Why defects, decay and fungi don’t automatically mean the end of a tree

There is a particular sequence of events that occurs surprisingly often.

Someone notices a hole in a tree.

Or a crack.

Or a fungus.

A photograph is taken.

Someone searches the internet.

Ten minutes later, a perfectly respectable mature tree has apparently become an imminent threat to civilisation and somebody is looking for a chainsaw.

Trees deserve a slightly more considered trial before sentence is passed.

A tree can be hollow, decayed, cracked, leaning, covered in fungi, shedding branches or simply very old and still have a perfectly acceptable future.

Equally, a tree that looks magnificent can occasionally contain a serious structural defect.

The interesting bit is working out which is which.

Decay doesn’t necessarily mean dangerous

Trees decay.

For people accustomed to buildings and engineering structures, this can be difficult to reconcile with structural safety.

If a steel column loses a substantial proportion of its section to corrosion, we understandably become concerned. A tree is different because it is a living structure.

Trees respond to injury and dysfunction by growing, adapting and compartmentalising.

A mature tree may contain a substantial central cavity while maintaining a functional shell of wood around it. New growth can progressively modify the structure in response to mechanical loading.

This doesn’t mean hollow trees are automatically safe.

It means that:

hollow = dangerous

is not a particularly sophisticated risk-assessment methodology.

The important questions concern the location and extent of decay, the remaining structure, the tree’s architecture, evidence of adaptive growth, loading, species characteristics and what would actually happen if something failed.

Then somebody finds a fungus

Few things accelerate the proposed removal of a tree quite like a mushroom appearing at its base.

Fungal fruiting bodies can provide extremely useful diagnostic information. Their significance varies enormously.

Different fungi colonise different tissues, produce different forms of decay and have very different implications for structural integrity.

The location matters.

The host species matters.

The extent of colonisation matters.

The tree’s response matters.

And identifying the fungus correctly is rather important.

A fungal fruiting body is therefore evidence.

It isn’t, by itself, a felling specification.

Sometimes fungal colonisation does indicate a serious deterioration in structural condition. Sometimes it supports continued monitoring. Sometimes further investigation is justified.

And sometimes the fungus is considerably more interesting than it is alarming.

What about cracks?

Cracks deserve careful attention because they can indicate movement or partial structural failure.

But once again, context matters.

Is the crack new?

Is it extending?

Does it close and reopen under wind loading?

Is there associated decay?

Are there included unions?

Has the tree produced adaptive growth around the affected area?

What is the architecture above it?

A substantial longitudinal crack in a heavily loaded stem over a busy road is clearly a different proposition from a stable historic defect in a lightly loaded tree standing in an area almost nobody enters.

The physical defect might be similar.

The risk isn’t.

The missing half of tree risk assessment: the target

This is one of the most frequently misunderstood aspects of tree safety.

A hazardous tree doesn’t necessarily create a significant risk.

For risk to arise, something of consequence normally has to be exposed to the hazard.

Consider two identical defective branches.

One hangs over the entrance to a busy school.

The other hangs over an inaccessible patch of scrub.

The probability of the branch failing might be identical.

The consequences are plainly not.

This is why competent tree risk assessment considers not only the tree and the likelihood of failure, but also what could be struck and how frequently the target is present.

A tree doesn’t become more structurally defective because somebody builds a conservatory beneath it.

The risk can nevertheless change considerably.

Risk isn’t the same thing as certainty

People understandably want certainty when dealing with large trees.

Unfortunately, trees haven’t agreed to provide it.

Nobody can inspect a tree on Tuesday and guarantee that it won’t fail on Wednesday.

Tree risk management is about making proportionate decisions based upon reasonably available evidence.

That distinction matters.

A large tree can never be made completely risk-free while remaining a large tree.

Branches occasionally fail.

Trees occasionally uproot.

Perfectly healthy trees can fail during exceptional weather.

The objective isn’t to eliminate every conceivable possibility of harm.

If it were, Britain would rapidly become considerably less wooded.

The objective is to identify material risks and manage them proportionately.

The chainsaw isn’t the only tool in the box

Suppose an inspection identifies a genuine concern.

