Article: Decaying Wood, Ancient & Veteran Trees in the Cyclic Life of Arboreal Ecosystems

Dead wood may well have recently died, and no longer part of the living tree or even attached to it, but we should not be calling it DEAD, because it’s DECAYING. You may think this is just another word for the same thing, but unlike Monty Python’s dead parrot sketch, the point is that dead wood is anything but dead. The description dead wood implies a static state, without the consideration for the process of decay, and the diversity of life forms involved.

It is the process of decay which is the focus here, the progression of use by different organisms. Some like their wood served up fresh with the sap still ebbing from its vessels, while there are those that prefer it when others have had their fill and all that is left is a mass of soft cellulose or brittle lignin. The diverse array of organisms that are involved in the breakdown of dead woody tissues is truly amazing. So much so that decaying wood can be considered a specialist habitat in its own right.

There is a growing emphasis on biodiversity and protected species, which is influencing a change in management strategies and a shift in long term objectives. However, now there appears to be a revolution afoot, with more and more people, and organisations, recognising the need to focus on a broader picture. In conservation the world over, the time and money has been invested in ‘fire fighting’, to protect and preserve endangered populations of particular species.

The solution is one that manages the system, rather than concentrating on its component parts, if we can maintain healthy ecosystems the biodiversity should take care of itself. However, we cannot and should not try to force long-term change, if we are to be successful in sustainable conservation our role needs to be one of encouragement and persuasion with a respectful appreciation for the diversity or organisms involved.

Historically, woodland managers have removed dead wood on the basis of hygiene, to protect the timber resource from what have traditionally been perceived as pests, like insects and fungi. This is also true of many, parkland and garden sites managed by arborists, where dead wood in trees is seen as a liability, and is removed for fear it may fall and injure someone. The result is that there is simply not enough decaying wood habitat to sustain populations of many key species of conservation importance and an integral part of the arboreal ecosystem.

Dead and dying trees play a vital role in the functioning and productivity of arboreal ecosystems through effects on biodiversity, carbon storage, soil nutrients cycling, energy flows, hydrological processes and natural regeneration of trees.

This is a point now generally recognised by most of us, but this has not always been the case. The generations of managers that have religiously felled and removed dead and dying trees, has left us with a huge shortage, which is likely to take decades to replace.

The generation gap is aptly demonstrated when we look at the rare species, which are associated with our ancient and veteran trees. Many of these are only found on sites where there has been a continuity of decaying wood habitat for hundreds of years. However, ancient trees may appear plentiful today, but for how much longer? Next time you visit a site containing ancient trees, look around at the rest of the wood or parkland, and consider where the next generation will come from.

The organisms that rely on decaying wood habitat are becoming increasingly isolated, in time and place. This is made worse by their lack of mobility, which means that the creation of an intermediary ‘bridge habitats’, is essential if these species are to survive. This is a fundamental part of our involvement in the sustainability of arboreal ecosystems and the maintenance of biodiversity.

Veteran or Ancient?

The terms ‘veteran tree’ and ‘ancient tree’ have been used interchangeably for some years now, but recently an effort has been made to clarify the distinction between the two terms.

Veteran:

The term veteran is used to describe the growth characteristics of a tree, and has no relation to age, other than the fact that old trees are more likely to be described as veteran.

Trees grow reactively, by producing additional ‘wound’ wood or reactive tissue where an injury or associated decay has weakened the residual strength of a stem or branch.

The physical signs or symptoms of injury are known as a tree’s veteran characteristics, and a veteran tree is one that has suffered to such an extent that its whole growth form has been permanently altered. The tree has become a veteran.

Ancient:

The term ancient is used to describe a tree’s age class. An ancient tree is considered to be one that has reached an above average age for its species. Such a tree may also be a veteran, due simply to the number and extent of veteran characteristics it has accumulated over a long life.

Ancient English oak (Quercus rubor) pollard,

Windsor Great Park, Berkshire, Endland UK

Decaying Habitats

There are two distinct types of decaying wood habitat, the first is associated with standing dead trunks, limbs or branches left around the outside of the tree, while the second is found within the trunks and branches themselves, where the decay forms cavities. It is important to be aware of this distinction because the habitats that are created are quite different and require specific techniques to recreate them.

Standing dead wood, whether as whole trunks or branches within the crown of otherwise healthy trees, is relatively easy to replace by the creative use of destructive pruning, ring barking trunks and branches or by the re-erection of logs. This type of decaying wood habitat breaks down from the outside in, providing a large surface area for occupation by invertebrates, fungi, lichens and mosses.

However, when it comes to the creation of the decaying wood habitat found within the trunks and branches of trees, the techniques involved are not quite so simple. The decaying wood inside living trees decomposes from the inside out, creates cavities, rot holes and hollow trunks, which are created by invertebrates and fungi, but go on to provide shelter for a diversity of birds, small mammals and reptiles.

Destructive Pruning:

May be used to create habitat in trees as part of a conservation project and involve techniques that will result in the creation of decay within the trunks and main branch structure of trees.

