Arboriculture & Urban Forestry 46(2): March 2020 1998). The annual mean roadside PM10 concentrations reached 60 µg/m-3 and 29 µg/m-3 and PM2.5 , respectively, in 2014 (Meteorological and Geophysi- cal Bureau, personal communication). The PM results were almost triple of the international guideline at an annual mean of PM10 at 20 µg/m-3 (World Health Organization 2005). The CO2 and PM2.5 at 10 µg/m-3 emission was 3.8 t/capita in Macau in 2013 (The World Bank 2015), bringing 2.2 million t/yr of CO2 emission. Founded by the Portuguese in 1557, the city has witnessed over five centuries of urban history. Despite its small size and compact development mode, the urban design has adopted the European tree-planting tradition in streets and public parks. The present city- scape has inherited some old trees planted in the early days. Trees aged 100 or more years with special histor- ical and other values were identified as Ancient and Precious Trees (APTs) in 2011 (Department of Gar- dens and Green Area and South China Botanical Gar- den 2013). Most of them are roadside trees dominated by Ficus rumphii, but the highest diversity and largest tree dimensions are found in parks and gardens (Zhang et al. 2017). In this study, 790 APTs in 63 species were recognized as heritage trees, excluding 3 with special historical value and 2 with incomplete records. Tree Dimensions Tree assessment was conducted from July 2011 to April 2012 by the Department of Gardens and Green Areas of the Civic and Municipal Affairs Bureau of the Macau SAR Government and the South China Botanical Garden of the Chinese Academy of Sci- ence (Department of Gardens and Green Area and South China Botanical Garden 2013). Tree inventory data included species, location, tree height (m), under- branch height (m), dbh (m), crown W–E and N–S width (m), age (year), and health conditions. i-Tree Eco Model To quantify the ecosystem services of air-quality improvement and carbon sequestration of the heri- tage trees in Macau, the i-Tree Eco v5.0 model was used (www.itreetools.org). The model enlisted field data of tree dimensions and conditions, together with local meteorological and pollution concentration data. This model has been widely adopted to estimate the amount of air-pollutant removal by urban forest, carbon storage, carbon sequestration, and other eco- system services, with recent implementation examples 111 by Kim et al. (2015), Selmi et al. (2016), Jayasooriya et al. (2017), and Riley et al. (2017). We assumed the heritage trees to have 100% live crown ratio, i.e., “Total Tree Height” equaled “Height to Live Top”; “Percentage Crown Missing” and “Crown Dieback” were set at zero; and “Height to Crown Base” equaled to the under-branch height. DBH was measured at a height of 1.37 m. We esti- mated the crown light exposure (CLE) through maps in the book The Charm of Trees, Ancient and Pre- cious Trees in Macao (Department of Gardens and Green Area and South China Botanical Garden 2013), supplemented by satellite imagery and Google Street View. Hourly air-pollution concentration (C) was recorded by Automatic Air Quality Monitoring Sta- tions in Macau, and the monitoring data were sup- plied by the Macao Meteorological and Geophysical Bureau. Concentrations of NO2 , SO2 , O3 , and PM10 from 1 June 2011 to 31 May 2012 were extracted to match the tree inventory period. The concentration of PM2.5 in 2014 was selected because such data were only recently recorded and released. Monetary Values of the Ecosystem Services Nowak et al. (2008) calculated the ecosystem ser- vices of urban forests for the United States in 2007; the externality per-tonne monetary values were: NO2 at US $9,906, SO2 O3 at US $9,906 (same as NO2 at US $2,425, CO at US $1,407, ), and PM10 at US $6,614. With a 19.6% inflation rate based on the Con- sumer Price Index of Macau in 2007 through 2012 (The World Bank 2017b), the adjusted per-tonne val- ues were: NO2 at US $11,848, SO2 at US $5,152, O3 at US $11,848, and PM10 $7,910. PM2.5 was assumed to be the same as PM10 at US $2,900, CO at US . Carbon value of $23/t with marginal costs of carbon dioxide emissions for 2001 through 2010 was used in the US (Nowak et al. 2008). We adjusted the value as $25/t in 2012 by an 8.8% inflation rate in 2010 through 2012 in Macau. These monetary values were multiplied with the benefits of air purification, total carbon stored, and sequestrated by the 790 heritage trees in Macau in order to estimate the ecosystem service value. Statistical Analysis Spatial distributions of old growths were associated with the tree habitats and land ownerships such as ©2020 International Society of Arboriculture
March 2020
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