The Bubbling Life in Dead Trees

Rotting logs, it turns out, play an important role in forest biodiversity and organic matter recycling.

Although few people can hear a tree fall in the forest, countless living things take notice of the event. Fungi dormant in the tree awaken to feed on it, joined by others crawling up from the soil.

The bacteria slide along the fungal threads to penetrate deeper into the log. Termites alert their colony mates, who gather en masse and devour the wood. Over time, the deadwood decomposes, nourishing new life along the way.

Decomposing wood, one of the hardest organic materials, is easy, but scientists still have much to learn about this vital ecological process. Some researchers are studying how fungi and other microbes digest wood, and how animals use this skill to their advantage.

Others are calculating the role of deadwood in recycling organic matter and stabilizing the global climate. What they are studying is beginning to expose the complex interactions that occur within dead trees.

"The fact that a tree is dead doesn't mean it doesn't fulfill a huge function in the ecosystem," says ecologist Amy Zann of the Cary Institute for Ecosystem Studies in New York State. "But in many forested areas around the world, the amount of deadwood is declining, and with it the vital role it plays."

When trees absorb carbon, some of it is used to make complex structural molecules such as cellulose, hemicellulose, and lignin.

"Concrete-like" lignin, the strongest of the three components, holds all the structures together. Cellulose and lignin, in particular, are "unbelievable evolutionary innovations," says mycologist David Hibbett of Clark University in Massachusetts. "They have so many beneficial properties for cell walls and are very difficult for the body to digest."

Only certain groups, such as fungi in the basidiomycete family, are able to break down these tough molecules. Some of them arrive on dead wood as floating or insect-borne spores, while others grow on it as branching filaments consisting of many cells or containing many nuclei and called hyphae. Some even doze in living trees, waiting to experience stress or die.

Such fungi penetrate with their long hyphae into openings in wood - insect tunnels or water-conducting plant tissues - or make their own pathways. White rot fungi secrete aggressive enzymes that break down lignin to reach other components, leaving behind white, stringy wood composed mostly of cellulose.

Wood rot fungi leave lignin intact, but secrete highly reactive molecules that break down cellulose, leaving behind a brown crumbly substrate. They can be very territorial.

If you look inside a rotten log, you can most often see black lines running through the wood. This is melanin - the same pigment that makes human skin darker - that fungi use to defend themselves against other, invading fungi. "These are areas where fungal mycelia are bumping into each other and essentially fighting for territory," Hibbett says.

Researchers have long believed that insects play only a relatively small role in wood decomposition, but they are major players, especially in the tropics and subtropics, and are responsible for about a third of deadwood decomposition worldwide, according to a 2021 study by ecologist Sebastian Seibold of Dresden University of Technology and colleagues.

These insects include wood wasps and various flies, weevils and other beetles, and of course termites, those extraordinary wood eaters. These insects speed up microbial decomposition by chopping wood into small pieces and also by digesting some of it themselves.

Few could do it without the cellulose-degrading microbes living in their gut, and none do it as efficiently as termites. These insects form huge colonies that fiercely defend their deadwood from other insects, said Jan Szobotnik, a termite ecologist at the Czech University of Life Sciences in Prague. Some termites "can completely eat a large tree within a decade," he says.

In ecosystems such as dry Australian savannahs, some termites even invade living trees, which is an unusual step because living tissues contain harmful defense compounds. These termites penetrate the roots and then chew their way into the dead wood of the tree, which may contain less of these chemicals.

"Most decomposition in the savanna occurs while the tree is still alive," says Zann, who co-authored a paper on deadwood and the carbon cycle in the 2024 Annual Review of Ecology,evolution and systematics.

Other termites use fungi to degrade lignin and grow them in the same way humans grow crops. For example, in Africa and Asia, some termites collect spores of white rot fungi from the environment and grow them on a comb-like structure built from dead plant material.

Termites care for the "comb," feeding it harvested wood and then eating the mixture after it is decomposed by fungi.

In turn, arboreal life forms feed other living things. Many species of beetles feed on the spores, mycelium or fungi of wood-destroying fungi, and some ants specialize in hunting and eating termites.

It is estimated that one-third of insect species in the forest feed on deadwood in one way or another, and these insects serve as food for other invertebrates as well as birds and bats. Rotting logs create excellent places for tree seedlings to grow, as well as for animal nests, dens and burrows.

"It's pretty clear," Seybold says, "that this is the type of habitat and resource we need to sustain this part of life on Earth.

When a log disappears, where does the wood eventually go? Wood-eaters use some of the carbon for energy, emitting carbon dioxide - and methane, in the case of termites - as a waste product.

Carbon is also used for construction: some termites use their lignin-rich feces to build nests and mounds. When these structures decompose, some of the carbon is released into the air and some is left on the ground, along with the remains of the wood. Together, these residues become part of the humus of the soil, helping to hold water and support soil-dwelling microbes, invertebrates and plant roots.

However, the amount of deadwood and its associated biodiversity has been severely reduced in many of the world's forests. Forests have become plantations where trees are harvested before their natural death; some foresters also remove deadwood to reduce fuel for forest fires.

Logs are also removed in the belief that they breed insect pests that attack living trees, but environmentalists say this risk is exaggerated. Deadwood is also taken away to be burned in industrial incinerators to produce bioenergy.

Scientist Steven Ellison of the University of California, Irvine, believes that while most of the carbon in deadwood is released into the air, some stays in the soil for more than a century. "The deadwood is your friend," he says. "You definitely want more of it, and you want it to stay around longer."

Signs of change are evident. In recent years, politicians in Europe and the US have begun to encourage the retention of deadwood - for example, forest management plans call for leaving it alone. In the US, deadwood is now making a comeback. But in some other parts of the world it is not yet, especially where there is a lack of awareness of its benefits.

And perhaps that's not surprising. While awareness of the important role of living trees for the health and biodiversity of the planet has been growing in recent years, dead trees are not becoming more popular. Yet these decaying logs are important to the natural circular economy of the forest, in which dead trees are recycled to create life.