Urban forests against heat waves in cities

Through the application of the urban climate model called “i-Tree Cool Air”, which integrates variables relating to temperature, humidity rates, type of land cover and vegetation densitythe researchers calculated the trees’ cooling capacity resulting from the combined processes of shading and evapotranspiration. To verify the effectiveness of the model, the working group composed of Theodore Endreny, Marco Ciolfi, Anna Endreny, Francesca Chiocchini and Carlo Calfapietra projected the tree cover scenario to 30 percent based on the weather and climate of the summer of 2003. Simulations indicate that a similar density of green areas in individual neighborhoods would have allowed the extreme thermal impact to be reduced by approximately 36 percenta parameter calculated taking as a reference the excess heat-related mortality recorded in the population aged 65 or over.

The distribution of thermal benefits is not homogeneous, but is mainly concentrated in urban sectors characterized by a high population density and a high rate of soil overbuilding. Regarding local microclimatic dynamics, Marco Ciolfi and Francesca Chiocchini explained: “The benefits would be particularly important in the most concreted and densely populated neighborhoods, where the urban heat island effect is more intense. The cooling produced also depends on the availability of water: in Mediterranean climates, which are increasingly subject to drought, it may be necessary to integrate green infrastructure and efficient water management systems to keep trees healthy without excessively increasing humidityThe study therefore highlights the need for an integrated approach that combines the planting of tree species with a sustainable management plan for water resources.

The introduction of urban forests also generates a series of collateral environmental benefits which directly impact the resilience of cities. The ecosystem services provided by the increase in tree stock include the reduction of air pollutants, the increase in carbon dioxide sequestration and the reduction of surface runoff of rainwater, thanks to the creation of more permeable surfaces. From an economic point of view, the value of these environmental benefits has been estimated on average at around 56 thousand dollars per year per square kilometer of tree-lined area. Theodore Endreny underlined the applicative prospects of the research: “Compared to the urgency of preparing cities for extreme heat, this research work proposes a concrete solution. Urban forests can become a solution for climate resilience, benefiting public health and quality of life, along with increased permeable surfaces and stormwater management systems. We plan to develop detailed models, in order to identify the neighborhoods designated for the home of new treesThe detailed mapping will represent the next step in planning urban forestry interventions on a neighborhood scale.