me. Wouldn't the nutrients absorbed into the tree have to flow against a vacuum to seep back into the soil? I wasn't sure, so I decided to find out how these trees are killed.

Dutch elm disease

Elm trees
2 are particularly suceptible to fungal invasion because of their large vascular systems3, necessary to pump up large amounts of water in the early spring to produce seeds before other trees have their leaves. This allows the elm seeds to get started before the other trees block all the light.

Dutch elm disease4 has devastated elm populations in the Midwest since it was first introduced to the U.S. in Ohio in the 1930s. The disease is actually a fungus spread by the elm bark beetle, which nests in the upper branches. The beetle makes incisions in the small branches near the top, where the fungus gets in and follows the trail of nutrients down the tree, leaving a tell-tale black mass in the tree's vessels. Unchecked, it will continue growing down into the root system where it can spread to nearby trees.

This works against the nutrient-release theory. The fungal mass that clogs the tree's vessels as the fungus moves along would not let air through to relieve the aforementioned vacuum. Also, such a nutrient release would rob the DED fungus of its sustenance and not allow it to progress into the elm's root structure.

My uncle dealt this theory another blow. A few years ago he cut down a completely healthy elm tree on his property. It was a tree standing alone, surrounded by grass and weeds, several feet from other trees. Three years later he found over a hundred morels around the stump.

Since cutting the tree down took away most of the nutrients that would, supposedly, have seeped into the ground, and nutrients released into the soil wouldn't likely last three years, what is going on?

Invasion Theory

Let's try to think like a mushroom. You are a mycelium, a mass of threadlike hyphae living underground engaged in a mycorrhizal coupling with the roots of an elm tree. It's a symbiotic relationship; you help break down nutrients for the tree during wet periods, and, in turn, the tree shares some of its stored nutrients with you during the hard times. The tree calls the shots, though, as its natural defenses prevent you from becoming a parasite like DED. These defenses are stronger here in the roots, where fungal invaders are common, than up in the branches where the elm bark beetle lives.

Then one year the tree dies. You can't tell why; the disease that killed it is still up in the top. All you sense is a cache of stored nutrients and the defenses keeping you from getting to them fading away. What do you do?

Mushrooms long ago learned to use nutrients stored in wood to produce fruiting bodies. A short walk in the woods virtually anywhere on Earth is likely to turn up a few native species. Could morels be exhibiting the earliest form of this behavior, demonstating the first step in the evolution of a mychorrhizal relationship into the exploitation of a new food source?

The mycorrhizal connection puts the morel in a very opportunistic position upon the death of the tree. The traditional theory holds that the mycelium sits and waits for the tree to release its nutrients. Invasion theory says it goes in and gets them.

Quick Step?

As Dutch elm disease has been in the country for less than 80 years, isn't this a rather large evolutionary step for morels to have taken in such a short time?

Yes it is, especially considering that morels started appearing in large numbers as soon as the big old trees started dying. This leads me to believe that this relationship closely mimics another morel-tree relationship — the burn-site morel.

Just like Dutch elm disease (or cutting down a healthy tree), wildfires attack the top part of trees. The fires often totally destroy or dry out the trees, leaving no nutrients to flow back into the soil, yet morels pop up around the charred stumps. The massive fruitings often found in these areas require a sudden, substantial food source. What else but the freshly killed roots could provide this source? Dutch elm disease, then, acts like a slow, selective wildfire, killing elms from the top down, allowing the morels time to feast on the roots.

In both instances, the amount of time between the death of the tree and the appearance of mushrooms is similar to the time between inoculation and production in commercially grown wood-borne mushrooms. In such species, the mycelium must first fully colonize the wood and doesn't start producing mushrooms until the nutrients begin to run out.

As morels aren't found growing directly out of the wood, they apparently still need a loose substrate to form. This is logical for a soil-based mushroom taking advantage of underground deadwood.

What About the Ower Method?

Much of what we know about the morel life cycle comes from research done at Michigan State University in developing a patented growing method
5 that apparently has failed to become commercially feasible. The impetus for the research was Ronald Ower, a student in San Francisco who, legend has it, first grew morels in a cardboard box in his kitchen.

Coming from California, these morels were most likely of the species morchella rufobrunnea, the only "yellow" morel the MDCP has identified from that state. While public details from the Ower research are sketchy, one report I read listed the species they were growing as deliciosa, a previous misidentification of rufobrunnea, according to the MDCP. This species is most commonly found in disturbed areas, such as areas in which landscaping work has been done, and are sometimes called "landscape morels." There is no recognized tree or plant association with this species.

