What happened to Oak 5552?

Sometimes, healthy and mature trees shed large branches during the summer for no apparent reason. This is what is known as Summer Branch Drop Syndrome and it is what happened to Oak 5552 in August of this year.

The two branches that fell showed no indication of structural weakness or fungal infection. It was not storm damage: the weather was calm and wind speeds were low. Nevertheless, August’s weather patterns may have been part of the problem.

Oak 5552 suffered Sudden Branch Drop Syndrome in August of 2021

There seems to have been little research into the phenomenon but Summer Branch Drop affects broadleaved trees, particularly oaks, and is associated with certain kinds of weather: a period of calm after a heavy rain shower that ends a time of drought. The relationship between the failing branch and this particular sequence of weather events is not fully understood.

It has been suggested that the trigger may be water stress caused by insufficient water during the period of drought; water stress is known to cause cracks in conifer stems and may be responsible for the same condition in broadleaved trees. The weight of the summer foliage and the almost fully developed acorns may have been increased so much by the rain shower that internal cracks, the result of the period of drought, proved fatal for 5552’s branches.

[1] Storm damage to an oak in the hedge between Sleepers and Cornfield in October 2019.
[2] The weight of leaves and acorns is implicated in SBD.

It is possible that the onset of decay, outwardly invisible, may have reduced the strength of the wood at the junction between mainstem and branches to the point where the added weight of the rain was sufficient to bring about sudden failure.

Another possibility might be that gases released inside the wood by bacteria expand abruptly in response to the rapid changes in temperature and pressure following a heavy rain shower, and fracture the wood. The branch’s failure is sometimes associated with explosive sounds that suggest this might be at least one of the possible causes.

Essentially, Sudden Branch Drop is a tree’s response to sudden changes in a time of hot weather and as our climate heats up and periods of extreme temperatures increase, we fear it will become more frequent.

Oak 5552’s lovely symmetry might be spoiled but it is still valuable real estate for the hundreds, if not thousands, of species of flora, fauna and fungi that inhabit the living tree and will soon move into the dead branches.

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