Underground Intimacies

© Else C. Vellinga
Original publication: Mycena News, November 2002

We are all familiar with Amanitas, Boletes and Russulas in the forest. These fungi not only are a colourful sight (and make our mouths water) but they are of utter importance to the trees. Underground they make contact with tree roots, forming a partnership, or symbiosis, in which they provide the tree with water and nitrogen, while the tree transfers sugars to the fungi. But these mushrooms are quite picky and choose their partners carefully. The Death Cap is found in our region almost exclusively with Coastal live oak; if we want a meal of King Boletes we look under pines, and for Chanterelles we'll search through the oak copses. Are Redwoods, California bay laurels and Bigleaf maples free from fungal helpers?

The answer on that question is a big NO! Around 80% of all plant species, in other words, grasses, herbs, weeds, and trees, do have fungi in their roots. But these fungi do not form the mushrooms we all love so much. These fungi are microscopically small, produce only hyphae (long thin fungal cells) and spores. They never reveal their underground existence on the forest floor. Again the plant and the fungus benefit from the symbiosis, but in this case it is mostly phosphate that the plant gets from the fungus. The fungi are known by several names: Endomycorrhizae, Arbuscular Mycorrhizal fungi (AM fungi for short), Vesicular-Arbuscular fungi, or by the taxonomic group they belong to: Glomales or Glomeromycota.

AM fungi grow inside the plant roots, and form there elaborate structures for exchange of nutrients. Little is known about the life of these fungi. Do they have sex? Probably not, but we are not 100 % sure. How long do they live? We do not know. Until 1953, people thought that the spores of those fungi were the eggs of nematodes! The spores are actually quite big, up to 1 mm in diameter. And the identification of the species is based on these spores: size, colour, shape, and the structure of the wall. A mere 150 species have been described in the whole world, based on those spore characters, and remember these 150 provide every plant species with phosphorus! Actually, this number is likely to be much too low and it tells us more how hard it is to recognize the species, as well as how few people have studied them. Nonetheless, compare this number with the thousands of Cortinarii and Russulas, which grow together with a handful of tree species. AM fungi have been around for a very long time. Fossil spores and hyphae have been discovered in Wisconsin that are estimated to be 460 million years old, much older than any fossil of a real mushroom. At that time there were no land plants as we know them now, only hornworts and liverworts. So, the fungi might have made it possible for plants to evolve, and to grow on the land.

Endomycorrhizal fungi are found in habitats as varied as dry sand dunes, redwood groves, chaparral, and tropical rain forests. You can get an idea about the number of different species and the structure of that fungal community by looking at the spores in the soil. Sieving the soil is one possibility, growing plants in the soil followed by examination of the roots is another. With the first method it is even possible to get an idea about the species composition of dry soils kept in a ‘herbarium’ for 50 years. Not all species form spores at the same time of year, so getting a complete picture of what is going on is not as easy as it looks. In a small abandoned field in North Carolina, researchers found, to their surprise, a total of 37 species of AM fungi. Only 50 species of herbs and grasses were counted in the same field. And some fungal species were very specific, growing only with one host plant, just as Leccinum scabrum only grows with birch.

In another field setting in Minnesota, plots with varying numbers of plant species (from 2 to 18) were investigated to see the effect on the number of AM fungi. And yes, the more different the plants were, the higher the number of fungal species. Also, in laboratory experiments, it has been shown that the success of grassland plant species often depends completely on the presence of AM fungi. No fungi, no growth. So the diversity of that grassland, and the structure of that plant community exists only because of the presence of AM fungi! This has led agricultural and horticultural enterprises to develop spore mixtures to enhance growth of crop or garden plants, and to use in habitat restoration projects. As you drive by Big Sur, and see tiny grass plants waving in the gales, reflect on the AM fungi helping them grow in those landslide areas!

Further information can be found at the following website:

Two books chockfull of information on mycorrhizal symbiosis:

An anthology of more specific articles (of which there are many):