From concrete to canopy: the quest for a greener Hong Kong —— A conversation with the world's leading expert in forestry research

Professor Jim Chi-yung, affectionately known as “Dr Tree”, is recognised internationally as a leading researcher, educator, and advocate for understanding, conserving, creating, and refining nature in cities. He was ranked first in the world in forestry research on both the “Single-Year” and the “Career-Long” impact lists of the World’s Top 2% Most-Cited Scientists 2024, repeating his same achievements in 2023. Stanford University compiled these lists.

Professor Jim holds the position of Research Chair Professor and Advisor of Geography and Environmental Science at the Department of Social Sciences and Policy Studies. He has published over 450 scientific papers and books and has accumulated more than 43 years of teaching and research experience in both conceptual and applied domains. He consequently stands out among over 35,000 forestry scientists worldwide. His focus is on how to conserve trees in a high-density urban environment.

Professor Jim not only pioneers research on stonewall trees in Hong Kong but is also the most active scholar in rooftop and vertical greening. He has earned the additional nickname of “Founding Father of Rooftop Greenery” for his role in researching and advocating for rooftop greenery. In this issue of FLASS FORWARD, the veteran professor discusses the city’s achievements and challenges in tree and urban conservation work, what Hong Kong should do to enhance its urban greenery further, and what motivates him to devote his life to research on tree, urban greening, and urban nature conservation.

A1: The greatest hindrance to tree conservation is land scarcity. The Hong Kong Government’s land use policy prioritises constructing buildings, roads, and other grey infrastructures. Due to this land use preference, very limited space has been allocated for pedestrian and greening usage, particularly noticeable in old districts. Consequently, there is insufficient space to grow trees and other vegetation in urban areas. 

Another problem Hong Kong faces in urban tree planting is the poor quality and insufficient volume of its soil. Generally, Hong Kong’s urban soil is excessively stony and sandy. It often lacks nutrients and has low water retention capacity. The common occurrence of underground utilities further hinders tree root growth. Beneath the dense road networks in Hong Kong lie a dense network of subterranean pipes, cables and related installations, making it difficult for trees to have sufficient soil volume to expand their roots.

Most developments in Hong Kong are constructed on reclaimed land. Construction waste, including rubble and debris from demolished buildings, is frequently mixed with soil and other materials to fill water bodies and create new land. While this practice helps manage construction waste, it can lead to exceptionally poor soil conditions for vegetation growth. Because of these acute challenges, many urban trees are in poor condition; some are even in terrible shape.

There is also a cultural dimension to the tree deficit. Many people in Hong Kong prefer covering exposed soil with concrete to preserving the original soil surface. Many owners of standalone houses and low-rise blocks in low-density residential areas have removed trees from their yards. Because of this, the original Kowloon Tong garden city with an extensive tree cover has been stripped of its charming sylvan endowment and become rather barren, like high-density neighbourhoods. In public parks across Hong Kong, concrete-covered football fields and basketball courts occupy a large portion of space. Without soil, how can we grow large trees? In contrast, parks in Singapore appear much greener as they strive to preserve the original soil, natural fauna and flora, and landscape.   

The thriving greenery in Hong Kong's country parks demonstrates that the city’s humid-subtropical climate is highly amenable to plant growth. Our rainfall, warmth and sunshine can support the growth of high-quality trees that provide ecological and aesthetic benefits. However, in decades of relentless urbanisation, urban infilling and intensification, tree conservation has been sacrificed for the overarching priority of economic development. While typhoons have uprooted some of our old trees, many others have fallen victim to inadequate tree-planting methods and management.

A2: Leadership is fundamental in urban conservation. Singapore’s founding father Lee Kuan Yew was visionary in many aspects. Since founding the city-state in 1965, Lee has cherished a vision to transform Singapore into a green city characterised by abundant and meritorious greenery and a clean environment. Lee was clear that a liveable city could not consist solely of grey infrastructure; it must also be accompanied by and intimately blended with green infrastructure. The “green city” vision sought to enhance the quality of life for Singaporeans and make the city an attractive destination for global tourists, foreign investments and talents. 

People travelling to the city centre via the East Coast Parkway (ECP) after landing at Changi Airport are often mesmerised by the stunning treescape of the highway and the adjacent East Coast Park. The huge rain trees (Albizia saman) along the ECP form a magnificent tree-lined boulevard, leaving an indelible first impression on visitors about Singapore. During the planning stage, authorities allocated ample space for planting tall trees, shrubs and flowers on both sides of the airport expressway as well as in the median divider. The ECP was designed not only for transport functions but also to provide visitors with an aesthetically pleasing experience.

