Oak Branch Failure, Summer Branch Drop?

Last week one of the large English oak Quercus robur trees within the grounds of our head office at Lockington Hall, Derby unexpectedly shed a large lateral branch from its northern crown.  As with all the trees in our grounds, this oak tree had been visually assessed from ground level, as part of our annual tree risk assessment carried out by our in-house qualified Arboricultural team.  Outwardly, the limb appeared sound with no visible indicators of potential to fail and 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 why this limb failed, we carried out an aerial assessment.  We suspected there may have been a partial historic rupture around the area of the union with the stem, which had not caused the limb to fall when it occurred.  This partial failure then led to the development of an internal spilt / crack allowing in water and a weakening not visible from the outside which, because of the natural weathering processes of freezing and thawing and 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 back up our suspicions.

The team concluded 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 (SBD)?

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 beyond that outlined the Arboriculture Research Note1.  VALID has recently published a handy and easy-to-understand guide about SBD, and managing the risk from it.  Here’s are the summary headlines.

  • The overall risk from SBD is ‘mind-bogglingly low
  • It’s a loose term for branches that fail after hot dry weather that have no obvious risk features
  • There’s no agreement about what SBD is or what are the critical factors that trigger it
  • We can’t tell which branches will or will not fail
  • Unless a tree has a history of SBD, the risk is Acceptable

With 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 risk features.

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

To establish the context, we know the overall level from SBD risk is mind-bogglingly low.

the annual risk of death or serious injury is less than a one in one hundred million.  That’s so low, we’re at greater risk for the few minutes it takes to cover about 3km/2mi on a drive, than we are from SBD for a whole year.4

We also know that an Arborist can’t tell the difference between a branch that has a high likelihood of failure from one that has a low likelihood of failure, before it fails.  What all this means if that unless you have a tree that has a history of SBD then the risk is Acceptable.  If the risk is Acceptable, then you don’t need to manage that risk any further.  However, if you have a tree that’s a repeat offender, then you need to manage the risk to an Acceptable level.  If you think you have a tree that’s a repeat offender, or are after some common sense tree risk management guidance, then get in touch with us.

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 VALID’s Summer Branch Drop Guide can be downloaded from their Risk Management page here: www.validtreerisk.help/Risk-Management