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Kevin Hoover

Newly clear cut forest

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I talked to one of the guys I deer hunt with today and he told me that the area behind camp (mixed oak hardwood forest) has been clear cut. That got me thinking. 

What happens to a mycorrhizal fungus when the tree that it is associated with is cut down?  Does it then put all its energy into producing mushrooms in order to disperse all the spores it can?  And if it does, does it do it only in its normal fruiting window or as soon as it realizes the tree is no longer providing nutrients, which may mean it may fruit out of season?

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Here in southwestern BC we experience clearcutting quite frequently.  From personal experience I've found that most fungi just die when their mycorrhizal associate is killed and do not produce any mushrooms.  The only exception that I can think of is morels, which sometimes do fruit after their host tree is cut down.  However, this is relatively rare and may depend on the time of year when the clearcutting occured and/or the tree species.  I've checked many forests in the spring after a clearcutting event and have had minimal success, essentially zero for conifers, which are the primary type of tree harvested here.  The one exception that I've noted fairly consistently is when the host tree is a cottonwood.  Practically the only yellow morels that I've found (not including burn morels) have been associated with either cut cottonwood stumps or cottonwood trees uprooted by wind events.  The productivity of the uprooted trees may depend on the time of year that the windthrow occured and, of course, will depend on whether morel mycelia are present.  I suspect that morel production will be more likely if the trees were downed during the summer or fall, the same time of year that forest fires occur.  Cottonwoods that have been cut down are able to produce morels for at least the following 2 years, probably because the tree does not die immediately after being logged; the roots survive for some time afterwards.  Fungi's response to clearcutting in eastern North America might be different, because many eastern forests are primarily deciduous.

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I would think that most of the associated symbiotic species would die.

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As mentioned earlier, morels appear to react to a loss/compromising of a symbiotic partner by initially producing lots of mushrooms within the time-frame of a small window. Examples seem to span a variety of species/situations. Here is eastern NA, Morchella americana (and probably also the similar cryptic species) produce the most mushrooms when an apparent associated tree dies or begins to die. Morels in area with elms seem to be "all or nothing". Under healthy elms mushrooms do not appear. But, in areas where the fungus is associated with elm, there are notable fruitings the first spring after the a tree dies. With apples trees, the tree often does off slowly, limb by limb. In this situation, morels appear in smaller numbers (than with dead elms) but for several years running, while the tree slowly dies.

In western NA, "Fire Morels" appear in massive fruitings the first spring after a forest fire in a pine forest. Interestingly, at least some of these species --there are several--- seem to fruit for an extended period of several months. Some western morel hunters believe that the extended fruitings are the result of spores germinating and producing new mycelium/mushrooms. I think another explanation is that the fungus is following the roots of the killed/compromised trees. In either case, a viable hypothesis amounts to the following: these Morchella species play more than one role in Nature, mycorrhizal associates of certain trees as well as a saprobic organism that feeds on decaying organic matter.

I have heard that areas of clear cut forest can produce bumper crops of morels... for a similarly limited period of time. Most of these reports seem to come form western NA.

But, what about other types of fungi? Is Morchella unique in this respect... a fungus that is programmed to engage in reproduction --ie. produce fruit bodies-- when a mycorrhizal associate is dies or is compromised? In my experience --which, when it comes to a big question like this is quite limited-- most mycorrhizal fungal species die when the symbiote dies, and production of fruit bodies then ceases. But I believe there may be two notable exceptions.

Here in NE PA for about 40 years I have been collecting Boletus edulis under planted Norway spruce, in many cases maybe 5-10 trees on a lawn. During the past 10 years or so, many of these trees appear to be reaching the extent of their life span. In cases where the trees have died --and then usually removed before they toppled-- there have been notable fruitings of B. edulis during the last few years of the trees' presence. Once the trees are gone, the boletes are gone.

Grifola frondosa --Hen of the Woods, Sheepshead-- is an interesting species. The mushrooms seem to begin to appear when the tree is very old (generally large oaks). Once the tree dies the mushrooms continues for several years, apparently feeding off the dead/decaying roots. As with morels, I wonder if this fungus plays both a mycorrhizal and saprobic role in Nature.

Additionally, I wonder if some fungi --most notably Morchella and Grifola-- also function as parasites when the host/partner is old/compromised.

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We had something like this happen in Nebraska after really severe and prolonged flooding in 2011.  A large amount of old huge cottonwood trees died from being under water for too long (3 uninterrupted months). The next year there weren’t any mushrooms in the flood plain but the year after was insane. My brother and I were literally crawling through a few areas because it didn’t pay to stand there were so many. As the cottonwoods died completely so did the flush. We were back to normal after just a few years. 

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