As trees age, fungi decay and hollow out the dead central heartwood. This is hugely beneficial for wildlife and other garden plants, but it is important to assess the strength of decaying trees that may fall or break.
Scientific name: Basidiomycetes and some ascomycetes
Plant affected: Aging trees
Main symptoms: Hollowing of central dead heartwood and appearance of fungal fruit bodies
Cause: Fungal enzymes breakdown lignin, cellulose and pectin
Timing: All year
What are heartwood fungi?
The heartwood of trees is not alive; this region of dead cells supports a tree and makes up the bulk of the weight of the trunk but it does not help the tree to survive or carry out processes necessary for life. Heartwood fungi are not plant pathogens; they are natural associates/partners of ageing trees. They shape new habitats and make available nutrient resources from dead heartwood. The rotting of the heartwood is a natural part of a tree's life. The hollowing out of trees provides important habitats and nutrient resources for many organisms and can support a large diversity of plant, microbial and animal life.
Spores of heartwood fungi need to reach the tree’s heartwood in order to become established. Spores can be blown in the wind or carried (e.g. by insects) into exposed heartwood through a wound. Once established, the fungus slowly decomposes the wood using enzymes that breakdown the tough heartwood tissue. By the time a fruiting body is produced and visible on the trunk of a living tree, the fungus will likely have been there for many years. They can live within a tree for decades causing no change to the health of the tree, and tree declines are often due to a secondary infection or environmental cause.
Trees with hollows may be more stable as they are less heavy, but they can also be more vulnerable to breakage and wind damage. It is good practice to have trees with heartwood fungi assessed by a qualified arborist to manage the risk of damage from falling trees or branches. However, it will benefit wildlife and garden health to keep decaying trees standing for as long as is feasible, and to retain fallen dead wood in a garden where possible.
Visual indications of heartwood fungi in trees may not be apparent until the later stages of decay. Below are some of the visual effects of heartwood fungi that you may see on trees:
- Presence of fruiting bodies: These usually appear on branches, trunks, buttresses or roots only once the heart rot is extensive. Fruit bodies appear in many forms such as crusts, fans, saddles, oysters and cap-and-stalk mushrooms.
- Presence of rot in the wood: Different fungal species perform different types of decay, categorised as either brown, white or soft rot. All rots make wood brittle, but white and soft rot bleaches the wood while brown cubical rot becomes dry, brown and cracks into cubes.
- Patterns in the wood: Dark lines or discoloured areas are produced by the fungal decay, by the fungal communities interacting with each other, the trees' natural response to presence of fungi, as well as fungal competition with bacteria. This happens in the earlier stages of heartwood rot and can be seen on cross sections of the wood.
- Central hollowing: As the heartwood decays, cavities open up that eventually turn into hollowed out areas of the tree. This rate of hollowing and age at which it normally occurs varies with species of trees and fungi. Aging, hollowing trees will gradually lose branches and overall structural integrity.
- Adventitious roots: Nutrient resources released during breakdown of heartwood become available to the tree, which they reabsorb by growing aerial roots into the hollow.
- Tree growth: Growth may actually increase as the heartwood fungi continue to make nutrient resources available to the tree, in some cases prolonging the tree’s life. Oak trees may develop buttress roots to help with their stability in response to heartwood decay.
- Growth of seedlings: Mycorrhizal networks can share the nutrient resources from the decaying heartwood to other plants, supporting the growth of tree seedlings.
- Increased wildlife: You may notice more vertebrate and invertebrate wildlife associating with the tree as the decay alters chemical composition of the dead wood, giving it a higher nutritional content for other organisms to access.
There are various ways of how heartwood fungi get into the heartwood of trees. They can enter through branches with or without heartwood, large dead branches with exposed heartwood that are still attached to the tree, wounds, bases of twigs, root grafts and sapwood of roots. Fungi feed on the heartwood of the tree while growing their mycelium through the wood. They exit the heartwood by producing fruiting bodies. These can appear on the main trunk, branches, often appearing on wounds where the heartwood is exposed. However, that does not mean that the spot where the fungus entered the tree and where it came out is the same as the time span between the two events can take many years. Others may occur at the base, such as the giant polypore (Meripilus giganteus) or dyer’s maizegill (Phaeolus schweinitzii).
The moment when a fungus produces its fruiting bodies depends on the species and its preferences for environmental conditions such as water availability, temperature and light. To produce fruiting bodies and spores a fungus needs to accumulate a lot of nutrient resources, especially nitrogen.
Heartwood fungi spores are present in the air whether or not a fruiting body is growing in your garden. It is natural that trees will form multiple associations with heartwood fungi throughout in their lives. However, the biggest influence a gardener can have to slow down colonisation of heartwood by fungi is to reduce unnecessary wounding of trees, e.g. by lawn mower damage or over-pruning, and when pruning is necessary to use good pruning techniques that allows wounds to heal over quickly. Wound paints are no longer recommended as this can delay wound healing and encourage colonisation of fungi in the exposed wood.
Hollow trees are not always weaker than those with intact heartwood, however, heartwood decay may create weak points in relation to the architecture of the tree. Such weaknesses may mean branches, or the whole tree, could be more at risk of falling or windthrow. Landowners are liable for damage caused to people of property from falling trees or branches. The appearance of a fruit body of a heartwood fungus indicates that there is decay in the tree, which may be at greater risk of falling or breaking. To assess this risk, we recommend hiring a Lantra-qualified Arborist to assess the structural strength of the tree. A well-trained arborist will be able to manage the size and shape of an ageing tree to preserve its life for as long as possible. This might involve reducing the crown size or removing specific high-risk branches. For assessments of tree health, or advice on suitable consultants and contractors contact:
The Arboricultural Association
Tel. 01242 522152
Hollow trees, fallen dead branches and standing dead wood provide valuable habitats and resources for wildlife. Provided a tree or branch has not died from a disease, such as honey fungus root rot or apple canker, keeping standing dead trees and fallen branches will maximise the benefits of your garden to wildlife. Various fungi will flourish from dead wood over time, which will also add aesthetic interest to your garden and contribute to improved microbial richness of your soil.
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.