130 Further Studies A tree increases its dimensions with age, and this pro- gressive accretion provides more and better ecosys- tem services, fostered by good management practices and a suitable growing environment. In this study, not all heritage trees showed a positive relationship between tree dimensions and tree age. Surveys have to be carried out to understand the growth rate of trees at different ages at remote and urban areas. Any estab- lished equations would help better modeling of pro- jected future ecosystem service provision. Besides employing the growth factors of old trees to improve carbon sequestration calculation, it can serve as a management tool to estimate changing growth space and associated tree-care requirements. i-Tree Eco provided a reasonable estimation of ecosystem services contributed by the heritage trees in Macau. However, relationships between trees and environment were complex and they tended to high- light the inherent limitations of the common models. Additional studies could be conducted to enhance the understanding of ecosystem services provided by heritage trees. Firstly, research on crown volume and biomass equations for species in south China will provide a more accurate estimate of their ecosystem services. Secondly, finding out growth factors at dif- ferent ages will refine the carbon sequestration calcu- lation for trees at various growth stages. Thirdly, the current monetary values of air-pollution abatement are limited to common pollutants, and the values vary greatly among study regions. Investigation of the marginal cost of a wider range of pollutants at regional and local scales could be attempted. Lastly, other hitherto widely neglected potential benefits of heri- tage trees, including environmental, ecological, social, and cultural benefits, could be evaluated and quantified. These suggested studies could begin with heritage trees and extend to other urban-forest com- ponents. The results could provide a comprehensive and objective management. basis to strengthen CONCLUSIONS The increasingly urbanized world population, driving rapid urban sprawl and in situ urban renewal and den- sification, especially in developing countries, can induce widespread degradation in urban environmen- tal quality. Climate change impacts, urban heat island effects, and air pollution have jointly aggravated the ©2020 International Society of Arboriculture decisions and Lai et al: Heritage Trees in Macau conditions of life in many cities. Stresses on human mental and physical health and other natural compo- nents embedded in cities call earnestly for effective solutions. Among different mitigating measures, urban trees and associated urban green infrastructures could contribute notably to nature-based solutions to drive cooling and cleaning in a cost-effective and sus- tainable manner. Understanding the air-pollution abatement, carbon sequestration capabilities, and the more critical ecosystem services offered by trees can inform tree planning, planting, care, and conservation. Whereas the environmental benefits of greenery have been widely advocated for and understood, the economic value has received less attention. Conver- sion of tree benefits to dollar terms can enhance assessment and appreciation of their benefits using a universal and layman language. The monetary value can also strengthen the justifications to deploy public funds in urban greening programs and put the requests on a level playing field in the keen competition for funding with other pressing public expenditure items. Decision makers, managers, and citizens will be bet- ter equipped to appreciate the important contributions of trees vis-à-vis other environmental protection and enhancement measures. The long-term monetary val- ues, in particular, can enrich debates and inform judg- ments in the contests between development and conservation. Using the i-Tree Eco model in the study site has several limitations. A localized database for evaluating values of heritage trees could be devel- oped. It is important to develop planning and conser- vation policies based on scientific research to justify the costs of management and conservation of heritage trees and urban trees in general. As the finest exemplars of the urban tree stock in terms of age, size, tree form, aesthetic qualities, genetic composition, growth performance, ecological benefits, biodiversity significance, historical connec- tion, cultural association, and sentimental attachment, heritage trees play a pivotal role in offering truly mul- tivariate if not outstanding services to cities. They are the remnant daughter population of the toughest con- stitution that has survived the harsh urban selective forces. Some of them have exceptional ability to tol- erate environmental stresses and human abuses. Some could attain heritage caliber due to survival in suitable places where they could escape damage or removal by changing land use or urban intensifica- tion. Some could be overlooked or ignored and left by default. Others could be given special protection
March 2020
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