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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. Pâques (France), Julien Saudubray (France), Marc Villar (France), Vlatko Andonovski (FYR Macedonia), Dragi Pop-Stojanov (FYR Macedonia), Merab Machavariani (Georgia), Irina Tvauri (Georgia), Alexander Urushadze (Georgia), Bernd Degen (Germany), Jochen Kleinschmit (Germany), Armin König (Germany), Armin König (Germany), Volker Schneck (Germany), Richard Stephan (Germany), H. H. Kausch-Blecken Von Schmeling (Germany), Georg von Wühlisch (Germany), Iris Wagner (Germany), Heino Wolf (Germany), Paraskevi Alizoti (Greece), Filippos Aravanopoulos (Greece), Andreas Drouzas (Greece), Despina Paitaridou (Greece), Aristotelis C. 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White ash has high levels of genetic diversity, heterozygosity, and allelic richness in its native range, with little evidence of inbreeding (Flower et al., 2018). Most genetic variation is within populations rather than between them, reflecting extensive gene flow typical of wind-dispersed, wind-pollinated tree species (Flower et al., 2018).
White ash has high adaptive genetic variation and expanded gene families linked to stress responses and defence, suggesting a flexible genome that has high resilience (Liu et al., 2025). The lineage has also undergone historical genome duplications and structural diversification, providing a rich reservoir of adaptive potential, including high salt tolerance (Liu et al., 2025). However, all this research has been conducted in the species’ own native range; no in-depth research has been conducted on the genetic diversity of white ash populations in Europe.
White ash has low genetic differentiation between populations and minimal genetic structure within its native range, with some populations having unique genetic variants (Flower et al., 2017; Flower et al., 2018). This lack of genetic structure is the result of widespread gene flow and effective dispersal of both seed and pollen (Flower et al., 2018).
Populations from different parts of the species’ range in North America differ in growth rates and growth habits, showing intraspecific adaptive variation despite the lack of genetic structure (Marchin, Sage, and Ward, 2008). For example, trees from the drier western range edge show greater drought tolerance than populations from wetter regions. This indicates that, while neutral genetic structure is weak, adaptive differentiation exists, caused by gradients in climatic factors such as precipitation (Marchin, Sage, and Ward, 2008).
The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2025.
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The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2025.
In North America, white ash is under threat from the emerald ash borer (Agrilus planipennis), which has caused widespread mortality and led to white ash being listed as Critically Endangered on the IUCN Red List alongside other ash species. The loss of ash trees could cost billions of dollars in its native range (Flower et al., 2017; Flower et al., 2018). Climate change adds further risk, with eastern populations potentially vulnerable to hotter, drier conditions and limited short-term acclimation capacity (Marchin, Sage, and Ward, 2008). These pressures threaten both population survival and genetic diversity. However, these are threats to the species in its native range; risks to white ash genetic diversity in Europe remain unknown.
To maintain white ash genetic diversity in its native range, research has suggested conservation should focus on protecting multiple populations rather than just increasing the number of individuals per population. Insecticide treatments could be integrated into in situ/ex situ programmes to mitigate threats such as emerald ash borer (Flower et al., 2018). However, there is a lack of population-level genetic studies in Europe, and further research is needed to understand the status, genetic diversity, and appropriate conservation strategies in the non-native range. However, measures being taken in its native range could be applied to European populations.
The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2025.
Further reading
Armstrong, J.E. and Funk, D.T. 1980. Genetic variation in the wood of Fraxinus americana. Wood and Fiber Science, 12(2): 112–120.
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
Flower, C.E., Aubihl, E., Fant, J., Forry, S., Hille, A., Knight, K.S., Oldland, W.K., Royo, A.A., and Turcotte, R.M. 2017. In-situ genetic conservation of white ash (Fraxinus americana) at the Allegheny national forest. In: R.A. Sniezko, G. Man, V. Hipkins, K. Woeste, D. Gwaze, J.T. Kliejunas, and B.A. McTeague, technical coordinators. 2017. Proceedings of a workshop on gene conservation of tree species—Banking on the future, pp. 165–169. General Technical Report PNW-GTR-963. Portland, OR, USA, US Department of Agriculture, Forest Service, Pacific Northwest Research Station.
Flower, C.E., Fant, J.B., Hoban, S., Knight, K.S., Steger, L., Aubihl, E., Gonzalez-Meler, M.A., Forry, S., Hille, A., and Royo, A.A. 2018. Optimizing conservation strategies for a threatened tree species: in situ conservation of white ash (Fraxinus americana L.) genetic diversity through insecticide treatment. Forests, 9(4): 202. https://doi.org/10.3390/f9040202
Marchin, R.M., Sage, E.L., and Ward, J.K. 2008. Population-level variation of Fraxinus americana (white ash) is influenced by precipitation differences across the native range. Tree Physiology, 28(1): 151–159. https://doi.org/10.1093/treephys/28.1.151
Liu, J.N., Yan, L., Chai, Z., Liang, Q., Dong, Y., Wang, C., Li, X., Li, C., Mu, Y., Gong, A., and Yang, J. 2025. Pan-genome analyses of 11 Fraxinus species provide insights into salt adaptation in ash trees. Plant Communications, 6(1): 101137. https://doi.org/10.1016/j.xplc.2024.101137
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