Solved! The Puzzle of Bark Inclusions in Trees

Some things in life remain a mystery, despite the major technical and social advances that are a notable aspect of our modern age: What became of Lord Lucan? Who discovered Marmite and thought it was edible? What shape is the Bermuda Triangle? We may never have robust and verifiable answers to such mysteries.

Similarly, although people have lived amongst and utilised trees for many thousands of years, there are many things we do not know or understand about them – in terms of their physiology, their interactions with other organisms, and why, for instance, they often take radically different forms even when growing in the same habitat. Science that investigates trees, their growth, form and physiological systems continues to expand our knowledge of these fascinating woody plants, but even some basic questions that a child might ask us about the trees we see each day remain unanswerable.

For example, what causes a tree to form a bark-included branch junction? Such potentially weak bark-included junctions (BI junctions) are a common feature in a wide range of tree species (Figure 1), yet the mechanism by which they form has not been satisfactorily explained. Well… that was true up until I found that mechanism, and wrote a short book about it (Slater, 2016).

Figure 1: A bark-included junction in a southern beech (Lophozonia alpina ((Poepp. & Endl.) Heenan & Smissen)). Branch junctions that include large layers of bark within them are structurally much weaker than those that are normally formed.

“So what is the primary cause of bark-included junctions, then?” – I am sure you will want to ask me – “…what is causing these malformations in our trees?”

Previous suggestions for the cause of BI junctions from a range of authors include:

• That BI junctions are formed because the bases of two or more branches above a junction have compressed together as they have grown. Nope – no-one has shown this to be a cause of BI junctions – it is an unproven theory, which now looks highly implausible.
• That where two branches arising from a junction grow at a tight angle in relation to each other, it will become a BI junction. Nah – one can easily find normally-formed branch junctions that form very tight angles and BI junctions with wide angles between branches (Figure 2).
• That the genetic make-up of the tree lends itself to forming weak junctions. Uh-uh – although the genetic make-up of an individual tree informs what form it seeks to develop, the actual form of a tree is affected by a wide array of environmental factors. This is easily seen by growing clones in different situations – some will generate BI junctions in their structure, others will not, even though they are genetically identical.

Figure 2: A: A tightly-angled tree fork in a sycamore (Acer pseudoplatanus L.) that is well-formed, with a good branch bark ridge formation. B: A wide-angled branch junction in Sorbus aria 'Lutescens', which contains included bark.

Actually, the answer is much more tangible and straightforward than you might at first think – and solving this puzzle represents an important breakthrough for arboriculture, informing how we should prune and manage our urban trees differently than is current recognised practice.

Branch Junction Anatomy 101

One needs to have a clear conception of the anatomy of branch junctions and how they adapt to static and dynamic loading in order to follow the logical argument that explains the formation of BI junctions. In this short article, I can only give a very brief summary of what is a fascinating topic – but we have a review paper on branch junctions coming out soon for anyone that wants more detailed information.

Two key components provide the majority of the bending strength for normally-formed branch junctions: a) dense tortuous wood that is formed under the branch bark ridge (BBR) of the junction (Slater & Ennos, 2013; Slater et al., 2014); and b) where one of the two limbs forming the branch junction is growing in diameter at a greater rate than the other, the base of the slower-growing branch becomes occluded into the faster-growing limb, eventually forming a knot (Figure 3).

 Figure 3: Differences in anatomy between codominant branch junctions (image A) and branch-to-stem junctions (image B). They share some features (e.g. (in red letters) p = pith, B = point where pith divides into two and (crucially) BBR = interlocking dense wood formed under the branch bark ridge), but the branch-to-stem junction (image B) also ends up producing a knot by the process of the stem occluding the base of the branch (G = point of "grain capture" where the grain either ends the base of the branch or deviates around the branch to remain part of the stem's tissues, C = where the stem occludes the branch, forming a branch collar).

Where bark becomes included within a branch junction, we can conceive it as the failure of the junction to develop sufficient tortuous wood under the branch bark ridge. Identifying that these important tissues under the BBR are either fully formed (in a normal branch junction), or they are partially to wholly absent (in a BI junction), is key to solving the puzzle of why BI junctions are generated as the tree grows. 

In our scientific investigations, we found the wood formed under the BBR, upon examination by dissections, electron microscopy and micro-CT scanning, to be denser than surrounding wood, having a much lower proportion of vessels to fibres in broadleaved trees, and that this wood grain takes on a convoluted twisting pattern, often incorporating whirled and circular grain patterns (Figure 4). It is this wood that forms the critical join for taking tension at the top of a branch junction, and its absence (in a BI junction) substantially weakens the junction. Several independent researchers have identified whorled grain under the BBR of branch junctions (Hejnowicz & Kurcryńska, 1987; Lev-Yadun & Aloni, 1990: Kramer, 1999; André, 2000), but we were the first to point out the important mechanical role that this twisted dense wood performs.

