THE CASEBOOK OF DR. D. R. SLATER - No. 5 - Tree Failures

In these latest examples from his casebook, Dr. Duncan Slater tells the stories of the failure of five trees, which failed in very different ways...

LIGHTNING DIDN’T STRIKE TWICE

?Trees are highly-evolved organisms: to destroy their structure often takes hurricane-force winds – unless they have a substantial defect in their structure…?

When I first came to Myerscough College to teach arboriculture, I used to use a semi-mature turkey oak (Quercus cerris L.) as pictured, in the College’s woodland garden, to demonstrate the damage that lightning can cause to a tree.?There was a long vertical strip in the exposed sapwood, running down the stem for several metres, which is a typical lightning scar one gets to see on struck trees.?Additionally, near the base of the tree, a large area of the bark had also been blown off by what is called ‘instantaneous gasification of sap”.?This occurs when the sap in the outer layers of the tree, when conducting the very high electrical current generated by the lightning strike, changes in an instant from a liquid into a gas, leading to a massive expansion in its volume over milliseconds.?The results of super-heating the sap in this way are explosive!?

Unfortunately, the size of the wound allowed in secondary decay fungi into this oak tree pretty quickly.?There were already long tiers of turkey-tail fungus (Trametes versicolor, syn. Coriolus versicolor) at the edges of the wound in 2006, which were related to large areas of white rot in the wood of this tree.?This suggests that the lightning damage was already several years old before I got to see this tree.?

As can be seen in the image, this tree failed only one year on from when I started at the College: the intensity of the wood decay and a strong rainstorm in June 2007 took the tree down by snapping its rotting stem.?It is often a combination of factors (in this case, lightning and stem decay) that lead to tree failure, because trees are normally very resilient to the physical forces acting upon them.

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THE BIG OLD BEECH

There was a time when every arboricultural student at Myerscough College knew a tree which had the simple, respectful name of ‘The Big Old Beech’.?This ‘Big Old Beech’ was monstrously large – being an edge tree to a woodland that was part of the original planting for Myerscough Hall, probably dating back to the 1770s.?Growing at the southern edge of the wood, this beech had gained massive dimensions – a trunk diameter of over 2.7 metres, a crown spread of over 35 metres and many side limbs that, of themselves, were as big as normal trees.?‘The Big Old Beech’ was greatly-favoured by our students for climbing purposes – but, when this tree showed signs of decay, we had to stop them accessing it for safety reasons.?

The swollen base of the trunk of the tree had, for many years, shown signs of Ganoderma fungal brackets – not very large ones and only appearing in a few of the flutings on its lower stem.?From an old branch pruning wound, it was clear that the trunk was pretty hollow – but ‘The Big Old Beech’ coped well with that incipient decay for more than a decade.?Then, in the spring of 2012, and quite unexpectedly, most of the crown of the tree only produced ‘micro-leaves’ – very small leaves of a chlorotic yellow colour. The ‘Big Old Beech’ was then very sickly and obviously in trouble.?That autumn, a large quantity of the fruiting bodies of the fungus Meripilus giganteus emerged around its base; something that had been previously seen on this tree.?

The tree died rapidly after that.?The image from June 2013 shows that there was almost no leaf coverage in the crown of the tree.?In hope of keeping the tree as a disintegrating remnant of the old estate, the arboricultural students built a post-and-rail fence around this tree, excluding college students, staff and visitors from under the potential fall zone of this massive tree and its now-dead branches.?We hoped that branches and limbs would drop as they rotted, and we might keep this famous tree as a natural standing stump.?Unfortunately, the rate of the root decay caused by the Meripilus infection proved too much, and ‘The Big Old Beech’ failed at its roots in February 2014.?I was probably one of the last people to see it still standing, as I stood in the rain and wind to listen to the dead branches in its crown clattering down earlier on the night that it fell.?

What a privilege to have been there – in the last hours of a great tree.?I know that I, and many of our students, will always fondly remember this ‘Goliath’ of a tree.?

DON’T LET IVY DRAG YOU DOWN

Winter storms are the ‘norm’ here, in the North West of England: our trees have to be tough and strong to survive them.?One trick that many trees have ‘up their sleeve’ is to be deciduous – and thus drop all their leaves in the winter months, which limits the extent to which the winter winds can blow them about.?Often, when winter storms hit our local area, it is notable that pine and Leyland cypress branches and stems fail quite often – as these trees are evergreen and still bearing foliage in the winter, unlike most of the broadleaved trees.?

