Urban trees play an important role in human society, by providing a range of ecosystem service benefits. Ecosystem services are defined as the direct and indirect contributions of ecosystems to human wellbeing, for example, carbon sequestration, pollutant adsorption, and stormwater runoff interception. The amounts of these so-called ‘regulating services’ are positively correlated to leaf area, and so large trees with large total leaf areas will produce greater benefits. However, it is common for urban trees to be pruned to meet modern requirements set by the electricity generation provider, wherein branches are removed to achieve statutory clearances between trees and the overhead utility wires, to prevent outages and faults in the electrical network. This sort of pruning can remove considerable proportions of live foliage.
Using iTree Eco, this research investigated whether utility wire pruning affected ecosystem service benefits for three regulating services (carbon sequestration, pollutant adsorption, and runoff interception). The 25 most common urban street trees in Auckland, New Zealand, were ascertained by examining a record of 10,323 tree records held in our database. A further 15 trees selected by Auckland Council’s urban forest manager were added to the list making a total of 40 urban trees. Mean mature dimensions (trunk diameter at 1.4 m, height, and crown spread) and standard deviations were ascertained for each species and 100 representative trees for each species were simulated with a normal distribution for each dimension in each species. Trees were stratified into small, medium, and large trees, being trees with a mean mature height of < 5 m, 5 m-10 m, and > 10 m, respectively. Two simulated utility wire pruning treatments were then established, being an L-shaped treatment, and a topped treatment, where an elliptical segment was removed from the latter, and half the elliptical segment was removed from the former. The percentage loss of crown volume for each tree was computed based on the two pruning treatments and the data were entered into iTree Eco for analysis of ecosystem services.
Utility wire pruning significantly reduced ecosystem service provision in large trees at p ≤ 0.05, but not in small or medium trees. Large trees that had been topped ($26.68) had significantly fewer total annual benefits than control ($46.11) and L-shaped trees ($36.97). The effect of tree size on ecosystem services was significant for all treatments. Large control trees had significantly greater ecosystem services than small control trees for all ecosystem services. Large control trees had significantly greater ecosystem services than small and medium trees for all ecosystem services. The differences in ecosystem service provision between trees in different size classes and between different treatments can be explained by differences in leaf area. Interestingly, large, topped trees still had significantly greater total ecosystem service benefits than small unpruned control trees. Local communities can still receive greater ecosystem service benefits by planting large trees that require ongoing utility wire pruning at maturity, than from small trees that do not.