February 15, 2012
Sean Fox
University of Guelph Arboretum

The continuing march of emerald ash borer (EAB) through Ontario’s urban forests and natural areas paints a grim picture for the future of ash trees (Fraxinus spp.). Is it truly the end for this versatile genus?

EAB certainly isn’t the first highly destructive forest pest to invade North America. With each instance, we understand more about the importance of biodiversity at the ecological level and genetic diversity at the species level.  When diversity is high, our forests have great potential for long-term recovery and resilience, even when our short-term view makes it seem hopeless. It is true, that these invasive pests are an enormous economic burden and strike a huge blow to the health and beauty of our cities.

Looking further ahead, history indicates that ash may continue to be a significant component of our forests. Ash trees are one of the first to establish on disturbed sites such as fields and newly developing woodlots.  This adaptability to exposed sites is also what has made them such popular street trees.  

If the majority of large ashes are killed, as happened to American chestnut with chestnut blight, how can we expect them to continue as an important future component of our forests?

Canopy openings that arise from heavy EAB feeding provide an ideal site for ash seedling regeneration. Seed released from ash trees in varying states of decline begin to form the next generation, as EAB moves on to larger trees in the next region. Unlike American chestnut, ash trees begin to reproduce at a young age. This will provide the opportunity for a second generation to become established before EAB cycles back through the area.

This scenario may be very similar to what we see with American elm (Ulmus americana). Dutch Elm Disease (DED) wiped out the majority of large elms, but as early succession species, the elms are able to reproduce early in age before succumbing to DED. Ash will likely follow a similar pattern in the short term. There will be an abundance of young trees, but large specimens will be a rare sight. Unfortunately, while young ash may still be part of fields and woodlots, ash in cities will be less common. Ravines, river edges and small woodlots hold some promise for modest regeneration, but not streetscapes.

There are currently no known truly resistant ash species. Several species from Asia are able to tolerate the insect by producing unappealing compounds and benefiting by the presence of natural antagonists. North American ash species have not had centuries to adapt through co-evolution as Asian species, however, initial reports indicate that blue ash (see urban tolerant trees article on page 19) seems to fare better under EAB pressure than other native species.  

Lab tests by the Canadian Forest Service and at Michigan State University in the U.S. confirm that blue ash is not a preferred species for EAB feeding, though it could still host the insect. Blue ash trees have commonly emerged healthy in areas where EAB has decimated other ash species. Reports from Windsor, state that only one out of several hundred exposed blue ash was lost from EAB, while nearly the entire population of black, green and white ash was killed.

A concern for blue ash is that once EAB has exploited most of its preferred ash species, it will begin feeding on this less preferred species. That threat is possible, however, there will likely always be younger, regenerating populations of other ash species that will continue to host EAB. As time passes, the development of natural antagonists to help control EAB levels also becomes more likely.    

What is the future of ash in our cities? In the short term, we can continue to expect high mortality within our provincial ash populations. The stress endured by ash caused on urban sites makes them even more vulnerable to EAB. It will be a long time before the typically used ash species will be replanted in urban areas.

We must not lose sight of nature’s survival policy: ecological biodiversity and genetic diversity within species. No matter how magnificent and suitable a tree may seem for our cities, we must plant a mix of species. A broader range of street tree species in the last half of the 20th century would have helped to lessen the devastation of DED, and now EAB.

Landscape Ontario’s list of 29 recommended urban tolerant trees provides alternatives to ash, as well as Norway maple cultivars that tend to dominate our cities.

For the complete list and the fact sheets on Landscape Ontario Growers’ Sector working group, Urban Tolerant Trees, go to landscapeontario.com/trees-for-urban-landscapes.