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Fraxinus pennsylvanica 

Scope: Global
Language: English
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Taxonomy [top]

Kingdom Phylum Class Order Family
Plantae Tracheophyta Magnoliopsida Scrophulariales Oleaceae

Scientific Name: Fraxinus pennsylvanica Marshall
Common Name(s):
English Green Ash

Assessment Information [top]

Red List Category & Criteria: Critically Endangered A3e+4ae ver 3.1
Year Published: 2017
Date Assessed: 2017-03-08
Assessor(s): Westwood, M., Oldfield, S., Jerome, D. & Romero-Severson, J.
Reviewer(s): Rivers, M.C.
Contributor(s): Miller, F.
Justification:
Fraxinus pennsylvanica (Green 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 green ash and shows no sign of stopping. EAB infests and feeds on all North American ash species it has so far encountered, but has a strong preference for Green Ash. 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 Green Ash populations. Green 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 Green 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 Green 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. pennsylvanica is assessed as Critically Endangered (CR) under criteria A3e+4ae.

Geographic Range [top]

Range Description:Fraxinus pennsylvanica is found across the US and Canada (WCSP 2016) and is the most widespread of the North American ash species. It ranges from Cape Breton Island in Northern Canada down to Texas and eastwards to Nova Scotia and northern Florida. It is hardy from zones 3-9 in North America. It has been introduced to several European countries. The extent of occurrence (EOO) of F. pennsylvanica, at the moment, is very large (at least 6,000,000 km2) and well over the threshold for listing under criterion B. Sources disagree on this species' presence in Mexico, The World Checklist of Selected Plants lists F. pennsylvanica as occurring in Mexico, other sources such as Natureserve and the USDA do not. For this assessment Mexican data points from GBIF were included in the EOO, making it considerably larger, but this did not change the overall outcome of the assessment as these populations make up only a very small portion of F. pennsylvanica's total population.
Countries occurrence:
Native:
Canada (Manitoba, Ontario, Saskatchewan); Mexico (Chihuahua, Coahuila, Durango, Nuevo León, San Luis Potosí, Sonora, Tamaulipas, Veracruz); United States (Alabama, Arkansas, Colorado, Connecticut, Delaware, District of Columbia, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New York, North Carolina, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, West Virginia, Wisconsin, Wyoming)
Introduced:
Austria; Belgium; Bulgaria; Czech Republic; Germany; Hungary; Poland; Romania; Russian Federation; Spain; Ukraine
Additional data:
Upper elevation limit (metres):3000
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:Fraxinus pennsylvanica is a common and widespread species throughout the 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, and especially of F. pennsylvanica, 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, although root sprouting in response to EAB in urban environments root sprouting in forests stands is uncommon and 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. pennsylvanica populations and is rapidly spreading across the majority of Green 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. pennsylvanica will not be impacted, but this is a very small portion of the population compared to F. pennsylvanica's vast range. Additionally there is currently no evidence to suggest that the rate of spread of EAB, or its impact on Green 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
Additional data:
Continuing decline of mature individuals:Yes
Extreme fluctuations:NoPopulation severely fragmented:No
Extreme fluctuations in subpopulations:Unknown

Habitat and Ecology [top]

Habitat and Ecology:Green Ash grows in moist bottom land or riparian environments, occasionally in swamps, thriving on fertile, moist, but well-drained soil. It tolerates a broad range of temperatures and precipitation and is probably the most adaptable of all the ash species. Green Ash is an indicator of the Sugarberry-American Elm-Green Ash forest cover type (Society of American Foresters Type 93) and is commonly associated with Acer negundo, A. rubrum, Carya illinoensis, Celtis laevigata, Liquidambar styracifolia, Platanus occidentalis, Ulmus americana, Salix nigra, Populus deltoides and P. tremuloides (Burns and Honkala 1990). Green Ash is a large (up to 21 m high), dioecious, deciduous, wind-pollinated tree, flowering between March and May (depending on the region). Fruits are elongated, single-seeded samaras that are wind or water dispersed (Burns and Honkala 1990). According to the US Forest Service's Forest Inventory Analysis (FIA), the three most dominant species of ash (Fraxinus pennsylvanica, F. americana, and F. nigra) together total nearly nine billion trees (14% of all woody species stems > 1 in dbh) in the forested lands of the contiguous US states (DeSantis et al. 2013). Green Ash is an especially important tree in the forests of the US Midwest, around the Great Lakes, where it can represent up to a quarter of the woody biomass of the forest. Green Ash seeds are an important food source for animals.
Systems:Terrestrial; Freshwater
Generation Length (years):40-50

Use and Trade [top]

Use and Trade: Fraxinus pennsylvanica wood is strong, durable, and shock resistant. As such, it is valuable for specialty items like tool handles and baseball bats. Its good form and suitability as a shade tree has made Green Ash a popular ornamental tree and it is widely planted as a parkway and street tree in urban and suburban areas throughout the US and abroad. 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).

Threats [top]

Major Threat(s): Emerald Ash Borer (EAB), Agrilus planipennis, is by the far the most serious and urgent threat to F. pennsylvanica 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. pennsylvanica. EAB feeds on ash species and exhibits a preference for F. pennsylvanica and a few other North American species, although 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 green 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 Green 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 Green 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. pennsylvanica may be safe from EAB, due to 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. pennsylvanica 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 Green 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 [top]

Conservation Actions: Because of 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 pennsylvanica is found in 159 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. pennsylvanica 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 pennsylvanica. The IUCN Red List of Threatened Species 2017: e.T61918934A61919002. . Downloaded on 21 October 2017.
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