Optimizing urban forests for climate resilience: shading, transpiration, and multilayered vegetation in mitigating heat stress across climates

Urban forests have emerged as a key strategy for promoting adaptive capacities of cities to climate change by mitigating urban heat island (UHI) and thus heat stress for humans. An empirical study in Würzburg, Germany (2018–2020) revealed a significant increase in extreme heat conditions, with nine days exceeding a Wet Bulb Globe Temperature (WBGT) threshold of 35°C in the treeless city center, compared to none in a treed suburban site just five kilometers away.

Further research in Munich, Germany, comparing two ecologically contrasting tree species, Tilia cordata and Robinia pseudoacacia, under varying meteorological and soil conditions highlighted that shading is the dominant cooling mechanism, particularly under high atmospheric aridity. In temperate climates with adequate soil moisture, lighter shaded canopies may enhance grass evapotranspiration, suggesting a preference for dense canopies over built environments and lighter canopies over grass surfaces.

To explore cooling mechanisms across climates, sensible and latent heat fluxes under tree canopies within various settings (park, street, square) were analyzed in Munich (temperate) and Beer Sheva, Israel (hot arid). Despite irrigation, trees in Beer Sheva exhibited higher hydraulic resistance, limiting transpirational cooling (regression slopes of 0.25–0.44 in Beer Sheva vs. 0.10–0.18 in Munich). Analysing the proportion of latent heat to total available energy, we found transpiration cooling was about 40% less significant at noon hours in summer in Beer Sheva than in Munich. These results suggest that with anticipated higher atmospheric aridity, shading will become even more important, thereby, denser tree canopies would be of higher importance.

Additionally, LiDAR scanning revealed that multilayered vegetation enhances cooling benefits more than twofold compared to single-layered vegetation. Thus, increasing vegetation complexity both horizontally and vertically can optimize thermal benefits through shading and transpiration across diverse climates, while also supporting biodiversity and fostering nature connectedness.