Implications for city greening
Cities and their rural surroundings are often quite different environments in terms of their tree composition. For Leipzig, the overlap between tree genera planted in the city and those growing in regional forests is small. Unlike Leipzig, the surrounding forests consist mostly of spruce and pine (https://bwi.info, data for Saxony, accessed 2020-03-07), that is, taxa of no or minor importance as allergen producers (cf. Table E1). By planting trees in proportions found in natural habitats and eliminating highly allergenic taxa47,48 city greening may become even more beneficial.
Attention to allergenic potential is also important when selecting tree species that are able to withstand the challenges posed by climate change. Resilient tree taxa 49-51 may prove detrimental for the development of allergenic diseases. Examples includeAlnus x spaethii 52 and Corylus colurna,a close relative of the major allergen source Corylus avellana,which despite their sturdiness should be reconsidered as future city trees.