Is there a better way to manage Hong Kong’s iconic, centuries-old stone wall trees?

Amid public safety concerns, officials are carrying out studies to find a more quantitative basis for managing the city’s more than 300 culturally sensitive trees  

PUBLISHED : Sunday, 29 April, 2018, 6:00pm
UPDATED : Monday, 30 April, 2018, 8:00am

Landscaping officials are studying the safety and structure of Hong Kong’s iconic stone wall trees in the hopes of finding a new way to manage them.

Typically banyans and many centuries old, the trees are common across the city and often get seeded in the gaps of old masonry walls by birds or bats. As they grow, their roots extend across or through the wall surface to secure their footing and absorb water and nutrients.

Under current Development Bureau guidelines devised in 2013, the trees are considered natural cultural assets requiring “special preservation measures”.

But Deborah Kuh, who has headed the bureau’s Greening, Landscape and Tree Management section since 2015, believed the measures should be reviewed amid growing public safety issues as well as climate change and more extreme weather patterns.

Kuh described the matter as “tricky” because much was unknown about “the relationship between trees and walls, and trees on walls on slopes”.

[The trees] grow on walls that were never really designed to support them
Deborah Kuh, Development Bureau

“[The trees] grow on walls that were never really designed to support them,” she said. “This magnifies our risk, but in the eyes of the public, these are valued assets. How do we start to balance their priorities and our priorities of managing public safety?”

In other parts of the world, stone wall trees were considered weeds, Kuh added. “They are an aggressive and invasive species, predominantly the ficus microcarpa banyan tree, and are grown by wild seeding.”

Yet the bureau official conceded that discussing safety or removal would be controversial, not least because many in the community harbour strong cultural attachments to the trees. In 2015, the Highways Department’s hasty removal of four century-old stone wall trees along Bonham Road caused a public uproar.

The decision came after two people were injured following a tree at the site crashing down during a rainstorm three weeks earlier.

A starting point for discussion could be how to redesign walls and improve landscapes and soil quality to accommodate such trees, Kuh said. The trees pose special dangers as they mostly grow alongside pavements and roads in the city, with typhoons or heavy rain capable of uprooting or peeling them off.

The Post understands 15 stone wall trees have been taken down over the past two years due to safety reasons and at least five have failed on their own. This brings the total number managed by the government to 319, as of last month. Most trees are located in Central and Western District on Hong Kong Island.

At least 32 are deemed “old and valuable” due to their rarity, cultural significance, or exceptional size or form. The bureau has designated two – one on High Street, the other on Caine Road – as afflicted with brown root rot, also known as tree cancer.

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Officials are commissioning a number of geotechnical and urban arboricultural studies to examine stone wall tree root anchorage and stability. They seek a more “quantitative basis” for assessing the trees’ interactions with stone walls.

“When you have quantitative data and scientific basis, it’s a lot more convincing than subjective analysis,” Kuh said.

A spokesman added the bureau was also “exploring smart sensor technologies to detect potential displacement” of tree trunks and identify risks earlier.

Chan Yun-cheung, the government’s former head of geotechnical engineering and a member of its Urban Forestry Advisory Panel, is conducting a study to set parameters for assessing root-wall interactions. 

From looking at the wide, interwoven layers of roots that envelop the wall, for example, one would be able to assess the tree’s strength and rigidity, he said.

The office is also consulting the panel, comprising local and overseas experts, on how to enhance stonewall tree management for further follow-up action. “[This] may include updating the Guidelines of Stonewall Trees,” the bureau spokesman said.

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Professor Jim Chi-yung, who studies trees at the University of Hong Kong and advocates their preservation, said studies were the right direction. But he hoped officials would not carry them out presuming trees were dangerous and then use the findings to justify removing more of them.

“This would be unscientific and irresponsible,” he said. “They are unique to Hong Kong and indeed a cultural asset. The failure rate of stone wall trees is actually lower than that of ground trees.”

Arborist and panel member Kevin Eckert believed that tree removal should be considered a last resort. “If it’s dead or dying and is not going to recover and is a high safety risk, take it down,” he said at a recent panel meeting.