Veteranisation:

Pruning techniques intended to prematurely ‘age’ a tree in a controlled and targeted manner to initiate the creation of habitat or stimulate the formation of a secondary crown

Natural Fracture Pruning:

Pruning techniques that mimic the natural branch loss that would occur following storm events, small diameter branches may be partially cut through from above and then ripped
off, by hand, from within the crown or by rope from ground level, seeking to leave a split or fractured branch end, and exposed heartwood, that may or may not be associated with an existing growing point.

Ancient English oak (Quercus rubor) collapsed pollard

Richmond Park, London, England, UK.

Creating the Habitat

Training as a practical arborist has progressed over the years, from the days of old when tree surgery work involved carting a hand axe and cross cut saw around the tree, through the era of flush cutting and cavity excavation, to the enlightenment of target pruning and an understanding of CoDiT (Compartmentalisation of Decay in Trees). However, modern pruning techniques may prolong the safe useful life of the trees in our parks and gardens, but they are threatening the sustainability of arboreal ecosystems, and potentially the life expectancy of the tree themselves.

There is a tendency to use pruning techniques, like reduction or thinning, to maintain trees in a particular form or shape. Our use of terminology is prone to describing a particular state, like dead wood for instance, rather than considering the process of decay, hence decaying wood. When we look at managing a process, the emphasis shifts, because this involves an understanding of how things change as they adapt and evolve within a natural system.

To create the bridge habitat so desperately needed by some of our rarest flora and fauna, we are going to have to adopt destructive pruning techniques, which will contradict much of our formal training as ‘tree care professionals’. However, our knowledge of tree biology is going to be essential, because if these methods are going to succeed we need to mimic the natural processes of tree decline, which is a slow progressional balance.

The term veteranisation is being used to describe destructive pruning methods, which accelerate the ageing process of trees, by inducing controlled stress. We do not have the knowledge or understanding to duplicate nature, because natural tree decline starts below ground, when the root system becomes exhausted and can no longer support a full crown of leaves. The transportation paths then start to break up and the tree progresses into a stage of retrenchment, like an army in retreat, resources are moved to a more central location.

The selective use of destructive pruning methods that involve natural fracture techniques and coronet cuts, encourages premature retrenchment, by reducing the crown area, while providing niche habitat for decaying wood organisms. This veteranisation of healthy trees is an essential part of the management of arboreal ecosystems, particularly in association with ancient decaying wood habitats where the generation gap is greatest. It can also be used instead of natural target pruning when managing hazardous trees, by reducing the potential for a lever arm to fail, while also retaining more structure within the trees crown.

Coronet Cuts:

The cut end of a reduced branch or a large stub that may be creatively cut into the form of a ‘coronet’ which is a man-made wound so-called because it resembles the appearance of a coronet, whilst it approximates to the appearance of a naturally fractured broken end. The siting of the cut is generally around a distance of five times the diameter of the branch from the branch union.

Retrenchment Pruning

This involves a combination of pruning techniques that are utilised to mimic the natural processes of aging whilst extending tree viability and retaining habitat features. The techniques seek to reduce the potential for a tree to collapse under its own weight due to excessive end weight on long or weakly attached limbs over a long period of time. Reduction in height, and weight, encourages the development of adventitious growth and the formation of a lower or secondary crown form equivalent to natural canopy retrenchment.

Ancient English oak (Quercus rubor) pollards following

retrenchment pruning, Elan Valley nature reserve, Mid Wales, UK

Sustainable Conservation

The creation of bridge habitats is a lengthy process, so consideration has to be given to the sustainability of the existing decaying wood, within our ancient arboreal habitats. As we are all aware the slow process of decay can significantly reduce the integral strength of trees, compromising their structural stability, ultimately leading to partial then total collapse. This is a natural progression and would not normally be a problem, but our obsession with the removal of, what has been perceived as, dead wood now means that for many organisms, there may be no where else to go.

Research into the sustainable management of ancient and veteran trees has been the focus of the Ancient Tree Forum (www.ancient-tree-forum.org.uk) in the UK, for over fifteen years now. A pruning method known as restoration pruning became a recognised system of trying to reinstate lapsed pollards, which had become unstable due to neglect. This involved the selective reduction work necessary to restore a more uniform and sustainable lower crown form.

There are some, who would express reservations about the use of the term restoration pruning. This is because it is in principle, a descriptive term for, a method of restoring, reinstating and imposing a physical state on the tree, which we perceive to be desirable with consideration to the management objectives of tree longevity and safety. However, ideas have evolved and a new term has been adopted, which was suggested by Paul Muir, of Treework Environmental Consultancy, that of ‘retrenchment pruning’, where the idea is to mimic the natural processes, encouraging a progression to a more sustainable structural form which considers the tree’s physiological systems.

