In the midst of Hong Kong's high-rises and concrete sprawl, where space for greenery is at a premium, there is a man who has spent the past 40-plus years preserving and creating pockets of nature in the most unexpected places.

Known affectionately as "Dr Tree", Jim Chi-yung stands out as a champion of Hong Kong's green heritage. His pioneering work in urban forestry has helped transform the city's skyline into a greener, more sustainable space, offering a model for other cities grappling with the environmental challenges of rapid urbanization.

"People often think of my field as purely technical, but urban forestry is an interdisciplinary science," says Jim, a research chair professor and adviser of Geography and Environmental Science at the Education University of Hong Kong, and a former head of the Department of Geography at the University of Hong Kong. "It demands that we balance conservation with the many competing needs of urban development."

In September, a ranking by Standford University named Jim the world's leading forestry scientist, out of more than 35,000 researchers, who not only retained his top spot in the "Single-Year Impact List", but also topped the "Lifetime Impact List".

On the rooftop of CLP Power Hong Kong Limited (CLP)'s Chui Ling Road substation in Tseung Kwan O lies Hong Kong's largest "sky woodland". The rooftop brims with native trees, its walls draped with climbing plants, creating a strikingly verdant contrast amid the cityscape.

A walk through this green canopy feels like entering a natural forest, a haven for urban wildlife in the heart of a bustling city — a brainchild of Jim's decades of research initially on soil science and later on arboriculture and urban ecology.

Jim's efforts are particularly evident in his preservation of "champion trees" — mature, often ancient specimens that form the backbone of Hong Kong's green heritage. Many of these trees, such as the century-old camphor trees on Haiphong Road, Tsim Sha Tsui, were threatened amid urban expansion and typhoons. Fortunately, Jim came to their rescue with his expertise.

"Champion trees are vital to Hong Kong's natural and cultural identity, and cannot be left unattended," Jim says. His advocacy also led to the creation of a government registry for ancient trees.