Published on 02/18/10

Fox-fire makes forests glow

By Kim D. Coder

The forests of a wet winter and spring are very active. Roots are growing and rains are helping decay last year’s woody debris. One of the most curious features of decay in spring forests is fox-fire, which are strange and eerie lights that illuminate the forest.

Fox-fire is not fire and is not associated with foxes. Fox-fire is a light source or glow on the ground; a by-product of rotting wood.

Not a fairy or a ghost

In the past, people thought fox-fire was a cold fire which burned old stumps in moist areas of oak and hardwood forests. This light has been attributed to fairies, ghosts and assorted other supernatural beings. The much less romantic and less sinister cause for these night lights is special fungi-rotting wood.

The lights glowing along the floor of a forest are called fox-fire, fairy-fire, or will o' the wisp. The scientific term for this glow is bioluminescence, which means light given off by living things.

The best way to see fox-fire is in old, moist oak woods where plenty of big dead limbs and old stumps litter the ground. Fox-fire can be seen in the spring as the forest floor warms. The light is so dim, many people never notice it. To see fox-fire, pick a night with no moon. Keep away from areas with artificial lights and do not use a flashlight. Your eyes must be well adjusted to the dark.

Fungi + rotting wood = light

The glow of fox-fire is powered by fungi consuming rotting wood. The fungi inside this wood produce light as a byproduct of growth. It is the mat of growing fungal strands, or mycelium, not the mushroom that usually glows. The most common luminous fungi is a root rot fungus found on many hardwood trees.

Light is produced when the fungi break down food materials within the wood in the presence of oxygen. Some of this food energy is captured to maintain fungal life, and some is lost as heat. In luminous fungi, a small amount of the food energy, usually lost as heat, is shifted into an enzyme system that emits light.

Usually bluish green

Fox-fire’s luminous glow is bluish green. The colors seen in the woods can be slightly different because leaf litter, woody debris and soil may filter or block some portions of the light. The color is also affected by an individual’s color vision at night. In general, color descriptions for fox-fire range from a stark blue to a sickly green.

Fox-fire is strongest in the early evening when temperatures are around 71 F. Both hot and freezing temperatures eliminate light production. The rotting wood needs to be moist, not soaked with water. Too much water keeps oxygen away and drowns the fungi. If the wood dries out, the light is extinguished and the fungus dies.

Fox-fire specters of dark spring nights can only be seen by those who look carefully in the forests after dark. Sometimes firewood cutters and late-night hikers notice this strange light. Most people never see fox-fire.

Kim Coder is a forester with the University of Georgia Warnell School of Forestry and Natural Resources.