The ECP is just one example of Singapore’s government's determination to achieve its high-calibre green vision. As visitors stroll through Singapore, they can easily find urban locations adorned with tall and luxuriant plants. While roadside trees in Hong Kong compete for limited space to grow, new construction sites in Singapore—made available when old buildings are redeveloped—are required to set back the building footprint to spare extra ground-level space for pedestrian usage, and extensive vegetation.  

In early 2021, Singapore announced an ambitious plan to develop Tengah in the western part of the city-state into an eco-smart city. Spanning a total area of 700 hectares, Tengah will resemble a “forest town” with automated trash collection, pedestrian-friendly zones, and extensive green spaces. Meanwhile, the city-state initiated the OneMillionTrees movement in 2020 to grow an additional million trees across Singapore by 2030, increasing the total number of trees to more than eight million. All these plans reflect its determination to morph from a garden city to a city in the garden.

A3: Route 3, running from Sai Ying Pun on Hong Kong Island to Yuen Long in the New Territories, is an engineering feat. It comprises a series of expressways, connecting roads, bridges, and cross-harbour and mountain tunnels. Completing such a complex engineering project within a short time is undoubtedly a remarkable achievement.

However, this achievement is less impressive in terms of greenery. A large portion of Route 3 appears grey due to a lack of roadside and median vegetation. When construction work on the Yuen Long-Ting Kau section of Route 3 was nearly completed, the private company responsible for constructing and operating this section belatedly considered improving the green appearance. Unfortunately, little could be done as the road design did not include enough space for extensive soft landscaping. This case reflects a lack of awareness in the commercial sector regarding urban greening.

Another example is the conservation of champion or heritage trees, notable for their size, age, ecological value, and cultural heritage. In the early 1990s, I initiated a study on champion trees in Hong Kong that led to the government's subsequent establishment of a registry for old trees. To acquire more resources for tree preservation, I suggested that the government could solicit help from the commercial sector, but this proposition was not materialised.

My studies inspired Singapore’s government to document its champion trees as well. Furthermore, Singapore’s government invited corporations to sponsor the protection of champion trees. Thanks to financial support from the private sector, Singapore has achieved more in champion tree protection than Hong Kong despite starting later.

These examples illustrate that the commercial sector can significantly foster urban nature conservation. Private companies conduct most building developments in Hong Kong. If they adopt a more positive green mindset, they could incorporate many more high-quality green components into their projects. Given their financial resources, knowledge, influence on consumer mentality, and capacity for innovation, commercial entities have ample opportunity to support and synergise with government-led green initiatives.

A4: To make our city greener, we need visionary government leadership. A clear vision and strong determination will create consensus among various stakeholders in society, including businesses, professionals, academics, and residents, towards achieving this common goal. It is also advisable to form partnerships among stakeholders in conservation projects such as champion tree protection and enhancement, highway greenery initiatives, rooftop and vertical greening, and so forth. Partnerships can leverage additional financial resources from the private sector while garnering community support and innovative ideas from academia and professionals.

We need to triple our efforts to narrow the gap with other forerunners in urban conservation; there is much we can do to catch up. First, we must pay more attention to every detail in our greening efforts. As urban space is scarce in Hong Kong, we must ensure that high-quality soil and planting materials are used. We should replace substandard soil with nutrient-rich alternatives; trees will never thrive without good soil.

Second, we should cultivate more local tree species. For instance, we can grow Scarlet Sterculia (Sterculia lanceolata, 假蘋婆), Rhodoleia (Rhodoleia championii, 紅花荷), Callery Pear (Pyrus calleryana, 豆梨), Tutcheria (Pyrenaria spectabilis, 石筆木), South China Rosewood (Dalbergia assamica, 南嶺黃檀), Hong Kong Magnolia (Magnolia championii, 香港木蘭), Manglietia (Manglietia fordiana, 木蓮), among others—all of which produce beautiful flowers. Native plants provide essential habitats and food sources for local wildlife, including birds and insects. These species have co-evolved with the plants over many millennia, serving as natural pollinators that enhance overall ecosystem health.

The urban greenery of Japan always amazes foreign tourists. They are awed by the Zen gardens, urban woodlands embedded in built-up areas, lush pedestrian walkways, and rows of cherry blossom trees lining city parks, roadsides and river banks. The widely practised formative pruning contributes significantly to these spectacles. Japanese arborists and gardeners invest considerable effort into cultivating tree saplings; at nurseries, they meticulously prune seedlings to direct their future growth before transplanting them to final planting sites. Most importantly, Japanese horticulturists take pride in their work. If we wish to catch up with other places regarding urban greenery efforts, we must take pride in our greening initiatives.

