When a Tree Falls in the Forest, Fungi Hear It

Trees die, some of them suddenly, when lightning strikes. Others, relatively slowly, as they are eaten from the inside by pathogens. Some die young, standing at the edges of the forests prone to winds or landslides. Others, big and old, stand the test of time until drought or pests take them down. When trees die, they fall, and new life is ushered in.

Fungi are key players in the afterlife of trees. These are the most important decomposers of wood. Trees that once stood over 40 m tall are reduced to dust thanks to the chemical prowess of fungi, and insects. However, fully breaking down a tree can take several years or even decades. During these years, various episodes of combat and coexistence occur between different fungi within the seemingly lifeless log as they eat away the wood. Besides, breaking down wood releases nutrients that other plants can use. About 8 per cent of carbon stock in the world's forests is stored in deadwood, which means wood decomposition rates will potentially impact the carbon cycle. 

Wood decay fungi are hard to miss on a hike in the forest while ambling along fallen trees. From bracket or shelf fungi to slime mould (not true fungi in the taxonomic sense), many fungi adorn dead trees. While we can see these conspicuous fruiting bodies (“mushrooms”), there is a lot more going on inside the wood. The majority of the fungal organism exists as a network of fine threads called mycelium, visible only under a microscope. As spores of the fungi arrive at the wood, they germinate, and the mycelium grows as the fungi derive their food by breaking down wood. To secure these resources, they fend each other off chemically through the mycelia.

Bracket fungi

Bracket or shelf fungi belong to a group of fungi called polypores and are some of the most important wood decomposers. They are characterised by a network of pores on the underside of the mushroom where spores are produced. In other gilled mushrooms spores are produced in the gills. Apart from some exceptions, they do not look anything like a typical mushroom with a stem and a cap, nor are they fleshy. Rather, they look like circular shelves (often) with a very small stalk. Hard and leathery, these shelves can grow quite big over many seasons; sometimes, they are seen on live trees too. 

Some common bracket fungi include Ganoderma, Trametes, and Lentinus species. The artist’s bracket (Ganoderma applanatum), with its large but dull-looking brown/grey shelf is easily spotted on dead trees. It is found throughout the year, even in dry and cold conditions. Other Ganoderma species have shiny, brownish-red fruiting bodies with shades of yellow and white. Some Ganoderma species also act as tree pathogens. The group Trametes are formidable wood decomposers found on many host trees. Turkey tail (Trametes versicolor) has vivid colouration of different zones on the shelf. Cinnabar bracket (Trametes sanguinea) is an eye-catching, bright orange polypore, while another species, Trametes flavida is a humble-looking pastel green shelf.

Some polypores do not look like a typical shelf. Groups, such as Lentinus species, are funnel-shaped with a visible stem. Yellow-stemmed micropore (Microporus xanthopus) is another such species usually found on twigs and smaller logs. 

Polypores and wood decay

Polypores play a key role in wood decay. Not all fungi can degrade lignin, a tough wood polymer generally resistant to microbes. However, polypores can perform this crucial function. These enzyme powerhouses break down lignin and cellulose, leaving behind a white mass of wood debris, and hence, they are called white-rot fungi. There are some non-polypores, too, which produce white-rot, including oyster mushrooms (Pleurotus species), split gills (Schizophyllum commune), and sulphur tufts (Hypholoma fasciculare).

More than one way to rot wood

Some fungal groups do not produce white rot but still aid in wood decay. These include some species of bonnet or helmet mushrooms, such as the bleeding fairy helmet (Mycena haematopus) and orange mycena (Mycena leaiana), which grow gregariously on wood stumps. The Xylaria group includes several species, most in the shape of a club, including dead man’s fingers (Xylaria polymorpha) and candlesnuff fungus (Xylaria hypoxylon). These fungi turn black and hard upon maturity. Stump puffball (Apioperdon pyriforme) grows in clusters on dead wood with a ball-shaped fruiting body that releases fumes of spores on contact. A type of cup fungus (Cookeina) species has a funky, cup-like fruiting body with spiky hair-like accessories. 

Living within the wood gives some fungi a head start. A curious aspect of wood decay is that it is said to be kickstarted by fungi already present within the wood when the tree was alive. These fungi are called endophytes (“living within plant tissues”). They mostly do not affect the tree when alive and turn to decomposer mode once the tree dies. Therefore, trees may contain the seeds, or rather, spores of their own destruction. These initial fungi also influence what fungi can further colonise the wood.

Different stages of decay

With ample resources available at the start of wood decay, various fungi can be found on dead wood. With time, easily degradable resources such as cellulose become depleted, resulting in fewer fungi remaining on the wood. Besides, fungi compete with one another for these resources. As white-rot fungi act to break down lignin, they pave the way for a suite of other fungi to colonise the wood again.

Scientists are still scratching the surface, trying to understand wood decay fungi and their complex lifestyles. When the lines between gentle inhabitants of the tree, disease-causing agents, and the ultimate decomposers of the wood blur, many questions about how these fungi interact with trees arise. Different fungal actors occupy dead wood at different instances of time. They arrive and depart, compete and coexist, while providing the crucial ecosystem service of nutrient cycling. Deadwood is more than a lifeless log. It is an important habitat for other plants, fungi, insects, and other animals, and management practices must aim to retain them in the forest.

This article is based on the author’s work in Pushpagiri Wildlife Sanctuary, Karnataka.