Fungi are organisms that cannot produce their own food through photosynthesis. Instead, they obtain nutrients from organic matter, making trees a rich source of material. The visible structures on trees, such as mushrooms or shelf-like growths, are only the reproductive parts of a much larger organism hidden within the wood or soil. Fungi are a natural component of the forest ecosystem, existing in diverse relationships with their host trees, ranging from mutually beneficial associations to destructive decay.
The Diverse Roles Fungi Play
Fungi interact with trees based on how they acquire nutrition. Saprophytes are the recyclers of the forest, feeding exclusively on dead wood or decaying organic material. These fungi secrete enzymes that break down complex components like cellulose and lignin into simpler forms, which they then absorb. This decomposition process is fundamental to nutrient cycling, returning elements stored in wood back to the soil.
Parasitic fungi derive their nutrients from living tissues, often causing diseases that can lead to damage or death of the host tree. These pathogens invade the tree’s living cells, sometimes blocking the flow of water and nutrients in the vascular system. Damage caused by parasitic fungi includes cankers, wilting, and rapid dieback.
The third category involves mutualistic fungi, commonly known as mycorrhizae, which form a symbiotic association with a tree’s roots. The fungus extends its hyphae into the soil, acting as an extension of the root system to collect water, phosphorus, and trace elements more efficiently. In exchange, the tree supplies the fungus with carbohydrates produced during photosynthesis.
Common Forms and Structures
The visible fungal growth on a tree is called the fruiting body, which serves as the reproductive structure. The majority of the organism exists as a vast network of thread-like filaments called mycelium, hidden within the wood, bark, or soil. Seeing a fruiting body indicates the fungus is already well-established internally.
One common visual form is the bracket or shelf fungus, also referred to as a conk or polypore. These are hard, woody, shelf-shaped structures that grow directly out of the trunk or branches. Unlike traditional mushrooms, many bracket fungi produce spores from tiny pores or tubes located on their underside, rather than from gills.
Mushrooms and toadstools are softer, stemmed fungi that may emerge from the ground near a tree’s base or directly from the wood. These forms are generally more ephemeral than woody brackets, often appearing quickly after rain and then decaying. Other appearances include crust fungi, which form thin, often colorful patches that lie flat against the wood, and sooty molds, which appear as black, powdery growths on leaves and branches.
Assessing Tree Health and Fungal Impact
The presence of a fungal fruiting body is a positive indicator that decay is occurring inside the tree, even if no other outward symptoms are present. The fungi that cause wood decay often penetrate the tree through wounds, such as broken limbs or poor pruning cuts, and begin to degrade the structural components of the wood. Decay fungi reduce wood strength by breaking down cellulose and lignin. Even a ten percent loss in wood weight can result in a seventy to ninety percent loss in wood strength.
A tree can appear healthy and green even while it is significantly hollowed out by heart rot. This is because the living tissues that transport water and nutrients are located in the outermost layer just beneath the bark. Fungi fruiting at the base of the trunk or near the root flare are particularly concerning, as they often signal decay in the large structural roots or the main stem. The location of the fungus determines the severity of the risk.
Fungi that are pathogenic, meaning they attack living tissue, present different visual symptoms beyond the fruiting body. Pathogenic infections often manifest as cankers, which are sunken, dead areas on the trunk or branches that may ooze sap. Other signs include wilting, premature leaf drop, or general dieback of the crown, which suggests the fungus is actively disrupting the tree’s physiological function. If a fruiting body is present on the lower trunk or roots, or if other signs of structural decay are observed, a professional arborist should be consulted to perform a decay assessment.
Safety Considerations
It is important to treat all wild fungi found growing on trees or in the surrounding environment as potentially toxic. The identification of fungi is extremely difficult for the average person, and numerous poisonous species closely resemble edible varieties. Cooking does not eliminate the toxins in some of the deadliest mushrooms, such as the Death Cap, which is responsible for the majority of fatal mushroom poisonings worldwide.
The best prevention against accidental poisoning is to follow the rule: “When in doubt, throw it out,” and never consume a wild mushroom unless it has been positively identified by an expert mycologist. Children and pets should be monitored closely in wooded areas to prevent them from touching or ingesting any fungal growths. Beyond ingestion, fungal spores are ubiquitous in the environment, and inhaling them can pose a health risk to sensitive individuals. Exposure to high concentrations of spores can cause allergic reactions, inflammation, and respiratory symptoms, especially in those with asthma or other respiratory sensitivities.