Fruits of the Forest

©Else C. Vellinga
Lab
Original publication: Mycena News, January 2007

When and where mushrooms fruit is one big mystery. Water and temperature are the main factors, but we do not have a formula to tell us when the boletes will show up at Salt Point. So many days after the first rain, a daily high temperature of x and a low of y, and then they should be popping up. No, it doesn’t work like that.

However, there is a small group of mushrooms that requires some other trigger than just moisture and the right temperature. Some will only fruit when ammonia is available. This can be in the form of a carcass, an animal latrine, or an old wasp nest. In an experimental plot at Salt Point State Park where urea was added to the soil, Tephrocybe tylicolor, a small greyish mushroom, responded immediately and its fruit-bodies appeared. This species normally grows on places where cows have peed, around carcasses, and on dung; but here it only grew on the plots treated with urea. The more urea, the better it did. Such mushrooms are called ammonia fungi, as they only form fruit-bodies where ammonia and similar chemicals are available in great quantities.

Tephrocybe is a saprotrophic species, but there are also ectomycorrhizal fungi that only fruit in the presence of ammonia. Hebeloma radicosoides from Japan is a striking example. Fruiting of such a species is rare, as there has to be both the host tree and the right amount of ammonia in the same place. It is a big, showy, yellow species, with a long “rooting” stipe and a fringed ring. Whereas most Hebeloma species have very well defined smells, such as earthy beet leaves, cacao, or orange blossom, H. radicosoides lacks any particular smell.

The Hebeloma species have been found on a wide range of strange habitats. They have been reported at the latrine area of a Boy Scout camp, a deserted wasp nest, and on raccoon dog excrement. They like a wide range of chemicals, such as urea, milk casein, arginine, and sodium glutamate.

Hebeloma radicosoides is so far only known in Japan, but its look-alike, H. radicosum, is widespread and known in Japan, Europe, and North America. Again, there is a rooting fruit-body connected to an animal midden, but it is inhibited by ammonia. This species has been found mainly on the abandoned latrines of moles. These latrines are in the ground, and there the fungus has its connection with the tree. The soil is full of fine roots with mycorrhizal tips and fungal mycelium. In more northern regions where moles do not occur, e.g. in Scandinavia, wood mouse middens (Apodemus) are an alternative.

Similarly, in a beech forest in Switzerland, the mushroom was found growing out of a wood mouse nest. Just like moles, the wood mice have their nests deep in the ground. The mushrooms start at the level of these nests and surface one to two feet above them. In other words, it is what we call a deep rooting species; but of course, it starts at the bottom and grows upwards, not like a plant, which sends its roots down.

These two Hebeloma species have been thoroughly investigated by a Japanese mycologist who traveled all over the world in pursuit of these mushrooms.

The two substrate types on which the mushrooms grow seem very similar—urea treatments (including carcasses and raw excrements) versus abandoned middens of moles and wood mice—but apparently there is a difference in the chemicals the fungus can absorb and use. Different fungal species, or even different strains of the same species, use a different form of nitrogen. Some species, such as H. radicosoides, are only able to use ammonia and its derivates; others will only thrive on nitrates. Another group of ectomycorrhizal fungi uses peptides or proteins as its sole nitrogen source. So here again, as in many other aspects of fungal life, much is possible.

In Europe, Hebeloma radicosum associates with deciduous trees (beech, etc.), which may be the reason that it does not occur in our area; but the wide array of burrowing little rodents here might invite other species. Perhaps we have not yet recognized the connection.

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