Historical drivers of street tree species selection: A comparative archival study of Canadian prairie cities

• We examined archival documents on street trees from five Canadian cities. • Desirable species were limited by their natural and nursery availability. • Aesthetics have evolved from large tree-lined streets to ornamental species. • Species-specific ecosystem disservices resulted in the shift from poplar to elm and ash. • Conflict with utilities contributed to the use of ornamental species over elm and ash. There is a long history of planting trees along streets in cities around the world. In many cases, only a few species were used at any given time, leading to localized monocultures that increase urban forest vulnerabilities. In North America, elm and ash were historically among the most dominant tree species, yet only a nascent body of literature has examined the factors that influenced their dominance. In this study, we examine historical street tree species selection in five Canadian prairie cities, an underrepresented geography in urban environmental history literature. Using grounded theory and inductive coding, we developed a conceptual framework that explains historical biophysical and socio-political drivers of tree species dominance. The first driver is desired tree species traits. These traits, in turn, are related to the second and third drivers , species-specific ecosystem services and disservices, which influence what species are planted, avoided, or removed once they reach maturity. The fourth and fifth drivers are species availability and species-specific pests and pathogens, which were the ultimate drivers determining what species were planted on city streets. In the five study cities, Winnipeg and Regina planted American elm and green ash soon after their planting programs began because they were readily available. Calgary, Edmonton, and Lethbridge originally planted native Populus species, but later shifted to the non-native American elm and green ash due to the ecosystem disservices of Populus species at maturity, including the production of fluff and conflicts between roots and underground utilities. Today’s street tree composition reflects these legacy planting decisions.