Cool Tree Architecture - A tree architecture typology to temper urban microclimates

As the ‘mainframe’ of green infrastructure, the urban forest is recognized as vital to the adaptation of cities to deal with urban heat extremes. But while it is common knowledge that trees contribute to cooling through shading, it remains unclear what each tree species cooling contribution is. Current indicators of potential cooling performances such as gap fraction and leaf area index overlook the more complex combination of traits of urban trees that lead to thermal amelioration. This paper elaborates on a study to develop a “Cool Tree Architecture Typology” (C-TAT), involving a structured and comprehensive overview of the physical characteristics that deliver the cooling performance of trees in urban environments. It hypothesizes by extension, that trees with a similar C-TAT profile have comparable cooling performances. The C-TAT framework builds on the foundations of tree architecture, a field of study that understands trees in terms of their physiognomy, morphology, and morphogenesis. A multi-year study was done of 69 similarly-aged young specimens of different tree species common to Cfb climate zone cities. These were installed in above-ground planter boxes in a paved courtyard area on the TU university campus in the city of Delft in the Netherlands. The results of the study show that six sub-traits together impact reflection, absorption and transmission of radiation. These include Crown Density, Wood Zoning, Foliage Porosity, Foliage Trans-luminescense and Wood Grain. Based on this order of traits an overall ranking of the trees was established, with the 69 species reduced to a total of 51 architectural types. An abbreviated type was also trialed, considering Crown Density and Foliage Trans-luminescence only as traits with the least crossover in terms of characteristics. This clustering resulted in nine architectural types.