The potential of the Handheld Mobile Laser Scanner (HMLS) tool ​ in urban forest planning to design ​canopy consolidation intervention

Abstract

Recent remote sensing technologies have been a breakthrough over the past three decades in urban green natural heritage planning. In particular, LIDAR Handheld Mobile Laser Scanner systems (HMLS) have proven invaluable for measuring various parameters of individual tree structure, results of considerable utility for urban monumental trees research. The estimated morphological variables of an individual tree contribute to understanding well its resilience and role within an urban landscape. Knowing the dimensional variabilities that contribute to the structural stability of each individual tree would enable the design of more consolidations targeted to the specific needs of the examined plants. The consolidation of trees refers to all methods of connecting or supporting the branches or stems of a plant that aim to reduce the likelihood of failure and/or the damage associated with a structural failure of the plant. In this study we focus the canopy stability of monumental trees present in two different historical gardens of Palermo (Italy) with the use of innovative technology of HMLS system. The HMLS scans were performed around two old growth trees of Ficus macrophylla subsp. columaris located respectively in the Botanical Garden and Garibaldi Garden (Palermo city, Sicily, Italy). A ZEB HORIZON handheld mobile laser scanner. was used for the survey. This species tends to drop aerial roots from its branches, which thicken into supplementary trunks upon reaching the ground, which help support the weight of its crown. Because of these characteristics, it was necessary to have full knowledge of canopy load distribution to identify the weakest points of the stems. The use of innovative terrestrial technologies LIDAR is the objective of this study. It aims to support the collection of preliminary data necessary for the design of a consolidation of the monumental trees canopy. The methodology used for extrapolating dimensional information and creating a digital model of the trees involves the application of specific computer algorithms. These algorithms provide valuable information on parameters such as the spatial distribution of tree stems under the canopy, the estimated total volume of the canopy, and the dimensional information of first-order branches. Accurate assessment of the condition of the vulnerable parts of the canopy is essential for planning effective future consolidation interventions. To this end, it is crucial to obtain comprehensive information about the actual state of these areas. Furthermore, the structural conformations of the trees were examined from the LIDAR scans for possible anomalies or defects, which are essential for the categorization of greenery according to failure propensity class (CPC) of the SIA (Italian Society of Arboriculture) protocol. Terrestrial laser scanning has therefore made it possible to obtain key biometric information on the above-ground parts of trees. They are essential for preparing any management measures, because they are a useful tool for future in-depth studies and analyses on the multifunctional role of city green.