Oak Branch Failure, Summer Branch Drop?

Last week one of the large English oak Quercus robur trees situated within the grounds of our head office at Lockington Hall, Derby unexpectedly shed a large lateral branch from its northern crown. As with all of the trees in our grounds, this particular oak tree had been visually assessed from ground level, as part of our annual tree hazard assessment carried out by our in-house qualified Arboricultural team. Outwardly, the limb appeared sound with no visible indicators of potential to fail and as such did not raise concern. There had been no evidence of fungal fruiting bodies, either within the crown or around the base.

To gain a better understanding as to why this limb failed an aerial assessment was carried out. It was suspected that there may have been a partial historic rupture around the area of the union with the stem which had not caused the limb to fully fail at the time and which resulted in development of an internal spilt / crack allowing ingress of water and weakening not visible from the outside which, due to the natural weathering processes of freezing and thawing as well as this year’s hot, dry summer, may have led to failure. However, the aerial inspection revealed there were no signs of wetness, mould or fungal mycelium to concur with this.

As such, the team conclude this is a case of a Summer Branch Drop resulting from the prolonged dry weather, higher than average temperatures and droughts followed by more recent heavy rainfall and gusting wind.

So, what is Summer Branch Drop?

Summer Branch Drop (SBD) has long been considered a phenomenon characterised typically by ‘apparently healthy and stable trees’ shedding ‘large limbs during the summer for no obvious reason’1 often with trees exhibiting no prior obvious visual defects or signs of reduced structural integrity and in almost all cases following extended dry periods in summer.

To date, there has been little research carried out into this phenomenon, although more recently awareness has been raised by Jeremy Barrell of Barrell Tree Consultancy who published a briefing note on UK Summer Branch Drop providing an informal summary of his research and findings. This guide highlights that ‘it is not yet possible to reliably identify the individual branches that may fail, there are emerging trends in the reported incidents that provide an indication of the most vulnerable tree types and the time that the risk of failure peaks’. As such professional practitioners only have these previous trends of the reported species types and the time of year when the risk of failure is more likely, to use as a guide.

The Briefing Note indicates apparent SBD incidents are more common:

  • In particular species types e.g. large mature oak, beech, horse chestnut and cedar trees; although it should be highlighted it isn’t just confined to these species
  • Following heavy rainfall after a period dry weather or drought lasting longer than three to four weeks
  • In the early afternoon, in calm conditions, immediately following rainfall after prolonged dry spells

 

In the case of our Oak tree, we believed this happened on Sunday 19th August. There has been a prolonged period of drought leading up to the event of around 5 weeks with temperatures peaking in the low 30’s (Celsius) and nationally in the first month and a half of summer only 47mm of rainfall2 occurring. This summer the UK experienced its driest start to a summer since modern records began in 19613. The evidence would therefore suggest that our diagnosis of SBD is highly plausible as it meets all the criteria and outwardly showed no visible signs of structural defects.

So, how can the risks associated with SBD be assessed and managed?

It is important to highlight that instances of Summer Branch Drop should not deter people from enjoying trees forming our landscapes, after all trees and indeed all living organisms pose a risk to some degree. However, duty holders have an obligation under the legal Duty of Care to ‘assess the risk of harm from tree failures’4 including SBD and ‘take proportionate measures’5, that would be expected of the prudent landowner to actively ‘reduce any significant risks’6. As highlighted in the Briefing Note on UK Summer Branch Drop, this is typically required for trees within ‘public parks, botanical gardens, temporary showgrounds, theme parks, highway boundaries, golf courses, and corporate/institutional facilities, etc’.

In practice, all of the hazard survey and assessments we carry out consider SBD and identify large mature trees of the species considered anecdotally to be the most vulnerable, assesses whether harm could potentially arise from such a branch failure and provides advice based on reasonable and proportionate recommendations to reduce any significant risk of harm.

In law, landowners and duty holders should be acutely aware of the tree stock under their control and have them regularly assessed by a qualified Arboricultural Consultant and placed under an appropriate inspection regime. Vulnerable trees should be identified and consideration made to the level of access and activity near those trees, so that any risks can be managed to acceptable levels. In the context of SBD, it is suggested that landowners and duty holders should be ‘particularly vigilant shortly after heavy rain follows a prolonged dry period’

Author: Callum Throw ND Arb, TechArborA, FPCR Principal Arboricultural Consultant – 01509 672772

References and Acknowledgements:

1Arboriculture Research Note: SUMMER BRANCH DROP, by K D Rushforth, amended by D Patch, Arboriculture Advisory and Information Officer

2,3 Met Office – Summer 2018: A possible record-breaker? August 2018.

(4,5,6,7) Briefing Note on UK Summer Branch Drop – Jeremy Barrell, 13th July 2018.

https://www.barrelltreecare.co.uk/assets/Uploads/BTC129-BriefingNote-SBD-2018.pdf – July 2018