124 Lai et al: Heritage Trees in Macau Table 4. The associations between the 10 most common genera and annual air pollutant removal indicated by Kruskal-Wallis test and their gross carbon sequestration and carbon storage per tree. Genus Albizia Bombax Cinnamomum Dimocarpus Ficus Litsea Plumeria Podocarpus Pterospermum Syzygium χ2 df P CO 30.4 48.8 47.1 43.6 45.0 32.4 16.1 88.6 78.7 36.5 341.4 9 < 0.001 O3 Air pollutants removal (g/tree/yr) NO2 SO2 115.5 236.5 244.5 277.8 355.2 139.3 64.6 336.8 299.5 168.6 142.4 9 < 0.001 236.6 347.7 347.4 284.2 320.3 246.1 119.6 689.8 613.3 253.0 83.8 9 < 0.001 22.7 74.2 69.7 97.5 124.2 33.1 15.1 66.2 58.9 49.0 134.4 9 < 0.001 4.1 3.9 4.3 3.4 3.5 2.5 1.2 2.9 6.4 2.8 81.1 9 < 0.001 PM2.5 PM10 Total 309.9 337.6 354.0 311.9 318.5 195.3 97.3 217.4 479.8 240.1 74.6 9 < 0.001 719.3 1,048.6 1,067.0 1,018.3 1,166.8 648.7 313.9 1,401.8 1,536.6 749.9 141.1 9 < 0.001 Gross carbon sequestration (kg/tree/yr) 47.3 47.0 39.8 35.2 26.3 41.5 23.4 9.8 32.7 40.2 68.4 9 < 0.001 Carbon storage (kg/tree) 2,591.3 3,299.8 3,505.5 1,325.3 5,093.6 2,252.0 1,202.1 418.2 3,805.9 2,636.6 99.7 9 < 0.001 results can be considered by decision-makers as a factor for species selection in establishing an urban forest that can maximize ecosystem services in terms of air-pollutant removal and carbon sequestration. However, it should be reminded that concentrating on a few species for the urban forest as a whole is unde- sirable due to limitations to biodiversity. The principal component analysis was used to investigate the relationships among carbon sequestra- tion, carbon storage, and air-quality improvement of the 10 most common genera. Two principal compo- nents had an eigenvalue larger than 1. PC1 explained 59% of the variance, mainly correlated with carbon storage. PC2 explained 20% of the variance, mainly correlated with O3 , NO2 , PM10 , and SO2 removal (Fig- ure 5). The trees clustered at relatively high carbon storage and average air-pollutant removal were mostly shouldered by Ficus spp. (Figure 5). A Ficus microcarpa with 25 × 30 m canopy scored the best performance by carbon storage and air purifying capacity, followed by a Pterospermum heterophyllum with 28 × 35 m canopy. Both trees were sited at the remote Ilha Verde. Without growing stresses of the dense urban center, these exceptional heritage trees had realized their potential biological capacity to pro- vide outstanding ecosystem services. Plumeria spp., common in urban parks and cemeteries (Zhang et al. 2017), had a lower capacity in carbon storage and air ©2020 International Society of Arboriculture purification. It might not be a suitable species for effective provision of ecosystem services. This study provided estimation of ecosystem ser- vices furnished by heritage trees. Pollutant removal varied among cities due to the amount of tree cover, tree age, size and performance, pollution concentra- tion, length of the in-leaf season, and meteorological variables that affect transpiration and deposition velocities (Nowak et al. 2006). Macau has to accom- modate a large human population relative to its tiny land area. With an excessively compact development mode, little land is available for urban greening. For the dispersed air pollutants that are difficult to scrub by artificial means, the meager amount of greenery offers some natural and sustainable abatement. Tree quantity, quality, dimension, and location would jointly regulate the efficacy of air-pollution mitiga- tion. Poor air quality in urban areas impinges on an exceptionally large number of citizen victims. The heritage trees, comprised of the largest and longest- living doyens of the urban tree stock, play a critical role in bestowing ecosystem services and environ- mental health on the city. Monetary Values Together, the studied trees generated US $8,692 mon- etary benefits per year, in which CO removal was US $180, O3 was US $2,810, NO2 was US $2,974, SO2
March 2020
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