通过遥感监测不同城市栖息地的城市树冠动态

[Objective]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.[Method]Using the Kowloon Peninsula, Hong Kong, as a case, this study divided the trees in the study area into three urban habitats: roadsides, residential neighbourhood, and parks, and natural habitats based on government and public data. The study compared four methods for segmenting individual tree canopies using WorldView-2 remote sensing data with sub-meter spatial resolution. Then, the individual canopies were combined with multispectral and high-temporal resolution Sentinel-2 satellite data to obtain the canopy growth dynamics for each season from 2016 to 2020. Three indicators, leaf chlorophyll density,woody branch density, and canopy water content, constructed based on the spectral characteristics of different components, were used to reflect the canopy condition.[Result]Among the four methods applied to tree canopy segmentation in urban areas, all three performance indicators of the combined VDVI and decision tree method were greater than all other methods, indicating that the quality of tree canopy segmentation using the combined VDVI and decision tree method was significantly better than the other methods tested in this study. A total of 47,428 tree canopies were identified in the entire study area based on this method. We then further explored the dynamics of Ficus microcarpa, the most widely distributed tree in subtropical cities, in different urban habitats under the influence of super typhoons at a five-year scale.[Conclusion]The study found that urban trees showed different canopy growth trends, and the condition of trees in the three urban habitats was worse than that of trees growing in natural terrain, indicating that the urban environment inhibited the natural growth of trees. In addition, the two super typhoons during the study period also provided evidence for the resilience of urban trees in different habitats. Before the typhoon, the indicators of trees growing in natural areas performed best. During the typhoon, the trees in natural areas suffered the most serious losses. In the recovery period after the typhoon, the trees in natural areas recovered faster than the trees in the other three urban habitats, indicating that natural trees have the strongest recovery capacity. At the same time, among the three urban habitats, the recovery capacity of trees growing on the roadside was the weakest. This study shows how remote sensing data can be used to better understand the long-term dynamics of tree canopies in different urban habitats, which is crucial for monitoring the dynamics of urban trees and taking management measures to maintain the health and growth of urban trees.