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Aspen boletes build physical connections for mutualistic exchanges with quaking aspen

Mushrooms abound on the eastern flank of Owl Creek Pass in Western Colorado in early August, particularly after an unusually rainy summer. Fly agaric (Amanita muscaria), the infamous and deadly red capped mushrooms with white spots, grow in dense clusters near the West Fork of the Cimarron River.

But many other mushrooms were unfamiliar to me and one of these stood out as the clear winner of the big and ugly category. The cap was slightly over six inches in diameter and brown to dark brown, with one small blue structure near the edge of the cap. The stem or stipe was thick, cream colored, with many small dark flecks called scabers. 

Professor Alisha Quandt, mycologist in the University of Colorado Boulder Department of Ecology and Evolutionary Biology, identified this as an aspen bolete, Leccinum insigne, and explained that it looked like it had begun to decay—fresh specimens have a flatter cap, colored orange or brownish yellow. 

This species was first recognized in 1966, so it is relatively new to science. The edibility of L. insigne is either variable, or people vary, for some poisonings have been reported, but others appreciate its taste. Another possibility is that two species, one edible and the other poisonous, are recognized as one species. 

The genus Leccinum is closely related to the genus Boletus but these genera are distinguished by scabers on Leccinumspecies but none on Boletus species. Leccinum and Boletus mushrooms do not have gills, those numerous and delicate flaps on the bottom of the cap of some groups of mushrooms. 

Instead, the underside of the cap appears first as a smooth surface, but closer inspection reveals a surface of tightly packed tubes with small pores. Both gills and tubes release  spores, microscopic analogues of seeds.  

The common name of L. insigne is the aspen bolete or orange-capped bolete, which is mycorrhizal on quaking or trembling aspen. Mycorrhizal fungi build mutualistic relationships with their host plants by wrapping their mycelia (analogous to roots) around the fine root fibers of its host to build a bridge for exchange. 

Aspen bolete (above) is the most common of more than 50 mycorrhizal fungi in aspen forests. Two king boletes appear at the top of the page. Photos by Jeff Mitton.

Fungal mycelia are much thinner than root hairs, providing a greater surface area for absorbing water and nutrients such as phosphorus and nitrogen, which are transferred to the plant. Photosynthesis in the plant leaves produces sugars such as sucrose and glucose, which are transported to the roots and then from roots to mycelia.

The genus Boletus includes the king bolete or porcini, Boletus edulis, one of the mushrooms prized most by people who search forests for edible mushrooms. It can be quite large, with a cap 12 inches in diameter and the cap and stem weighing more than four pounds. It forms mycorrhizal associations with approximately 30 conifer and deciduous species and is native to Europe, Asia and North America.   

Studies of mycorrhizal fungi in the northern Rocky Mountains found that more than 50 species were associated with quaking aspen and noted that the aspen bolete was the most characteristic of aspen stands. A laboratory study of aspen seedling growth compared uninoculated seedlings with seedlings inoculated with one of five native mycorrhizal species. 

Growth measurements showed that five of six mycorrhizal fungi improved seedling growth by factors of 2 to 4, an enormous boost. Mycorrhizal associations are common—most plants have these beneficial associations with fungi.

A mycorrhizal fungus can establish bridges to several plants, and biologists have found that mycelial connections may also serve to shuttle warning signals between plants. For example, when a plant is being attacked by aphids, it releases volatile organic compounds (VOCs) that attract predators of aphids. Plants connected to the embattled plant by mycelia but not attacked by aphids also release VOCs, a preemptive defensive action triggered by a chemical alarm.

Every once in a while, while walking through an aspen forest, it is insightful to remember that you might not be walking amongst trees, but within an individual, a clone. Many ramets or stems sharing a root system are physically connected, like the fingers on your left hand are joined at the palm. 

An aspen forest may also be composed of many clones, which are separate individuals. But the insight that an aspen forest contains more than 50 species mycorrhizal fungi, aspen bolete prominent among them, connecting different clones and shuttling VOC messages, is epiphanous. 

Aspen forests in the West Elk Mountains, Ruby Mountains and Ragged Mountains contain many clones and extend for miles, but all are conceivably connected by the rhizosphere.