Measures to safeguard three rare 'wall trees' in Kennedy Town affected by a railway project should become a benchmark for similar structures across the city, a specialist said. MTR Corp contractors are monitoring the stone walls in Forbes Street and the old trees growing on them, as construction of the West Island Line is under way nearby. Professor Jim Chi-yung, of the University of Hong Kong, who devised a tree protection system for the MTR Corp, said the wall trees were 'world-class urban living heritage'. 'The wall dates back more than 140 years. Nowhere else can you see such large and old stone walls with so many magnificent trees in such a clustered pattern,' Jim said. The site of the stone walls was earmarked for the new line's Kennedy Town station, but green groups and district councillors campaigned against the plan in 2005. The MTR Corp then decided to move the station eastward and southward to a public swimming pool, and relocate the pool at a cost HK$600 million. The walls are made of volcanic rock and support 22 Chinese Banyans and five Japanese superb figs, four of them listed as 'old and valuable trees'. Jim said the trees were protected by three measures that went beyond government requirements. A tree protection zone prohibits excavation or construction work within four metres of the front of the wall and eight metres behind; government guidelines normally require just a two-metre rear buffer. A protective ground shield is laid on the ground of any adjacent works to stop wastewater penetrating the soil. A baseline study was conducted by MTR Corp contractors before work started to record tree features for later comparisons, and monthly visual assessments have been conducted since 2009 to monitor changes. Those checks are supplemented by daily instrumental monitoring to determine more subtle impacts, such as groundwater levels, wall tilt, and soil contamination. All works on the trees were supervised by the Leisure and Cultural Services Department and an independent tree specialist. 'Some of these precautionary, assessment and monitoring measures are Hong Kong's first and they are worthy of adoption as a benchmark for the protection of other wall trees in Hong Kong,' Jim said. Another potential target for the measures would be a wall tree at the Former Police Married Quarters on Hollywood Road, Jim said. The site is set to be converted into a creative industry hub and a wall with six Banyans up to 80 years old.