Street Trees In High-Density Urban Hong Kong

Palms and conifers

The few palms include the common Phoenix roebelenii and Archontophoenix alexandrae, and the less common Livistona chinensis, Roystonea regia and Phoenix hanceana. The subtropical climate of Hong Kong is well suited for palm growth, but only P. roebelenii and A. alexandrae are widely used. Because the final stature of the former rarely exceeds 5 m, it fits cramped roadsides. For lack of local nursery space, most of the palms come from mainland China or Taiwan. The slow-growing conifers, including Juniperus chinensis, Araucaria excelsa, Podocarpus macrophyllus and Thuja orientalis are uncommon. All had been on private property and became pavement trees after road realignments.

Native and voluntary species

Street tree diversity is governed by natural and cultural factors.

The indigenous tropical woodlands have a high inherent diversity. Urban encroachment into the woodlands at the city fringe could leave a few relic trees which happen to fall on future sidewalks. Most rare native species are in sloping areas formerly covered by woodlands that are believed to have been left. The notable examples are the weed trees Litsea glutinosa, Macaranga tanarius, Schefflera octophylla which are commonly found in local woodlands. Other voluntary species include Celtis sinensis, Broussonetia papyrifera, Ailanthus fordii, Hibiscus tiliaceus, Ficus variegata and Bischofia trifoliata which are sparsely cultivated. Many of the woodlands species cannot survive the drastic changes in habitat conditions associated with urbanization. The successful species provide a potential pool from which candidates can be selected for street planting to enrich diversity. A systematic evaluation (2, 3, 4, 13) of these rare native trees is worthwhile.

Exotic garden species

Some of the exotic species were previously cultivated in private grounds where tree selection followed the preferences of the property-owners or managers. Site redevelopment and the associated lotboundary readjustments and road-widening exercises could transpose these private trees into the public roadside space. Some prominent examples are Mangifera indlca, Euphoria longan, Syzygium samarangense, and Litchi chinensis (fruit trees), also Araucaria excelsa, Podocarpus macrophyllus and Morus alba. None of these species was planted by the government at roadside sites. Some of these popular south China garden species prove to be rather suitable by virtue of their tree form and physiological tolerance to street planting. More widespread planting of, for example, M. indica, E. longan, and S. samarangense should be encouraged.

Favoured species

The introduced Phoenix roebelenii and Cassia surattensis, both with low stature and narrow crown spread appropriate to street use, are widely planted recently. Their visual impacts are more important than their ameliorative effects on the harsh urban environment. Although Aleurites moluccana is the most abundant street tree in Hong Kong, in our study area other species are more common. Its ease of propagation, rapid growth rate, vigor in resprouting, dense foliage, compact crown, and quick greening effect make it popular with the city‘s tree managers, but its soft wood is prone to typhoon and pest damage. Bombax malabaricum, another common fast-growing tree, is poor for street planting, especially at exposed sites, because of brittle branches. Bauhinia blakeana, a common street tree selected as the “city tree” of Hong Kong mainly for its beautiful flowers, has straggly growth and weak branches. Melaleuca leucadendron, a sturdy tree whose columnar form is suitable for streets is gaining in popularity.

Out-of-favour species

Ficus microcarpa, a large, beautiful, hardy, long-living, evergreen indigenous species has fallen out of favour. It was planted exclusively from the 1840s, when the city was founded by the British, until the 1930s. It becomes too big for the high-density modern Hong Kong. Its wide crown and its aggressive searching roots render it suitable for street planting only in ample roadside spaces. Many large specimens were sacrificed as the city expanded. Most of these trees have reached full size, and many show signs of decline. Some survivors are stranded incongruously in narrow streets, abutting adjacent buildings. Excessive pruning or lopping often were carried out to prevent obstruction to traffic or building redevelopment (Fig. 1). These maltreatments, plus senescence, leave few good trees of this species.

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Figure 1

This Ficus microcarpa (Chinese Banyan) had been lopped so excessively that the effort to keep it is rendered meaningless.

This long-lived species once gave a unique character to the city. The tree is revered, even worshipped, by many traditionally minded old people. Felling an old F. microcarpa, however diseased or dangerous, evoked strong sentiments, but few young ones were planted. Its normal lifespan (several hundred years) is much longer than the city‘s regeneration cycle. Hence trees planted in the past, that suited the site then, became out of place as the city evolved. Because it would be unthinkable to have no F. microcarpa along Hong Kong streets, adequate space should be opened to plant this species.

In addition to F. microcarpa, species popular in the past that now are rarely planted include Ce/tis sinensis (due to large size), Delonix regia (due to large size, tendency to damage pavement, and brittle branches), Acacia confusa (due to straggly habit, low branches, and aggressive roots), and Crateva religiosa (no apparent reason)—a beautiful tree relatively free from troubles.

There is a recent tendency to choose introduced, small trees. Excessive reliance on a handful of well-tried species can increase the hazards of any epidemic. For lack of research on the suitability of tropical species for street planting, the use of any new species is only trial and error. Attention should be paid to maintenance costs.

Limited growth space

Lack of space for street trees limits urban planting in Hong Kong. High-density, high-rise buildings stretch from city center to fringe. Most sites have a 1 00% building-and-pavement coverage. This drab environment pervades the city, except a few residential neighbourhoods. Lack of growth space is acute in the older part planned when controls were lax and when standards were low. New developments have more planting spaces.

