To learn more about the map elements, please download the "Pan-European strategy for genetic conservation of forest trees"
This distribution map has been developed by the European Commission Joint Research Centre (partly based on the EUFORGEN map) and released under Creative Commons Attribution 4.0 International (CC-BY 4.0)
Caudullo, Giovanni; Welk, Erik; San-Miguel-Ayanz, Jesús (2017). Chorological maps and data for the main European woody species. figshare. Collection. https://doi.org/10.6084/m9.figshare.c.2918528
The following experts have contributed to the development of the EUFORGEN distribution maps:
Fazia Krouchi (Algeria), Hasmik Ghalachyan (Armenia), Thomas Geburek (Austria), Berthold Heinze (Austria), Rudi Litschauer (Austria), Rudolf Litschauer (Austria), Michael Mengl (Austria), Ferdinand Müller (Austria), Franz Starlinger (Austria), Valida Ali-zade (Azerbaijan), Vahid Djalal Hajiyev (Azerbaijan), Karen Cox (Belgium), Bart De Cuyper (Belgium), Olivier Desteucq (Belgium), Patrick Mertens (Belgium), Jos Van Slycken (Belgium), An Vanden Broeck (Belgium), Kristine Vander Mijnsbrugge (Belgium), Dalibor Ballian (Bosnia and Herzegovina), Alexander H. Alexandrov (Bulgaria), Alexander Delkov (Bulgaria), Ivanova Denitsa Pandeva (Bulgaria), Peter Zhelev Stoyanov (Bulgaria), Joso Gracan (Croatia), Marilena Idzojtic (Croatia), Mladen Ivankovic (Croatia), Željka Ivanović (Croatia), Davorin Kajba (Croatia), Hrvoje Marjanovic (Croatia), Sanja Peric (Croatia), Andreas Christou (Cyprus), Xenophon Hadjikyriacou (Cyprus), Václav Buriánek (Czech Republic), Jan Chládek (Czech Republic), Josef Frýdl (Czech Republic), Petr Novotný (Czech Republic), Martin Slovacek (Czech Republic), Zdenek Špišek (Czech Republic), Karel Vancura (Czech Republic), Ulrik Bräuner (Denmark), Bjerne Ditlevsen (Denmark), Jon Kehlet Hansen (Denmark), Jan Svejgaard Jensen (Denmark), Kalev Jðgiste (Estonia), Tiit Maaten (Estonia), Raul Pihu (Estonia), Ülo Tamm (Estonia), Arvo Tullus (Estonia), Aivo Vares (Estonia), Teijo Nikkanen (Finland), Sanna Paanukoski (Finland), Mari Rusanen (Finland), Pekka Vakkari (Finland), Leena Yrjänä (Finland), Daniel Cambon (France), Eric Collin (France), Alexis Ducousso (France), Bruno Fady (France), François Lefèvre (France), Brigitte Musch (France), Sylvie Oddou-Muratorio (France), Luc E. 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Genetic diversity of green ash in Europe remains poorly studied. However, it will be affected by the number of introduced genotypes, the origin of genotypes, whether they are from wild or cultivated varieties, and potential hybridization with other ash species (Floren, Horchler, and Müller, 2022). The origin of green ash genotypes in Europe is uncertain.
In its native North American range, green ash shows high genetic diversity, high allelic richness and observed heterozygosity, and low population differentiation. Most genetic variation is found within populations, typical of wind-pollinated trees with high seed and pollen dispersal (Hausman et al., 2014; Huff et al., 2022). While European populations may reflect similar trends, their diversity is reduced due to founder effects and limited introductions (Floren, Horchler, and Müller, 2022).
In Europe, green ash is primarily found in urban plantings and riparian habitats. Its distribution is patchy and closely linked to human introduction. Its genetic structure in Europe is expected to be less complex than in its native North American range, due to founder effects and the limited number of introduction events (Floren, Horchler, and Müller, 2022).
Studies in North America reveal moderate genetic differentiation between populations, with three distinct genetic clusters and two ancestral lineages (Hausman et al., 2014). These groupings reflect historical glacial refugia and subsequent hybridization (Huff et al., 2022). However, genetic distance is not correlated with geographic distance, suggesting that structuring results from restricted gene flow caused by land-use fragmentation or glacial history (Hausman et al., 2014). Low levels of genetic differentiation have been observed within riparian habitats, because of rivers allowing long-distance dispersal and gene flow, a pattern also seen in European ash (Fraxinus excelsior) (Hausman et al., 2014).
