Normative tree crown insecuring (Normative Baumkronenverunsicherung)
Recently, I received a phone call from an arborist who saw a crack at the base of a limb of a large old beech tree. Because it sounded dangerous, I drove there right away. The arborist was correct to be scared: the smallest of the three big limbs had a nearly horizontal crack and moved a bit in the light wind, connected to the other limbs with cables. So, I asked the arborist to get his lift and equipment and used my traffic cones to provisionally close the nearby public road (and to keep people away from the tree). Minutes later, we stood about 30 meters away from the tree on the road and suddenly, after a gust of wind, the trunk (diameter 130cm ≈ 50") broke at the base of the 3 limbs in front of our eyes (about 1m above the grafting point). This failure could easily have killed several people on the property and on the road.
The beech had a fairly new-looking “dynamic” crown-cabling system, obviously installed following the German Tree Care Standard “FLL-ZTV”. Unfortunately, this is not the only such incident, as I was shown other trees with cracks at the base of the corresponding limbs after the installation of “dynamic” crown-cabling systems (as the FLL-ZTV suggests since the 1990s). Because of such accidents, some innovative arborists developed modern static cabling systems already years ago, usually to be installed low above problematic forks (with great success).
According to the FLL-ZTV, “dynamic” crown-cabling has to be installed on 2/3 height of the limbs and shall allow their natural movements, but prevent breaking. After this concept was presented in the 1990s, world leading neutral and independent biomechanical experts, such as Prof. Dr. H.-C. SPATZ (Freiburg University, Germany) stated that these “dynamic cabling” systems cannot fulfil the proclaimed purpose (“the concept does not correspond to the physics”). Nevertheless, science was dismissed and the "dynamic" crown-cabling was repeatedly written into the FLL-ZTV (and Millions of cables have been installed since then according to the manufacturers):
Scientific work by KEN JAMES and others showed that the uncoordinated movement of limbs (and branches) dampens a significant proportion of the wind load. If ?dynamic“ crown-cables were to allow these (significant!) natural movements on the desired height (2/3), they would have to be installed with such a large sag that this would not only look like a monkey swing, but would also not prevent break-out. Interestingly, the frequently published sketches of the FLL-ZTV do not show any sag at all but just a shock-absorber in a straight line (on 2/3 height of the limbs). The sag of the systems installed in reality is thus generally relatively small (because Germans tend to strictly follow laws/standards/rules). In consequence, these “dynamic” systems go on tension very early in wind - creating mechanical stresses at the base of the limbs the trees are not used to. An this can lead to cracks and failures as shown here and in other cases.
Interestingly in 2009, GüNTER SINN confirmed H.-C. SPATZ’s criticism: “The short expansion of a few centimeters of the dynamic crown-cabling systems used in practice are not adapted to the elastic deflection of the swinging tree parts.” [SINN had introduced the idea of pulling tests (originally born in forestry science in the 1960s) to urban trees in the later 1980s.]?
When the German FLL tree-care standard (“ZTV”) was to be renewed in 2017, virtually all members of the committee said that most of the very many “dynamic” crown-cabling systems are installed wrongly or not correctly maintained (too lose, too tight, too deep, too high, or overgrown/ingrown, …). The inventor of the dynamic cabling, who was also present in the corresponding committee session and sitting beside me, said it is impossible to write the standard’s text as to how such wrong installations could be avoided in the future - instead, the competent arborist shall decide at the tree how to put the system in. Obviously, this can hardly be the basis of a normative specification - but, when one of the committee members then said: these "dynamic” cabling systems had been introduced in the 1990s for preventing the tree-damaging "1/3 topping of crowns according to Mattheck", the session chair agreed and took a vote. The committee then decided by majority to include the “dynamic cabling” in the new FLL-ZTV again. Scientific evidence, showing that this kind of "dynamic crown-cabling" cannot fulfill the alleged purpose (but can be biomechanically harmful and therefore more than just a waste of money) was once again dismissed. The practical experience that most "dynamic" cabling-systems are installed wrong, was again ignored and not addressed.
