Urban trees and their ecosystem services

Urban forests and trees provide a range of benefits, called ecosystem services. A subset of these are regulating services, including carbon storage and sequestration, stormwater runoff attenuation, and urban heat island mitigation. The focus of this report was to quantify the degree to which trees contribute to these regulating services. Moreover, the factors that influence trees’ contribution were explored. These aims were achieved by reviewing the scientific literature pertaining to these topics. The review methodology resulted in roughly 100 scientific articles split across the three regulating services. These articles were used to quantify and qualify the role of trees with respect to carbon storage and sequestration, stormwater runoff attenuation, and urban heat island mitigation. The review showed that above-ground carbon storage density for trees averaged 11.5 kg of carbon per square meter of tree canopy cover (range 1.7–28.9 kg C m-2 ), while total carbon (above and below ground) storage density for trees had an average value of 7.95 kg/m2 (range 0.8–36.1 kg C m-2 ). Trees also reduced stormwater runoff, primarily by intercepting between 9% and 61% of total rainfall. Finally, ground surface temperatures were 0.6–22.8°C and air temperatures were 0.8–7° cooler beneath trees than in surrounding non-treed environments. The variation in carbon storage and sequestration, stormwater runoff attenuation, and urban heat island mitigation was shown to be related to the quantity of trees (e.g., tree density or canopy cover), their configuration (fragmentation, clustering), and their structural characteristics (e.g., height, crown volume and shape, stem diameter, leaf area or density, wood density), the latter of which is influenced by tree species and age. More trees or tree cover, in clusters, with greater total biomass and wood density, will improve the regulating services researched in this report. In contrast, development intensity and impermeable surfaces (buildings and/or pavements), which are associated with reduced tree cover, threatened the provision of carbon storage and sequestration, stormwater runoff attenuation, and urban heat island mitigation by trees