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Imaging technology makes tree maintenance more efficient

Is a hole in the trunk of a tree a sign of decay so advanced that the tree could collapse at any time?

Sometimes. But it's also true that some trees that look ready for the axe could survive for many years to come - and that others that appear perfectly healthy on the outside may in truth be very sick and just about ready to fall.

Diagnosing trees is a tricky business. Just ask the Leisure and Cultural Services Department (LCSD), which is responsible for over 800,000 trees in Hong Kong's urban areas.

To help its Tree and Landscape Team see past the surface to the interior cavities and cracks that may signal trouble, LCSD has imported from Europe the last word in arboreal imaging technology - a sonic tomograph.

"This device can see inside a tree like the computer scanner that doctors use to examine human bodies," Mr Lawrence Cheung Yiu-kong, Chief Leisure Manager (Passive Amenities), explained.

"Sometimes, it's difficult to tell accurately the internal condition of a tree from its appearance, or by experience alone," he said.

"For example, the size and location of a hole indicate the scale and degree of deterioration. With this new tool, we can get more information from the inside of a tree to decide whether it is healthy or not.

"On the one hand, it enables us to decide whether a sick tree is suffering from structural stress. If it is dying, it is better to remove it before it becomes a danger to passers-by.

"On the other hand, the device will prevent us from felling trees that have only minor problems.

"The traditional way to check the health of a tree is to drill into the trunk. However, the information obtained is limited to the area where the drill travels and doesn't show the whole picture of the stem cross-section. This method not only causes damage to the tree, it also fails to accurately reflect the reality of the tree.

"The sonic tomograph overcomes all these shortcomings. It takes only 20 minutes to get the stem cross-section data."

The sonic tomograph consists of 12 box-like sensors connected by electronic lines. As a first step, the operator measures the perimeter of the trunk, fixes the sensors around the tree with plastic tape and plants metal nails in the tree surface at designated points to connect the sensors.

Next, the operator knocks three times on each nail with a hammer, making sure that other nails are receiving the sound waves.

After summarising all the data, the computer projects an image of the stem cross-section on the screen, using different colours to distinguish areas of decay.

"It may sound complicated, but our tree and landscape staff have received professional training on how to use the equipment," Mr Cheung said.

"We also tested the device on a tree that was about to be felled. We first examined the tree with the sonic tomograph. Then we cut down the tree and compared the actual cross-section with the tomograph image.

"The image generated by the machine was amazingly similar to the actual stem cross-section," Mr Cheung said.

And so, with the help of the latest technology, staff of the LCSD will be more effective in maintaining trees in parks and on roadsides. These trees will grow healthier and stronger, contributing to the greening and beautification of Hong Kong.

Ends/Sunday, January 25, 2004
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