Cool Tree Architecture: A Descriptive Framework for a Tree Architecture Typology to Temper Urban Microclimates

As the elementary unit of the urban forest, trees temper thermal extremes in urban microclimates through shading and evapotranspiration, and by altering the movement of air. Metrics on shade performances of different species, however, are currently limited, which can be remedied by the development of a method to describe the range of species and cultivars via a structured overview of physical characteristics impacting radiation reflectivity, absorptivity, and transmissivity. This paper proposes a descriptive framework based on the concept of “tree architecture,” which has developed into a recognized field of plant study from the perspective of their physiognomy, morphology, and morphogenesis. The framework describes various architectural sub-traits within the overall trait categories of Crown, Wood, and Foliage. The descriptive framework can be used to develop a “Cool Tree Architecture Typology” (C-TAT), in which trees can be organized into similar types based on common physical characteristics. Further elaboration of sub-traits using observations of trees in controlled field laboratories resulted in new derivative classes for use as key in classifications for the C-TAT. The C-TAT can be used to organize the many species and cultivars occurring in, for example, Cfb Atlantic climate zone cities, to a lesser number of architectural types. This allows for more rapid evaluation and cooling performance calculations of tree inventories and can also be of value in assisting tree managers to propose more accurate thermal performance standards for trees in urban projects. The elaboration of tree architecture from an urban microclimate perspective complements existing elaborations and approaches in the field of tree architecture.