The innovative 4-D monitoring of urban trees based on ALS LiDAR

The spatial distribution of urban green spaces is crucial for the sustainable provision of ecosystem services in a way that ensures even and equitable access for urban residents. In order to quantify urban green infrastructure, direct field measurements, topographic maps, aerial or satellite orthophotomaps were most often used. They usually provided only 2-D information, which does not fully characterize the spatial structure of high vegetation. The photogrammetric studies of urban high vegetation on CIR stereo -aerial photographs have been effectively supplanted by analyses of 3-D point clouds derived from Airborne Laser Scanning (ALS LiDAR). After their normalization, it is possible to derive biometric characteristics of individual tree or bio-groups of high vegetation. On the basis of point cloud analyses or CHM (Crown Height Model) models it is possible to extract information on the number of trees (crown segmentation), heights (e.g. percentile analyses), horizontal closure of tree canopy, penetration rate, 2-D and 3-D crown area, crown length, etc.
The purpose of presented study was to determine the 4-D changes that have taken place in the green infrastructure of the city of Krakow. Changes in horizontal (2-D) as well as vertical (3-D) space of the high vegetation class from ALS LiDAR point clouds for area of 326 km2 of Krakow area were analyzed. ALS time-series were registered in 2006 (18 pts/m2), 2012 (12 pts/m2), 2017 (12 pts/m2) and 2021 (30 pts/m2). The results obtained show an ongoing increase in the 2D area of high vegetation (>2m AGL) from 5.569,69 ha (17.04% of Krakow) in 2006 to 9.206,27 ha (28.17%) in 2021. The average annual increase in the (2-D) area of tree crowns over the 15-year period was 242.44 ha. During the same period, the 3-D surface of tree crowns (CHM) increased by about 47% from 10.131,85 m2 (2006) to 14.983,86 m2 (2021). The average annual increase of CHM was 323.47 ha. Trees that were not removed grew in height on 8.443,23 ha which for the 15-year period gives: 6.75m (0.45 m/yr). On an area of about 1.929,62 ha, a decrease in tree height was recorded (average -7.11m), which in most cases could mean tree removal, crown breakage or dying of older trees. Height loss of tall vegetation in the largest area (2.984,54 ha) was recorded for the period 2012-2017 when there was legislative turmoil and the obligation to apply for permits for tree removal on private land was temporarily lifted. Detailed voxel analyses conducted for the Planty Park (approx. 44 ha) showed a change in tree crown volume by about 21%, i.e. from 149.627 m3 in 2006 to 181.399 m3 in 2021, with the 2-D area increasing by about 14.6% ( from 18,45 to 21,15 ha). The extremely detailed results that can be obtained from ALS LiDAR point cloud time series analyses can be of great value to city rulers in monitoring and managing the city's green infrastructure (public and private) including controlling compliance with the law in protecting urban greenery providing numerous ecosystem services invaluable to gigapoli residents.