Crown reaction and acclimation to cyclical V-trimming of city trees: An analysis using terrestrial laser scanning

Abstract

Trimming is an important practice for reducing potential contact between trees and power lines. V-trimming occurs when a tree is located directly under the electrical wires and results in the formation of a bilateral crown, but not much is known about a tree’s reaction and acclimation to such a repeated stress in an urban context.Using Terrestrial Laser Scanning (TLS), we present a study that focuses on documenting (i) short term effect of V-trimming on the tree structure, through the quantification and analysis of the dispersion of trimming induced branch loss and subsequent growth reaction, and, (ii) long term acclimation (i.e. changes in biomass location) of tree structure to repeated unidirectional trimming. A voxelisation method was used to derive space exploration metrics from TLS data based on explored volume quantification and voxels dispersion within the tree crown.Our results show that V-trimming induces a significant decrease in explored crown space volume (12.8% on average) but that this loss is regained by trimmed trees within only 1year following trimming thanks to a rapid regrowth rate. This was supported by an analysis of radial growth that showed that the growth of trimmed trees was greater than non-trimmed trees although this tendency was not statistically significant. In our study this regrowth was achieved without suckering; instead the regrowth mainly occurred within the crown periphery. We also observed that trimming had a significant influence on the way trees explore space with their crowns. While non-trimmed trees explored space preferentially toward a South direction, trimmed trees explored space in directions perpendicular to the wires (East and West). We also observed that crown biomass was located more in the extreme crown periphery in trimmed trees compared to non-trimmed trees.