Tree care saves lives

July 13, 2019

Tree tech:

Tree experts use tomography to check the structural condition of trees, by tapping on nails equipped with sound sensors.

Trunk health:

The resistograph works by drilling a tiny needle connected with a sensor into the tree. It measures the correlation of drilling resistance to wood density to detect decay, stages of rot and cavities.

Top shot:

Drones enable inspections from high angles, eliminating the dangers associated with climbing trees.

Mitigation measures:

To enhance public safety, tree experts sometimes need to prune trees that show problematic signs, including dead, diseased or crossing branches.

Green scene:

The Development Bureau has replaced trees destroyed by last year’s super typhoon at the Tseung Kwan O seaside with Fox Tail Palms in accordance with the Street Tree Selection Guide.

Download Video| Transcript

Voiceover:
With tens of millions of trees in Hong Kong, the Government takes proactive steps to enhance public safety before the start of the wet season. It’s a job that requires experts in the tree management departments, who are busy carrying out "body checks" for trees.

That process calls for the most innovative tools that have the unique ability to review and assess the health and structural condition of trees.

Leisure & Cultural Services Department Leisure Manager (Tree) Clarence Chan: 
Tomography works on the principle that sound waves take different times through wood with different density. The method is to install metal nails with receivers around the tree. By tapping on the nails, the sound waves generated will be received by different receivers. After the processing of a computer, an image showing the cross-section of the tree can be shown. And we can make use of the graph to analyse the structural condition of the tree. 

Resistograph is an electronic device to assess the internal condition of a tree. It has a tiny steel long needle drill connected with a sensor. When the drill is driven into the tree, the resistance will be recorded. Then the recorded resistance can help us to analyse the structural condition of the tree. 

Voiceover: 
In addition to scrutinising the grounds around trees, drones are deployed to enable inspections at the highest of angles. While all means are used to protect and keep trees healthy, there are times when removing and replacing trees is necessary.

Leisure & Cultural Services Department Leisure Manager (Tree) Clarence Chan: 
The mitigation measures include pruning, pest and disease control and also attaching a cable to enhance the tree support. If no other viable measures can be used, we will consider removing the dangerous tree to enhance public safety. 

Voiceover:
Maintaining trees is indeed a science. For example, the Government plants different species along streets with the strategic goal of improving ecological health and minimising tree risks. Some of the dangers include trees falling due to decay or infestation by pests or diseases along with falling branches. 

This is why the Development Bureau has published the Street Tree Selection Guide.  It features 80 less commonly used tree species that are suitable for street planting in Hong Kong. It’s used as a reference guide for government departments and anyone else interested in planting unique tree species.

Development Bureau Landscape Architect Allen Cheung:
The objective is to create a systematic method for street tree selection. It is aiming to allow the designers to understand the considerations and criteria in street tree selection.
The aim is to explore the possibility of planting a higher diversity of tree species and planting the suitable native species, in order to improve the resilience of our urban forests and in a way to promote the vegetation diversity under the principle of "right tree, right place", and in the long term, it will help to (improve) the ecological health and also minimise tree risks and maintenance pressure. 

Voiceover:
Hong Kong was hit by a super typhoon last year that caused the destruction of more than 60,000 trees. The Development Bureau has replanted specific species of trees according to the Street Tree Selection Guide.  

Development Bureau Landscape Architect Allen Cheung: 
Here, it’s next to the seaside, with strong winds, therefore we should select some tree species with high wind tolerance. Therefore, we selected the Fox Tail Palm. 
It is an exotic evergreen palm species. They do not have obvious flowers, but their leaves and tree form are very unique and attractive. And their essential attribute is also very suitable for street planting. For example, they are resistant to pests and disease, they have a high wind tolerance, drought tolerance and they are also easy to maintain.

With tens of millions of trees in Hong Kong, the Government takes proactive steps to enhance public safety ahead of the wet season. It plants different species along streets with the goal of improving ecological health and minimising tree risks.

Some of the dangers include trees falling due to decay or infestation by pests or diseases along with falling branches.

Diagnosing trees is no easy task. Some may look ready for the axe, but can survive for many years to come, while others that appear perfectly healthy on the outside may in truth, be very sick and ready to fall.

Maintaining trees is indeed a science. Just ask Leisure & Cultural Services Department Leisure Manager (Tree) Clarence Chan, who has managed trees for almost a decade.

“When we conduct tree inspections, we will first take a look at the tree’s crown, leaves, branches, main trunk and roots, to see if there are any abnormal conditions.

“We will check its crown density, leaf colour and size; whether cross branches and dieback are present; any cavities on the trunk and pests and diseases.”

Innovative tools

If a tree’s health looks questionable, that is when advanced equipment such as resistograph and tomography, which can see inside a tree, are used, said Mr Chan.

The goal is to inspect the tree’s interior for cavities and cracks that may signal trouble.

When using sonic tomography, tree experts install metal nails equipped with sound receivers around the stem of the tree. By tapping on the nails, sound waves are generated. From those sounds, a two-dimensional image revealing the cross-section of the tree is shown on a computer.

Different zones of the tree are colour-coded and indicate degrees of decay.

“Tomography works on the principle that sound waves take different times through wood with different density,” Mr Chan explained.

However, tomography cannot be applied to smaller trees. When faced with that situation, Mr Chan said tree experts turn to another useful instrument called the resistograph, which drills a tiny needle connected with a sensor into the wood.

It measures the correlation of drilling resistance to wood density. That resistance is used to analyse the structural condition of the tree. The tiny hole closes itself up without any damage to the tree.

Bird’s-eye view

In addition to scrutinising the trees at ground level, drones are deployed to enable inspections at the highest of angles.

“Drones can reach above the top of the tree crown to gather more information for tree assessment, at the same time eliminating the risk of climbing up the tree,” Mr Chan said.

While all means are used to protect and keep trees healthy, there are times when removing and replacing trees is necessary.

“The mitigation measures include pruning, pest and disease control, and also attaching a cable to enhance the tree support. If no other viable measures can be used, we will consider removing the dangerous tree to enhance public safety,” Mr Chan added.

Planting plan

Along with the maintenance of trees, the Government plants different species along streets with the goal of improving ecological health and minimising tree risks in certain areas.

This is why the Development Bureau has published the Street Tree Selection Guide.

It features 80 less commonly used tree species that are suitable for street planting in Hong Kong and is used as a reference for government departments and anyone else interested in planting unique tree species.

Development Bureau Landscape Architect Allen Cheung said the objective of formulating the guide is to introduce a systematic method for choosing trees that will thrive in the right area.

“The aim is to explore the possibility of planting a higher diversity of tree species and planting the suitable native species, in order to improve the resilience of urban forests and in a way to promote the vegetation diversity.”

Hong Kong was hit by a super typhoon last year that destroyed more than 60,000 trees. The Development Bureau has replanted specific species of trees according to the guide. At Tseung Kwan O’s seaside, the Fox Tail Palm was planted.

Mr Cheung said the palm tree is very suitable for street planting, resistant to pests and disease and has a high wind tolerance.

Taking into consideration the environment of each street is the most crucial aspect of selecting trees, he added.

The guide also includes a data sheet that breaks down each tree species and their resilience to various urban planting environments, distinctive characteristics, species features, and a lot more.