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Fraxinus caroliniana 

Scope: Global
Language: English
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Taxonomy [top]

Kingdom Phylum Class Order Family
Plantae Tracheophyta Magnoliopsida Scrophulariales Oleaceae

Scientific Name: Fraxinus caroliniana Mill.
Infra-specific Taxa Assessed:
Common Name(s):
English Carolina Ash, Water Ash

Assessment Information [top]

Red List Category & Criteria: Endangered A3e+4ae ver 3.1
Year Published: 2017
Date Assessed: 2017-03-30
Assessor(s): Westwood, M., Oldfield, S., Jerome, D. & Romero-Severson, J.
Reviewer(s): Rivers, M.C.
Contributor(s): Miller, F.
Justification:
Fraxinus caroliniana (Carolina 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 Carolina 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 ash populations. Closely related species of ash (Green and Black Ash) are 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 it is likely that for Carolina Ash 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 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. The general consensus is that EAB will spread across much, if not all, of the eastern, central, and southern US. It is possible that up to half of Carolina Ash's native range may fall outside the suitable habitat for EAB, being too tropical for the insect to survive and undergo an overwintering phase of its life cycle. This would indicate a >50% decline of the species, which passes the threshold for Endangered (the category given), however, there are uncertainties in the projected decline and the decline may be as great as 80%, which would qualify the species for a Critically Endangered category. As such, a population decline of at least 50% over the next 100 years (and likely much faster than that) due to EAB infestation is a conservative estimate of threat. Therefore, F. caroliniana is assessed as Endangered (EN) under critera A3e+4ae.

Geographic Range [top]

Range Description:Fraxinus caroliniana occurs throughout Southeastern USA and the island of Cuba (WCSP 2016). It can be found from southeastern Virginia down through southern Florida and over to eastern Texas. Its extent of occurrence is around 1,500,000 km2. Being the southernmost ranging of the eastern US ash species, it is hardy down to zone 10B in the US. In Cuba there are records of  F. caroliniana from Villa Clara and Matanzas provinces in West-Central Cuba (Nesom 2010).
Countries occurrence:
Native:
Cuba; United States (Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, Texas, Virginia)
Additional data:
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:Fraxinus caroliniana, Carolina Ash, is a fairly widespread species in certain wetland habitats throughout the southeastern United States. 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). Studies conducted on the closely related Black (F. nigra) and Green (F. pennsylvanica) ash indicate that 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.). Additional studies have shown that Black and Green ash seeds only remain viable in the forest seed bank for up to eight years (Burns and Honkala, 1990, Klooster et al. 2014, Aubin et al. 2015). 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. caroliniana populations in North Carolina and other states where it has begun to encounter this species, and is rapidly spreading across the majority of Carolina Ash's range, without any treatment or remediation available. It is possible that the southernmost populations of Carolina Ash may be too tropical for EAB to survive, but it is very likely that at least 50% if not up to 80% of the range of Carolina Ash will be suitable for EAB, and thus that those trees will be rapidly killed by EAB infestation (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:Unknown
Extreme fluctuations in subpopulations:Unknown

Habitat and Ecology [top]

Habitat and Ecology:Fraxinus caroliniana, also known as Carolina Ash or Water Ash, is found in wetlands, swamps, bottomlands and riverbanks throughout the southeastern US and in two provinces in Cuba. It is sometimes completely submerged under water. It is a medium sized tree, growing usually to 30 feet, occasionally to 60 feet or more. It often develops multiple trunks. Leaves are pinnate and opposite and fruit is a winged samara (University of Florida Extension). Seeds of Carolina Ash are eaten by waterfowl, which also use the tree for roosting.
Systems:Terrestrial; Freshwater
Generation Length (years):40-50

Use and Trade [top]

Use and Trade: Wood from Carolina Ash is light and weak, so it is not utilized commercially. Occasionally wood may be used for fuel or pulp. Carolina Ash is occasionally used in horticulture.

Threats [top]

Major Threat(s): In 2016 Fraxinus caroliniana subsp. cubensis, which is endemic to one province on the island of Cuba, was listed as Critically Endangered (CR) - B2ab(ii,iii) because of over exploitation as a source of timber, habitat fragmentation, and deforestation (Gonz├ílez Torres et al. 2016). However, for the species as a whole, Emerald Ash Borer (EAB), Agrilus planipennis, is by the far the most serious and urgent threat to F. caroliniana 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), including seven of the 10 US states in which F. caroliniana is native. EAB feeds on ash species and exhibits a preference for the closely related 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. 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 (diameter at breast height) - well before trees reach reproductive maturity (McCullough et al. 2008, Aubin et al. 2015). While most research on EAB impacts on its host has been conducted on F. pennsylvanica, populations of F. caroliniana are being equally decimated by the pest (https://www.nrs.fs.fed.us/disturbance/invasive_species/eab/control_management/biological_control/). 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 of the closely related Black and Green ash have shown that seeds only remain viable in the forest seed bank for two to three (at most eight) years (Aubin et al. 2015). 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). One possible explanation of EAB persisting in forests after local Green Ash (it's favored host species) 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). It is possible that the southernmost extent of F. caroliniana in the US (in central and southern Florida) and its small range in Cuba (>10,000 km2) may be be too tropical for EAB to survive and have an equally lethal impact on F. caroliniana populations there (Liang and Fei 2014). However, at a minimum it is likely that EAB will cover at least 50% of F. caroliniana range (reaching Endangered under A3e+4ae), and it is possible that EAB will reach 80% of its range (Critically Endangered under A3e+4ae). Taking all of these observations into account, it is clear that EAB will rapidly kill all living F. caroliniana trees greater than 2 cm dbh that it encounters across the 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 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 threat (J. Romero-Severson and F. Millerpers. comm.).

Conservation Actions [top]

Conservation Actions: In Cuba where the subspecies Fraxinus caroliniana subsp. cubensis is considered Critically Endangered, efforts have been made to collect and establish cubensis seedlings in the Matanzas Botanical Garden (JBM), as well as population reinforcement of the wild populations (Gonz├ílez Torres et al. 2016). Due to the great ecological and economic value of ash trees (and the cost of removing dead ash trees), much research and management effort related to EAB 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.). However, most of these studies have focused on the much more commercially valuable Green and White Ash. For a detailed review of of the history, biology, ecology, impacts and management of EAB see Herms and McCullough (2014). As of yet, none of these efforts have uncovered a solution to halting the spread and destruction of EAB. Currently, 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 and southern 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 caroliniana  is reported in 21 ex situ collections worldwide according to PlantSearch (BGCI 2017), whereas F. caroliniana subsp. cubensis is not reported in any ex situ collection. 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. caroliniana 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 caroliniana. The IUCN Red List of Threatened Species 2017: e.T63004A96445289. . Downloaded on 21 November 2017.
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