Arboriculture & Urban Forestry 46(2): March 2020 under the aegis of enabling public policies or legisla- tion, often in response to people’s quests to preserve them as living landmarks or cultural or religious sym- bols. In most cities, only a small cohort of urban trees could soldier on for a century or more to reach extraordinary size. Their cumulative sizeable dimen- sions and relative vigor offer a high capacity for eco- system services. Their persistence in the urban milieu provides ample hints to optimize tree growth and management in cities. Their numbers may not be large, but their individual and collective environmen- tal contributions, measured mainly by crown size, leaf area, and trunk diameter, can be appreciably higher than ordinary urban trees. An in-depth study of this special cohort can lay a foundation for a compre- hensive assessment of the whole urban tree stock. Past studies of heritage trees tended to focus on botan- ical and ecological domains. Evaluation could be expanded to cover their varied ecosystem services and economic valuation. Contributions of urban trees have been studied in many cities, but few have focused the scope squarely on the most outstanding tree subset. The results of this study have provided an approximation of air-quality improvement and gross carbon sequestration by heri- tage trees using the well-established and well-tested i-Tree Eco model. Despite some data inadequacies and estimation uncertainties, some useful results have been generated. They demonstrate convincingly that heritage trees can be effective in removing air pollut- ants and sequestrating and storing carbon, which can bestow considerable monetary values in the long run. The benefits were several times higher than ordinary urban trees. A clairvoyant strategy can be distilled from the findings with far-reaching consequences. Keeping more urban trees healthy and vigorous can be extremely worthy of the extra efforts and invest- ments. By extending their life span and sustaining their performance, more individuals can be ushered to the superior heritage tree cohort, thus significantly raising overall services and values of the urban tree population. The findings, therefore, highlight a new dimension in the interpretation of heritage tree bene- fits in urban areas. Especially for compact cities where planting sites are inadequate, tree management should concentrate on nurturing a meritorious cohort of outstanding elites, rather than providing a multi- tude of commonplace products. LITERATURE CITED Albani M, Medvigy D, Hurtt GC, Moorcroft PR. 2006. The con- tributions of land-use change, CO2 fertilization, and climate variability to the Eastern US carbon sink. Global Change Biology. 12:2370-2390. Becker N, Freeman S. 2009. The economic value of old growth trees in Israel. Forest Policy and Economics. 11:608-615. Beijing Garden Administration Bureau. 1992. Famous ancient trees in Beijing. Beijing (China): Beijing Publishing House. Bolund P, Hunhammar S. 1999. Ecosystem services in urban areas. Ecological Economics. 29:293-301. Cekstere G, Osvalde A. 2013. A study of chemical characteristics of soil in relation to street trees status in Riga (Latvia). Urban Forest & Urban Greening. 12:69-78. Chen S, Chen B. 2012. Network environ perspective for urban metabolism and carbon emissions: a case study of Vienna, Austria. Environmental Science & Technology. 48:4498-4506. Commission of Audit of Macao SAR. 2010. Commission of audit releases performance audit report on greening works carried out by the Civic and Municipal Affairs Bureau. Macao (China): Civic and Municipal Affairs Bureau. [Accessed 2018 Jul 2]. http://www.ca.gov.mo/en/press_release/press_release_detail .php?id=91&kd=2&Page=1 Department of Gardens and Green Area and South China Botan- ical Garden. 2013. The charm of trees, ancient and precious trees in Macao. Macao (China): Department of Gardens and Green Areas, Civic and Municipal Affairs Bureau of Macao SAR. Fragkias M, Lobo J, Strumsky D, Seto KC. 2013. Does size matter? Scaling of CO2 One. 8:e64727. emissions and US urban areas. PLoS Hirabayashi S. 2014. i-Tree Canopy Air Pollutant Removal and Monetary Value Model Descriptions. [Accessed 2018 Jul 2]. https://www.itreetools.org/canopy/resources/i-Tree_Canopy_ Air_Pollutant_Removal_and_Monetary_Value_Model_ Descriptions.pdf Horikoshi M, Tang Y. 2016. ggfortify: Data Visualization Tools for Statistical Analysis Results. [Accessed 2018 Jul 2]. https://cran.r-project.org/src/contrib/Archive/ggfortify Howarth RB, Farber S. 2002. Accounting for the value of eco- system services. Ecological Economics. 413:421-429. Jayasooriya VM, Ng AWM, Muthukumaran S, Perera BJC. 2017. Green infrastructure practices for improvement of urban air quality. Urban Forestry & Urban Greening. 21:34-47. Jim CY. 2003. Protection of urban trees from trenching damage in compact city environments. Cities. 20:87-94. Jim CY. 2004. Spatial differentiation and landscape-ecological assessment of heritage trees in urban Guangzhou (China). Landscape and Urban Planning. 69:51-68. Jim CY. 2005. Floristics, performance and prognosis of historical trees in the urban forest of Guangzhou City (China). Environ- mental Monitoring and Assessment. 102:285-308. Jim CY. 2018. Protecting heritage trees in urban and peri-urban environments. Unasylva. 69:66-74. Jim CY, Chen WY. 2008. Assessing the ecosystem service of air pollutant removal by urban trees in Guangzhou (China). Journal of Environmental Management. 88:665-676. 131 ©2020 International Society of Arboriculture
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