Better Environment for Better Urban Trees in Hong Kong  

Project Title: Better Environment for Better Urban Trees in Hong Kong

Applicant: University of Hong Kong

Total Approved Grant: $1,286,084 (ECF&WWGF: 50/50)

Duration: 01/10/97 to 31/03/2001

Project Status/Remarks: Completed

Scope: To evaluate the above - and below-ground environmental attributes associated with the growth in the stressful urban habitats, with a view to finding a package of recommended measures to improve our urban tree stock in both quality and quality terms.

Summary of the Findings/Outcomes:
A total of 120 soil samples from sites scattered in a wide range of roadside and greenspace sites in the built-up areas of Hong Kong have been identified for detailed study. Topsoil and subsoil samples were collected to analyze a wide range of physical and chemical properties. Large-scale roadside planting programmes initiated recently required opening many tree pits at pavement locations. The excavation provided access for a comprehensive study of edaphic limitations to tree growth. The results have been analyzed and interpreted in detail, and papers have been written to report on the results with respect to tree growth in the stressful environment of urban Hong Kong. Some interesting and useful data have been obtained with reference to the soil reaction, soil texture, soil structure, soil porosity, soil moisture, soil temperature, soil volume, soil nutrient contents and availability. Arboricultural practitioners have derived a broad spectrum of practical recommendations, which serve to improve urban soil management for better urban-tree performance, for adoption.

The monitoring of soil temperature and moisture variations within a one-year-cycle at 30 roadside-tree pits and 30 landscape-tree planters has been successfully completed. A stratified random sampling selected the subset of planters in a dense built-up district mainly according to planter geometry, and the tree pits. A Time Domain Reflectometry (TDR) miniature moisture probe was lowered into a PVC-access tube, to monitor volumetric soil moisture at 5-cm depth intervals once every two weeks for one year from June 1998 to June 1999. Miniature water-proof temperature loggers were buried at different depths in tree pits and tree planters to monitor soil temperature over the same period. In selected locations, a similar data-collection exercise will continue to understand the relevant regime over a longer period. The large volume of data has been partly analyzed in detail to judge if more monitoring work should be conducted in the future, and if so, whether the mode of data acquisition should be modified. The results have provided for the first time such time series data for a humid-tropical city in relation to tree growth.

Tree growth in urban Hong Kong faces a myriad of constraints due to excessive development density and the ubiquitous soil disturbance. Almost all tree sites have been contaminated by construction rubble, with plenty of cement, concrete, plaster, mortar, brick and asphalt fragments, as well as a host of other artifacts. Only a few sites could inherit some natural soil in the subsoil layers. Most soil sites have very shallow slum, often with less than 70cm depth of usable soil. The soil volume is also laterally confined, being trapped below the narrow sidewalk between the carriageway and the adjacent building. The underground space for roots to explore is extremely limited. The proliferation of underground utilities, boulders, old building foundations and old road beds, imposes rather immutable physical hindrances to root development. There is a need to improve site conditions and to replace the poor site soil, and these requirements could be more rigorously introduced into the local landscape practice. The present approach of accepting whatever materials are left at the planning site is not conductive to healthy tree growth.

The lack of a regime of separation between the planning sites and buried services is a major obstacle to roadside planting in urban areas. The frequent trenching to reach the utility lines is the major cause of root injuries. A guideline to minimize root damages due to road construction work and in general construction activities could provide much relief to a key factor of tree decline in the city. The extremely poor soil structure and elevated bulk density reflect the common occurrence of compaction. The forces responsible for compaction are engineering specification to press the soil so as to meet a specified load-bearing capacity, and mainly in the topsoil due to human-foot trampling and rain-splash packing of soil particles. The lack of organic matter and other aggregating agents does not provide mitigating factors to counter the degradation of soil structure. The addition of soil amendments to improve soil conditions is rarely practiced.

