Trees in the city: measuring rather than assuming

Urban greening: between promise and reality

In the face of accelerating climate change, the greening of cities is an obvious response to urban overheating. But behind the obvious, an essential question remains: how effective are trees in terms of cooling urban spaces and thermal comfort?

Contrary to the simplistic rhetoric that equates tree planting with guaranteed environmental results, scientific studies provide a more nuanced vision, based on the measurement and analysis of local contexts.

Three determining factors: climate, urban form, species traits

The thermal effects of urban trees are not automatic. Their ability to reduce heat islands depends on three interdependent parameters:

• local climate: average temperature, air humidity, intensity of solar irradiance
• urban morphology: building density, street orientation, sky view factor
• tree characteristics: foliage type (deciduous, evergreen), leaf density (LAI, LAD), height, crown shape

Together, these factors determine the effectiveness of the shading, evapotranspiration and ventilation mechanisms that enable plantations to modulate temperatures at pedestrian level.

What the scientific literature shows: quantified results

A recent meta-analysis, published in Communications Earth & Environment (Li et al., 2024), synthesizes the results of 182 studies covering 110 cities and 17 climate types. This review offers the best available insight to date into the actual thermal efficiency conditions of urban plantations.

Temperature reduction (ΔTair) measured :

Note: in dense urban forms (LCZ 1-3), efficiency is lower, with the risk of a reverse effect at night (heat trapping).

Factors amplifying the cooling effect :

• mixed species (deciduous + evergreen) in temperate, continental and tropical climates
• Dense, well-developed canopies (high LAI / LAD)
• installation in open areas with good air circulation

Limiting factors :

• stomatal closure (closing of leaf pores) in prolonged heatwaves
• young plantations (very limited effectiveness before maturity)
• over-dense, compact plant morphology with poor night-time ventilation

Don't confuse vegetation with biodiversity, or planting with climate efficiency

The choice of tree species, their density, their layout and their suitability to the climatic context are essential technical parameters. To equate tree planting with automatic biodiversity production or a universal thermal solution is dangerously simplistic.

Biodiversity, like climate performance, cannot be decreed: it must be designed, measured and assessed using appropriate methods.

Evaluating strategies, certifying results: the role of the independent third party

In this context, independent evaluation of greening strategies is essential to move away from the realm of intentions and towards that of real effectiveness. Performance indicators, measurement methods (ΔTair, UTCI, PET), contextualization of results: these are all levers that can be used to adjust choices and avoid false promises.

As an independent certifier, IRICE's actions are in line with this logic of methodological rigor, serving to ensure the robustness of environmental approaches adopted by project owners, developers and local authorities.

Conclusion: act with method, not slogans

Trees are a valuable lever for action. But like all levers, their effectiveness depends on the conditions in which they are used.

Planting is not proving. Measuring, adapting, certifying: this is how credible strategies to combat the urban heat island are built.

References

Li, H., Zhao, Y., Wang, C., Ürge-Vorsatz, D., Carmeliet, J., Bardhan, R. (2024). Cooling efficacy of trees across cities is determined by background climate, urban morphology, and tree trait. Communications Earth & Environment, 5:754. DOI : https://doi.org/10.1038/s43247-024-01908-4