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Fungi assist in decomposition, team with plants to boost absorption

In late summer, after a couple of rainy afternoons, I happened to see several huge mushrooms under a pine tree at the Cary Institute. Mushrooms can be a little deceptive because they appear so suddenly, often seemingly overnight. They are also deceptive because they are only a small part of a much larger organism, most of which remains hidden from view.

Mushrooms are the fruiting bodies of certain fungi. Unlike the growth of most other organisms, mushrooms grow by pumping water into cells that are already formed. Because only water is required for the growth of the pre-formed cells, mushrooms can grow very fast. This is different from growth by cell division, which requires a lot of energy. Using water to pump up the mushroom explains why they make sudden, wet weather appearances.

The fruiting bodies we see above ground are only a small part of the whole fungus. The body of the fungus consists of a network of almost microscopic threads that penetrate the soil, rotting wood, or other organic matter.

The fungus obtains its nutrients and energy from organic matter. Fungi feed on organic matter from many sources. Some break down dead plant material, some attack living plants, others form beneficial partnerships with trees, shrubs and herbaceous plants.

The hidden bodies of fungi can be quite large, in spite of the fact they are made of small threads of cells. Based on the area it covers, an individual Armellaria ostoyae fungus in Oregon's Malheur National Forest is said to be the largest organism in the world. It is estimated to cover 2200 acres and may be more than 2,000 years old.

Fungi do important ecological work. One unique thing they do is break down wood into its component chemicals. Wood contains lignin, which is a remarkably difficult compound to decompose. Some fungi have the biochemical tools to digest lignin and convert it into simpler compounds and molecules that are then put back into circulation.

Another special job some fungi perform results from a unique partnership with higher plants. Together, fungi and plant roots form a new structure - a mycorrhiza - that is very efficient at absorbing nutrients and water from the soil. Partner plants benefit from increased nutrients and a better water supply, while the fungus gets direct access to organic carbon, which it cannot make on its own. So fungi play an important role in the nutrient and carbon cycling machinery of ecosystems.

Mushrooms appear from the body of a fungus when it has captured enough extra energy to form the fruiting structure and to produce millions of spores. Reproduction is an energetically expensive endeavor. Without sufficient energy stores generated by either their own photosynthesis, consuming other organisms, or a few other specialized chemical pathways, organisms cannot successfully reproduce. This is why mushrooms are not always present.

Mushrooms also don't last long once they are produced. They appear, produce spores, release those tiny spores into the wind, and often wither away within days. One reason mushrooms must do their work quickly is that they are at risk of becoming high value food for insects, snails and vertebrates. Coming and going quickly may be a mechanism for mushrooms to avoid being found, eaten, or damaged before their precious cargo of spores is produced and shipped off.

The poisonous nature of some mushrooms is also a likely defense against predators such as squirrels and chipmunks that would impair the reproductive output of the fungus. This protective mushroom poison is the reason why people must not eat wild mushrooms unless they are absolutely sure that they have a non-toxic one in hand.

Next time you see a mushroom, consider the larger part of the organism you can't see, the struggle to protect and release spores that can carry on that organism's genetic legacy elsewhere, and the ecological work it represents.

steward pickett
Plant Ecologist

Steward Pickett is an expert in the ecology of plants, landscapes, and urban ecosystems. The founding director of the Baltimore Ecosystem Study (1997-2016), his research focuses on the ecological structure of urban areas and vegetation dynamics, with national and global applications.