Natural Bracing in Trees - ISA Article 2016

Page 1 Natural brace in rowan (Sorbus aucuparia) which has resulted in the formation a bark-included junction below it. The ability of plants to adjust their growth and shape due to perceiving strain is a well-known phenomenon: some authors favour the term ‘mechanoperception’ (Telewski, 2006) but the original scientific work to verify the response of plants to mechanical perturbation was by Jaffe (1973) who termed the response of the plant as ‘thigmomorphogenesis’. This can be translated as ‘the origin of the shape is through touch’. The word is a bit of a mouthful, but the conception behind it is clear (that plants sense strain and adapt their growth accordingly) and the evidence for this effect is very self-evident if one visits coastal trees and contrasts their form to Natural Bracing in Trees - ISA Article 2016 Duncan Slater PhD Published Mar 24, 2017 + Follow Top Content People Learning Jobs Games Get the app Natural Bracing in Trees - ISA Article 2016 https://www.linkedin.com/pulse/natural-bracing-trees-isa-article-2016-... 1 of 9 3/12/2026, 3:57 PM Page 2 those growing in more sheltered locations (Fig. 1). Figure 1: A yew tree (Taxus baccata) with a highly one-sided crown shape, growing on the very wind-exposed coast of north Lancashire, England, 2016. A simple principle one gains from an understanding of thigmomorphogenesis is that, to be sufficiently strong, component parts of a woody plant (the twigs, branches, stems and main roots) need regular perturbation: in essence, trees need a “physical work-out” on a regular basis to be strong. A tree’s “work-out” comes from two main types of loading: static loading (e.g. from the tree’s own weight or from snow-loading) and dynamic loading (e.g. from moving and bending in the wind). Evidence in-the-field that a lack of exercise leads to the formation of weak tree structures is easily shown in young trees that are over-supported by their staking system, resulting in poor development of the trunk of such a tree (Fig. 2) (Watson & Himelick, 2013). Natural Bracing in Trees - ISA Article 2016 https://www.linkedin.com/pulse/natural-bracing-trees-isa-article-2016-... 2 of 9 3/12/2026, 3:57 PM Page 3 Figure 2: A Norway maple (Acer platanoides) that was over-supported in the nursery in which it was grown, so the trunk is not strong enough to bear the growing crown of the tree – Myerscough College, England, 2008.  From understanding that simple principle, one can then come to a realisation that it is not only over-rigid artificial staking that can be responsible for the creation of weak structures in a tree. One of the most common causes of poor structural elements forming in the crown of a tree is when a tree’s branches or stems create a ‘natural brace’ which straddles across a branch junction, resulting in that junction becoming a bark-included junction (Fig. 3). This weakened structure is the result of the branch junction being held static or with highly restricted movement because of the natural brace formed above it. In a similar way, in our paper on modified branch junctions (Slater & Ennos, 2016), we found that by statically bracing young bifurcations of hazel (Corylus avellana) they started to weaken. Natural Bracing in Trees - ISA Article 2016 https://www.linkedin.com/pulse/natural-bracing-trees-isa-article-2016-... 3 of 9 3/12/2026, 3:57 PM Page 4 Figure 3: A bark-included branch junction formed in a grey alder (Alnus incana), due to the presence of a natural brace above restricting the movement of the junction. The lower white arrow identifies the position of the bark- included junction, the upper white arrow identifies the natural brace – Lancashire, England, 2016. In a recent survey I carried out of 575 branch junctions formed in a mix of broadleaf trees in a linear woodland in Lancaster, England, a very strong association (93%) was found between the formation of a bark-included branch junction and the presence of natural braces acting above such junctions. Natural braces can take several forms: I have found nine distinct forms of natural brace in trees: but the most common ones are fused branches, entwining branches and crossing branches (Fig. 4). As a result of this branch-on-branch contact higher up, the branch junction formed below gets a deficit of exercise and the branch bark ridge does not form properly. Instead, a bark-included junction will form in this situation if the junction is effectively held static for many years and little to no gravitational loading is experienced across that junction either. This latter point helps to explain why most bark-included junctions occur where two upright co-dominant branches rise up in a near-vertical orientation from the junction. Figure 4: Three common types of natural bracing found in trees that can result in the formation of a bark-included junction lower down in the tree: A: Fused branches; B: Entwining stems; C: Crossing branches.  If one of the branches that forms the natural brace above a bark-included junction is pruned out or is shaded out as the tree develops a mature crown, the bark- included junction created below the brace is suddenly opened up to movement that it has not experienced for some time - potentially for decades if the natural brace has been in place that long. The loss of such a natural brace can result in failure at the weakened bark-included junction in the next strong wind. If that branch junction does not immediately fail, it would normally start to bulge at its sides (perpendicular to the plane of the bifurcation), as the tissues are now reacting to the heightened strain levels in that location, as predicted by the process of thigmomorphogenesis. When one sees a bulging bark-included junction, one is seeing the tree trying to repair the self-induced structural fault by producing dense Natural Bracing in Trees - ISA Article 2016 https://www.linkedin.com/pulse/natural-bracing-trees-isa-article-2016-... 