Figure 4: Whorled grain formed at the apex of a branch junction in oak (Quercus robur)

Shigo described the wood formed in the axil of branch junctions as ‘compacted xylem’ and gave it no mechanical role in his branch attachment model (Shigo, 1985) – this is clearly an error when one sees how important the tissues under the BBR are for providing strength to a codominant branch junction (Figure 5) and one cannot logically argue that the BBR tissues provide an important join in codominant branch junctions but contribute no join in branch-to-stem junctions. We have verified our new model for branch attachment with a series of wood tests, identifying that this “BBR wood” has very different mechanical properties from xylem in the neighbouring stems and branches (Slater & Ennos, 2015; Ozden et al., 2017).

Figure 5: A very tight angled union, but yet it is normally formed, with a long branch bark ridge. A tight angle to the junction is not a primary cause of bark inclusions.

From observation and research by other authors (e.g. Zimmermann, 1978), the wood formed in the axil of a bifurcation, under the BBR, is very poor for sap conductance – indeed, in some of our samples, relatively large areas of this BBR wood contained no vessels, and those vessels we did find were very small, distorted and often terminated or formed circular patterns. This tissue is not formed to provide a decent rate of sap conductance through it: it is wood that is primarily formed for its mechanical role in holding the branches of a junction together. The growth of this “BBR wood” is stimulated by static and dynamic loading to the junction. The absence of such stimulation would result in these conjoining issues not forming. Essentially, hold a branch junction still and keep the arising branches near-to-vertical, and a BI junction will form.

Natural Bracing in Trees

Through a large-scale cohort survey, of 575 bifurcations in a range of tree species, I identified a key feature associated with BI junctions that was acting to hold them static. Where BI junctions were not bulged (Figure 6a), 93.9% of such junctions were associated with what I have termed a ‘natural brace’ – a branch configuration or other naturally-occurring feature that substantially restricted dynamic movement occurring at the branch junction. Where BI junctions had noticeable bulging (Figure 6b), such natural bracing was absent in 93.2% of cases.

Figure 6: A: Bark-included junction with no discernable bulging. B: Bark-included junction with overt bulging.

Natural braces take a number of forms, including entwining stems, fused limbs, crossing limbs, climbing plants and complex braces that involve other trees or other objects (Figure 7). They all act to prevent the branch junction from experiencing normal movement – and with that loss of ‘exercise’, a weak junction is subsequently formed – especially where the two or more branches arising from the junction rise vertically up, leading to little or no gravitational loading to the junction too.

Figure 7: Three of the ten types of natural brace - A: Entwining stems, B: Crossing branches, C: Climbing plant.

From this survey, it became clear that natural bracing was the primary cause of BI junctions in this large cohort of trees. Since then, I have found this relationship to be a common feature in hundreds of other trees I have examined at numerous venues in the UK, Ireland, Italy, the USA, Hong Kong and Singapore. This relationship is summed up in Figure 8.

Figure 8: The process of the formation of a bark inclusion by natural bracing, the loss of that brace through self-shading, and the subsequent bulging of that junction is outlined in this diagram. It is clear in this sequence that the branch junction is most vulnerable to failure at stage 2 - not due to a large bulge.

Implications for Arborists and Arboriculturists

I soon realised the importance of this finding and started to share it by working with the UK’s Arboricultural Association to disseminate this information via training workshops based in the UK. To date, I have trained over 1,250 arborists and arboriculturists in natural bracing and how it is associated with the formation of BI junctions in trees.

The good news is that we can do something about the problem of BI junctions, now we have this knowledge of the primary cause. There are a number of consequences for arboriculturists, tree surveyors and arborists from this research and I outline some of these here:

We can carry out formative pruning in an amenity tree to prevent unwanted natural braces from forming. If we prune a young tree to ensure that all the main branch junctions are free to move under wind loading, then few to no BI junctions will form in the tree. So, if we act in the early growth stages of a tree, we can prevent a large number of BI junctions from occurring in our urban trees and produce well-formed specimens.

To assess the safety of a BI junction, it is now essential to look at the crown structure of the tree above such a junction. No tree should be condemned just on the basis of the presence of a BI junction, now that we know that they are formed primarily due to natural bracing – and the BI junction in the tree being assessed could be quite firmly braced and thus unlikely to fail at the time of inspection.

Arborists should be trained to avoid cutting out natural braces from mature trees where they are associated with large BI junctions, for such tree work opens up a weak BI junction to movement that it may not have experienced for many years (Figure 9). Currently, it is quite common for some natural bracing structures, such as rubbing branches, to be a target for removal during tree surgery operations, but this approach needs to change, as such work can result in trees being more likely to fail after pruning than beforehand.

Figure 9: Example of a cut-out brace when some utility line clearance work was done in 2017. The removal of the natural brace (a crossing lateral branch) has opened up a bark-included junction to movement - and potentially to failure.

