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Alders in the genus Alnus are common wetland, riparian, and cool-climate trees or shrubs across large parts of North America. For allergy sufferers, they matter because alder is one of the earlier tree-pollen sources of the year, especially in the Pacific Northwest, coastal Alaska, the Great Lakes, the Northeast, and streamside habitats in the East and Southeast.^1,2

Maple trees in the genus Acer are among the most widespread deciduous trees in temperate North America. Across the United States, Canada, and Puerto Rico, maples are common in forests, parks, and residential landscapes, making their pollen a regular component of the early-spring airborne allergen environment. The genus contains roughly 110–120 species globally, with about 12–15 native species in North America and several additional ornamental species planted widely in urban landscapes.¹