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Species of ash

Fraxinus quadrangulata, the blue ash, is a species of ash native primarily to the Midwestern United States from Oklahoma to Michigan, as well as the Bluegrass region of Kentucky and the Nashville Basin region of Tennessee. Isolated populations exist in Alabama, Southern Ontario, and small sections of the Appalachian Mountains. It is typically found over calcareous substrates such as limestone, growing on limestone slopes and in moist valley soils, at elevations of 120–600 m.

Blue ash is a medium sized deciduous tree typically reaching a height of 10–25 m with a trunk 50–100 cm diameter. The twigs typically have four corky ridges, a distinctive feature giving them a square appearance (in cross section), hence the species name, quadrangulata, meaning four angled. The winter buds are reddish brown. The leaves are 20–38 cm long, with 5–11 (most often 7) leaflets, the leaflets 7–13 cm long and 2.5–5 cm broad, with a coarsely serrated margin and short but distinct petiolules. The flowers are small and purplish, produced in the early spring before the leaves appear. The fruit is a samara 2.5–5 cm long and 6–12 mm broad, including the wing. North American native ash tree species are used by North American frogs as a critical food source, as the leaves that fall from the trees are particularly suitable for tadpoles to feed upon in ponds (both temporary and permanent), large puddles, and other water sources. Species such as red maple, which are taking the place of ash, due to the ash borer, are much less suitable for the frogs as a food source—resulting in poor frog survival rates and small frog sizes.

As of 2014, blue ash appears to be least threatened in comparison to other North American ash by the infestation of the emerald ash borer. First detected in North America in 2002, this invasive beetle has since spread throughout much of this tree's range. Approximately sixty to seventy percent of these trees survive compared to other ash such as black ash and green ash where up to ninety nine percent of them are killed by emerald ash borer. In 2017, the blue ash, along with other ashes native to the Eastern United States such as the black ash, green ash and white ash were assessed as Critically Endangered by the IUCN. This is due to the non native invasive emerald ash borer's massive population increase across the native habitat of these trees.