Detecting and measuring fine-scale urban tree canopy loss with deep learning and remote sensing

Abstract

Urban trees provide a multitude of environmental and amenity benefits for city occupants yet face ongoing risk of removal due to urban pressures and the preferences of landowners. Understanding the extent and location of canopy loss is critical for the effective management of urban forests. Although city-scale assessments of urban forest canopy cover are common, the accurate identification of fine-scale canopy loss remains challenging. Evaluating change at the property scale is of particular importance given the localised benefits of urban trees and the scale at which tree removal decisions are made.

The objective of this study was to develop a method to accurately detect and quantify the city-wide loss of urban tree canopy (UTC) at the scale of individual properties using publicly available remote sensing data. The study area was the city of Christchurch, New Zealand, with the study focussed on UTC loss that occurred between 2016 and 2021. To accurately delineate the 2016 UTC, a semantic segmentation deep learning model (DeepLabv3+) was pretrained using existing UTC data and fine-tuned using high resolution aerial imagery. The output of this model was then segmented into polygons representing individual trees using the Segment Anything Model. To overcome poor alignment of aerial imagery, LiDAR point cloud data was utilised to identify changes in height between 2016 and 2021, which was overlaid across the 2016 UTC to map areas of UTC loss. The accuracy of UTC loss predictions was validated using a visual comparison of aerial imagery and LiDAR data, with UTC loss quantified for each property within the study area.

The loss detection method achieved accurate results for the property-scale identification of UTC loss, including a mean F1 score of 0.934 and a mean IOU of 0.883. Precision values were higher than recall values (0.941 compared to 0.811), which reflected a deliberately conservative approach to avoid false positive detections. Approximately 14.5% of 2016 UTC was lost by 2021, with 74.9% of the UTC loss occurring on residential land. This research provides a novel geospatial method for evaluating fine-scale city-wide tree dynamics using remote sensing data of varying type and quality with imperfect alignment. This creates the opportunity for detailed evaluation of the drivers of UTC loss on individual properties to enable better management of existing urban forests.