Effect of vegetation biomass structure on thermal performance of tropical green roof

The passive cooling effect of green roofs in humid, tropical Hong Kong was investigated with reference to three vegetated plots, grass, groundcover herb, and shrub, with contrasting growth form and biomass structure and a bare control plot. Temperature was monitored at 15-min intervals for a year at seven levels: high (H) at 200 cm, middle (M) at 60 cm, low (L) at 20 cm, surface, soil, rockwool (water storage), and roof-tile surface. The findings indicated the crucial roles played by biomass quantity and structural complexity in passive cooling functions. Temperature variations of vegetated roofs occurred mainly during the day, with lower maximum and minimum than the control, but they did not cool air at night better than the control. Control and grass surfaces were warmed above the ambient temperature, but groundcover and shrub surfaces followed the ambient. Despite complex biomass structure, shrub created the most extreme diurnal air temperature regime. Despite simple biomass structure, grass cooled air more effectively than groundcover and shrub. Four anomalies in the vertical temperature profile were detected. First, the grass roof cooled daytime near-ground air to create a suspended temperature inversion. Second, the stagnant air within the shrub biomass trapped heat to generate a daytime canopy temperature inversion. Third, the elevated branch-foliage biomass of groundcover and shrub brought passive cooling to form a perched thermal discontinuity. Fourth, the air gap of the plastic drainage layer arrested downward heat transmission in all vegetated plots to form a subsurface thermal discontinuity. The findings provide hints on species choice and design of green roofs. © 2011 The Author(s).published_or_final_versionSpringer Open Choice, 21 Feb 201

Multipurpose census methodology to assess urban forest structure in Hong Kong

Fulltext link: http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=3073&Type=2Surveys of urban forests in the compact city environment of Hong Kong were initiated in 1985 and regularly updated thereafter. Roadside trees were evaluated first in a tree census and reported in this article followed by urban parks, public housing estates, and special habitats such as old stone walls or special specimens such as heritage trees. The survey method aimed at collecting comprehensive data to echo both tree conditions and tree-environmental interactions. Detailed information was gleaned, with the help of well-trained assistants, on tree sites, tree growing space, tree structure, and tree defects and disorders. A field record form was designed, pilot-tested, and refined to solicit responses to multiple choices or direct measurements to minimize subjectivity and errors in data recording and entry. The study also identified potential planting sites, registering suitability for tree growth, site characteristics, and dimensions. Data fields were designed to be quantitative or convertible to ordinal ranks to facilitate statistical analysis. Locations of trees and planting sites were marked on large-scale maps to permit spatial analysis. Besides statistical analysis, community ecology attributes and custom-designed indices were used to assess urban forest structure. The multipurpose method could be appropriately adjusted for use in other compact city areas. © 2008 International Society of Arboriculture.link_to_subscribed_fulltex

Physical and chemical properties of soils in an urban park in Hong Kong

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Impacts of intensive urbanization on trees in Hong Kong

Trees in cities face a severe limitation of plantable space and an exceptionally stressful growing environment. In Hong Kong, shortage of developable land has relegated trees to a lower priority and intensified urban impacts on them, relative to other cities. The vicissitudes of urban growth and trees since the founding of Hong Kong are reviewed, and eleven specific conflicts between urbanization and trees in Hong Kong are described. Redevelopment of existing buildings has raised site coverage by impervious surface and taken away ground-level planting space within and around affected lots. Infilling of relatively low-density areas mainly for government and institutional land-uses has increased development density and removed existing greenery and planting spaces. Road construction and improvement has widely damaged roadside trees and removed valuable and conspicuous greenery. Proliferation of underground utilities has fuelled the contest for usable space and precluded planting in many places. Widespread and frequent roadside trenching, associated with utilities and the laying of cable television and telecommunication networks has incurred massive root damage at roadsides. Poor soil quality commonly beset by chemical and physical constraints has caused chronic poor tree performance. Intrusion into urban parks and other green enclaves by buildings and roads has usurped the limited stock of green spaces. Encroachment into peri-urban woodlands has deprived the city of fringing mature greenery with conservation, landscape and amenity worth. Plantable space in reclaimed lands has been intensively used with inadequate allocation for trees. Protection and preservation of champion specimens has lacked statutory means and a coordinated policy. Reinforcement and demolition of stone walls has destroyed many large trees on unique mural habitats. Quality of arboricultural practice is poor, particularly in the private sector. Possible solutions to these limitations in Hong Kong are suggested, and have implications for other cities facing similar problems.published_or_final_versio

Assessing the Cooling Effects of Different Vegetation Settings in a Hong Kong Golf Course

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Seasonal and Diurnal Thermal Performance of a Subtropical Extensive Green Roof: The Impacts of Background Weather Parameters

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Floristic and habitat differentiation of heritage trees in Hong Kong’s urban forest

Session UA1: Urban arboriculturepostprin

Fragmentation and rehabilitation of urban forests in relation to new town development

Theme: Connecting the Street Tree to the ForestHong Kong has experienced fast population growth in recent decades that demands corresponding intensification of land use in existing city areas and its countryside envelope. New towns have been built since the 1970s on previous rural lands occupied by villages, farms and adjacent hillslopes, in tandem with new lands created by reclamation from the sea using earth fills. Due to the rugged hilly topography, new town development involves sprawling up the slopes which may contain valuable forests. The massive urban growth of nine new towns accommodating 2 million people has brought extensive intrusion into farmlands and natural hill slopes, and imposed widespread influence on pre-urbanization natural and cultural vegetation. The conservation policy emphasizes protecting areas of high ecological value, which include woodlands dominated by native and mature trees. Recently, the urban planning mindset and practice have shifted to a sympathetic and synergistic attitude towards the natural landscape, especially at the city-countryside interface. The study aimed at tracking the changes in the distribution and pattern of forest canopy cover before and after the inception of Tai Po new town development in 1979 using sequential aerial photographs, maps and documents. The following aspects of the urban tree cover were explored: nature and magnitude of the positive and negative vegetation changes, geometric pattern and distribution of forest cover dynamics, fragmentation and coalescence of forest patches, present forest condition and performance, and factors and processes leading to vegetation modification and restoration. The spatial variations in forest cover were characterized by patch geometry and size, and evaluated in relation to the different stages and forms of new town development. Both the core urban areas (urban forest) and the countryside hinterland (peri-urban forest) were studied. The concepts of precision land use zoning, green infilling and assisted relay floristic in urban forestry work were proposed. The implications of the findings to the preservation and creation of high quality nature in compact urban development were discussed. The forest restoration could contribute to carbon sequestration and climate‐change adaptation in the quest for sustainable development. The findings and implications of the case study could throw light on nature conservation and restoration associated with new towns developments in the developing world.published_or_final_versio

Rapid changes of precipitation pH in Qinghai Province, the northeastern Tibetan Plateau

Rainfall monitoring programs were conducted in two industrial cities of China's Qinghai Province, Xining and Germu, in some periods of the 1980s and 1990s. The results show that the natural precipitation in this area is originally alkaline. Compared with the late 1980s records, pH values declined significantly from approximately 8 in the 1980s to below 7 in mid-1990s. Such rapid and drastic changes were attributed to fast industrial development that released a large amount of pollutants. Subsequent tough control on pollutant emission partly restored pH values back to above 7 in the late 1990s. The pH and rainfall chemical analyses indicate that alkaline rain in this continental arid region is caused by airborne dusts which originate from local alkaline soils. With decrease of pH value, the total ionic concentration of rainwater is increased because acids were added to the rainwater. © 2002 Elsevier Science B.V. All rights reserved.postprin