A5: Grass and shrubs are the main vegetation used in traditional rooftop greening. I have introduced native trees to create self-sustaining and biodiverse spaces, advising on dozens of such projects in Hong Kong. One notable project is CLP Power’s Chui Ling Road substation in Tseung Kwan O, where the station’s rooftop garden is lush with interlocking native trees, and its walls are adorned with climbing plants, making it the largest “sky woodland” in Hong Kong.

Regarding vertical greenery, many shopping malls in Hong Kong utilise potted plants to decorate their walls. However, this approach incurs high maintenance costs. Without proper management, potted plants can easily wither and die. Nature offers a wealth of climbing plants, also known as climbers—species that climb tree stems, rocky cliffs and large boulders. They thrive well on the abundant vertical surfaces in cities, including building and free-standing walls. Instead of placing numerous potted plants on walls, why not plant climbers at wall-toe level and allow them to grow upwards to cover the walls? As their root systems are accommodated in the soil on the ground level, gardeners can conveniently water and fertilise them just as they do for other ground-growing plants.

A detailed study on climbers I completed for the Drainage Services Department found that more than twenty species are suitable for growth in Hong Kong. Climbers such as Wisteria (Wisteria sinensis, 紫藤), Honeysuckle (Lonicera japonica, 金銀花), and Rangoon Creeper (Quisqualis indica, 使君子) are woody, perennial, evergreen and flowering climber species. Some climbers can grow up to 10 metres tall within two years, offering attractive natural covers for otherwise dull concrete walls and fences.

However, despite how quickly climbers can grow, it takes time to cover a wall fully. Unfortunately, many Hong Kong people embrace a fast-food culture and do not have the patience to wait a few years to achieve pleasant natural shields. The commercial sector often desires to install a vertical green wall overnight, leading to a common preference for greening walls with potted plants. We must realize that urban greening, whether embellishing our roads, parks, rooftops or walls, cannot be rushed. It requires years of patience and care before we can enjoy the benefits.

A6: Every autumn, many Hong Kong residents travel to mainland China, Japan and South Korea to admire the fall foliage or flock to Tai Tong to appreciate the Sweet Gum (Liquidambar formosana, 楓香) trees to appreciate the colourful display of red leaves. Unfortunately, they have to travel abroad or squeeze into a tiny area in Tai Tong to enjoy nature’s magnificent autumn colour; this should not be the case.

Local species such as Chinese Tallow (Sapium sebiferum, 烏桕), Common Elaeocarpus (Elaeocarpus decipiens, 杜英), South China Honeylocust (Gleditsia fera, 華南皂莢), Tutcher's Maple (Acer tutcheri, 嶺南槭), also exhibit stunning red foliage. Exotic species such as Japanese Maple (Acer palmatum, 雞爪槭) and Crape Myrtle (Lagerstroemia speciosa, 大花紫薇) (Terminalia catappa, 欖仁) can be planted to provide autumn red foliage too. Chinese Tallow tree with a graceful tree form and leaves has particularly high ornamental value. Its leaves are green during summer but undergo dramatic colour changes in autumn, turning yellow, orange, red, and even purple. These trees can grow tall and broad and are easy to cultivate, making them excellent candidates to enrich the seasonal colour changes in our urban landscape.

Another underrepresented local species is Rhodoleia (Rhodoleia championii, 紅花荷,  紅苞木 or 吊鐘王). When in full bloom, its clusters of dangling flowers rival the beauty of cherry blossoms. Often referred to as “Hong Kong Rose” by foreigners, it is a favourite among United States and Australian gardeners. Different species and varieties of Azaleas (杜鵑花) also thrive well in Hong Kong. Some years ago, my friends and I proposed to the government to cultivate various species and cultivars of Azaleas on a hill in Tung Chung. If adopted, this idea could provide the impetus to transform Hong Kong into a city of Azaleas, significantly boosting its image to residents and tourists.

Some exceptionally attractive exotic flowering tree species can flourish in Hong Kong. In recent years, many residents have taken a liking to the Golden Trumpet Tree (Tabebuia chrysotricha and Tabebuia chrysantha, 黃花風鈴木). Other members of the genus with equally charming blooms include Rosy Trumpet Tree (Tabebuia rosea,  紅花風鈴木) and Purple Trumpet Tree (Tabebuia impetiginosa, 紫花風鈴木), and White Trumpet Tree (Tabebuia roseo-alba, 白花風鈴木). As spring arrives, when these deciduous trees have not sent out new leaves,  masses of flowers begin to emerge. The charming and dense clusters of blooms present spectacular yellow, red, purple or white hues to adorn our urban landscape. As Hong Kong’s climate and soil are suitable for growing trumpet trees, we can plant more.

