It can be a slightly sombre moment when I have to fell a tree. Though always a last resort and with the obvious consideration for public safety taking precedence, it can still produce ambivalent feelings. Without an access route in the vicinity of the tree failure zone, the said tree would be left to do its own thing, at its own pace, on its own terms. But this is part of my job and needs must. One positive from this though is that it gives you the opportunity to form a clearer picture of what was going on inside the tree and more often than not, a cross-section will offer an insight into the complex and dynamic relationship between trees and fungi.

The variation in fungi types is incredibly vast but for this blog I will focus solely on the tree rotters, organisms that directly cause the wood tissues to decay and break down in variegated ways. The relations between trees and fungi is not yet fully understood but is gaining more and more traction due the fact the subject offers endless learning and is quite simply downright magical. Trees are thought to communicate with each other through fungal webs. Fungi help to transfer carbon-containing materials between trees and in many cases form one part of a symbiotic or mutualistic partnership with trees. Most of the fungi we see above ground is only the ‘fruit’ part of the whole organism. Fungi connects with tree roots through its tiny mycelial threads underground and through this the fungi can provide the tree with nutrients and minerals, whilst the tree can transfer sugars to the fungi from its own photosynthesis. The name for this fascinating relationship is mycorrhiza and through this network trees are able to warn each other of potential threats, whether that be disease, drought or insect attacks.

What we see above ground in the case of trees and fungi can be just the tip of the iceberg, but is equally as engrossing.

With some species of fungi, the relationship isn’t quite as straight forward. Whilst some can prolong the life of a tree, others will accelerate it. These species don’t just simply rot a tree, they will do so using various devised methods and most are specific to certain individual tree hosts, adopting a particular colonisation strategy in order to achieve a successful outcome.

A brown rot fungi (e.g. chicken of the woods, birch polypore) will attack the white cellulose strands of the tree tissue, eventually leaving behind only the brittle brown lignin of the wood, making the tree unstable. Whereas a white rot fungi (e.g. dryad’s saddle, artist’s bracket) will solely target the brown lignin of a tree - the material that gives plants and trees their stiffness, thus leaving behind only the white cellulose. Both variations of rot compromise the tree in different ways.

In addition there are also those that rot the white cellulose and brown lignin simultaneously such as the shaggy bracket which is often found on ash and the soft rot fungi brittle cinder fungus (Kretzschmaria deusta) which can be a slow death sentence for lime or beech. Finally there are also saprophytic fungi which aren’t of any real significance in arboricultural terms, but feeding on dead wood they are a useful indicator of what you may or may not have to do with branches in terms of pruning for safety.

Whether a fungus is causing a brown, white or simultaneous rot, they all adopt their own colonisation strategies for penetrating the tree tissues and recycling the nutrients from the decaying wood. Highly specialist species’ such as Ganoderma (there are over 80 species!) and chicken of the woods attack the heartwood of trees through wounds in the sapwood and/or cambium layer of the tree. The tree wound is colonised by the spores of these fungi. Non-specialist opportunists like dryad’s saddle will colonise only the sapwood of a tree through wound exposure, again through the spreading of its spores by the wind. Some latent or specialist opportunists such as birch polypore are already present in the tree, potentially from seed and wait for the perfect moment to colonise the tree when it ages and becomes weak from an oxygenised open wound.

The dreaded honey fungus, in particular Armillaria mellea uses the strategy of active pathogenesis, meaning this fungus is capable of entering the tree tissues by their own volition. They do not require an entry point or wound to do so. Chemicals from the tips of their underground rhizomorphs (they look like bootlaces) are exuded to break down root or bark tissues in the tree. This makes the penetration of honey fungus an absolute nightmare for trees as they just aren’t able to cope with it and can’t ‘compartmentalise’ the affected area for self-preservation.

Time to ramp up the science…

The Compartmentalisation of Decay in Trees or ‘CODIT’ as first coined by plant pathologist Alex Shigo is utterly astonishing. In a nutshell this is how a tree shuts off an area of itself that is under threat and creates a sealed cone shape on the inside of e.g. a wound, rather than actually healing itself. It achieves this using a multitude of strategic techniques and growth stages.

There are three ways in which a tree responds to wounding. The first is the reaction zone, where it strengthens its existing barriers to decay. The second is where it will create new barriers for protection, known as the barrier zone and thirdly the tree will continue to grow as part of its overall defence system.

Delving into this a little deeper, the tree does something amazing in the form of four growth walls of protection as seen in the diagram above. The walls are the way in which the tree seals off the affected area to protect itself from the spread of further decay.

Wall 1 is the weakest of the four and resists decay vertically by using cells of the xylem vessels (tissues in the sapwood that transport water from the roots to the crown) and blocks them with tyloses (cell contents that enter the xylem vessel and prevent the movement of water). Bear in mind that trees being opened up to oxygen in the form of a wound can be bad news for trees, as pathogens can enter. Trees such as cherry or conifers use gums and resins rather than tyloses to block up wounded areas.

Wall 2 is a moderately strong structure and resists the spread of decay inwards by using cells of the annual ring of the tree (parenchyma) and phenolic compounds (phenols) as a defence mechanism.

Wall 3 is very strong and resists the spread of decay sideways by using cells of the rays and phenols. Rays are responsible for the growth in trees and run vertically, helping to transport nutrients.

Wall 4 is very strong physiologically but weak structurally. It resists the spread of decay outwards using cambial cells and the highly toxic chemical suberin that trees can produce in certain environmental conditions. Woundwood grows as an outside barrier until the wound is fully occluded (covered). You may have noticed wall 4 in action when looking at an old wound or prune where the tree is trying to cover it up with woundwood. How successful this is depends on how good a tree pruner you are!

If a tree successfully uses its four walls of compartmentalisation against decay, it will have sealed off the issue from the rest of the tree and can then happily survive. The fact that trees have evolved to do all of this themselves is just mind-blowing to me. They truly are a dynamic living system and the ways in which trees and fungi interact with each other in so many different ways is majorly impressive.

The effects that some of the rot fungi have on trees may in some ways seem a little brutal, but they are an integral part of nature and most trees can live with them for a long time. Without the existence of fungi to recycle and atrophy materials, we would be kilometres high in dead bodies, plant matter and who knows what else. The rot they cause in trees creates a whole host of new and niche habitat in woodpecker holes, roosting sites for treecreepers, nest cavities for owls, homes for bats and pine martens as well as key nutrients for countless numbers of invertebrates. All vital, all linked intricately to the world of trees and fungi. This all takes time though and this is one of the reasons why ancient trees, hundreds of years old are irreplaceable.

So the next time you spot a tree that has toppled over, have an investigate around any visible annual rings and see if you can spot internal barrier walls to try and piece together what was going on beneath the surface. Being able to look beyond the bark opens up a whole new world of wonder, never mind that which is going on concurrently under the ground.

Trees and fungi, the legitimate dynamic duo of the natural world.

Thanks for reading. I’ll be off now to get these annual tree inspections finished.