4 of 9 3/12/2026, 3:57 PM Page 5 wood with an interlocking wood grain pattern to the sides of the seam of bark. This repair may be successful or the branch junction may fail. This process of natural brace formation and bark-inclusion development is outlined in Figure 5. Figure 5: A diagrammatic representation of the process of the formation of a bark-included branch junction. Stage 1: A natural brace restricts movement and a bark-included junction is formed. Stage 2: The natural brace (in this case, a lateral branch) is shaded out (or potentially is pruned out). It is at this stage that the junction is most vulnerable to failure. Stage 3: The junction starts to form bulges either side due to the movement it is now experiencing – this is a form of repair of the branch bark ridge that failed to form at Stage 1. Stage 4: A good proportion of bark-included junctions do not fail and can complete their repair to make a satisfactory branch junction. In the light of this scientific finding, the targeting of crossing and rubbing branches by arborists for pruning out of a tree is both a right and a wrong decision – that is to say, it is very much to do with the timing and context. When two small branches have just started to come into contact within the crown of a young or semi-mature shrub or tree, then good formative pruning practices should apply, and the arborist should prevent that contact from happening through pruning (Gilman, 2011).  However, very often, the arborist is asked to do tree work on a tree that has had little formative pruning work for many years – and the removal of a branch that forms part of a natural brace above a bark-included junction may well result in that junction having a much higher likelihood of failure, as the arborist has essentially opened up a weak junction to much more movement and dynamic loading than it has previously adapted to. As an old and rather rotund man, it is like asking me, all of a sudden and with no preparatory training sessions, to run a marathon. That would probably cause some of my own components to fail – ankles, knees, hips, lungs or heart – as they have not experienced that level of mechanical strain in many years. That’s not an experiment I am yet ready to undertake, as a sedentary academic! In the same way, think of the poor bark-included junction, held static for decades, weakened by lack of exercise, suddenly released to major wind loading by severing the bracing branch out of the crown in three seconds flat with a chainsaw! Natural Bracing in Trees - ISA Article 2016 https://www.linkedin.com/pulse/natural-bracing-trees-isa-article-2016-... 5 of 9 3/12/2026, 3:57 PM Page 6 This short article has been written to highlight to tree surgeons, arborists and arboriculturists that this effect of ‘natural bracing’ is very common in a wide range of broadleaved and coniferous trees. All professional arborists should be aware of this effect of holding a branch junction static via natural bracing and they should be able to come up with suitable remedies to this situation. In particular, the solution is often not to prune out crossing branches in mature trees, but to leave them in place. I intend to carry out further scientific assessment of the phenomenon of natural bracing in trees, to inform good tree care practices. Please note that the result of removing an important natural brace in a mature tree, or it naturally being shaded out and dying, can be catastrophic! (Fig. 6) Figure 6: Failure of a co-dominant branch junction in a beech (Fagus sylvatica) which contained a large seam of included bark, Liverpool, England 2016. References Gilman E F (2011) An illustrated guide to pruning; 3rd edition; Independence, Kentucky US, Cengage Learning. Jaffe M J (1973) Thigmomorphogenesis: The response of plant growth and development to mechanical stimulation; Planta 114, 143-157. Slater D and Ennos A R (2016) An assessment of the ability of bifurcations of hazel (Corylus avellana L.) to remodel in response to bracing, drilling and splitting; Journal of Arboriculture and Urban Forestry 42, 355-370. Telewski F W (2006) A unified hypothesis of mechanoperception in plants; American Journal of Botany 93, 1466-76. Watson G W and Himelick E B (2013) The practical science of planting trees; Champaign, Illinois: ISA. Acknowledgements With thanks to Laura Power for the drawings of trees and natural braces. *** THIS ARTICLE FIRST APPEARED IN THE ISA AREA NEWSLETTER, DECEMBER 2016 ***  Natural Bracing in Trees - ISA Article 2016 https://www.linkedin.com/pulse/natural-bracing-trees-isa-article-2016-... 6 of 9 3/12/2026, 3:57 PM Page 7 To view or add a comment, sign in More articles by Duncan Slater PhD Like · Reply Like · Reply 1 Reaction Like · Reply 1 Reaction See more comments Bianca Thomas 9mo I love this article. So useful thank-you! I started observing this a while ago, and thought there must be some association so I've always left fused branches in place in mature trees. Really nice to read an article on it! Darren Kilcoyne 8y Hi Duncan Me personally as an arborist have witnessed this phenomenon many times throughout my daily works and agree with your scientific research, in this case I have always left the natural brace as I understood that was the trees motive to support its weakly established branch union with included bark or compression fork, in this particular case I would do a reduction where possible on the supported codominant stem to reduce the stress on the compression fork and to assist in reducing the pressure on the Union in related to the law of the lever arm potential Is this a correct action Please advise Dr. LN Vemuri 8y Beautiful article, enjoyed reading......... though possess very minimal knowledge in this domain! 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