城市树木作为应对气候变化和环境恶化等人类社会面临的各种挑战的绿色处方越来越受到关注。然而,没有城市树木的健康成长,它们不能为任何环境服务,社会,和可持续的经济利益。通过监测冠层发育,可以检测不同城市栖息地的树木生长动态,并执行适当的管理方法。利用九龙半岛,香港,作为一个案例,本研究探讨了遥感数据如何帮助监测和了解异质城市生境对树冠动态的影响。比较了基于WorldView-2卫星图像的四种算法,以优化冠层分割。然后将单个树冠与Sentinel-2卫星数据集成,以获得2016年至2020年每个季节的树冠生长动态。应用三个指标来反映树冠状况,包括荧光校正植被指数(FCVI,跟踪叶片叶绿素密度),土壤调整后的总植被指数(SATVI,测量木质树枝和树枝的密度),和归一化差异物候指数(NDPI,捕获冠层含水量)。并指定了城市树木的四个异质栖息地。结果表明,城市树木表现出不同的冠层生长状态,从自然地形降序,公园,住宅区,到道路边缘,这表明城市栖息地显著抑制了树木的生长。此外,2017年和2018年分别发生了两次超级台风,对树冠造成严重破坏。树的相关弹性各不相同,将冠层生长状态的顺序与道路上的冠层生长状态相呼应的顺序几乎没有弹性。这项研究表明,如何使用遥感数据来更好地了解大规模异质城市栖息地的长期树冠动态,这是监测和维护城市树木健康和生长的关键。
Urban trees have attracted increasing attention to serve as a green prescription for addressing various challenges facing human society like climate change and environmental deterioration. However, without healthy growth of urban trees, they cannot service any environmental, social, and economic benefits in a sustainable manner. By monitoring the canopy development, the tree growth dynamics in different urban habitats can be detected and appropriate management approaches can be executed. Using the Kowloon Peninsula, Hong Kong, as a case, this study explores how remote sensing data can help monitor and understand the impacts of heterogeneous urban habitats on tree canopy dynamics. Four algorithms based on WorldView-2 satellite image are compared to optimize the canopy segmentation. Then the individual tree canopy is integrated with Sentinel-2 satellite data to obtain canopy growth dynamics for each season from 2016 to 2020. Three indicators are applied to reflect tree canopy status, including the fluorescence correction vegetation index (FCVI, tracking leaf chlorophyll density), the soil adjusted total vegetation index (SATVI, measuring the density of woody branches and twigs), and the normalised difference phenology index (NDPI, capturing canopy water content). And four heterogeneous habitats where urban trees stand are specified. The results revealed that urban trees show varying canopy growth status, in a descending order from natural terrains, parks, residential lands, to road verges, suggesting that urban habitats curtail trees\' growth significantly. Additionally, two super-typhoons in 2017 and 2018, respectively, caused serious damages to tree canopy. Relevant resiliency of tree varies, echoing the sequence of canopy growth status with those in road verges the least resilient. This study shows how remote sensing data can be used to provide a better understanding of long-term tree canopy dynamics across large-scale heterogeneous urban habitats, which is key to monitoring and maintaining the health and growth of urban trees.