|Scientific Name:||Fraxinus quadrangulata Michx.|
|Red List Category & Criteria:||Critically Endangered A3e+4ae ver 3.1|
|Assessor(s):||Westwood, M., Oldfield, S., Jerome, D. & Romero-Severson, J.|
Fraxinus quadrangulata (Blue Ash) 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 blue 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 Blue Ash populations. Blue 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 Blue Ash seedlings that remains is quickly infested as they reach a suitable size for 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. quadrangulata is assessed as Critically Endangered (CR) under criteria A3e+4ae.
Fraxinus quadrangulata, or Bue Ash, is found across Central US (WCSP 2016) from Michigan to Tennessee and from Arkansas to Ohio. At the edges of its range (e.g. Wisconsin, Arkansas) it has some scattered, disjunct populations. It is hardy from zones 4-7.
Native:United States (Alabama, Arkansas, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Mississippi, Missouri, Ohio, Oklahoma, Tennessee, Virginia, West Virginia, Wisconsin)
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Fraxinus quadrangulata (Blue Ash) is a widespread species throughout the central United States. The US Department of Agriculture (USDA) estimates that as many as eight billion ash trees (from all Fraxinus species) live on US timberlands. However, populations of ash trees 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 epicenter of the original EAB introduction (near Detroit, Michigan) 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. quadrangulata populations and is rapidly spreading across the majority of Blue Ash's range, without any treatment or remediation available. There is currently no evidence to suggest that the rate of spread of EAB, or its impact on Blue 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:||Fraxinus quadrangulata, Blue Ash, occurs in upland forests on dry, rocky habitat, in limestone glades and on limestone bluffs. It thrives on alkaline soils and is one of the most drought tolerant ashes, being adapted to drier sites than other eastern North American ashes like Pumpkin Ash or Carolina Ash. Trees produce perfect flowers (somewhat unusual for North American ash species) followed by wind-dispersed, winged samaras. Trees are large, growing to 15-25 m at maturity (Missouri Botanical Garden Plant Finder). Associated species include Acer saccharium, Tilia americana, and Juglans nigra.|
|Generation Length (years):||40-50|
|Use and Trade:||Like Green and White Ash, Blue Ash wood is strong and of good quality for uses like tool handles and furniture. Blue Ash has been used in the nursery trade in the past, but horticultural use is rapidly declining because of EAB.|
|Major Threat(s):||In 2014 Blue Ash was listed as threatened in Canada due to its small population size in Ontario as well as declines from over browsing by White-tailed Deer and the invasive pest Emerald Ash Borer (COSEWIC 2014). Emerald Ash Borer (EAB), Agrilus planipennis Fairmaire, 1888, is by the far the most serious and urgent threat to F. quadrangulata (Blue Ash) 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 virtually 100% of the native range of F. quadrangulata. EAB feeds on ash species and, although it exhibits a preference for F. pennsylvanica (Green Ash) and F. americana (White Ash), it thrives equally well on F. quadrangulata (Anulewicz et al. 2008). 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.), and F. quadrangulata populations have been especially effected because of the nearly complete overlap of their range with EAB. 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 ash 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 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 EAB's favored hosts of Green and White Ash disappear from the landscape, Blue Ash is likely to be its new preference. Taking all of these observations into account, it is clear that EAB will rapidly kill all living Blue Ash trees greater than 2 cm dbh that it encounters across the entirety 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 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. There are 90 ex situ collections in botanic gardens and arboreta worldwide (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. quadrangulata outside of the US and Canada represent important germplasm stores for this rapidly disappearing species.|
|Citation:||Westwood, M., Oldfield, S., Jerome, D. & Romero-Severson, J. 2017. Fraxinus quadrangulata. The IUCN Red List of Threatened Species 2017: e.T61919112A61919114.Downloaded on 21 October 2017.|
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