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Tree Felling

Written by John James, posted on Monday, August 12, 2019

Tree felling is always an emotive subject and not one we undertake lightly. We will only ever fell a tree for good arboricultural reasons or because it represents a significant safety hazard, and with an estimated visitor footfall in Christ Church Meadow of in excess of one million people annually it is something we take very seriously.

All our trees (over 1,200) are surveyed by a professional tree surveyor on a bi-annual basis who then produces a list of recommended works which are prioritised according to urgency. The majority will pass with only a brief “no work required” in the comments box, whereas others may need ‘crown lifting’ to clear vehicles and passers-by, others will need branches removing or reducing in order to prevent them splitting out, dead wood removal is a frequent recommendation and so on.

Beech with large tear out cavity on the main trunk.I sometimes think being a tree surveyor must be a little bit like a detective, looking for clues as to possible points of failure in the tree and then following them up to see where they lead.

A good case in point was a large beech tree growing alongside the Cherwell Path opposite the Boat House Island. Historically, this tree has suffered the loss of a large limb, leaving a large tear out cavity on the main trunk. We have noticed that this cavity has supported the growth of an elder bush and a bramble plant for quite some time, which is never a good sign, but on the last survey some black crusty growth was noticed. This led to further investigation up a ladder, which confirmed that the tree was being colonised by a fungus – Kretzschmaria deusta a soft rot fungus which can lead to catastrophic brittle failure, never a good thing next to such a popular walking route.

On its own, spotting this fungus is not enough to condemn the tree, so specialists were brought in to carry out further testing. This consisted of using a Picus Sonic Tomograph to take the equivalent of an x-ray of the trunk. This is a specialised electronic instrument which can 'look' internally into a branch or tree trunk and display a computer-generated image of its condition. It achieves this by measuring the speed that sound travels through the wood in a number of different positions and directions. Sound travels fastest through solid wood. Decayed wood will slow its path. By measuring the speed that sound takes to pass through a tree, an idea of its condition can be obtained.

Typical tomograph set up showing the sensors and nails which are driven a short way into the tree and which are struck with a hammer to send sound waves through the trunk.The tomograph consists of 8 to 14 sonic sensors. These sensors are spaced out evenly around the circumference of the trunk. They detect stress waves induced by manual impact propagated through the wood. Time-of-sound-transmissions are used to generate two-dimensional pictures that document decay and cavities.

The sounds are generated manually by tapping on a number of metal nails with a hammer. Special sensors fixed around the stem read the interval the sound takes to travel through the wood. Once all nails have been tapped, and recordings taken, the computer software works out a visual image that requires professional assessment to assess decay.

Resistograph being used at the base of a tree to investigate suspected decay.To back this up, further testing was done with a resistograph which is a long micro drill that records how easily it penetrates the wood, again indicating the amount of decay. This confirmed the results shown by the tomograph.

In order to investigate further, more resistograph readings were taken above the main cavity, but below an old pruning wood. The reason for doing this was that the old pruning wood had been made very close to the trunk, a practice now frowned on by arborists since it maximises the size of the wood and removes what is known as the branch bark ridge, which is the point at which the wound will be at its smallest and will therefore be more likely to heal quickly. The results at this level were even more worrying, showing further decay with only a 10 to 14cm outer layer of undecayed timber.

At this point it was clear that something serious had to be done to the tree before it collapsed. Consideration was given to heavily reducing the tree rather than felling it, but the general opinion was that any reduction large enough to eliminate the risk of collapse, would reduce the trees canopy and area of photosynthetic material so much that it would be unsustainable and even more detrimental to the tree’s health and likely to accelerate its decline. This left the only safe and viable option being to fell the tree.

Having had the tree removed we will replace it with a new beech tree in the autumn.

Computer image indicating the actual areas of decay shown in the tree trunk when it was felled.In this image, left, you see how closely the computer image generated by the Tomograph, very closely matches the actual areas of decay shown in the tree trunk when it was felled. The green lines on the photograph of the felled trunk, numbered 1 and 2 represent the positions for the two resistograph readings.


Click images for larger versions.


Resistograph results can be found in the gallery below.