116 Two principal components (PC1 and PC2) were iden- tified with an eigenvalue larger than 1. PC1, explain- ing 48% of the variance, was mainly correlated with leaf area, followed by crown area. PC2, explaining 21% of the variance, was positively correlated with dbh but negatively correlated with age. Considering all heritage trees, the associations were weak between tree dimensions and age, and among tree dimension attributes. The distribution of the heritage trees did not show distinctive clusters with reference to tree habitats, but a few trees from Figure 2 were characterized by older age and larger leaf area with lower PC1 and PC2 fac- tor scores. These trees were located at the remote area where more natural site conditions permitted attain- ment of maximum potential biological dimensions. Meanwhile, trees at urban parks had a lower PC2 fac- tor–scores value, implying that they were older but with a relatively thinner tree trunk (Figure 2). It could be explained by a divergent genus composition and close planting of trees (Figure 1). Further analysis is needed to understand tree dimension and age among species. Accordingly, the study selected the 10 most common heritage tree genera (≥ 12 trees/genus) to compare their dimensions with age. For the heritage trees from the 10 most common genera, only Ficus had a positive correlation of height, crown area, dbh, and leaf area versus age. Bombax, Dimocarpus, Podocarpus, and Syzygium only increased their dbh with age. No tree dimensions of Albizia, Litsea, and Plumeria were significantly correlated with age (Table 2 and Figure 3). Tree dimensions were expected to increase with age and to provide more ecosystem services. However, trees ≥ 100 years old at high-density areas suffered from confined growing space or poor growth conditions, which restricted extension of branches and height. For some old and weak trees, the crown might have Lai et al: Heritage Trees in Macau experienced retrenchment. Some trees only increased their dbh but not their crown. Better management practice and growth-site protection can allow trees to increase their dimensions and ecosystem services. Overall Ecosystem Services Total air purification by heritage trees amounted to 841.7 kg/yr due to combined pollutant removal by absorption and deposition (Table 3). Street trees accounted for 36.0% and natives 38.9% of removal. No significant differences in total pollutant removal were found between street trees and non-street trees or between native and exotic species (independent samples t-test, P > 0.05). The total capacity of air-pollutant removal depended on individual pollutants, with NO2 the highest. The heritage trees totally intercepted 251.0 kg of NO2 , 237.3 kg of PM10 , 237.2 kg of O3 being , 78.6 kg of SO2, and 2.7 kg of PM2.5 per year. Ficus contributed 61% of total air pollutant removal due to a large number of individuals, followed by 5.6% of Cinnamomum and 4.6% of Dimocarpus. Although the dimensions of some trees were lim- ited by age, overall heritage trees still provided effi- cient and active sites for pollutant deposition or absorption. Although tree age would reduce leaf growth and lower functional efficiency of individual leaves, older trees have a larger total leaf area per tree to raise the aggregate leaf area of the whole cohort (Stephenson et al. 2014). A large aggregate leaf area of heritage trees could compensate partly for their general decline. These trees were found to have a rather high capacity and value in air-cleansing capa- bility. Common species in Florence, Italy, could remove 4.5 g/tree/yr (Carpinus spp.) to 362 g/tree/yr (Pinus spp.) in total air pollutants (NO2 , SO2 , CO, O3 , and PM10)(Paoletti et al. 2011). In nearby Guangzhou city, 1,794,455 urban trees could remove 312 t/yr of Table 2. Pearson correlations of tree height, crown area, dbh, and leaf area with tree age of the 10 most common genera of the heritage trees. Height Leaf area Albizia Bombax Cinnamomum Dimocarpus Ficus -0.334 Crown area -0.205 dbh -0.445 -0.218 (*P < 0.05, **P < 0.01) -0.066 0.164 0.697** 0.214 0.102 0.346* 0.738** 0.190 -0.199 -0.081 0.458** -0.129 0.183** Litsea 0.118 0.220** -0.190 0.252** 0.219** 0.429 0.128 Plumeria Podocarpus Pterospermum Syzygium -0.225 0.320 -0.118 0.152 0.195 -2.390 0.703* -0.279 0.406 0.557* 0.402 0.609* 0.279 -0.234 0.608 -0.200 ©2020 International Society of Arboriculture
March 2020
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