Microclimatic differences and their influence on transpirational cooling of Tilia cordata in two contrasting street canyons in Munich, Germany

- Urban microclimate can influence the cooling effect of urban trees.
- Paved circular street canyon reduces the wind speed but increases soil temperature.
- Trees grown at open green square provide better cooling during the day.
- However, nocturnal cooling was stronger for trees at the circular paved square.

Urban trees can help to mitigate the urban heat island through evapotranspiration. However, growing conditions in cities are heterogeneous and micrometeorological conditions in street canyons can have a large impact on a tree's transpiration. Here we investigated a common urban street tree species Tilia cordata of different ages and sizes, planted in two contrasting street canyons in a densely built neighbourhood within the centre of Munich, Germany: Bordeaux Platz, an open green square (OGS), and Pariser Platz, a circular paved square (CPS) with similar aspect ratio ≈ 0.5. The experiment was carried out during the exceptionally hot and dry summer of 2015. The open green square showed significantly higher wind speed and vapour pressure deficit but lower soil temperature, less negative soil moisture potential and also a smaller wind tunneling effect compared to the circular paved square. All these variables showed strong relationship with the sapflux density (Js) of trees grown at these two sites. They explain almost 78% of the variation in Js. On average Js values of the trees at Bordeaux Platz peaked to 0.30-0.35 ml cm−2 min−1 during the day compared to the peak values of 0.20-0.23 ml cm−2 min−1 for trees at Pariser Platz. Consequently, trees grown at the open green square showed higher peak cooling of 2.3 kW tree−1 or 82 w m−2 than at the circular paved square (1.9 kW tree−1). Interestingly, nocturnal cooling was higher for trees at the circular paved square with 18% of daytime transpiration compared to 2% for trees at the open green square. The study gave new insights into the cooling benefits of urban trees planted in open green squares compared to closed and paved squares and its temporal variation.