Tree Pulling comes to the UK
Tree Pulling was the star turn at the Tree Statics and Dynamics seminar held on July 21 and 22 2003 at The National Arboreturn, Westonbirt. Speakers from the UK, the Czech Republic and as far away as Australia discussed approaches to understanding and evaluating hazards in trees.
21 & 22 July 2003: The National Arboretum, Westonbirt
A capacity audience of some 70 delegates witnessed the first public UK demonstration of the technique, part of the ‘Statics integrating Method’ originated by Lothar Wessolly of the University of Stuttgart. This system, remarkably, appears to have been in use in Europe for nearly 20 years. The pulling test is a critical part of the method, applying a small, precisely measured load to the tree in order to evaluate the likelihood of the trunk to break, or the tree to uproot.
The test was demonstrated on a mature Silver Birch near the spectacular new Great Oak Hall at Westonbirt – the delegates’ home for the two days of the seminar. Two small electronic devices were strategically attached to the trunk, including an Elastometer pinned to the lower trunk to measure microscopic alternations in the marginal fibres immediately below the bark and an inclinometer attached to the very base of the tree (thereby uninfluenced by stem flexing) to measure the degree of tilt or lifting.
A measured winch load, using an in-line ‘Dynamometer’ typically to 1-2 metric tonnes, substituted wind load on the crown. Practitioner Jarek Kolarlk, and Arboricultural Consultant from the Czech Republic, and Dr. Peter Horacek, Head of the Wood Science at Mendel’s University, Brno, appeared well used to jibes about pulling an arboretum specimen over and, unlike some of the onlookers, were unsurprised by no visible movement of the tree.
The exciting bit over, back I the hall a myriad of detail was fed through the computer programme to determine breaking and uprooting resistance of the tree and to calculate the “factor of safety’, Key basic parameters included wind force (calculated at gale force based on a ‘100 year return-period storm’), load bearing geometry of trunk and crown and the structural properties of green wood (wood strength being derived in continental Europe from the Stuttgart Table of measurements for different species). Other recorded details factored into calculations include – tree height, trunk diameter, crown surface area, (including eccentricity and centre of gravity), aerodynamic crown drag, wind loads, (adjusted for geographical, topographical and seasonal factors). The crown geometry and disposition is digitally represented and superimposed on to a photograph of the tree, and recommended reductions can then be graphically illustrated.
Team outside the Great Oak Hall Right to left: Neville Fay, David Lonsdale, Ken James, Jarek Kolarik, Peter Horacek, Mike Ellison, Geoff March
While Peter Horacek comprehensively described the mathematics behind the process, he also very ably used other means to illustrate this – by using volunteers to break standardised wood samples by pulling, snapping and twisting to demonstrate the significance and nature of wood failure from shear action.
It can justifiably be claimed by Jarek Kolarik that the Statics Integrating Method (SIM) is the only evaluation method that directly tests the likelihood of a tree to uproot. Other methods, including drilling, will inevitably involve some subjective input, even if made in conjunction with a thorough visual tree assessment. The system is claimed to be completely non-invasive, with the applied load coming well within safety (i.e. non-reversible damage) limits.
Where the assessment determines that the stability of the tree falls outside safety criteria, an accurate recommendation may be made for remedial work. This is calculated to reduce the extent of the crow (and its geometry) to return the stability of the tree within acceptable levels. Where such means are not considered achievable, tree removal may be necessary. Minimal crown reductions (involving as little as 1-2m pruned from the crown periphery) can achieve significant improvements in stability. This is because wind speed (and therefore wind-load) increases with height above ground level and small modifications in crown geometry and the eccentricity can significantly reduce wind-included torsion effects.
A number of ‘what ifs’ could be sensed spinning through the minds of the audience and were raised with the lecturers. These included possible accelerated re-growth or increased crown density following crown reduction, the presence of brittle decay (where little distortion may occur just prior to breakage), saturated ground leading to reduced soil cohesiveness and problems of restricted access for the pulling test. David Lonsdale chaired the valuable discussing session and skillfully raised these and other issues. However, the two Czechs smiled disarmingly and while perhaps agreeing to some of the reservations pointed to the high level of empirical success of the method – over 5000 pulling testes, with trees currently standing being a testimony to the system’s success, following quite exceptional continental storm conditions. The proof of this pudding does indeed appear to be in the eating.
Two impressive examples were given of how the technique can be used to good effect. Developers damaging tree roots on construction sites can be obliged to either pay the amenity valuation sum of the damaged tree or pay for a pulling test and, as Jarek put, it, pray for a good result (this clearly appealed to the local authority delegates!). The test results might then indicate the degree of remedial work necessary to improve stability. What other method could test a tree where, for example, root severance had been permanently covered over?
As with all systems, there are limitations – the safety of the crown (the branch framework) and the dynamic load (build-up of multiple stresses) cannot be measured.
On another site where some mature oak trees had fallen close to housing during the severe flooding a year ago in the Czech Republic, there was great pressure from local residents to have the remaining trees removed. Pull tests demonstrated a high safety limit, and the trees were saved. Such measurable results offer the possibility to cut through the cloud of emotive reaction to provide a clear, and justifiable, basis for decision-making.
As with all systems, there are limitations – the safety of the crown (the branch framework) and the dynamic load (build-up of multiple stresses) cannot be measured. The location of the tree may make testing impossible. By no means least, is the cost – a typical single test taking at least 2 hours for two people. Despite this the ‘wow’ factor was certainly generated, and many of the delegates came away eager to learn more and to find cases for testing the system. This may be the first occasion, but it is certainly not the last word on this subject in the UK.
So why has this method not been used in the UK to date? The answer appears to be partly due to the fact it is not sold as a commercially available product. EU practitioners are specially trained within a certified user-group, meeting annually to compile test results and continually update and evaluate the system. Jarek emphasised that ‘off the peg’ equipment carries the danger that the method would be misapplied and discredited (has this happened with some of the hand held devices?). More prosaically, failure of SIM to cross the channel is in part due to our language barrier – all the detailed literature to date is in Czech or German. To quote Jarek once again, ‘not every-thing of importance is in English!’ But we are reminded that in the dim and distant past, Wessolly published an article in the Arboricultural Journal (1989, Vol 13 pp. 45-65, Sinn, G, & Wessolly, L.)
The seminar also introduced the concepts behind Tree Dynamics, very ably described by Ken James, from the University of Melbourne, Australia, who gave a delightful and stimulating presentation. Coming from an engineering background, he left the audience eager to learn more about the subject. With the use of a 5 ft piece of dowel he illustrated the principle of ‘mass damping’ in branch behaviour. He showed how wind-induced branch bending absorbs wind energy and proposed that trees have evolved so that trunk and branch sway are designed to reduce any predisposition to express a natural frequency in winds. This reduces any tendency of the tree to behave as an ‘inverted pendulum’ which would otherwise lead from progressively amplified sway to eventual failure.
Mike Elison, Arboricultural Consultant, gave a thorough overview of his sophisticated quantified Tree Risk Assessment method, to place diagnostic systems in the decision making context for risk management.
Throughout the seminar David Lonsdale provided great guidance and served the audience well, eliciting clarification from the lecturers while drawing together the threads of the various issues presented.
Reproduced from the Arboricultural Association Newsletter Issue 122