Targeted irrigation to improve the cooling potential of urban trees as a nature-based solution: Insights from a field trial in Hanover

• First experimental field trial to measure the climatic benefits of urban tree irrigation. • Irrigation increases the benefits of trees as nature-based solutions. • Tree irrigation leads to air temperature reductions up to 0.67°C and humidity increase up to 5.98 %RH. • Diverse behaviour of soil water potential under urban trees due to different irrigation strategies. With regard to the rising temperatures due to climate change and the intensification of the urban heat island effect (UHIE), nature-based solutions (NbS) are becoming increasingly important. In particular trees can mitigate the effects of the UHIE by shading and through transpiration. As trees suffering from heat and drought decrease their cooling properties under stress, this study investigates the effects of supplementary irrigation to increase the cooling capacities. The research focus is on how different irrigation practices can directly support the cooling effects of young urban trees as a NbS. The novelty of our study is the direct field trial comparison of irrigation quantities in combination with the quantification of their microclimatic effects. From May to September 2024, we observed three groups of Tilia cordata (unirrigated, monthly irrigated, weekly irrigated) in the city of Hanover, Germany, and measured air temperature and relative humidity (RH) within the tree crowns. The results show, that an investment in tree irrigation results in an increase of cooling capability of up to -0.67°C and up to +5.98 %RH on hot days over 30°C. Urban tree care is more than just keeping trees alive by minimizing water use, trees are a valuable resource that we can benefit from by using water wisely. Future-oriented tree irrigation, which not only ensures the survival of the trees but also promotes their natural functions, optimises the urban climate in a natural way.