|Scientific Name:||Fraxinus americana L.|
|Red List Category & Criteria:||Critically Endangered A3e+4ae ver 3.1|
|Assessor(s):||Jerome, D., Westwood, M., Oldfield, S. & Romero-Severson, J.|
White Ash (Fraxinus americana) is the most common and useful native ash of the US but is never a dominant species in the forest. Fraxinus americana is suffering the devastating impact of a recently introduced invasive pest, the Emerald Ash Borer (EAB), that has rapidly spread across much of the native range of White Ash and shows no sign of stopping. EAB infests and feeds on all North American ash species it has so far encountered. The nature of the infestation (larval feeding in the phloem) effectively girdles trees as small as 2.5 cm diameter at breast height (dbh) (many years before reproductive maturity), leading to death within five years of infestation. EAB therefore causes virtually 100% mortality of White Ash populations. White Ash is unable to persist for very long through vegetative reproduction and seeds only remain viable in the seed bank for 2-3 (rarely 7-8) years, so regeneration after EAB infestation is minimal or nonexistent. Furthermore, EAB persists in forests in low population densities after major ash population crashes, so the orphaned cohort of White Ash seedlings that remains is quickly infested as they reach a suitable size for EAB infestation. Although niche, dispersal and climate change modeling studies for EAB in North America have been conducted, results have been conflicting and subject to model uncertainties. While some studies have indicated that a very small portion of White Ash's native range may fall outside the suitable habitat for EAB, all authors agree that the overwhelming majority of ash populations will very quickly be overcome by EAB infestation. As such, a population decline of at least 80% over the next 100 years (and likely much faster than that) is assumed. Therefore, F. americana is assessed as Critically Endangered (CR) under criteria A3e+4ae.
|Range Description:||Fraxinus americana is native to Central and Eastern US, and has been introduced to the state of Hawaii (WCSP 2016). It grows from near sea level in the southeastern Coastal Plain to about 1,050 m in the Cumberland Mountains and up to 600 m in New York's Adirondack Mountains. White Ash grows naturally from Cape Breton Island, Nova Scotia, to northern Florida in the east, and to eastern Minnesota south to eastern Texas at the western edge of its range (Burns and Honkala 1990). Sources disagree on this species presence in Mexico, The World Checklist of Selected Plants (WCSP 2016) lists F. americana as occurring in Mexico, but other sources such as NatureServe and the USDA do not. For this assessment Mexican data points from GBIF were included in the estimation of the extent of occurrence, making it considerably larger, but this did not change the overall outcome of the assessment.|
Native:Canada (New Brunswick, Nova Scotia, Ontario, Prince Edward I., Québec); Mexico (Aguascalientes, Chihuahua, Coahuila, Durango, Nuevo León); United States (Alabama, Arkansas, Connecticut, Delaware, District of Columbia, Florida, Georgia, Hawaiian Is. - Introduced, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, Tennessee, Texas, Vermont, Virginia, West Virginia, Wisconsin)
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Fraxinus americana is a common and widespread species throughout the United States, although it is never a dominant species in the forest. The US Department of Agriculture (USDA) estimates that as many as 8 billion ash trees (from all Fraxinus species) live on US timberlands. However, populations of ash trees, including F. americana , have been rapidly declining since the introduction of the invasive Emerald Ash Borer (EAB), Agrilus planipennis Fairmaire, 1888, from Asia to Detroit, Michigan in the 1990s (Haack et al. 2002). Studies have shown that EAB can spread rapidly, infest both healthy and stressed trees, and that ash mortality across an entire forest stand exceeds 99% within six years of EAB infestation (Knight et al. 2013, Klooster et al. 2014, McCullough et al. 2008). EAB can infest ash saplings as small as 2 cm dbh - well before trees reach reproductive maturity (McCullough et al. 2008, Aubin et al. 2015). Persistence of infested trees through epicormic or root sprouts is not a viable mechanism for survival, since these sprouts often show low vigour and only persist for a year or two before the entire tree eventually dies (Klooster et al. 2014, J. Romero-Severson pers. comm.). Further studies have shown that ash seeds only remain viable in the forest seed bank for two to three (at most seven) years (Klooster et al. 2014). Recent studies of forests at the epicentre of the original EAB introduction (near Detroit, MI) revealed that even up to 12 years after initial outbreak, and at least seven years after >99% ash mortality in the region, a residual EAB population still had a stronghold on the forest, infesting nearly 20% of regenerating stems (Aubin et al. 2015). EAB has decimated F. americana populations and is rapidly spreading across the majority of White Ash's range, without any treatment or remediation available. EAB does need a period of cold to survive, so it is possible that the southern most populations of F. americana will not be impacted, but this is a very small portion of the population compared to F. americana's vast range. Additionally there is currently no evidence to suggest that the rate of spread of EAB, or its impact on White Ash mortality, will decrease significantly. Therefore, under the precautionary principle, it is assumed that there will be at least an 80% population size reduction within the next 100 years (J. Romero-Severson and F. Miller pers. comm.).|
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||White Ash is a major component in the forest cover type White Pine-Northern Red Oak-Red Maple, but is also found in many other forest cover types. White Ash has demanding soil fertility and soil moisture requirements. These requirements may be provided by soils derived from a variety of parent materials-limestone, basalt, shale, alluvium, and fine glacial till. It grows best on rich, moist, well-drained soils to medium size. Seeds of White Ash are eaten by the Wood Duck, Bob White, Purple Finch, Pine Grosbeak, and Fox Squirrel (Burns and Honkala 1990).|
|Generation Length (years):||40-50|