However, even a deciduous tree can find itself bearing evergreen leaves in the winter in a way the tree itself could not anticipate.?Ivy (Hedera helix L.) is a common coloniser of trees in woods, hedgerows and gardens – and can develop a large crown of foliage, especially when colonising a declining deciduous tree.?

The image here shows a large and old alder (Alnus glutinosa (L.) Gaertn.), with a well-developed inner crown of ivy.?This climbing plant not only adds to the weight and the wind loading to the tree, but it also acts to obscure structural defects that might be present.?In this case, the tree was in decline due to basal decay – and it fell in the spring storms we experienced in March of this year.?

Ivy-clad trees can often be given less attention by tree inspectors and by our students, for their problems are hidden and there is something about the ivy’s presence that tends to put them ‘into the background’ for many people, visually-speaking, especially when that tree is one amongst a group of other trees.?However, my own experience is that one’s time is well-spent checking out the structure of ivy-clad trees: quite often, you can prevent a tree failure before it happens, by doing such rigorous checks.?

TAKING A STAND

It is now a common practice in UK arboriculture to create ‘monoliths’ or standing stumps.?I’m not really sure why they are called ‘monoliths’ – as this literally means ‘a single standing stone’ – but I am a big fan of this practice where the whole tree cannot be retained (very often due to extensive root, basal or stem decay in a large tree). Monoliths are potentially a great haven for wildlife – particularly xylophagous insects - and thus make great feeding venues for woodpeckers.?

However, as is often the case in arboriculture, practice becomes common before any science develops to back it up.?In particular, what types of tree make the best and long-standing monoliths – and what sizes and heights work the best??To the best of my knowledge, there are no standard guidelines for creating monoliths in the UK, so you get to see all sorts of variations in size, shape, location and species used.?

From time-lapsing quite a few monoliths in my local area, it is becoming clear that tree species with more durable woods (e.g. oak, sweet chestnut) reliably make long-standing monoliths – which is unsurprising – but, also, creating a monolith that is relatively short also keeps it standing for longer.?For example, a monolith made out of a beech tree, if cut to only 4 metres in height, will often last over 15 years – cut it to 8 metres in height, and, due to weight and leverage, it’s likely to fall in less than ten years.?

My time-lapsed image here shows a sycamore monolith created in a local nature reserve.?This was cut to be 11 metres tall, and, due to basal decay, fell only a few years after its creation.?It can seem better for nature conservation reasons to keep as much of the tree as possible when ‘monolithing’; however, I suspect, if we do some science on this topic, that shorter monoliths may be proven to provide a better habitat for longer to more wildlife – basically because they stay standing up for longer.

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THE CHEESE-CUTTER

Trees often fail due to a combination of factors – and those factors can include activities that we do as arborists, tree planters or gardeners.?It is quite possible to prune a tree so that it fails, for instance, by crown lifting it excessively, by cutting out a natural brace that prevented movement at a bark-included junction below it, or by heavy crown thinning.?

Failures can start even in young trees by making poor choices for tree support.?Pictured is a pine tree in a tree collection – but, to establish the tree in this spot, those planting it have opted for guy wires as a support system, with a bit of hosepipe around the wire.?This is quite an effective system for providing initial support – but potentially a very damaging system if it gets left on the tree for too long.?

In the image here, you can see that this wire-based support system was totally neglected and the wires started to get occluded into the stem of the growing pine tree (see image June 2007).?If the wire had got fully occluded into the stem of the tree, it wouldn’t have been a major mechanical flaw – but, as the wire starts to throttle the trunk, this becomes a potential point of mechanical failure.?

For this tree, the ‘stress notch’ caused to the stem by these two wire loops occluding at the same point along the stem proved too much for it: the tree failed in less than two years after my initial image was taken.?The pine’s top completely broke off in the wind, ruining this rare specimen.?

Unfortunately, I’ve seen this type of tree failure far too often, typically relating to cheap tree ties or when wire is used for tree support.?Pretty much all tree support systems can cause damage to trees if they are left in place for too long, but wire has a greater chance to cause this sort of damage to young trees: it’s like a cheese-cutter – and the poor tree is the ‘cheese’!

* This article first appeared in the January 2019 edition of Pro Arb Magazine *