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Morton Arboretum Researchers Reveal Bleak News for America's Ash Trees: 5 of 6 species Critically Endangered

A dead tree along a residential street.
Ash species have been decimated by the invasive emerald ash borer insect.
September 14, 2017

A new assessment of North American ash trees for the International Union for Conservation of Nature (IUCN) Red List of Threatened Species™ has found that five of six of the United States’ most prominent ash species are under threat of extinction, listed as Critically Endangered – only one step away from going extinct.

Researchers at The Morton Arboretum, in collaboration with the IUCN Global Tree Specialist Group and the University of Notre Dame, conducted the assessments for the IUCN Red List, examining six prominent ash trees native to the eastern United States: white ash (Fraxinus americana), Carolina ash (Fraxinus caroliniana), black ash (Fraxinus nigra), green ash (Fraxinus pennsylvanica), pumpkin ash (Fraxinus profunda), and blue ash (Fraxinus quadrangulata). All of these once-plentiful American ash species have been severely impacted by the invasive emerald ash borer beetle (EAB), putting them on the threatened species list.

Out of the five Critically Endangered ash species, three (Fraxinus pennsylvanica, F. americana, and F. nigra) have been the most dominant species of ash in the U.S. according to data from the U.S. Forest Service.

“Ash trees have been  essential to plant communities of the United States and have been a vastly popular horticultural species, planted by the millions along our streets and in gardens. The likelihood that we are losing more than 80 percent of these trees has, and will continue to, dramatically change the composition of both wild and urban forests,” said project lead Murphy Westwood, director of global tree conservation at The Morton Arboretum. “Now, our challenge is to figure out what will fill those gaps and how the community dynamics of those forests will change.”

An Ongoing Threat
All of the native ash species in the eastern U.S. are being decimated by EAB, which arrived in Michigan from Asia in the late 1990s via infested shipping pallets. Fast-moving and devastating to susceptible host trees, EAB has already destroyed tens of millions of trees throughout 30 states and Canada, and is continuing to move across the country, with the potential to decimate over 8 billion ash trees. As the beetles feed on their host trees and spread rapidly, they can kill nearly an entire forest stand of ash within six years of infestation.

Recent ecological risk assessments have found that it’s impossible to know how far north, south, and west EAB could spread. Furthermore, whatever peripheral regions of ash's range that may currently be too cold for EAB to thrive may become more suitable for EAB as the global climate warms and EAB is able to migrate northward at a faster rate than its long-lived ash hosts. Southern U.S. populations of ash may be safe from EAB, due to the need for a cold period in the beetle’s life cycle; however, this portion of the overall ash population is very small.

Moving Forward
Due to the great ecological and economic value of ash trees and the cost of removing dead ash trees from urban areas, much research and management effort has been underway in multiple sectors, including government agencies, local municipalities, universities, horticulture, and botanical gardens including The Morton Arboretum. The disappearing ash trees provide a valuable case study from which government agencies, land managers, researchers, botanic gardens, and the general public can learn.

The final Red List report is available on the IUCN’s website, at iucnredlist.org.

About the IUCN Red List of Threatened Species™
The IUCN Red List of Threatened Species™ is widely recognized as the most comprehensive, objective global approach for evaluating the conservation status of plant and animal species. From its small beginning, The IUCN Red List has grown in size and complexity and now plays an increasingly prominent role in guiding conservation activities of governments, NGOs and scientific institutions. The introduction in 1994 of a scientifically rigorous approach to determine risks of extinction that is applicable to all species, has become a world standard. In order to produce The IUCN Red List of Threatened Species, the IUCN Global Species Programme working with the IUCN Survival Commission (SSC) and with members of IUCN draws on and mobilizes a network of scientists and partner organizations working in almost every country in the world, who collectively hold what is likely the most complete scientific knowledge base on the biology and conservation status of species.