Most pavements are too narrow (less than 2 m) for street planting. The common practice of building above them prevents planting in much of the city. Thus most of the trees are along a few of the roads. Of the street trees, 60% are separated from their nearest neighbour by less than 4 m, and 84% by less than 8 m (Fig. 2a). In all, less than 5% of the trees are solitary (no neighbour tree within 16m). Most such isolated trees are mature or old, private garden relics or remnants. Because of narrow sidewalks, 75% of the trees are less than 4 m from the curb (Fig. 2b). With little onstreet parking, fast moving vehicles, including double-decker buses, graze the tree crowns. Although most sidewalks with trees are wider than 2 m, 16% of the trees are found in sidewalks narrower than 1 m or where no sidewalk is present (Fig. 2c). The Highways Department normally does not allow planting where sidewalks are less than 2 m wide, for sake of road safety and pedestrian flow.

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Figure 2

The frequency of trees by the distance to the nearest tree, the distance to curb, and the width of pavements with trees.

Types of growth space

Although nine types of street growth space are common in Hong Kong, about half of the trees are found in one of the three types of planting strips (Fig. 3). Most of these strips are narrower than 2 m but they are, by local standards, very generous. Most of these strips are along the few roads through low density areas, or new roads where space was reserved, by new planning standards, for amenity planting. Very few roads have more than one line of trees. About 16% of the trees are reared in pits, most of which are undersized and not equipped with grilles.

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Figure 3

The frequency of trees by nine types of growth space.

Concrete was paved all the way to the trunk of 7.5% of the trees, leaving no soil surface for water and air exchange. This negligent failure to open tree pits or to use grilles is baffling. The remaining trees grow in stony and shallow soils, or in concrete planters which are getting popular where there are underground restrictions mainly due to the masses of buried services (including electricity, telephone, traffic control, water, gas, and sewers).

Figure 4 shows the trees‘ growth positions. Most are on property or curb side of the sidewalk. But some straddle the sidewalk obstructing pedestrians due to road alignments (Fig. 5). About 13% are in the few recently built central dividers.

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Figure 4

The frequency of trees by seven types of growth position.

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Figure 5

This Ficus microcarpa (Chinese Banyan) has been severely deformed and left trapped on a narrow sidewalk without a tree pit as a result of building redevelopment.

Results on proximal landuse show that government and institutional is the most frequent. Other landuses (including commercial, residential low and medium densities, open space, and transpor-tational) have roughly equal shares, except high-density residential areas which lack growth space, and housing estates, where most trees are away from the roads.

Tree structure

Tree sizes are given in Figure 6. Half the trees are less than 4 m tall, since many were planted in the past decade. Two recently planted species, Phoenix roebelenii and Cassia surattensis, have small final size. Very few trees exceed 12 m height, mostly Ficus microcarpa but some Celtis sinensis and Aleurites moluccana. The dbh has a similar pattern: two-thirds are less than 20 cm (Fig. 6b). And 62% have crowns narrower than 4 m (Fig. 6c). The small crowns result also from space limitations along streets. Often, large trees are heavily pruned to fit.

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Figure 6

The frequency of trees by three structural attributes of height, dbh, and average crown diameter.

Large street trees are rare due to senescence, diseases and urban redevelopments. The new trees, small to medium in final size, cannot replace them. Thus the sylvan character of the city declines. Since most of the plantable space is now used up, future planting must be in new developments or renewed districts.

Foliage, trunk and branch problems

Of the 930 trees studied, 611 had defects and disorders, an average of 1.8 items per tree. Hong Kong typifies the wide range of tree problems (Table 2) of any high-density environment (1, 20). Over half the trees showed foliage symptoms (9, 21): discoloration, spots, wilting, insect feeding, and defoliation, often quite acute. Without diagnosis, hostile environment (14, 21) and injuries by people were assumed to open them to attacks by pests and pathogens (11, 12) which led to decline and dieback. The fact that no systematic pathological study of street trees has ever been attempted in Hong Kong hinders proper tree care.

The trees urgently need expert attention: nearly 40% have structural defects. Some have forms unsuitable for streets (curved or crooked trunk, low branching habit, heavy branches, close branching intervals, V-shaped crotches, or straggling branching). Most of these problems were exacerbated with age, without shaping in the nurseries or later (8, 9, 10). The trees with split trunk and branches, plus those with large cavities, wounds and broken branches, are hazards.

Less serious, but not minor, defects include avoidable wounds from construction (16, 19). Canker and wood-decay pathogens enter the uncared-for wounds. Some trunks are willfully damaged by nails and other sharp implements.

Root problems

Slightly over 10% of the trees have exposed or girdling roots often with cracked or heaved sidewalks. Such trees have aggressive, thick roots (6, 17) and are in undersized tree pits or strips, or compact soil (7, 8, 22, 23). Unfortunately, some of these species are common and a few are still actively used: Acacia confusa, Aleurites moluccana, Bombax malabaricum, Crateva religiosa, and Delonix regia. The problems arise when nursery stock grows in small containers. The roots should be spread out during transplanting. Excavations and grade changes also damage tree roots.

Physical restrictions

Above-ground space is very restricted for 7% of the trees. Physical obstacles include buildings (especially with awnings), vehicles, signs and lamp-posts (Fig. 1 and 5). The trees‘ crowns are incomplete. Wrong species, mis-siting, typhoon damage, construction, and general decline and dieback also cause this. Building or road development often has priority over trees‘ crowns (Fig. 1 and 5). Selection and siting cannot preclude landuse change.

Tree care

Most trees have low to medium vigor, and many have acute untreated problems. Without prompt care, minor problems become major and irredeemable, because of shortage of skilled arborists. The tree staff has hardly any time for remedial or preventive work. There is no inventory of trees with problems and no regular schedule of tree inspection. This lack makes planning tree work almost impossible. Heavy investments in tree planting waste away, leaving eyesores, management liabilities, and hazards. Eventually, typhoons purge the weak and diseased trees.