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27 Nov 2023

Veteran tree management – a new kind of woodpecker

Written by Chris Wardle, Gardens and Designed Landscape Manager (Aberdeenshire and Angus)
A very large tree lies fallen on the ground beside a thin wire fence.
A veteran tree that fell during Storm Arwen
Tree management is an inexact science, but the use of technology is starting to assist those who manage our tree landscapes, helping them to make the correct decisions to ensure safety as well as the longevity of our precious woodlands.

Veteran tree management is a vital part of the work of the National Trust for Scotland. From north to south and east to west, almost all of our key gardens and designed landscapes (as well as countryside properties with natural woodlands) have a degree of tree cover. Each year Trust staff undertake tree safety management and tree inspections to assess the health and wellbeing of our trees, so that our visitors are kept safe and we also protect the health of the trees themselves.

Part of our work in caring for the environment is to keep our trees standing as long as possible. Trees inherently become more interesting as they age – more fungi, insects and birds are attracted to live on, in and around them. However, it is a balancing act to keep them standing and safe at the same time. Although many staff are trained to carry out visual tree inspections, it is an inexact science due to the fact that trees are living organisms. They react differently in all locations due to the type of soil, the climate, the tree species itself, wind conditions and many other environmental factors.

We haven’t had the ability to look inside the trees themselves with x-ray vision ... until now! We’ve recently purchased new equipment and trained some people to help us ‘look’ inside trees using a technique called sonic tomography.

A blue monitor screen is strapped to a mossy tree trunk with blue canvas straps. Some numbered metal pins are spaced around the trunk.
Our PiCUS machine in place on a tree trunk at Crathes Castle Estate

This new equipment is called PiCUS and takes its name from the Latin for a woodpecker (Picus picus), which also translates as ‘tap tap’. The equipment allows the technician to send sound waves through a tree using sensors and special hammers; from these readings, they can then build an image of the internal structure of the tree.

First of all, we collect some key data about the tree such as its species, height, spread and overall health. We then input this into the computer.

A man stands about 20m from a large tree and holds a device up towards it to gather measurements.
Taking some initial measurements of the tree

Next, the geometry of the tree is assessed, using more specialised callipers. Once the geometry has been calculated, the sensors are placed around the tree.

Once the sensors are in place, the technician uses a special hammer to send sound waves through the sensor points all around the tree. They are recorded by the special data collection device which then builds a picture of the tree’s internal components and their relative state of health. Sound waves move faster through good wood and more slowly through soft or damaged wood.

At the end, an image is produced that gives a visual representation of what may be happening internally in the tree.

A man taps an electronic stick against a tree trunk. The trunk has a blue monitor attached to it.
Using the hammer to send sound waves through the tree

Of course, the skill then comes in the interpretation of the image, which can be difficult at times! There is a quick image that appears on the machine, which can show if there is a potential problem. In this case, we have an additional diagnostic test that can be used – using a special drill that works on the resistance of the wood of a tree. The drill produces a graph of the wood structure. This would be the last line of testing though as it is intrusive.

A close-up of a tablet-style screen strapped to a tree trunk. The screen shows a cross-section of a trunk with various sections in different colours.
The PiCUS screen gives an initial idea of the condition of the tree.

After all tests are complete, the technician uses a computer programme to help interpret the data and to create high-resolution imagery to help us make decisions. Sometimes, if a tree is particularly special or difficult, we will involve a consulting arborist in the final decision-making process to ensure the best outcome for the tree, for safety and for reasons of landscape importance or wildlife value.

This new technology and investment in skills development for our teams will ensure that we continue to try and make the best decisions for the trees in our care.

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