Fantastic fungus find

Shoestring-like rhizomorphs spread from tree to tree

By BBC News Online's Jonathan Amos

Researchers in the US have found what is probably the largest living organism on Earth.

It is a fungus that is growing through the earth and roots of trees in the Malheur National Forest in the Blue Mountains of eastern Oregon.

Scientists say it covers 890 hectares (2,200 acres) of land - an area equivalent to about 1,220 football pitches.

The fungus is called Armillaria ostoyae, but is more popularly known as the honey mushroom. This particular specimen is calculated to be about 2,400 years old, although it could be two to three times this age.

"This fungus lives in a below-ground habitat, spreading very slowly outward from tree to tree along roots or by growth through the soil using special shoestring-like structures called rhizomorphs," said Dr Catherine Parks, from the US Department of Agriculture Forest Service's Pacific Northwest Research Station.

"The fungus is visible in the clusters of golden-coloured mushrooms occasionally seen in the fall on the forest floor that represent just the tip of the iceberg in regard to its true size and impact upon the forest."

Culture studies

The fungus will attack the roots of a range of tree species. When foresters cut into an infected tree they find spreading white filaments, mycelia, which draw water and carbohydrates from the tree to feed the fungus.

This interferes with the tree's own absorption of water and nutrients and eventually leads to death.

Until now, the largest known organism was another Armillaria ostoyae found infecting ponderosa pine in eastern Washington State in 1992. It covered 600 hectares (1,500 acres) near Mount Adams.

Co-researcher Dr Tina Dreisbach said lab studies had shown the fungus to be a single individual.

"We took hundreds of samples and compared them to each other in the culture plate," she told the BBC.

"If they grow together in the culture plate they are determined to be the same individual; if they form a gap between each other or ignore each other, they are determined to be different individuals. And we had hundreds of these pairings that showed this was indeed one huge individual."

Nutrient recycling

The fungus is important to forest ecosystem processes. By killing trees, it opens up gaps in the forest that allow different species to move in.

"The fungus encourages nutrient recycling, so if a tree dies it goes back into the soil and provides nutrients for the trees that come up in its place," Dr Dreisbach said.

"It also provides habitats for animals. For example, dead trees that are still standing will rot out in the middle and animals such as woodpeckers can come and make their homes there."

The huge size of this fungus may be related to the dry climate in eastern Oregon, Dr Dreisbach said. Spores have a hard time establishing new organisms, making room for the old-timers to spread.

To minimise tree mortality near the fungus, forest managers looking to protect their timber production will plant less susceptible tree species such as western larch and ponderosa pine, and harvest susceptible hosts such as Douglas fir and true fir during thinning.