Nectar Sources

A large proportion of the decay process is performed by juvenile invertebrates (grubs), which survive in the shelter of the decomposing wood, which provides them with all the nutrients they need to develop. However, when they leave the decaying wood as adults, they need a source of nectar to provide them with sufficient energy to fly, mate and disperse the population to the next available decaying wood habitat.

Nectar provides an energy-rich food, which can rapidly be assimilated and used to fuel flight, and pollen is a protein- rich food, which aids egg production. Flowering trees and shrubs are by far the most important sources, although other plants can also be very popular. It is therefore important to consider retaining and planting suitable species in association with decaying wood habitats that are part of an integrated conservation project.

Noble Chafer (Gnorimus nobilis) beetles on elder flowers

How much Decaying Wood and Where

An alliterative phrase adopted and promoted by Ted Green, is ‘sustainable, successional, structural, supply of decaying wood’, which sums it up neatly, although the implications may not be immediately obvious. However, it is clear that, an arboreal ecosystem needs just that, if it is to support a diversity of organisms, and maintain ecological integrity. It is a description of the level that needs to be achieved if our creation, management and maintenance of decaying wood habitat are going to be anywhere near natural.

It is however, difficult to accomplish something even near a natural state, when we have no real idea what that might be like, since it infers the absence of human manipulation. We therefore face a challenge where the ultimate goal is unobtainable, so it is important that our aims are based on viable benchmarks. This is exactly what Jill Butler, Fred Currie and Keith Kirby have attempted to do with a paper called ‘There’s life in that dead wood – so leave some in your woodland’ published in the Quarterly Journal of Forestry April 2002 (Vol.96 No.2 131-137).

The arboreal ecosystem relies on a sustainable supply of decaying wood, because the process provides a range of habitat types, thats are utilised by a large number of different organisms, which are in turn responsible for a particular stage of decomposition. It is therefore an absolute necessity that there is enough decaying wood around to provide the range of conditions needed to support these organisms.

To achieve a sustainable supply of decaying wood, without the necessity to keep importing new material to a site, we have to encourage a successional ecosystem. It is fundamental part of managing decaying wood habitat, that there is the diversity of niches, available at any one time, to support the full range of organisms associated with decaying wood.

Finally, we have to appreciate that arboreal ecosystems have multiple levels, and the creation, management or maintenance of this habitat needs to work in a structural way. It is not sufficient to have a sustainable, successional, supply of decaying wood on the ground, in piles of logs or brash wood. There needs to be decaying wood in all of the following places: dead limbs on living trees; decay columns in trunks and main branches; rot holes in standing trees; sap runs from decaying cavities or recent wounds; dead bark on standing trees; standing dead trees; fallen trunks and large branches; fallen small branches and twigs; dead tree stumps and old coppice stools; exposed root plates of wind blown trees; decaying wood in water causes; and it is important to have all of the above in a diversity of locations, and conditions, in full sun, dense shade and various stages in between.

Therefore our management goal is a Sustainable, Successional, Structural, Supply of Decaying Wood

Summary

The recognition that decaying wood habitat is a dynamic system of processes, which are a constantly evolving part of the arboreal ecosystem, is an important step towards its successful and sustainable management. It is also a demonstration of how the terms we use can influence our perception of the management objectives. Our role as arboricultural managers is one of careful guidance, to encourage and support natural processes, not to impose a physical form or state, to fit our ideas of what is right.

We must strengthen our recognition for the fact that trees live within a different time frame to us mere humans. Their living processes are almost the ultimate in sustainability, to a point where, in the right circumstances, they have the capability to attain immortality. We therefore have a responsibility to consider the management impacts of tree life spans measured in hundreds of years, and in some cases millennia. The implications of this are that the component parts of arboreal ecosystems can undergo cyclic fluctuations, which are measured in centuries.

The knowledge we use to develop tree management strategies, must have a depth of understanding that considers the tree’s interrelationship with its environment and other organisms, included within a broad arboreal ecosystem. It is also essential to have an appreciation of the ageing process of trees and be aware that different management methods are needed, which are sustainable in the context of tree longevity.

Conclusion

If sustainable conservation is to work we need to move away from management strategies that concentrate on individual species, and embrace an ecosystem based approach. This is needed, not least, because it would help define some common objectives for the various wildlife conservation organisations. As we are now, each group has it’s own goals and it is common knowledge that these conflict and are in many cases counterproductive, often canceling one another out.

These are not new ideas, and there is an evolution towards ecosystem based management, with the concepts of ecosystem health and sustainability becoming strategic goals. However, it has taken us decades to get to this stage.

In conclusion it is clear that we need to think more carefully about the far- reaching effects and repercussions of our management decisions. However, this is hardly a new concept. Aldo Leopold proposed the following metaphor in an essay he wrote in 1949, called ‘The Land-Health Concept and Conservation’, which was published for the first time in a book called ‘For the Health of the Land’ in 1999.

The biotic clock may continue ticking if we.
1 - Cease throwing away the parts.
2 - Handle it gently.
3 - Recognise that its importance transcends economics.

4 - Don’t let too many people tinker with it.

Andrew Cowan