Arboriculture & Urban Forestry 46(2): March 2020 Arboriculture & Urban Forestry 2020. 46(2):109–134 URBAN FORESTRY ARBORICULTURE Scientific Journal of the International Society of Arboriculture & Heritage Trees in Macau: Relationships Among Biomass Structure, Age, and Ecosystem Services By Po Ying Lai, C.Y. Jim, and Hao Zhang Abstract. Older trees in good health are expected to provide more ecosystem services and equivalent economic values due to their large size. The relationship of tree dimensions, respective tree height, crown area, diameter at breast height (dbh), and total leaf area vis-a-vis age were studied for 790 heritage trees ≥ 100 years old in Macau; 50 genera and 63 species were represented. Seven out of ten common genera showed no significant increase for all tested parameters except increase of dbh with age. Other factors, such as condition and geometry of growing spaces, controlled the performance of heritage trees, as well as the realization of their biological potential size, with implications on the provi- sion of ecosystem services. The effects of these heritage trees on air-quality improvement and gross carbon sequestration were quantified by the i-Tree Eco model. Overall, 806.8 kg of air pollutants were removed annually, with benefits valued at US $8,091. The heritage trees stored 3,041 t carbon in total and sequestered 842 kg carbon/yr, equivalent to US $601 in annual benefits. The values were much higher than ordinary urban forest trees. Ten common heritage tree genera were ranked by their capacities for air quality improvement, carbon storage, and seques- tration. The findings can serve as a decision tool for heritage tree management and conservation and to estimate potential ecosystem services of established trees. Keywords. Air-Pollutant Removal; Carbon Sequestration; Carbon Storage; Ecosystem Service; Heritage Tree; Monetary Value. 109 INTRODUCTION Rapid and intensive urbanization often aggravates environmental degradation. The impacts could be ameliorated by preserving or creating natural ele- ments subsumed under urban green infrastructure. Quantifying and monetizing the benefits can help us to understand and advocate for the multiple ecosys- tem services of urban vegetation. In recent years, a rising number of studies have quantified the benefits and services of trees in mitigating environmental problems (Roy et al. 2012; Mullaney et al. 2015). They tended to focus on two key environmental functions, namely air-pollution abatement and carbon sequestration. Trees are effective in capturing air pollutants such as nitrogen oxide (NOx), sulfur dioxide (SO2 ), ozone and PM10 (O3), and particulate matters (PM) including PM2.5 to enhance citizen well-being (Nowak et al. 2006; Rao et al. 2014; Vranckx et al. 2015). A model- ing study estimated that urban trees in the United States could remove a large amount of air pollutants (711,000 t/yr of NO2 , SO2 , CO, O3 , and PM10 ). The improved urban air quality was valued at US $3.8 bil- lion/yr (Nowak et al. 2006). Urban trees in the city of Guangzhou, China, could remove 312 tonnes (t) of NO2 , SO2 annually (Jim and Chen 2008). Urban areas are significant emitters of CO2 , and PM, equivalent to US $11,000 value with emission in a city. The total carbon stock in urban trees of the United States in 2005 was esti- mated at 643 × 106 ) during photosynthesis and storing carbon in biomass (Nowak et al. 2013), thereby reducing net CO2 t, with 18.9 × 106 t net annual carbon sequestration. The carbon storage and sequestration benefits were valued at US $50.5 billion and US $2.0 billion respectively (Nowak et al. 2013). The urban forests in the city of Shenyang, China, stored 337,000 t of carbon valued at US $13.9 million, with a seques- tration rate of 29,000 t/yr valued at US $1.2 million (Liu and Li 2012). ©2020 International Society of Arboriculture impacts on climate change (Chen and Chen 2012; Fragkias et al. 2013). Trees can effectively serve as a carbon sink by fixing atmospheric carbon dioxide (CO2
March 2020
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