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In view of these facts and the obvious evidence, it is not surprising that I have not yet found any publication proving that dynamic crown-cabling according to the specifications of the FLL-ZTV works as claimed. The publications presented so far as alleged proof of their suitability may represent correct research, but they are usually irrelevant in this context because they answer other questions or deal with young, thus other kinds of trees. It is already clear in advance, for example, that loosely installed holding/catching cables have no effect on the sway behavior of the corresponding branches. This is no proof that "dynamic" crown-cabling installed between limbs in old tree crowns on 2/3 height above “problematic/defective” forks according to the FLL-ZTV fulfills the declared purpose. The reference to such publications is therefore a smokescreen to distract and further mislead arborists.
The content of the German FLL-ZTV standard referring dynamic crown-cabling is therefore since the 1990s not only put in place without any proven technical/scientific basis, but against known scientific findings and practical facts. In this respect, it is more than just surprising that the European Arboricultural Council (EAC) in its recent corresponding standard took over the unproven and obviously dysfunctional concept of “dynamic crown-cabling” from the German FLL-ZTV:
When you look at all these facts and specific incidents, you wonder what else has to happen before the arboricultural standardization organisations wake up and act.
For crown cabling, there are provenly good (and cheaper) solutions available since long (static and holding/catching systems). So, there is no need to hold on to concepts that are never proven and obviously dysfunctional, potentially harmful and unnecessarily expensive. Interestingly, the same applies to tree-safety assessments and evaluations, where obviously wrong (and never proven) methods are promoted even by professorial “experts”, written into standards and thus frequently applied worldwide (by correspondingly misguided arborists).
When I look at all this, I wonder when arboriculture will finally leave behind the economically and ecologically harmful decades of unproven fairytales and costly lies in order to finally grow up to be taken seriously as a profession. This would be relatively easy to achieve: when the established professional organizations or donation funds would ask competent, independent scientists to check the many yet unproven but established concepts of arboricultural practice in question (dynamic crown cabling, tree-safety evaluation, soil-improvement, …) there would be a set of solutions on the table soon (and comparatively cheap). However, it is important to contract really competent and really independent scientists. We have already too many “professorial experts” repeatedly claiming no conflict of interest in their publications but always (at least implicitly) praising applications of products of the same companies (what reveals their unclaimed one-sided economic interests). Fortunately, there are still some great scientists without economic interests in arboriculture. So, there are options for this profession to grow into a positive direction.
Arboriculture Consultant and Tree Risk Expert, teaching in Asia, Europe, N. America
2 小时前Important insights for Support System considerations...
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1 周Inspirierend
Owner at ArborCare and Consulting
3 周I think what is needed is each arboricultural expert must searck their hearts and minds for their biases and as an industry we need to focus on education in ethics as much as we do the science of trees. Though ethics statments are taken in professional organizations - it is left untouched in practical application and educational events.
Tree Pruning Specialist
1 个月This post is a reminder that there are so many “unproven fairy tales” to be looked at. Like all 1/3 “rules”. None of them are scientific. Not even the 1/3 marker that distinguishes a proper heading cut from a removal cut. Which is a reminder there’s no scientific meaning to the 1:1 marker that divides a removal cut from a reduction cut. All these are just markers. Many, without scale, have limited to zero meaning.
Tree Pruning Specialist
1 个月I think “dynamic” systems aren’t ideal in many cases. Possibly a few limited scenarios. Mostly I think steel lasts much longer. The two basic systems function fairly similarly assuming they don’t fail. So a steel system in this Beech may also have caused uniform movement and failure? the dynamic has minor stretch and adds shock absorption, but not dynamic in the way that was intended. And yet even with steel the wood in the system also reduces shock. For example I’ve never seen a tree do the “karate” effect where the cable stops the stems and one breaks above the cable This supports the idea that steel doesn’t dysfunction in that manner. While I agree dynamic cables don’t work as intended, I think they generally succeed, up until they become degraded by weather or squirrels, which is often a long time. I think it is extremely rare that a cabled tree has this weak of a spot below the weak fork. I think dynamic systems prevent far more failures than they cause. I also suspect steel would prevent failure at a higher rate, especially given a long duration after installation. I don’t think synthetic systems are wasted money or causing failures in general but I agree that steel is generally money better spent.