110 Although trees can provide considerable values as ecosystem services, on a per-tree basis the highly var- ied benefits increase with tree size. Large and old trees are recognized as keystone components in fur- nishing landscape value and multiple ecosystem ser- vices (Manmoud et al. 2015). The unique ecological, landscape, and amenity services of large and old trees can hardly be substituted by young ones (Le Roux et al. 2014). In New York City, the overall net benefits of urban trees were quantified at US $5 to $9 for a small tree, US $36 to $52 for a medium tree, and US $85 to $113 for a large tree (McPherson et al. 2007). The capacity of trees to remove air pollutants increased with tree canopy cover and total leaf area (Mullaney et al. 2015). Large trees could remove 60 to 70 times more air pollutants than small ones (McPherson et al. 1994). Carbon storage and seques- tration also increased with tree size. Large trees had carbon storage rates 1,000 times and carbon seques- tration rates 90 times those of small ones (McPherson et al. 1994). Large trees have been identified as key carbon-storage entities in natural forests. Preserving large trees with a trunk diameter at breast height (dbh) > 1.1 m was recommended to improve above- ground biomass balance (Sist et al. 2014). Mean- while, Albani et al. (2006) argued that mature trees have reduced net annual CO2 uptake and hence they no longer served as significant recurrent carbon sinks. Even though large and old trees can provide greater ecosystem services, few trees could persist for a long time in a stressful urban environment (Jim 2004). Shading imposed by high-rise and densely packed buildings, confined physical growth space above- and belowground, impermeable urban surface, air and soil pollution, and frequent modifications of adjacent buildings and roads inflict many acute and chronic challenges on urban trees (Jim 2003; Cekstere and Osvalde 2013; Mullaney et al. 2015). The small cohort of trees that can survive decades or centuries of stresses and remain strong and vigorous are often respected and protected as heritage trees (Jim 2005, 2018). Such exceptional trees may embody remark- able values in culture, history, ecology, landscape, and amenity (Beijing Garden Administration Bureau 1992; Jim and Zhang 2013) to justify special care and conservation. Unfortunately, continued urban expansion, rede- velopment, and densification have often degraded physical and physiological habitat conditions to ©2020 International Society of Arboriculture Lai et al: Heritage Trees in Macau induce a decline in heritage trees (Le Roux et al. 2014). Recognizing their important ecosystem ser- vices denotes a key step towards their conservation and restoration in the changing urban fabric (How- arth and Farber 2002). Because of the uniqueness of heritage trees, studies have quantified their ecosystem services and equivalent monetary values. In Israel, trees older than 100 years provided an annual value between €2.35 to €19.9 million through a contingent valuation study (Becker and Freeman 2009). Quanti- fication of the multiple benefits of old growths can inform policymakers in resource allocation for enhanced care and preservation. This study aimed at estimating the capability of heritage trees in Macau to remove air pollutants and sequester carbon. As tree size was one of the major parameters in estimating the capacity to provide eco- system services, tree dimensions of the heritage trees were evaluated against their age. Understanding their potential growth and benefits could provide reliable information to guide decisions on their conservation or restoration in Macau, and the findings can be applied to other Chinese and developing cities. Decision-makers could have scientific, objective data to make judg- ments on the trade-offs among expenditures in pres- ervation, restoration, and alternative regulatory, policy, or management actions. The results could enhance people’s awareness of the realistic values of heritage trees to strengthen their support of relevant official policies. METHODS Study Area The Macau Special Administrative Region (SAR) sits at the west side of the Pearl River estuary at the south coast of China (22°N, 113°E). The humid sub- tropical climate is dominated by the Asian Monsoon system. Macau has experienced rapid economic growth in recent decades. Its gross domestic product (GDP) increased over seven times in the past 20 years, attaining US $44,800 million in 2016 (The World Bank 2017a). The population of 644,900 in 2016 (Statistics and Census Service 2016) dwelt in a tiny territory of 30.3 km2 . An adverse consequence of rapid economic growth and high population density is extensive environmental degradation. In the 1990s, 40% of daily average SO2 Chinese National Primary Standard (Mok and Tam concentration exceeded the
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