Can removing pavement help urban trees cool public squares as the climate warms?

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Trees help mitigate the urban heat island effect, but climate change and extensive soil sealing increasingly challenge their growth and cooling capacity.

This study examined the combined effects of surface sealing and future climate conditions on tree growth and microclimatic cooling across 25 public squares in Munich, Germany, including urban tree performance under RCP 8.5 climate change. Its design combined process-based modelling with empirical air-temperature measurements.

Using the process-based CityTree model, the researchers simulated biomass increment per unit basal area and transpiration-driven cooling under current climate conditions (1991–2020) and future conditions (2081–2090). The future simulations followed Representative Concentration Pathway (RCP) 8.5 from the IPCC Fifth Assessment Report (AR5). To characterize current shade-cooling patterns, the simulations were complemented by air-temperature measurements collected with iButton sensors during one summer season.

Under current conditions, the model indicated that reducing soil sealing by 20% increased biomass increment by 21.5% and canopy cover by 25%, alongside a 13.2% increase in cooling efficiency through transpiration.

Under the future RCP 8.5 scenario, predicted biomass increment declined by 16.5% and canopy cover by 15%. The cooling improvement from soil unsealing weakened to about 4%, reflecting constraints from higher temperatures and reduced water availability. The abstract also reports projected declines of 16.5% in modelled growth and 5.6% in transpirational cooling under this future scenario. It attributes the weaker cooling benefit of unsealing to drought-induced stomatal closure limiting latent heat flux.

The empirical measurements showed substantial differences in shade cooling between squares. At Alpenplatz, with 56% canopy cover, air temperatures dropped by up to 2.7 °C during peak hours. Heavily paved Bordeauxplatz and Marstallplatz had smaller reductions of 1.4 °C and 1.8 °C, respectively. The abstract identifies canopy density as the primary driver of pedestrian-level cooling in these measurements, with air-temperature reductions reaching 2.7 °C.

The findings highlight the pivotal role of soil permeability and spatial design in maintaining tree vitality and microclimatic regulation in densely built urban environments.