Cheng works as an engineer, while freelancing as an arborist in his spare time.

He is one of 566 people in Hong Kong who are qualified arborists, having obtained his certificate from the International Society of Arboriculture in the United States.

"What does an arborist do?" Video by Hedy Bok

An arborist's work is important because while trees offer shade and greenery, a sick one is unstable and can be dangerous.

This was tragically demonstrated in August 2008, when a fungus-infested coral tree collapsed in Stanley, killing a 19-year-old woman and exposing government failings in assessing tree risks.

In response, the Development Bureau set up a Tree Management Office that now employs eight arborists.

A spokeswoman said their responsibilities include tree inspections, risk assessment, maintaining records and handling complaints and enquiries.

But tree management and maintenance works are carried out by various government departments, which altogether employ more than 180 people with arboricultural qualifications in roles such as forestry officers, landscape architects and leisure service managers.

While the government oversees trees on the street or in public parks, Cheng's company Tree Master offers assessment of trees on private land.

His consultations involve advice on pruning, pest control, use of fertiliser and transplanting.

A common way of stabilising a dangerously leaning tree is by securing it with a cable around its trunk.

This technique was applied on a grand scale to the century-old "King Banyan" beside the football pitch in Kowloon Park which was unstable due to brown root rot disease.

The tree is now blocked off from the public and is surrounded by a tall metal frame with cables holding the branches and trunks in place.

"It's amazing that they did something on such a large scale to save this tree," Cheng said.

While the King Banyan appears to be returning to health, the disease is believed to have spread to a neighbouring banyan tree that collapsed in Nathan Road last July, injuring five pedestrians.

A hole in a tree is another symptom of illness, Cheng said.

Such cavities - popular as a home for birds - are caused by an improperly pruned branch that allow bacteria to enter and damage the tree's interior.

"While you may have seen people fill up the tree holes with concrete, it is unhealthy for the tree," Cheng says, advising instead to place a cover over the cavity to prevent water and small animals from getting inside.

While pointing out heritage trees in Kowloon Park that are more than 100 years old, Cheng said trees growing to that age in Hong Kong would become rarer because of construction work.

"A lot of developers do not consider the existence of trees, they just want to remove them and plant new ones elsewhere to make up for it," he said.

It's also not ideal for a tree to grow in an urban area.

"When a tree is growing on a concrete pavement, its fallen leaves cannot be turned into a nutritious fertiliser, and its roots may be damaged by construction work," he said.

While heritage trees are protected under the government's old and valuable trees registry, they are not immune to construction damage.

Jim Chi-yung, a geography professor at University of Hong Kong and a member of the government's expert panel on tree management, studied 50 heritage trees lost between 1993 and 2003 and found that they died mainly due to roadwork and construction activities damaging their roots and disturbing the soil.

Romantic couples and hikers can also be culprits.

Cheng advises couples to think twice before perpetuating their love by etching their names on a tree trunk, complete with a heart pierced by an arrow. "The etchings may go through the tree skin and into the trunk, exposing the tree to bacteria," he says.

As for hikers, Cheng advises against snapping branches off to use as walking sticks because the exposed bark and damage to the trunk may also introduce bacteria into the tree.