The impact of pruning and mortality on urban tree canopy volume

Urban trees provide a wide range of ecosystem services for city residents, with tall, mature trees with wide crowns generally regarded as preferable. The tree biomass which is responsible for shading, pollution removal, rain runoff retention etc. gets periodically reduced by the municipal tree management practice of pruning. This is a necessary activity, which reduces the risk of infrastructure damage and falling branches, but many estimates of ecosystem service provision in cities do not consider its impact explicitly. Tree mortality is also higher in cities, preventing trees from attaining and remaining at large sizes. This study used extensive field measurements of tree structure to estimate the impact of pruning on 8 tree species in two Italian cities: Taranto and Florence. Crown widths were reduced by 1.6 m on average, however there is large variation between species variation with branches more often being removed for thinning crowns resulting in larger gap fractions, which increased by 15% on average. No significant differences were observed for crown widths or gap fraction between trees pruned 3 and 4 years previously, suggesting that tree crowns structurally recover from pruning after 3 years. A deterministic model revealed that current urban forest pruning rates (every 6 years) and mortality (1%) may create a situation in which a city dominated by the species studied benefits from 93.5% of the maximum ecosystem services possible. This work will allow more nuanced estimates of urban forest services to be calculated.