Every early summer, the Cassia javanica var. indochenensis (節果決明) at Murray Hotel in Central bursts into clusters of light pink and white flowers that attract eager visitors to take photos. Species like Flame Bottletree (Brachychiton acerifolius, 槭葉蘋婆), Flame of the Forest (Delonix regia, 鳳凰木), and Rain Tree (Albizia saman, 雨豆樹), have majestic tree form and outstanding flowers to present a marvellous tropical garden scenery. They are suitable for planting in parks, gardens, and broad roadside planting strips. Unfortunately, these trees have not been widely planted in Hong Kong.  

A7: I have pioneered several blue sky research projects, including studies on stonewall trees, and rooftop and vertical greenery in Hong Kong. Stonewall trees grow on the vertical surfaces of old stone retaining walls, originally constructed to stabilise the cut slopes formed during urban development in Hong Kong’s hilly topography. These trees take root in the soil held behind the masonry structure to obtain water, nutrients and anchorage to thrive.

When I first ventured into this research field, neither the Hong Kong people nor the government acknowledged its significance. Some years after my pioneering work in this field, researchers in other places began studying woody plants on stonewalls. When overseas scholars visited me in Hong Kong, I often took them to see our fabulous stonewall trees. They were amazed that Hong Kong had such a unique, world-class, and spontaneous urban ecological gem.

As an academic, I constantly remind myself that I need to be at the forefront of knowledge, a place where being lonely should be a part of my work. It has been thirty years since I conducted research on stonewall trees. Over the decades, I have witnessed more people becoming interested in this natural-cum-cultural wonder of our city. Hong Kong residents are becoming more appreciative of my research and conservation efforts; they now see stonewall trees as part of the community’s collective memories and integral to the city’s historical heritage. This shift in attitude has brought me satisfaction and happiness. I am particularly gratified to see the connection between the town and the gown.

Of course, I felt disappointed when authorities did not consider my research and advice regarding tree and urban nature conservation. However, academics should not be short-sighted; we must always contemplate our research impact from a global perspective. I often meet people at international academic conferences who are eager to understand how my research can help greening efforts in their cities and countries. I am glad my research findings are noticed, adopted and applied in other countries.  

I believe I have diligently discovered, managed, and shared knowledge. The discovery of new insights and sharing them with others brings me great satisfaction. I will not feel lonely in the extensive international scholarship and professional practice sphere as long as I can continue my research.

A8: Imagine an urban landscape where the sides and medians of highways, parks, public spaces around buildings, and atriums of shopping malls are transformed into vibrant green belts and enclaves. Picture roundabouts adorned with flowering trees, pedestrian walkways lined with lush shrubs and colourful plants, and promenades that invite leisurely strolls amidst blooming flowers and expansive grasslands. Envision serene ponds and other blue features integrated into these spaces. This green urban landscape provides habitats not only for wildlife but for human beings as well. It makes a city liveable and sustainable.

Lush environments appeal to us because of our innate connection to the natural world. Trees, plants, and flowers create habitats for birds, insects, and other wildlife, connecting humankind with Mother Nature. If our parks spare an expansive area for forest landscape, like parks in Paris, London, Berlin and Tokyo, Hong Kong people will have high-calibre green refuges to retreat from the hectic life and restore their minds and body in the natural ambiance. People feel an indescribable satisfaction and joy when strolling under trees in the company of charming blossoms.

In addition to providing comfortable shade and visually appealing landscapes, greenery offers numerous benefits. Besides a purely economic perspective, a green space encourages people to do exercises embraced by natural companions, such as stretching or strolling, which can enhance  physical and mental well-being. Improved health translates to lower medical expenditures and increased productivity. The multiple benefits of urban greening can be translated into monetary terms to convince decision-makers to invest in more and better greeneries.

Research indicates that exposing children to green environments can stimulate their intellectual abilities, improve attention spans, and reduce aggressive behaviour. In the long run, they can achieve more academically and become a more capable and productive workforce. This, in turn, is beneficial to the economy.

A green environment can also aid in restoring health. Medical studies have shown how hospital settings affect patient recovery; patients recovering from surgery tend to heal faster when they can see leafy trees and plants through the window from their beds. They report less pain and experience fewer complications compared to those who view only buildings outside their rooms.

Furthermore, a green city is more sustainable. Trees and plants absorb carbon dioxide and air pollutants while releasing fresh oxygen, improving air quality in urban areas. Planting more trees and plants can can cool down the environment, reducing electricity consumption, decreasing greenhouse gas emissions, and lessening its negative environmental impacts.