Researchers’ Fierce Drive to Save Ash Forests
Invasive species of fungi and insects have decimated ash trees in North America and Europe over the past few decades. Researchers in both continents have been feverishly working to map the genomes and disease pathways of these trees to breed trees that are resistant to these pests.
Ash trees are among the most dominant trees across the U.S. and account for up to 25 percent of the trees that are located in big cities. While there are an estimated 7 billion ash trees in the U.S., experts believe that many will be killed off in the next few decades because of a non-native beetle called the emerald ash borer.
First appearing in the U.S. in 2002, the emerald ash borer is a slender beetle that is a native of East Asia. It is believed to have early arrived in the U.S. in packing material and has devastated entire ash tree populations in Detroit. This pest has now spread to 22 states, including New York, Missouri, Tennessee and numerous others.
Europe has also been battling its ash tree disease called ash dieback. This condition is caused by an infestation of the fungus Chalara. Researchers from 34 different countries focused on the fungal infection in a four-year effort to determine how to combat it. The fungus originated in East Asia and spread to Eastern Europe in the early 1990s. Since that time, it has spread across Europe. By 2012, as much as 90 percent of the ash tree population of Denmark had succumbed to ash dieback.
Researchers mapped the genome of the Chalara fungus and then studied the ash trees of Europe. They found that some ash trees in the United Kingdom were more resistant to Chalara than in other parts of Europe. One team of researchers from Denmark, the UK and Ireland began sequencing the genome of the ash tree in 2013. Their work in mapping ash tree variations across Europe and determining that some UK trees were more resistant was considered an important step toward breeding trees that are resistant to Chalara.
One problem that was identified by the European researchers is that ash trees that are more resistant to Chalara are less resistant to infestations of the emerald ash borer. Trees in Denmark that were resistant to Chalara were found to produce chemicals that made them less resistant to the emerald ash borer. Researchers now are working to find ways to breed ash trees that are resistant to both pests.
The devastation of ash trees in the U.S. by the emerald ash borer has left cities struggling with multiple problems. When the emerald ash borer is first introduced to a new area, half of all of the ash trees will die in about eight years. The trees that remain will succumb within three more years, leaving cities to struggle with increased runoff of stormwater, disposing of millions of dead trees and liability issues.
Some cities spray the ash trees with insecticides, which are costly but active for about three years. With strategic applications, the ports can then give themselves more time to address the problem through planting programs and establishing partnerships with companies to process the lumber from diseased trees. In New York, for instance, companies that specialize in distressed tree milling have gained considerable traction in the market.
U.S. researchers are working to develop hybrid ash trees that are resistant to the emerald ash borer. They have found that few ash trees in Asia are susceptible to the beetle because they co-evolved. For example, the Manchurian ash is much more resistant to the emerald ash borer than either the white ash or the green ash. Researchers have also found that there is a small subset of ash trees that remain alive in areas in which more than 98 percent of the ash tree population has died. They are reviewing the phenotypes of these trees to identify those that are resistant and those that are susceptible.
The U.S. Department of Agriculture has been conducting cross-pollination studies of ash trees since 2005 to develop hybrid trees that are resistant to the emerald ash borer. The government is aided in its efforts by researching seedlings in its nursery laboratories. The USDA hopes to introduce hybrids that contain the Asian trees’ resistant genes so that ash trees may again flourish across the U.S. and in urban landscapes.
Trees that are resistant to diseases, infestations, and weather are important for the maintenance of ecological systems. Some nurseries offer a variety of different resistant trees that can withstand drought, deer, and other difficulties. Nurseries also support governmental and university researchers in their efforts to develop hybrid species by donating plants. For example, one nursery in Tennessee called the TN Nursery donates seedlings and plants to nonprofits, governmental organizations, and university researchers.
While there have been promising developments in research in both the U.S. and Europe for developing resistant ash trees, it may take years before the population’s rebound. Communities across the nation may want to focus on replanting programs through which they can introduce greater diversity in their extant tree populations. Mapping the disease pathways and the tree genomes are only the beginning.