The researchers found that the morel organism cycles between its mycelial form and over-wintering structures called sclerotia. While it is often taught that the sclerotia are necessary for ascocarp (mushroom) production, the omission of this step in addendums6,7 to the Ower patent call this into question.

But is this the only way morels grow? There are obviously varying processes involved, given the species' different fruiting habits. I believe these may signal fundamental differences in the way divergent species form ascocarps.

One difference I suggest is that wood-using morels do this by sending hyphae from the colonized root through a thin layer of soil, bypassing the formation of sclerotia. It is this close relationship between the elm root and ascocarp that I was seeking, and may have found, in my field study.

While sclerotia are likely formed yearly during the morel's mycorrhizal relationship with the elm, the elm itself is possibly used as the over-wintering structure after its death. This difference may help explain the failures of others who tried to employ the Ower method with morel species other than rufobrunnea.

From the MDCP results, it seems that one of the most striking differences between the various species is the mycorrhizal relationships they set up. Different species are found to be associated with specific trees, often to the exclusion of other morel species. As the Classic American Yellow has claimed the elm, it is the morel affected (to our benefit) by Dutch elm disease. If a similar disease were to strike hickory trees, it is likely a different morel species would become more abundant.

I believe a study of burn-site morels would back up the MDCP results by showing a strong consistency between the species of morels found and the species of the remains of the trees around which they are found.

While some species seem to be specialists, the Classic American Yellow is an adventurer. According to the MDCP, it is found in "landscape morel" conditions as well as in association with various trees. This suggests that it is capable of both methods of fruiting, and therefore may be a parent species of the more specialized varieties. If this is true, I think we will find that it is the same as, or is very closely related to, the established European species morchella esculenta.

What About Fruitings Around Live Trees?

I haven't studied this phenomenon as closely, but I do have some thoughts.

If morels, as I am suggesting, are often stimulated to fruit by the death of a root system with which they are in contact, can they be likewise stimulated by the death of individual roots?

Trees in the wild face many adversities that they sometimes fight by allowing parts of themselves to die. This is usually indicated by a dead branch in the treetop; beech trees, for example, quite often have visibly dead sections in them. Other trees, too, while seemingly healthy, sometimes sport lifeless branches you can find if you look closely enough. These may also signify a corresponding dead spot in the root structure.

I have found (actually, my father found it) only one live-tree morel since I developed this theory. It was under an ash tree, and yes, the tree did have a dead branch in it. But this is by no means proof — it needs to be corroborated many times over and be present in a large percentage of live-tree morel finds before it even becomes a worthwhile observation.

So, if you're game, the next time you find a live-tree morel, look up in the tree and see if there are any dead spots. Let me know what you find.

Patches of Jerusalem artichoke, a doubly misnamed perennial plant native to North America, have been shown to produce morels. The plant is well known for its edible tuber, a thick root which stores nutrients for the plant during the winter. It doesn't take too much imagination to apply invasion theory to explain these fruitings.

Application

I've been conducting experiments to try to use this theory to develop a viable growing method for morels. I prefer to keep the details and results of these experiments to myself, for now. If I am successful, I will detail my method here — once I have a patent pending.

As mentioned before, though, I am pursuing this as a hobby, and my methods are therefore low-tech. If you have access to a suitable lab and want to help with this project, or if you would like to comment on my study and/or theory, please e-mail me.

Reader Comments

Links to additional information:
These Web sites are not connected with this site, and their inclusion
does not indicate their endorsement of my theories.

1. North American Morels in the MDCP, Kuo, M. (2004, June). Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/morels/mdcp_legend.html

2. About Elm Trees, www.elmcare.com

3. Important Facts About Trees, www.elmcare.com

4. How to Identify and Manage Dutch Elm Disease, Linda Haugen, Plant Pathologist with the USDA Forest Service, Northeastern Area State and Private Forestry, St. Paul, Minnesota

5. Cultivation of morchella, Ronald Ower, et al. June 17, 1986
(FYI — Patents only last 17 years, so you can use this cultivation method as a starting point for your own experiments.)

6. Cultivation of morchella (addendum 1), Ronald Ower, deceased, et al. July 19, 1988

7. Cultivation of morchella (addendum 2), Ronald Ower, deceased, et al. Sept. 19, 1989


© June 18, 2005 - Todd Kiefer