Green ash is an outcrossing, dioecious, wind-pollinated, diploid, hardwood tree that shows minimal levels of inbreeding (Hausman et al., 2014; Wu et al., 2019). The fruits of all ash (Fraxinus) species are samaras, winged seeds that allow wind and water seed dispersal (Abhainn et al., 2024). Dispersal along rivers has allowed the rapid spread of green ash in Central European floodplain forests where this species is not native (Abhainn et al., 2024).
The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2025.
Green ash has been widely planted in Europe for about 80 years in urban, riparian, and forestry settings because of its tolerance to cold, salt, and floods, its fast growth, and easy clonal propagation. In its native range, extensive clonal plantings of a small number of cultivars have led to cultivar gene flow into natural stands, risking dilution of local genetic diversity and suppression of population differentiation (Abhainn et al., 2024). High dispersal and weak genetic barriers mean clonal cultivars can swamp local provenances with non-local pollen and seed (Abhainn et al., 2024).
Green ash has been planted in Europe as an alternative to other ash species that have been heavily damaged by ash dieback, such as European ash. However, green ash also faces threats from emerald ash borer (EAB). Some varieties of green ash resistant to EAB have been identified. Resistant genotypes and varieties of green ash could be used for restoration and selective planting to conserve genetic diversity and replace lost trees in both Europe and North America; this is important given the economic value of green ash (Huff et al., 2022).
The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2025.
EAB causes rapid mortality in green ash, risking loss of locally adaptive genotypes and rare alleles in the species’ native range; it also poses a serious threat to European populations (Hausman et al., 2014). American ashes, including green ash, have mild susceptibility to ash dieback from Hymenoscyphus fraxineus, adding a second major threat to genetic resources (Huff et al., 2022).
Green ash could be used as a rescue species for declining European ash ecosystems but is itself vulnerable to EAB and few genetic resources or breeding programmes exist for it in Europe (Wu et al., 2019; Floren, Horchler, and Müller, 2022).
Management of green ash in Europe could include the establishment of ex situ germplasm collections, investment in genetic research, and breeding to identify or develop EAB-resistant genotypes before widescale restoration or planting (Wu et al., 2019; Floren, Horchler, and Müller, 2022). However, few resources are available for genetic studies and improvement of green ash (Wu et al., 2019).
The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2025.
Noble Hardwoods Network: Report of the third meeting
Further reading
Schmiedel, D. 2010. Fraxinus pennsylvanica in the forests of the floodplain of the central Elbe River, Germany. Berlin, Weißensee-Verlag.
Taylor, S.M.O. 1972. Ecological and genetic isolation of Fraxinus americana and Fraxinus pennsylvanica. PhD thesis. Ann Arbor, MI, USA, University of Michigan.
References
Abhainn, E.A., Shirley, D.L., Stanley, R.K., Scarpato, T., Koch, J.L., and Romero-Severson, J. 2024. Gene flow from Fraxinus cultivars into natural stands of Fraxinus pennsylvanica occurs range-wide, is regionally extensive, and is associated with a loss of allele richness. PLOS One, 19(5): e0294829. https://doi.org/10.1371/journal.pone.0294829
Floren, A., Horchler, P.J., and Müller, T. 2022. The impact of the neophyte tree Fraxinus pennsylvanica [Marshall] on beetle diversity under climate change. Sustainability, 14(3): 1914. https://doi.org/10.3390/su14031914
Hausman, C.E., Bertke, M.M., Jaeger, J.F., and Rocha, O.J. 2014. Genetic structure of green ash (Fraxinus pennsylvanica): implications for the establishment of ex situ conservation protocols in light of the invasion of the emerald ash borer. Plant Genetic Resources, 12(3): 286–297. https://doi.org/10.1017/S1479262114000033
Huff, M., Seaman, J., Wu, D., Zhebentyayeva, T., Kelly, L.J., Faridi, N., Nelson, C.D., Cooper, E., Best, T., Steiner, K., and Koch, J. 2022. A high‐quality reference genome for Fraxinus pennsylvanica for ash species restoration and research. Molecular Ecology Resources, 22(4): 1284–1302. https://doi.org/10.1111/1755-0998.13545
Wu, D., Koch, J., Coggeshall, M., and Carlson, J. 2019. The first genetic linkage map for Fraxinus pennsylvanica and syntenic relationships with four related species. Plant Molecular Biology, 99(3): 251–264. https://doi.org/10.1007/s11103-018-0815-9
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