Description and habitat

Blue Ash (Fraxinus quadrangulata) is a riparian tree species widely distributed across the Long Point Basin. Similarly to Ontario’s other ash species, Blue Ash can attain a height of 25 m or more in ideal growing conditions. It is one of five species of ash in the Long Point Basin – these include White Ash (Fraxinus americana), Green Ash (Fraxinus pennsylvanica), Black Ash (Fraxinus nigra), and Pumpkin Ash (Fraxinus profunda). As a genus, ash trees are distributed widely across the Northern Hemisphere. All ash trees in Ontario have oppositely arranged, pinnately compound leaves

Blue Ash is readily distinguished from the other ash species by it’s twig structure. The twigs have four ridges, giving them a square-like appearance. Because of this unique feature, Blue Ash is quickly identified. The leaves of Blue Ash, 20-40 cm long, each contain 5-11 leaflets, measuring 7-13 cm long with coarsely serrated margins. The bark is grey, comprised of scaly plates, unlike that of White Ash and Green Ash, both which have a regular pattern of ridges and furrows. The flowers appear in May, each comprised of both stamens (male structures) and pistils (female structures), allowing for self-pollination, in contrast with the dioicous White Ash and Green Ash populations. In September, the seeds ripen, each measuring 2.5-5 cm, including the wing, which obscures most of the seed.

In Ontario, Blue Ash is only found in the southwesternmost areas of Ontario. It is most common in riparian areas and other bottomlands. The Long Point Basin represents the eastern range limit for this species, with the only natural population (as known) in Catfish Creek Valley in the western part of the region. Other Blue Ash populations in Ontario are found along the Thames and Sydenham Rivers, at Point Pelee National Park, and on Pelee Island, where it is most abundant. Blue Ash natural distribution extends southward to Tennessee and Missouri.

In it’s natural riparian habitat, Blue Ash is most readily encountered in mesophytic bottomland forests with White Ash, Green Ash, Bitternut Hickory (Carya cordiformis), Shagbark Hickory (Carya ovata), Basswood (Tilia americana), Black Maple (Acer nigrum), Burr Oak (Quercus macrocarpa), Red Oak (Quercus rubra), Black Walnut (Juglans nigra), and Hackberry (Celtis occidentalis). It is rarely found in Beech (Fagus grandifolia) – Sugar Maple (Acer saccharum) forests, but these represent dispersal from core populations, rarely found from major rivers. This species is intolerant of the growing conditions characteristic of swamps or oak savannas. On the limestone alvars of Pelee Island, Blue Ash grows with Chinquapin Oak (Quercus muehlenbergii), Black Walnut, Hackberry, Basswood, and Shagbark Hickory.

Wildlife importance

Each autumn, Blue Ash seeds are consumed by wide diversity of wildlife. The twigs are also a valuable food source for white-tailed deer (Odocoileus virginianus). Blue Ash is also utilized by dozens of insect species. These include many butterflies and moths, including polyphagous species like the Hickory Tussock Moth (Lophocampa caryae), White-marked Tussock Moth (Orgyia leucostigma), Polyphemus Moth (Antheraea polyphemus), Cecropia Moth (Hyalophora cecropia), Luna Moth (Actias luna), Red-Spotted Purple Butterfly (Limenitis arthemis), Mourning Cloak Butterfly (Nymphalis antiopa), and the Eastern Tiger Swallowtail (Papilio glaucus).

Emerald Ash Borer 

Since the early 2000s, Ontario’s ash species are threatened by an invasive beetle from northeast Asia, Emerald Ash Borer (Agrilus planipennis, subsequently EAB). Blue ash is unique among Ontario’s Ash species that mature individuals have largely survived Emerald Ash Borer. It has a resistance rate at around 70-90%.

Originally introduced to Detroit and Windsor on shipping cargo, EAB has since spread across the Great Lakes Region. In the Lake Erie Region, EAB has killed over 85% of mature Ash trees. White Ash has slightly more resistance than Green Ash or Black Ash, and in some of the woodlots, there are healthy ash trees that have survived EAB. Several insect parasitoids have been introduced in an attempt to control EAB populations. This includes four wasps – Tetrastichus planipennisi, Spathius agrili, Spathius galinae, Oobius agrili. Wasp larvae consume the eggs or larave of EAB, resulting in both EAB mortality and large population increases of the biocontrol waps. In combination with natural predators (including Woodpeckers, Picidae, which can consume >50% of all EAB larvae), native parasitoids, and innate host tree resistance EAB, the future goal is to reduce the EAB population so that native ash trees can naturally regenerate into the forest canopy without experiencing EAB-induced mortality. Ash tree saplings remain abundant in the understories of many forests.

Fun facts

• Blue Ash gets its species name quadrangulata (meaning four-angled)  from the twigs, which are usually square in cross-section. This is the most reliable method of identification! 
• When the inner bark is crushed and exposed to air, it reacts to produce a blue-colored dye. This gives Blue Ash it’s name!
• The ability of Blue Ash to grow on limestone alvars comes from a tolerance for alkaline soils with a high pH level. Blue Ash is also quite drought tolerant, despite being a mesophyte!
• Ecologists believe that the heightened resistance of Blue Ash trees to EAB is due to a higher tannin content and a distinct phenolic profile. These chemical defenses are believed to act as toxins to EAB larvae, often unable to complete proper development. Additionally, Blue Ash is comparatively distantly related to other ash species.

 

Written by Matthew Palarchio, HBSc in Environmental Sciences, Western University.

Photos by Matthew Palarchio 

 

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