|Use and Trade:||White Ash wood is tough, strong, and highly resistant to shock. It is used for a variety of applications, including bows, baseball bats, tool handles, guitars, veneers, and joinery (Burns and Honkala 1990). Due to mortality caused by the Emerald Ash Borer, municipalities are now spending billions of dollars removing dead ash from communities across the US and Canada (Kovacs et al. 2010).|
|Major Threat(s):||Emerald Ash Borer (EAB), Agrilus planipennis, is by the far the most serious and urgent threat to Fraxinus americana and, most likely, all other North American ash species. EAB is a flat-headed borer from the Coleoptera: Buprestidae insect group that is native to China, Japan, Korean Peninsula, Mongolia and eastern Russia. EAB was first discovered in Detroit, Michigan in 2002 (Haack et al. 2002), having likely arrived in the 1990s in infested shipping pallets or crates from Asia. EAB has spread rapidly (through natural dispersal and inadvertent human-mediated dispersal) since its first detection in Michigan. As of January, 2017, EAB has been detected in 30 states in the eastern and central US and two provinces in eastern Canada (USDA Animal and Plant Health Inspection Service), which represents the vast majority of the native range of F. americana. EAB feeds on ash species and exhibits a preference for F. americana, although some sources suggest that White Ash is slightly less preferable (Aubin et al. 2015) than Green Ash, while others have found the species to be indistinguishable in preference (Klooster et al. 2014, DeSantis et al. 2013). All North American species tested so far are highly susceptible to EAB infestation (Anulewicz et al. 2008, J. Romero-Severson and J. Koch pers. comm.). EAB larvae feed on and create extensive galleries within phloem tissue of ash trees, effectively girdling the trees and rapidly cutting off vasculature. EAB can spread rapidly and kill almost 100% of an entire ash stand within six years (Knight et al. 2013, Klooster et al. 2014, McCullough et al. 2008). EAB can kill ash saplings before they reach reproductive maturity and residual EAB populations have been found in forests up to 12 years after the initial outbreak, outlasting Black Ash's seeds which can only survive in the seed bank for 2-3 (rarely 7-8) years (Klooster et al. 2014). One possible explanation of EAB persisting in forests after local White Ash populations are obliterated is host switching; in addition to being able to survive on all North American ash species so far tested, EAB was recently discovered living on White Fringe Tree (Chionanthus virginicus), another member of the Oleaceae family (Cipollini 2015). Moreover, in EAB's native range in Asia, other tree genera such as Juglans (walnuts) and Ulmus (elms) are suitable larval hosts. As of yet, ecological niche modeling studies and climate change predictions of future potential distributions of EAB in North America have struggled with uncertainty, ambiguity, and produced conflicting results (Sobek-Swant et al. 2012, Liang and Fei 2014, DeSantis et al. 2013), such that a recent ecological risk assessment declared that it is impossible to know how far north, south, and west EAB could spread, and so considered all populations of all North American Fraxinus species vulnerable to EAB infestation (Wagner and Todd 2016). Furthermore, whatever peripheral regions of White Ash's range that may currently be too cold for EAB to thrive (e.g. DeSantis et al. 2013) 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 populations of F. americana may be safe from EAB, due to a need for a cold period in its life cycle, however, this portion of the population is very small. Taking all of these observations into account, it is clear that EAB will rapidly kill all living F. americana trees greater than 2 cm dbh that it encounters across the vast majority of the species' range. Furthermore, EAB can survive at low densities in infested forests of decimated ash populations, on alternative host plants, ready to infest any regenerating White Ash sprouts until the trees finally die off and the seed bank is depleted. Therefore, until there is convincing evidence that EAB will be stopped before wiping out the entirety of the ash population in North America, it must be considered an imminent and all-encompassing threat (J. Romero-Severson and F. Miller pers. comm.).|
|Conservation Actions:||Due to the great ecological and economic value of ash trees (and the cost of removing dead ash trees), much research and management effort is underway in multiple sectors, including government agencies, local municipalities, universities, horticulture, and botanical gardens. Through these efforts over the past 15 years, we have gained extensive knowledge of how the Emerald Ash Borer (EAB) infests and kills ash trees, EAB life cycles and dispersal rate, and factors influencing ash susceptibility in the wild. Many different EAB detection and management methods have been trialed and improved, including biological control and insecticides. Research into EAB and ash genomics has advanced. Extensive lab studies have also been conducted to test host susceptibility and to develop breeding programmes for the handful of "lingering ash" that have been detected in the wild, which may harbor some EAB resistance genes (J. Koch and J. Romero-Severson pers. comm.). For a detailed review of of the history, biology, ecology, impacts and management of EAB see Herms and McCullough (2014). However, so far none of these efforts have uncovered a solution to halting the spread and destruction of EAB. As of yet, land managers and conservationists are simply managing forests and urban areas to slow the spread of EAB and minimize costs. The same stakeholders are also promoting a public awareness campaign to prevent human-mediated EAB spread through actions like moving infested firewood to as yet uninfested locations. Countywide quarantines have been in effect across dozens of states in the eastern US to prevent wood movement. Further research is needed to better understand the future spread of EAB and the impact of climate change on both ash and EAB distributions. Fraxinus americana is found in 187 ex situ collections according to PlantSearch (BGCI 2017). However, unless the trees are treated with insecticide, they will be equally susceptible to EAB infestations. Botanic gardens and seed banks growing or holding F. americana outside of the US and Canada represent important germplasm stores for this rapidly disappearing species.|
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|Citation:||Jerome, D., Westwood, M., Oldfield, S. & Romero-Severson, J. 2017. Fraxinus americana. The IUCN Red List of Threatened Species 2017: e.T61918430A61918432.Downloaded on 22 July 2018.|
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