The next question shouldn’t automatically be:

“When can we fell it?”

There may be other options.

Depending upon the tree and circumstances, these might include pruning to reduce mechanical loading, removing a particular defective branch, reducing part of a crown, installing a support system, altering access beneath the tree, moving a target, undertaking further investigation or establishing an appropriate inspection regime.

Sometimes quite a modest intervention can materially change the risk.

That is particularly important for veteran and ancient trees.

Aggressive pruning intended to make an old tree “safe” can itself cause serious physiological and structural consequences. Removing the tree obviously preserves even less of it.

Good management asks what intervention is actually necessary.

Not what intervention is most dramatic.

Sometimes the tree really does need to come down

There is an opposite mistake too.

Not every tree can, or should, be retained.

Trees eventually decline.

Some defects progress beyond the point at which reasonable management can provide an acceptable outcome. Severe structural deterioration combined with a significant target may leave few sensible alternatives.

There are situations where felling is entirely justified.

The professional skill isn’t in finding a reason to retain every tree.

Nor is it in finding a defect that justifies removing one.

It is in distinguishing between the two.

Development creates another version of the same question

“Does that tree really need to be cut down?” also arises constantly on development sites.

Sometimes the answer is yes.

A poor-quality tree occupying the only practical position for access may contribute little to the finished development. Retaining it at enormous design cost simply because it currently exists can be bad planning as well as bad arboriculture.

At other times, an apparently inconvenient mature tree may be one of the most valuable features of the site.

Moving a building, altering an access or redesigning parking may retain decades of accumulated landscape value.

This is why tree surveys undertaken early in the design process are much more useful than surveys commissioned after everything else has been designed.

It is much easier to design around a worthwhile tree than to persuade a worthwhile tree to move three metres to the left.

They are notoriously uncooperative in this respect.

Old trees aren’t defective young trees

Veteran trees deserve particular mention.

Features that might be regarded as defects in younger trees can be characteristic components of veteran trees.

Cavities, deadwood, decay columns, retrenchment, broken branches and exposed heartwood can provide exceptionally valuable habitat.

An old tree may therefore simultaneously possess significant structural defects and enormous ecological, historic and landscape value.

Managing such a tree requires more thought than simply applying the standards expected of a young highway tree.

Sometimes the right solution is to manage the surroundings rather than repeatedly cutting the tree.

Move a bench.

Divert a path.

Reduce access beneath a particular part of the crown.

Accept deadwood where there is no meaningful target.

The safest tree is theoretically one that has been felled and removed.

It is also no longer a tree.

Investigation should answer a question

Modern arboriculture offers plenty of impressive equipment.

Decay-detection devices can investigate internal wood condition. Sonic tomography can produce colourful images. Resistance drilling can provide information about changes in wood resistance. Detailed measurements can help characterise defects.

These tools can be extremely useful.

But equipment doesn’t replace professional judgement.

Before investigating a tree, there should be a question worth answering.

If knowing the extent of internal decay would alter the management decision, further investigation may be justified.

If the result couldn’t possibly change what happens next, producing an attractive multicoloured tomogram may achieve little beyond increasing the invoice.

Investigation should reduce relevant uncertainty.

Otherwise it is just expensive tree photography.

So, does it need to be cut down?

Perhaps.

But first we need to know rather more than the fact that somebody has found a crack, cavity or mushroom.

What species is it?

How old is it?

What condition is it in?

What exactly is the defect?

Is deterioration active?

What parts could realistically fail?

What could they hit?

How frequently is that target present?

What would the consequences be?

What value does the tree have?

Can the risk be reduced without removing it?

And crucially, would the proposed intervention actually improve matters?

Those questions turn an observation into an assessment.

There is a substantial difference between the two.

A good arboricultural consultant shouldn’t be looking for reasons to condemn trees.

Neither should they be looking for reasons to retain them regardless of condition.

The job is much less ideological and rather more useful:

understand the tree, understand its surroundings, assess the actual problem and recommend a proportionate response.

Sometimes that recommendation involves a chainsaw.

Quite often, it doesn’t.

Christopher Payne | Arboriculture Consultant