The excessive contents of stones and sand fraction, however, imposed a limit to the extent of particle packing, and hence prevented the bulk density from reaching the high level sometimes detected in urban soils in other cities. The pores in the coarse matrix could not be filled completely by small amount of fine materials, thus allowing avenues for air and water to move rather freely. The coarse texture is therefore a blessing in disguise - whereas it deprives the soil of the needed nutrient supply and cation exchange sites, it helps to maintain an open soil skeleton to ensure adequate aeration and drainage, and to furnish enough and amenable rooms for root growth. The loosened backfill did not engender more porosity for plant-available moisture. The root-ball soil supplied with the transplanted tree is consistently better in terms of porosity and nutrient status. A departure from the deeply ingrained practice of minimal soil amelioration is needed. The study could be expanded in the future to assess different methods of physical and chemical amendments of soils at tree planting sites.

Moisture content follows seasonal rainfall regime with marked variations between sampling periods. Moisture increases with depth in most planters, with indication of bottom ponding especially in wet season. Moisture ranges from near saturation to between wilting point and field capacity. Moisture contents are found to fluctuate widely over short periods and water depletion below the wilting point could occur soon after a rainfall episode. Moisture deficit occurs in the dry season as well as during the intervals between rainfall events in the wet season. The multiple implications on soil water management and planter design for urban trees are explored. Further findings on soil temperature, moisture and structure, on increasingly specialized aspects of the research, are being processed.

Most soils have unusually high pH, going up to as much as 10. The source of alkalinity is the ubiquitous construction rubble that finds its way into almost all urban planting sites. Such a high level of alkalinity could modify the soil chemical environment to restrict the availability of some nutrients for tree growth, especially manganese and iron. As most humid-tropical plants in Hong Kong are adapted to an acidic soil reaction, the unnatural alkaline soil does not augur well for vigorous tree performance. The available contents of nitrogen and phosphorus drop below the low-level thresholds expected of humid-tropical soils, and are likely to act as limiting factors on tree growth. The cation exchange capacity as well as base saturation are both well below the levels of poor soils. The lack of both organic and inorganic colloids restricts the exchange capacity. Some soils have elevated concentrations of heavy metals, indicating pollution of the soil mainly due to the wet and dry deposition of atmospheric particulates at roadside habitats.

Hong Kong's urban areas have pervasive high-density and high-rise development with meager niches for amenity vegetation. Tree planters provide possibilities to insert greenery in locations where above-ground space is adequate but soil volume is restricted. This method of planting has been under-used locally, and could well be more liberally used in both outdoor and indoor locations. The present research findings pinpoint the inadequacies of existing planter practices and suggest directions for alternative planter design, and planter soil composition and management. The choice of suitable tree species for use in planters could also be considered.

In the entire core urban area of Hong Kong, only 372 existing trees of high calibre, labeled as champion specimens, have been identified in a city-wide tree survey. About half of these trees are situated in public parks, and the remaining is trapped in poor and cramped sites situated in old neighbourhoods. Due to natural hazards (mainly typhoon) and lack of care, some 30 champion trees have been lost and another 25 are in a bad state of health in the last four years alone. Such a rate of attrition to the city's precious natural heritage is alarming, and it calls for some urgent measures to arrest further decline and demise. The detailed database on this elite member of the urban tree stock could help to formulate a proactive management plan to ascertain their protection and long-term survival.

The multiple range of physical and soil constraints to urban trees are compounded by the lack of suitable planting sites. The urban morphology is excessively high-density and high-rise, leaving scant interstitial space at the street level for the introduction of greenery. The recent urban renewal has resulted in higher development intensity, and as a result many existing planting or plantable sites were squeezed out. The tendency to infill the lower density government, institutional and community land-use sites has exacerbated the plight. Overall the tightly packed town plan, with no requirement to set back buildings at road frontage and to provide ground-level green spaces, forms the fundamental constraint to attempts to increase the quantity of greenery in the city. The need for a coordinated tree-planting and landscaping plan for the whole city cannot be more emphatically stresses. Proactive measures could be adopted to earmark sufficient and high quality planting sites at roadsides and other suitable off-road sites to effect a gradual improvement in the greenery endowment of the city. A partnership amongst government, business and citizens, with proper coordination and cooperation, could help this worthwhile cause.