Recent storm damage to horse chestnut tree ref 11

There have been comments and complaints online about work done to a tree in the recreation ground following a large limb loss.
The Parish Council uses a pool of tree tree surgeons depending upon availability, price and complexity of the work involved. On this occasion, work was carried out by Jenks Oxford, who have been carrying out tree works for the Council for several years now and had recently been awarded the contract to manage all the councils trees in the village per a tree survey carried out in November 2018. Jenks are Arboricultural Association Approved and carry out work for the County Council as well as a variety of major companies.
As a result of the complaints, the Parish Council asked the company it uses for tree surveys to visit and assess the work. Sarah Venners is not connected to any of the tree surgeons that we use – it is standard practice to employ one firm to carry out surveys and another to carry out the work – and her report is as follows. Given Sarah’s conclusions, the council is happy that the work has been carried out to standard.
Given the urgent nature of this case, I popped out earlier this afternoon to see the tree. I have the following comments to make which I would like you to relay to the concerned residents and to the Parish Council on Wednesday evening:

T11 is a mature Horse Chestnut. The survey carried out in November 2018 recorded the tree as having a crown break at 2m with multiple large limbs from this point with good unions. Deadwood was observed throughout, but the crown was in good form and vigour (despite previous pruning to reduce limbs overhanging the field). Given its form of multiple branching habit from 2m, it is highly likely that this tree was once historically managed as a pollard – where the branches are cut back to a knuckle every 5 – 8 years to promote the grown of a dense regrowth of foliage and branches. The branch attachments from the pollard knuckle are much weaker than if the tree is left to grow in a natural form and therefore pollarding needs to be done on a cyclical basis in order to minimise the risk of branch failure. Horse Chestnut timber is not very dense, is structurally quite brittle and degrades quickly, so it is likely that this management practice on Chestnuts was only really done for amenity purposes and to keep the tree small and manageable.

T11 is what is known as a ‘neglected pollard’. These are commonly seen in urban environments and the risk associated with these types of trees is high, given the targets around them. The canopy of this tree overhangs the Recreation Ground and a road and as typical of neglected pollards, its branches were very heavy around the edge of the pollard knuckle. Failure in pollards often involves snapping of top-heavy new branches and splitting at the pollard point – and it is the latter that has happened to T11. The failure to keep the tree re-pollarded has enabled it to develop larger and heavier scaffold limbs from the pollard point, one of which has catastrophically failed – causing the limb loss that can be seen today.

Management recommendations in November 2018 for this tree were to have the entire crown reduced by 2-3 metres within 24 months of the date of the report, because the wood of a Chestnut is very brittle and suffers readily from summer branch drop and wind loading failure. A crown reduction (like Jenks have now done) would have reduced the weight of the scaffold limbs and relieved the loading on the unions at the pollard point, potentially preventing the failure that has now happened. The recommended work would have been scheduled in by Dorchester Parish Council for the next tranche of tree works this autumn (2020).

Unfortunately, we have had some periods of unusually high, prolonged winds this year and it is therefore likely that in conjunction with the very dry spring period of April and May, the tree became drought stressed at a time when it would have been building up its carbohydrate reserves for the year and this has resulted in a phenomenon known as Summer Branch Drop – where the heaviest and weakest limbs simply drop without warning.

Given the failure that has occurred, Jenks Tree Surgeons were called in to make the tree safe. Jenks are an established and reputable tree survey company that are qualified and knowledgeable when it comes to tree science. They understood that this tree needed to be made safe and by only tidying up the limb that failed would have left the other limbs exposed to an altered wind loading which would likely see the same failure happen again to another large branch in the next period of high winds. By removing the weight from the whole crown, Jenks have likely prevented the same failure happening on other scaffold limbs and thereby enabled the crown to still retain some of its form in the landscape. As they have left enough leaf area where they were able to, the tree still has some reserves to enable it to respond well to this work and we will see over the next few years, the sprouting of prolific new growth from the cut points. This re-growth will need to be managed going forward as a pollard on a cyclical bases. I can assure you that the work that has been carried out is entirely appropriate and correct and in accordance with British Standards 3998:2010. The work they have done has in fact saved the tree from having to be reduced completely to a monolith and has likely extended its life cycle by another 20 years or so. If they hadn’t reduced the whole crown, other limbs would have failed like the one that did last week. The work that has been done was the only sound arboricultural management option to make the tree safe whilst giving it a chance to be retained and recover.

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Posted on July 7, 2020 in Events in and around Dorchester

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