Influence of evaporative cooling by urban forests on cooling demand in cities

Trees provide important ecosystem services to urban human society. Their absence can lead to more pronounced environmental and social consequences, for example the urban heat island effect. Evapotranspiration (E-t) from trees reduces air temperature in the urban microclimate by converting sensible heat to latent heat. Quantification and valuation of the ecosystem services provided by urban trees is important for improving cost-benefit evaluations in support of protecting tree planting and maintenance budgets and, thus, for building climate change resilience into cities. Inclusion of E-t cooling could improve ecosystem service valuation models by producing a more complete picture of the benefits that urban trees provide to society. This study explores two approaches for evaluating climate regulation as an ecosystem service of urban trees. Firstly, an enthalpy-based approach was adopted to valuate latent heat of evaporation from tree transpiration (in three case study urban forests) by equating it to an equivalent service from an active direct evaporative cooling system. Secondly, energy savings to air-conditioned buildings was modelled using TRNSYS and TRNFLOW simulation programs with and without air precooled and humidified by urban trees. Trees are shown to provide substantial urban cooling with an annual valuation of 84 pound m estimated using the enthalpy-based approach, or ranging from 2.1 pound m to 22 pound m using TRNSYS and TRNFLOW dynamic simulation programs; both for inner London case study. The latter savings arose from a modelled 1.28-13.4% reduction in air-conditioning unit energy consumption. Challenges around assumptions of homogeneity in both built form and urban forest canopy effects are discussed. The case study examples highlighted differences in E-t cooling between tree species, with Castanea saliva, Prunus avium, Quercus petraea, Platanus hybrida and Fagus sylvatica typically providing more E-t cooling than any of the other tree species commonly found in urban forests. The research highlighted a shortage of published E-t data, particularly for urban environments.