COLUMBUS – What will happen to North American forests if emerald ash borer (EAB) spreads out of control?
What types of wooded areas are most susceptible to this exotic killer of ash trees?
These are some of the riddles Ohio State University researchers are trying to solve in a unique study of the borer’s environmental impact.
Where does it kill? “We know that emerald ash borer has the ability to kill ash trees in the urban environment – places like golf courses, residential areas and strip malls,” said Annemarie Smith, a graduate research associate with the university’s Ohio Agricultural Research and Development Center.
“Our study is looking at the insect’s ability to kill ash trees in a forested environment.”
The most serious environmental threat now facing U.S. forests, the borer was first discovered in North America in the Detroit area in 2002 and has since been found in Ontario, Maryland, Indiana and Ohio.
An accidental import from northern Asia, the metallic-green beetle, Agrilus planipennis, has killed or infested 15 million trees in Michigan.
In Ohio, some 200,000 ashes have been destroyed as part of the Ohio Department of Agriculture’s plan to contain and eradicate the pest in the state.
Field work. Smith travels every week from Columbus to southeastern Michigan to conduct field work for the project.
There, she and center technician Diane Hartzler hike through research plots delimited at several parks and recreation areas, most of them in Oakland and Livingston counties.
The plots lie in the heart of Michigan’s core infestation zone, where the borer damage is so advanced that no eradication efforts are taking place.
One of Smith’s objectives is to determine whether or not different forested areas are equally susceptible to the borers’ attack.
Study plots show variations in the species of ash that are present and also in the density of ash trees that make up those wooded areas.
The plots also have different soil and moisture characteristics – there are stands in riparian areas, where the soil tends to be wet, and also in dry, upland sites that are very well drained.
“After the first field season, data indicate that there doesn’t seem to be any variation in susceptibility among the different forest sites,” Smith said.
“Ash mortality was high across all the stands that we sampled last year. It doesn’t appear that any environmental factors influence the rate of invasion in those stands.
“For example, ash mortality is high in stands where there are just a few ash trees as well as in stands where there are lots of ash. We will continue collecting more data this year to see if the pattern holds up.”
Black ash is dying faster. Smith said that while all three native ash species in the area – black, white and green – have shown mortality as a result of the borer infestation, data from last year indicate that black ash is dying at a faster rate.
“We have seen the highest mortality in black ash trees,” she said. “It’s still not clear why, but we have several hypotheses to explain this.”
One possibility, Smith explained, is that the insects may simply prefer black ash and are able to differentiate between the three species, choosing to lay their eggs on those trees.
The second hypothesis is that the insects may be drawn to stands where black ash typically grows – riparian areas with wetter soils.
“These riparian sites have an inherent edge where the forest meets the water, which means more available light. Marshy sites tend to have more open canopies, which also means more light,” Smith said.
“And beetles in the genus Agrilus are known to be drawn to high-light environments. So that may be the reason that black ash is more susceptible to EAB.”
The third possible explanation is that black ash’s phloem tissue – or inner bark layer, where the borer larvae feed, cutting off the pipeline that carries nutrients and water – may be softer, and therefore easier for the insects to chew through and develop faster.
This, Smith said, might be due to the fact that the black ashes are growing on wetter sites.
Impact on forests. Another major objective of Smith’s study is to determine what effect wide-scale ash mortality caused by the borer invasion will have on forest communities and how the composition of those stands is going to change in response to that loss, said Dan Herms, Smith’s advisor.
Smith is doing this by looking at the saplings that are pre-established in the understory of the forest, as those trees are the ones likely to be recruited into the canopy.
“Which species will replace ash? What will the future forests look like? Will they be as desirable?” asked Herms, Ohio’s leading emerald ash borer researcher and member of the National EAB Science Advisory Panel.
“This has important implications for how the invaded forests should be managed.”
For more information, visit www.ashalert.osu.edu.