Arboriculture & Urban Forestry 46(2): March 2020 , and PM in total (Jim and Chen 2008). Each tree was able to take away 174 g/tree/yr of air pollut- ants. In this study, the large heritage trees with high leaf surface area could remove considerably more air pollutants at an average of 1.0 kg/tree/yr, equivalent to an aggregate of 806.8 kg/yr by the whole lot of 790 trees. SO2 , NO2 The generally large size of the heritage trees could store a considerable amount of carbon in biomass. Carbon stock in tissues could be continually enhanced by annual accretion. In this study, the heritage trees removed carbon from the air at a rate of 24.0 t/yr in total (Table 3). The average carbon sequestration was 30.4 kg/tree/yr on average and reached 47.3 kg/tree/ yr for Albizia spp. The results were significantly higher than the overall 12 to 21 kg/tree/yr in US urban forests (Vaccari et al. 2013); 23 to 33 kg/tree/yr of species with the highest sequestration rate (Eucalyp- tus leucoxylon and Ficus rubiginosa) in Perth, Aus- tralia (Saunders et al. 2011); and 26 to 34 kg/tree/yr in Florence, Italy (Paoletti et al. 2011). Gross carbon sequestration of the i-Tree Eco model was deter- mined by the differences in carbon storage between year x and year x + 1. The tree biomass in year x + 1 was calculated from year x taken as the base growth rate. The exceptional biomass of heritage trees con- tributed to the high annual increment. Although most heritage trees in Macau are in good health conditions, many large and old trees are at risk of dieback and decline worldwide. However, even if a heritage tree is retrenched, it could still provide considerable carbon storage and other ecosystem services that are superior to urban trees in general. A study examined more than 670,000 trees from more than 400 species for the growth rate of old and large trees (Stephenson et al. 2014). The results showed that the growth rate of many species increased with dimensions, indicating that old and large trees did not only act as a large carbon sink, but they were also able to continuously fix a large amount of carbon in comparison with smaller trees. This finding implied a notable potential for heritage trees in Macau to fix car- bon despite their old age. The carbon storage by heritage trees in Macau was 3,040.8 t in total (Table 3), with an average of 3.8 t/ tree. Street trees accounted for 43.8% and natives 31.3%. The genus Ficus, with particularly bulky trees with a mean dbh of 158.8 cm and a mean height of 13.8 m (Table 1), predominantly accounted for 73.7% 117 of the total carbon storage. The second and third ranks were Cinnamomum and Syzygium, which con- tributed merely 5.1% and 3.3%, respectively. Significant differences in carbon sequestration and carbon storage between the street and non-street trees, and exotic and native species (independent samples t-test, P < 0.05) were likely due to the domi- nance of exotic Ficus rumphii mainly planted along roadsides. Macau has a record of 488,364 trees (Commission of Audit of Macao SAR 2010), of which less than 0.2% (790 trees) reached the age of ≥ 100 years. With better tree management and growing environment, more trees could reach larger dimensions in the future to offer more ecosystem services. Ecosystem Services by Genus The study selected the 10 most common heritage tree genera (≥ 12 trees/genus) to compare their capacity in providing ecosystem services (Table 4). A Kruskal- Wallis H test showed statistically significant differences in all pollutant removal, carbon sequestration, and carbon storage between genera, with mean values shown in Table 4. The annual air-pollutant removal on a per-tree basis was the highest in Podocarpus for CO (88.6 g/ tree/yr); Ficus for O3 for NO2 (689.8 g/tree/yr); Ficus for SO2 yr); Pterospermum for PM2.5 (355.2 g/tree/yr); Podocarpus (124.2 g/tree/ (6.4 g/tree/yr) and PM10 (479.8 g/tree/yr). The most capable genera for total pollutant removal were Pterospermum and Podocar- pus, reaching about 1.5 kg/tree/yr (Table 4). Overall, the air purification ability of heritage trees was nota- bly higher than urban trees in general. In comparison, a study in the nearby city of Guangzhou found total pollutant removal of only 174 g/tree/yr (Jim and Chen 2008). Carbon storage on a per-tree basis of the 10 most common heritage tree genera was the highest for Ficus spp. (> 5,000 kg/tree) which differed consider- ably from other genera except for Pterospermum (about 3,800 kg/tree)(Figure 4). Podocarpus had the lowest carbon sequestration rate and were signifi- cantly different from the remaining nine common genera (Figure 4). Despite a smaller dbh (Table 1) and the least carbon storage in biomass, Podocarpus managed to fix 10 kg/tree. This study compared the capacity of different tree genera in providing environmental benefits. The ©2020 International Society of Arboriculture
March 2020
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