The Sad Loss of Our Ash Trees
I was tempted to title this post “Kiss Our Ash Goodbye?”, but that sophomoric tone does not seem appropriate when I think about the sad loss of our ash trees to the Emerald Ash Borer in the coming years. EAB was recently confirmed in Buffalo, Washington and Seward counties and forest health experts agree that the insect has likely already spread much farther than that. As EAB continues its inexorable march across the state, now’s a good time to remind ourselves that over 40 million ash trees could be lost in Nebraska in the coming years! This includes more than a million trees in communities and millions more in shelterbelts and other planted landscapes.
Some magnificent green ash on my block in Waverly. My neighbor is not happy about EAB.
What will be lost? Since EAB’s arrival in the US in 2002, it’s been easy to dismiss ash trees as being unworthy of discussion, let alone for planting. Green ash especially is considered kind of a “junk yard dog” of trees, being tough as nails, but not as refined as other species. And yet in Nebraska, green ash has been casting significant shade in communities and giving stature to shelterbelts across the state for many decades. The canopy of more than a million community shade trees will be lost in the coming years which will mean hotter summer temperatures, more energy consumption, more stormwater runoff, lost wildlife habitat and fewer trees for kids to climb!
Ecological Impact. The ecological impact of losing native ash trees across North America is going to be significant. Ash species provide critical habitat for a wide variety of wildlife including many important insects and birds. And the loss to native frogs could be especially hard-felt. Ash leaves, with their low tannins, are an important food source to frog tadpoles when they fall into ponds and other water bodies. Ironically, researchers believe that the low-tannin content of native ash species also makes them more susceptible to EAB feeding.
Ash Species in Nebraska. About 13 species of ash are known to occur naturally in the US with two species being native in Nebraska (green ash and white ash). It’s interesting that the most common US species are named by color including green ash, white ash, black ash and blue ash, all of which can be found in Nebraska. At least two foreign species can also be grown here. A quick look at some of the ash species found in Nebraska:
Green ash (Fraxinus pennsylvanica) is named for its uniformly-green leaves and is by far the most common ash tree in Nebraska. It occurs naturally across the state being found most commonly in wet floodplains but can also be found in drier upland sites, including in the Pine Ridge. Green ash is one of the most common shade trees planted across the state and is one of our most reliable species for yellow fall color.
White ash (Fraxinus americana) is native to southeast Nebraska and is named for the whitish underside of its leaves. It’s best known by several popular cultivars that turn purple-red in the fall, including the variety ‘Autumn Purple’ which was planted in great quantities across eastern Nebraska in recent decades. This fall color will be sorely missed. White ash is also the preferred lumber for baseball bats! Oh my.
Blue ash (Fraxinus quadrangulata) was given its common name for a blue dye made from its inner bark. The scientific species name quadrangulata refers to its young twigs that are distinctly squarish, or four-sided. Although not common in Nebraska, there are many fine specimens growing in eastern communities. It’s important to note that blue ash shows some resistance to EAB (see discussion that follows).
Black ash (Fraxinus nigra) is named for its black buds and is native to wet ground across much of eastern Canada and the northeast US. It’s very cold hardy and is sometimes planted in Nebraska. It was prized by native Americans for basket weaving.
Common Ash (Fraxinus excelsior) is native to much of Europe and is occasionally planted in eastern Nebraska. The variety ‘Hessei’ is unique in that it has simple leaves rather than the compound leaves of most other ash species. Several of these trees were planted in Lincoln decades ago and often confuse people trying to ID trees. This species also has black buds.
Manchurian ash (Fraxinus mandshurica) is native to northeast Asia and Japan and is closely related to black ash (Fraxinus nigra) which it easily hybridizes with. The species has dark buds and is considered very cold hardy. The variety ‘Mancana’ has been planted in some locations in Nebraska and is distinguished by its very formal, upright branching structure.
EAB Resistance and Survivor Trees: Although most ash trees are considered highly susceptible to EAB attack, two species stand out with some potential resistance. Blue Ash (Fraxinus quadrangulata) is reported to be moderately resistant to EAB with about 30-40% mortality reported in its native range. The species is also adaptable to higher pH soils, which Nebraska has plenty of. There are also mixed reports that Manchurian ash, especially the cultivar ‘Mancana’, may be resistant to EAB. Manchurian ash and EAB co-evolved on the Asian continent so it would make sense that the species could develop at least some genetic resistance to the borer. Adventurous tree planters can keep trying these trees with caution and we’ll be keeping an eye on them for EAB resistance in the coming years.
Finally, researchers in the upper Midwest and northeast US are noticing some survivor trees of green ash, black ash and white ash. It’s not yet clear if some of these survivors are truly resistant to EAB or have just lucked out. But these survivors are being studied and it’s hoped that they will reveal genetic mechanisms for survival that can potentially be used to resurrect these species in the future. Let’s cross our fingers for that.
Green Infrastructure Coordinator
Nebraska Forest Service & Nebraska Statewide Arboretum
University of Nebraska-Lincoln
203G Forestry Hall, UNL 68583-0815
August 7, 2020