From my research, it follows that BI junctions with large bulges should not necessarily be a sign that major remedial tree works are needed, as some previous authors have suggested. Further scientific study is needed, but my own observations are that some large bulges formed at BI junctions are associated with on-going cracks at the junctions’ apex (Figure 10a) and other bulged junctions are not cracked and may well be mechanically stable (Figure 10b). Given the current prevalence of this dogma that a large bulge at a BI junction means it is unstable, I think it will take many years to change this illogical position and train arborists to differentiate those bulges with cracks from those that are potentially stable. I think it is very important that this aspect of the research is taken forward, for arborists are almost certainly targeting some trees for heavy pruning or removal wholly unnecessarily, based upon no scientific evidence or logical reason, just a previous suggestion that a bulged BI junction means an unstable junction. Arboriculture should be more science-based and less opinion-based in these important technical areas.

Figure 10: For branch junctions with large bulges associated with them, we will need to differentiate between those which are bulging in response to cracks (A) and those that are stable through the production of dense wood to support a junction that has, late in the development of the tree, lost its natural brace (B). The old rule of 'Big Ears = dangerous junction' is clearly not correct once one understands the role natural braces play in junction morphology.

For more information about tree junctions and natural bracing, my publication on this topic is available from the Arboricultural Association at the (hopefully) reasonable price of ￡15.00 (Slater, 2016). I am also running many workshops, to train those managing or working in trees about the effects of natural bracing, so that arborists, tree surveyors and consultants can make more informed judgements about BI junctions and whether there is a need for intervention or not.

As a consequence of running these workshops, I have met many people who were at first sceptical of my finding, but they mostly change their minds when I show them tree after tree after tree with this relationship between natural braces and BI junctions, and then subsequently they find that relationship for themselves too, in trees they come across in their work. Figure 11 shows just a few of the examples I have found: in truth, having only found this relationship last year, I already have a library of over 700 natural braces associated with BI junctions in trees, which perhaps persuades you this is a very common feature of trees that you will come across again and again after you have been trained to look for it. And the training is needed, as it can really change how trees are assessed and managed: it’s a “mini revolution” in terms of our understanding of tree management, for which I am happy to be the key provocateur.

Figure 11: Small red arrows mark bark-included junctions, small green arrows mark the location of natural braces. I have collated over 700 images of natural braces now - and am still collecting more. This common phenomenon needs to be shared and understood across the whole arboricultural community, for this knowledge informs better tree management.

The puzzle of why BI junctions form is now substantially solved – and this means we can manage trees in a more informed way from now on. I am enjoying this new knowledge a great deal myself – but I enjoy even more the process of passing that knowledge on, and seeing so many ‘light bulb moments’ in those attending my tree fork workshops. Does this mark the end of all the mystery around BI junctions, then? – No – this finding just leads to more questions: e.g. Why have trees not evolved out of forming crossing branches and stems? How was this simple relationship missed for so many years by other researchers? What are the other (minor) causes of bark inclusions? What has Lord Lucan and Marmite got to do with any of this? 

Such is the progress of the science of trees – once one puzzle is solved, it gives rise to further questions that need answering. Useful knowledge often develops in this way.

Duncan.

*** THIS ARTICLE FIRST APPEARED IN THE "TREES MATTER" MAGAZINE, NEW ZEALAND, SPRING EDITION 2017***

References

André J P (2000) Heterogeneous, branched, zigzag and circular vessels: unexpected but frequent forms of tracheary element files: description, localization, form; In: Savidge R., Barnett J. and Napier, R. (ed.s) Cell and molecular biology of wood formation; Oxford, Bios Scientific Publishers, 387-395.

Hejnowicz Z & Kurcryńska E U (1987) Occurrence of circular vessels above axillary buds in stems of woody plants; Acta Societatis Botanicorum Poloniae 56, 415-419.

Lev-Yadun S & Aloni R (1990) Vascular differentiation in branch junctions of trees: circular patterns and functional significance Trees: Structure and Function 4, 49-54.

Kramer E M (1999) Observation of topological defects in the xylem of Populus deltoides and implications for the vascular cambium; Journal of Theoretical Biology 200, 223-230.

Ozden S, Slater D & Ennos A R (2017) Fracture properties of green wood formed within the forks of hazel (Corylus avellana L.); Trees: Structure and Function 31 (3), 903-917.

Shigo A L (1985) How tree branches are attached to trunks; Canadian Journal of Botany 63, 1391- 1401.

Slater D & Ennos A R (2013) Determining the mechanical properties of hazel forks by testing their component parts; Trees: Structure and Function 27, 1515-1524.

Slater D, Bradley R S, Withers P J & Ennos A R (2014) The anatomy and grain pattern in forks of hazel (Corylus avellana L.) and other tree species; Trees: Structure and Function 28, 1437-1448.

Slater D & Ennos A R (2015) Interlocking wood grain patterns provide improved wood strength properties in forks of hazel (Corylus avellana L.); Arboricultural Journal 37, 21-32.

Slater D (2016) Assessment of Tree Forks: Assessment of Junctions for Risk Management; Arboricultural Association.

Zimmermann M H (1978) Hydraulic architecture of some diffuse porous trees; Canadian Journal of Botany 59, 2286-2295.