Urban afforestation in compact cities – 02/23/2024 – Claudio Bernardes

Daytime microclimatic conditions are affected especially by solar radiation, and the microclimate in specific areas of cities has very noticeable effects, especially with regard to thermal comfort.

To adequately address future urban climate conditions, landscape planners and real estate developers need reliable information about the conditions that affect changes in microclimate in order to create an urban design component suitable for implementing the built environment, urban vegetation and of street trees.

The application of nature-based solutions in cities to mitigate the harmful effects of urban overheating and make cities more resilient to heat waves has gained the attention of urban planners and researchers in recent decades.

In this context, appropriately designed road afforestation projects can become useful tools for planning neighborhood climate, resolving interactions between buildings, trees, the road system and the atmosphere.

To better understand these effects, researchers from the Institute of Environmental Sciences at the Autonomous University of Barcelona, ​​in Spain, developed a study to evaluate the effects of street trees on the microclimate in compact cities, where the high density of urban residents and intense traffic activities require urban planners to focus on urban design to improve thermal regulation.

Measurements were carried out in the city of Barcelona during the summer, on cloudless days, along two parallel streets with different tree densities but similar geometry, recording ascending and descending radiation fluxes, air temperature and humidity.

These data were used to evaluate the combined impacts of vegetation and buildings on the urban microclimate.

When comparing air temperatures observed at pedestrian level between the two streets, air temperatures up to 2.7 °C higher were observed in the street with low tree density compared to the one with denser trees.

Depending on the wind conditions, a considerable increase in the surface temperature of the pavement was observed, with maximum daytime temperatures around 10°C higher in the street with lower tree density than those measured in the street with greater tree density.

A greater flux of shortwave radiation (of solar origin) was found in the street with lower tree density compared to the other (150 W/m²). The temperature of the walls of buildings was approximately 2 °C lower in streets with more trees.

The researchers concluded that the main mechanism that induces thermal cooling of streets derives from the shading of ground surfaces, especially in the case of artificial materials such as asphalt and concrete, which have high heat retention, and the corresponding redirection of latent heat via transpiration.

Human thermal comfort can be estimated mainly from the meteorological conditions around the pedestrian, that is, air temperature, water vapor pressure, wind speed and mean radiant temperature. The study also evaluated the impact of trees in this regard.

The results presented indicate that street trees produce clear positive effects on radiation and air temperature. These effects can change the meteorological conditions perceived by a pedestrian and improve their thermal comfort.

The positive effects of road afforestation on the microclimate are quite evident, but it is up to planners to study the best ways to achieve perfect integration between the urban tree ecosystem, roads and buildings, in order to obtain the best possible results from this combination, especially for urban spaces that require greater compactness.

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