Tree colonisations - Which trees should you be looking at?

Autumn is the common time for fungi to produce their fruiting bodies. Wood decay fungi are no different. Everyone involved in managing areas of trees with any form of public interaction should be aware of the need to check trees and react accordingly. Guy Watson of Certhia Consulting Limited gives some advice.

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It is important to remember that not all fruiting bodies represent a danger and, in some cases, wood decay fungi can be seen as beneficial - so please do not over-react. Take some advice to get a correct diagnosis and consideration of the implications.

We now refer to a 'colonisation' of a living tree rather than an infection. It is a normal consequence of ageing and wounding in trees. It can be seen as beneficial as it releases otherwise locked up nutrient in the non-functional central areas of the tree, the heartwood. This produces hollowing, another natural process, and one that may give the tree an increased flexibility without loss of strength. We are all familiar with the concept that a tube is stronger than a solid. It allows host relationships with other flora and fauna, including micro-organisms, invertebrates and higher order mammals that rely on these specific environments. In the UK, the high proportion of old, ecologically and historically valuable trees is due mostly to this process. These are our ancient and veteran trees.

The fungal community decays its preferred wood component, altering the structure and digesting the nutrient it needs to survive, grow and reproduce. The fruiting body is the external reproductive growth that produces the masses of spores that are released into the atmosphere. Removal of the fruiting body will not prevent the fungus continuing to digest the wood and will obstruct any further investigation.

Wood is principally lignin and cellulose: the lignin provides the solid, non-compressible element that allows the tree to grow to the height that it does. But, on its own, it is a very brittle component. The cellulose introduces an element of elasticity that allows the lignin to bend and return to its original form without snapping. The cellulose also allows the lateral (side) branches to be tensioned and supported.

Different fungal colonisations remove different elements of the wood. If the fungus removes the cellulose and leaves the lignin (known commonly as "brown rot"), the structure can become brittle and prone to unpredictable failure. If the lignin is removed leaving the cellulose ("white rot"), the structure can gain increased movement and suffer a "ductile" failure where fibres flex and separate. In this case, the tree may well provide additional clues as to the amount of decay and likelihood of failure. Lignin removal, white rot, often leads to a growth response from the tree, in the form of extra growth at the weaker areas. These typically, and fortunately, are often at, or at least in the same locality of, the fruiting body. Adaptive growth can be seen as extra wood being laid down at the points of increased flexibility. Swellings, 'bottle butt', where the first metre of the trunk above ground level has an obvious increased diameter, can be that indicator. Increased angular buttressing from above a normal buttress flare may also indicate a growth response from altered wood. These indicators may well appear before any fruit body is produced and are constantly visible.

The major indicator of colonisation, and the only real way to identify the species involved, is by spotting the fruiting body. There are two principal fruiting strategies: perennial and annual fruiting. Perennial fruit bodies, as the name suggests, exist on their host throughout the year and are therefore woody or corky structures. These should be more easily seen, although some appear right down at ground level and may have a soil like colouration, absorb surrounding vegetation and can be overlooked. Some produce masses of spores when active, generally through the summer and into the autumn.

These may be very obvious, for example, Perenniporia fraxinea, a basal decay fungus of Ash, and other broadleaves, produces bright white spores which resemble several burst bags of icing sugar around the fruiting body which makes it very noticeable, generally in the summer. The annual fruit bodies are more short-lived and, whilst active, may only be emerging, producing spores and degrading in a matter of a few weeks. Some produce fruit bodies that are more persistent and, after spore production, may remain as an inactive fungal structure for a year or more. These, particularly the transient fruit bodies, are more likely not to be seen. It is therefore important that occasional checks are made around trees that may be colonised so as not to overlook any.

So which trees should you be looking at? In general, the older the tree the more likely there are going to be things wrong with it. Larger, older trees are more likely to be colonised than younger trees. The consequences of failure are also generally greater. Trees with low vitality - typically twig die back at the ends of the branches - may indicate an issue and these trees should be checked carefully. In summer, the leaves may be smaller, yellower or the density less than expected. Trees that have had a history of previous large branch removal, or that have been "topped", are more likely to show dysfunction and therefore lead to simple colonisation and significant decay. Those trees that do not form a durable heartwood (for example Poplar, Beech, Maple and Lime) are particularly prone to this and huge columns of decayed wood can be found, which in other trees may be restricted in extent.

