The Artist's Fungus

I have always loved wandering through the deciduous forests of the Great Lakes region. Each season, each day, even each hour, sauntering through one can reveal another surprise or new discovery to the focused eye. This day, I had come upon a large woodlot while strolling along the banks of the Speed River, outside Guelph, Ontario.

For some time, I had searched for a passage through the dense cedar growth at the edge of the river into the woodlot. Finding one at last, I entered a small clearing where a few trees had been cut years ago. On the other side of the clearing, I spied several large growths on a dead maple trunk. From that distance, they appeared to be rather soft and fleshy, like mushrooms, but this impression soon proved false. In fact, they had a woody body as hard as the tree itself.

The growth was the saprophytic fungus known as Ganoderma applanatum, one of the woody fungi found in forests around the world. Its bright white underside often served as a canvas for pioneer America painters, thus it was given the popular name of artist's fungus.

The upper portion of this fungus is either reddish brown or cocoa brown in color and may reach 45 cm (18 inches) across but most are in the 15-30 cm (6-12 inches) range. The lower surface when young and in rapid growth is a pure white which turns dark brown with age or injury. Its shape is like a fan whose handle grows from the trunk of the tree, or like a kidney. The body near the tree is the thickest and narrows down toward the outer edge, a wedge composed of layers representing each year's growth; the oldest tissue is found on the top, the youngest below. By counting the layers like the rings of a tree, the age of the fungus can be determined. The average life span is five years but some old Ganoderma may reach ten years.

Unlike most other fungi, Ganoderma remains visible all year long. It has an annual cycle of activity including growth, reproduction and winter dormancy. Each spring, a thin layer of tough brown tissue forms on the underside of the fungus body. A new layer of spore-producing tubes arises from this tissue and completely covers the previous year's growth (thus producing the rings). In May with the setting of spore tubes and ripening of spore cells, the discharge of spores begins and continues until autumn.

The release of spores goes on night and day at a steady rate and is scarcely affected by changes in temperature, humidity or sunlight. The number released is astronomically high. The spore tubes are packed closely together numbering nearly 2,000 per square centimetre (12,000 per square inch). Each spore tube contains a cell which liberates spores at a rate of ten per minute. Thus, a fungus with an undersurface area of about 500 square centimetres (one half square foot) -- not unusual for a 3-year-old fungus -- discharges 7 trillion spores in six months! This annual spore production is exceeded only by the Giant Puffball which sends out its spores in one giant explosion.

Because of this massive output of spores, the species could conceivably germinate in every suitable spot in a forest. It is thus difficult to imagine that in the life of a given Ganoderma, only one of its spores will ultimately be successful in establishing a new colony. For successful colonization of a tree, the spore must fall within a large wound, such as a broken root or broken branch, and the wind-borne spores will rarely travel more than half a mile from its parent through a dense woods before depositing on some surface.

After the spore nestles into a tree wound, the fungus germinates and begins to spread into the wood. The fungus can only move through the heartwood and outward through the sapwood if the wood in these areas is already partly rotted by other species of wood-rotting fungi or bacteria. After emerging through the bark, the Ganoderma produces the broadly attached, woody fruit body in which the spore tubes form.

The alignment of the long narrow spore tubes must be nearly vertical for the spores to drop out. As small a deviation as one degree would cause the spores to strike the tube walls and adhere there, thus clogging the tube. The rigid fruit body must therefore resist the forces of wind, snow, hail and falling branches, as well as the weight of small animals such as squirrels, and remain unmoved. The sturdy attachment of this fungus is apparent to anyone who attempts to remove one from a tree. The small, young fruit bodies require the use of a hammer or axe to dislodge them; larger ones require a person's weight.

The artist's fungus is found on more than 50 species of the hardwoods and is especially common on old and damaged beeches. With the first hard frost of Autumn, all spore production stops and the artist's fungus becomes dormant until the establishment of a new spore tube layer the following spring. Throughout the winter months, the woody body stands against the forces of winter -- silent and solid.

Keith C. Heidorn, PhD
May 27, 2001