How good are containerized trees for urban cooling?

The capacity of urban trees in mitigating urban heat is well-known. As space is often limited, one feasible option for increasing the urban green would be containerized plants. Nevertheless, for optimizing the vitality and benefits, detailed knowledge on tree growth reactions in different types of containers is missing. We designed an experiment with two commonly planted but ecologically contrasting urban tree species Tilia cordata and Platanus x hispanica planted into the ground and in containers according to four different planting types, with or without drought stress. Along with the meteorological variables, continuous soil moisture and temperature at 25 cm depth, sap flow, as well as measurements of leaf physiological responses i.e. stomatal conductance, mid-day leaf water potential and chlorophyll content were measured three times on sunny and warm summer days during 2020 and 2021. P. hispanica showed more than double diameter increment at breast height in the ground than in containers; however, the growth trend was relatively better for T. cordata in containers. While comparing different container types and species reactions, it was clear that soil temperature within the plastic containers were significantly higher, whereas insulation is not enough to reduce either the temperature or slowing down the soil drying out. Where both the species showed lower stomatal control over atmospheric demand, P. hispanica showed leaf transpiration energy loss of around 300 W m⁻² when planted in the ground and T. cordata trees around 260 W m⁻² when planted in non-insulated containers, which are comparable to the energy loss from the street trees. Therefore, a strategy of mixed planting with faster growing species such as P. hispanica that provide stronger cooling at the initial stage in the containers to be complemented and eventually replaced with medium growing species T. cordata for relatively longer time period could be suggested.