Fruiting bodies will generally be associated with wounds on the trunk or branches, or commonly around and in the buttress (the lowest part of the trunk). In particular, tucked away in the clefts between the buttresses. Make sure that these areas are looked at fully. One particular fungus, Kretzschmaria deusta, sometimes known as Cinder Fungus, produces very small easily overlooked fruiting bodies that resemble charcoal for the majority of the year, going grey with white edges when active during the summer and autumn. They are nearly always hidden deep in the buttress clefts and only spread out slowly from these areas over time. The size of the fruiting body is no real expression of the amount of decay within a tree. Just because it is a small body doesn't mean there is no need to assess the tree.

The tree will generally respond to wounding. Initial defence will include flooding the area with moisture to prevent oxygen ingress. Oxygen is vital to allow fungal activity. Where trees have been inappropriately wounded, for example, large branch removal close to the trunk or stems topped with no or an inappropriate growing point, then the tree will not be able to prevent oxygen entering the wood structure. The columns of water of the xylem vessels will collapse. Imagine holding a full hose of water with your thumb over the end. Lift that up and release your thumb, the water collapses and air rushes in to replace it.

This is what happens internally in the tree and results in xylem dysfunction, often seen on the underside of larger cuts as dead, non-functional bark and running down the stems or branches from that. This area provides an easy colonisation point for fungal species.

A well growing tree will try to compartmentalise any dysfunction and decay. Various "walls" within the wood structure provide barriers to the spread of these. However, some are stronger than others; some fungal species have a higher decay potential than others; and some tree species have a greater decay resistance. Some fungi will simply break through barriers.

This highlights the need to seek proper and competent advice. Not all trees with a fruiting body need work, let alone felling. Some trees with certain fungal colonisations can co-exist for centuries. It is therefore vital that a competent tree inspector is engaged to assess the tree, identify the fungus involved and consider its general impact on the host species. It is important for an owner to check their trees for fruiting bodies as part of their general responsibilities. If a fruiting body is found, please leave it in situ and seek further advice. Photograph it for the record as it may be that some other well-meaning but ill-informed individual may remove it. Or, if it is an annual body, it may be so transient that it has degraded beyond certain identification by the time a professional can be engaged.

Each tree that has a fruiting body should be assessed individually. There are no hard and fast rules relating to colonisation, species relationships and extents of decay. There is no doubt that decay may, at some point, change the wood properties to the extent that affected parts of the tree, or even the whole tree, will fail. An assessment will consider all the external indicators to attempt to determine the extent of decay. Crucially, it should also consider the progression of decay based upon fungal species-host relationships and the vitality of the tree. Simple diagnostic techniques of probing and hammer sounding may help in that process. If concern exists and it is vital to know more, then other diagnostic equipment may be required.

This will require further input, inevitably increase cost, but allow a more detailed and reliable decision to be made. (More on this another day…)

One of the big decision drivers revolves around the consequences of failure. If the tree is in a low use area and, in fact failure will result in little or no harm, then there may be little need to do further work. If, on the other hand, predicted failure would result in potential harm, for example, adjacent buildings, especially occupied ones, the risk of harm would inevitably rise and some form of mitigation would be necessary. This may be regular pruning to remove leverage and prevent likely failure, target manipulation to reduce the consequence where possible or a combination of both.

If the target cannot be manipulated and the condition of the tree causes a real possibility of harm, then the only course of action may be complete removal. Obviously, any work to a tree with cavities, hollows or other potential habitat value must be checked for protected species prior to works. A professional tree inspector will be aware of these issues and provide a base line recommendation. However, if it is likely that the tree is acting as a habitat then other professionals, probably ecologists, may be required to arrange the appropriate licences and to carry out the mitigation works required for potential loss of habitat.

Old trees are very valuable ecologically and may hold legally protected species. Clearly, there is a requirement in law to be reasonable in managing our responsibilities. If in doubt, seek professional advice. The Arboricultural Association runs the Registered Consultants scheme which follows a rigorous assessment process. Once accepted, all recipients of Registered Consultants status are required to meet prescribed business and professional standards. Clients can be confident they will receive expert and objective advice from consultants bound by rigorous codes of ethics and professional conduct. Lantra Awards provide a Technical Award in Professional Tree Inspection for competent arboriculturists to validate competency in this area.