Pruning intensity of street trees and associated effects on ecosystem services

Urban street trees provide multiple ecosystem services to city residents. In the Taipei city of Taiwan, street tree pruning is periodically applied due to disastrous prevention of typhoons or storms. To understand how pruning intensity affects the value of ecosystem services, we evaluated the changes of ecosystem services provided by a total of 87,014 street trees in Taipei in terms of pollution removal, carbon storage, gross carbon sequestration, and runoff avoidance. The current status of each ecosystem service was calculated using i-Tree Eco developed by US Forest Service based on the street tree inventory conducted by Parks and Street Lights Office, Taipei City Government during 2015 to 2017. Inventory information included tree species, diameter at breast height (DBH), tree height, and their locations. To simulate pruning intensity from 10% to 100%, we adjusted the crown missing rate from the current canopy cover estimated by DBH and tree height and quantified their associated effects on the ecosystem services. Then, for comparison purposes, each ecosystem service was transformed into monetary values using US market value of water, carbon, air pollution removal, and electricity. Our analysis showed that the Taipei street trees currently hold a relatively stable age structure with lower risk of disease or pest outbreak. These trees were estimated to deliver ecosystem services of equivalent value of 5.6 million USD, to which 4.97 million USD was contributed by carbon storage. Based on the pruning intensity simulation, we suggest a 20% or lower pruning intensity considering street trees’ impairment and physiology, to maximize the ecosystem service values. We also recommend landscape managers to monitor and assess the growth and health of the street trees to promote sustainable development in the Taipei city.