When people come across a fungus growing on a tree, they all-to-often assume that
the fungus is somehow damaging or injuring the tree, but this is far from true. In
reality the majority of fungi visible on trees are part of the natural recycling process
and are in fact beneficial to the tree. Fungal fruit bodies typically emerge from dead
woody tissue and not from living tissue.
Cuboidal rot, Laetiporus sulphureus in the top of an oak
The growth of trees involves developing a new annual ring of living tissue around the
trunk and branches. As the annual rings accumulate over time so the inner ones die
and become dysfunctional. Only around 20-30 of the outer annual rings of the tree
consist of living, functional tissues. The death of the older, inner rings is genetically
programmed. So, as trees grow and age, they increasingly build up a central core of
dead woody tissues. These are not accessible to the tree in which they occur as,
despite originally producing the wood, trees cannot break down and re-use the dead
tissues. This is where specialist wood-decay fungi come in. These fungi colonise the
dead woody tissues and break it down into materials that the tree can once again
assimilate.
Once the central core of the tree has been decayed by fungi the tree may respond
and produce aerial roots inside the decayed wood and begin to resorb the nutrients
released by the fungi – the tree benefits from the action of the decay fungi. The trunk
eventually becomes hollowed. The process continues as the older undecayed
annual rings also die and become available for decay.
Fistulina hepatica growing from dead oak heart wood
Before the older, inner annual rings die the tree is able to withdraw the more
accessible components but it is unable to reassimilate the main structural
compounds of lignin, cellulose and hemi-celluloses. Lignin provides compressive
strength and cellulose tensile strength – rather like modern reinforced concrete.
Cellulose is readily digested by wood-decay fungi but not all can digest the lignin.
Lignin is left behind by the so-called brown-rot, or red-rot fungi such as Chicken-of-
the-Woods Laetiporus sulphureus and Beefsteak Fungus Fistulina hepatica but
white-rot fungi such as Ganoderma australe and Inonotus hispidus can break down
cellulose too. The process of fungal decay varies with fungus species, some causing
simultaneous rot of lignin and cellulose, while others digest the lignin first and the
cellulose later. The relationships between the many species of wood-decay fungi, the
dead woody tissues and the tree species can be very complex. Oaks, for example,
lay down phenolics within the woody tissues which act to slow down the speed of
decomposition, and Chicken-of-the -Woods appears to be prefer dead wood which is
rich in phenolics as there is much less competition from other fungal species.
A good case can be made for hollowing being beneficial to the tree structurally as
well s nutritionally. Accumulation of dead woody tissues creates a lot of dead weight
inside of a tree and this makes a tree more top heavy, more prone to collapse when
winds unbalance the tree. In contrast, a hollow cylinder is a strong shape and a
relatively light load.
Ganoderma australe on old beech
The tree is largely in control of fungal decay as living tissues are normally too wet
and do not contain enough free oxygen for fungal growth. However, the balance may
be tipped in favour of fungal growth when trees are under severe stress and some
fungus species are capable of exploiting the situation and expanding into the
stressed tree tissues. The main cause of stress tends to be people and it is trees in
the built environment that are generally the most stressed. Stress can arise though
soil compaction around tree roots, chemical damage to the living soil in which the
tree is growing and physical damage to the roots from trenching and building.
In conclusion, most fungi should be regarded as good for trees, and wood-decay
fungi are recycling the dead woody tissues and releasing nutrients back into the
system where the tree can once again use them. And where would cavity-nesting
birds and roosting bats be without fungi hollowing out the dead tissues?
This article was written by Keith Alexander, 13th August 2024
Photos by Keith Alexander
Further reading:
Marren, P. 1991. Bracket fungi and their role in nature. British Wildlife 3 (1): 1-9.
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