Chapter 7, Allergenic potential of street trees in Boston, Massachusetts

While urban ecosystem services have gained widespread attention in recent years, nuisances and disservices by urban ecosystems are rarely being addressed. One such nuisance is the release of pollen with allergenic relevance into the ambient air by urban trees. In this study, we analysed Boston's street tree inventory in terms of allergenic potential, identifying allergenic relevant species with the help of literature and the latest findings in allergen research. In addition, a three-dimensional microscale tree pollen dispersion for Quercus (oak) was simulated using the Quick Urban & Industrial Complex (QUIC) dispersion modelling system for a small subset of the urban body (ca. 1.2 km2). Approximately 17% of Boston's street trees have allergenic relevance, mainly belonging to the Quercus (oak) and Ulmus (elm) genera. An additional 36% hold a high potential, dependent on the sex of individual trees, mainly belonging to the Acer (maple) and Fraxinus (ash) genera. While the diversity of street tree species is high, a few species like Acer platanoides (Norway maple) dominate. The simulation revealed that street trees have a strong local impact: zones of moderate and high pollen levels can be found in close proximity (up to 115m) to the emitting trees and low concentrations up to 700m away. However, wind direction, wind velocity, and building density and alignment have a strong influence on local concentrations. We suggest that allergenic potential should be taken into consideration when planting trees in cities. Increasing the evenness of the street trees would help to avoid over-abundance and reduce exposure to single pollen allergens. Some species should be avoided altogether.