Optimizing reduction pruning under electrical lines: The influence of tree vitality before pruning on traumatic responses

Abstract

Reduction pruning of main tree stems is commonly performed in the maintenance of an electricity distribution network to encourage the growth of scaffold limbs away from the wires. Understanding both the initiation and growth of epicormic branch after reduction pruning interventions, as well as the area of discolored wood at the cutting point are important for optimizing the pruning cycle and maintaining safe trees. In this study, we investigated post-pruning traumatic responses in relation to tree vitality before pruning in 116 hackberries (Celtis occidentalis L.) and 86 green ash (Fraxinus pennsylvanica Marsh.) growing in a controlled environment of a nursery. Four years before pruning, each tree of both species was assigned a degree of damage commonly occurring in urban areas, including root injury, defoliation, and stem injury, for a total of 18 treatment combinations arranged in a random block design. In both species, trees that had lower vitality prior to pruning had a reduced number, height, and volume of the epicormic branches at the cutting point two years after main stem pruning. Interestingly and contrary to expectations, a positive relationship was found between the area of discolored wood at the cutting point and tree vitality prior to pruning. However, below a vitality threshold, the area of wood discoloration was exacerbated by crown dieback and retrenchment. Celtis showed a less efficient ability to compartmentalize and produced fewer epicormic branches compared to Fraxinus, and epicormic branches were of reduced height and volume when low growth was observed. From a management perspective, although both species can survive after several damage events and after reduction of the main stem, the results suggest to conduct an assessment of tree vitality before pruning to identify trees with lower vitality and better predict the return pruning time.