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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
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The Mediterranean Basin is a biodiversity hotspot with an important role in preserving unique species and genetic diversity. Horse chestnut, an endemic species, shows moderate genetic diversity comparable to other Mediterranean woody endemics (Walas et al., 2019). The species' restricted and declining range has resulted in reduced genetic diversity and an excess of homozygotes, suggesting inbreeding (Walas et al., 2019). This is supported by evidence of bottlenecking in Balkan populations, consistent with the long-term range contraction that has occurred since the Last Glacial Maximum (Walas et al., 2019). However, the maintenance of higher-than-expected genetic diversity of the species is the result of current populations being remnants of larger populations that have persisted for a long period in climatic refugia (Walas et al., 2019).
Despite its limited and fragmented native range, horse chestnut shows moderate to high levels of genetic diversity comparable to that found in tree species with wider ranges, with high within-population diversity (Prada et al., 2011; Walas et al., 2019). Past reductions in the species’ range, persistence in climatic refugia, and isolation by distance have shaped its strong spatial genetic structure. Gene flow has been limited, with distinct populations resulting from low genetic mixing (Walas et al., 2019). Walas et al. (2019, 2021) identified five main genetic clusters, strongly influenced by geography and environmental variables, with northern and southern populations and marginal populations being very genetically different. Environmental differences do not appear to be key drivers of genetic differentiation, with genetic clustering being closely related to mountain barriers and range distribution (Walas et al., 2019, 2021).
Horse chestnut is insect pollinated and seeds are dispersed by gravity. As a result, gene flow is short ranged, especially when compared with wind-pollinated species. The tree’s native range is in areas with many topographic barriers such as mountains, and this limits genetic connectivity between populations at the landscape level (Walas et al., 2021).
The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2025.
Horse chestnut can hybridize with several North American Aesculus species. In Europe, the main hybrid is the red horse chestnut (Aesculus × carnea), a cross with red buckeye (Aesculus pavia) (Thomas et al., 2019). Genetic diversity is higher in horse chestnut than in red horse chestnut, while urban cultivars show especially low diversity (Thomas et al., 2019).
Human-mediated expansion and cultivation across Europe since the sixteenth century has given horse chestnut a range beyond its native distribution. Domesticated populations originate from northern Greek and Albanian populations; despite this history of cultivation, the species has retained high genetic diversity across its range (Prada et al., 2011; Walas et al., 2019).
Horse chestnut had a broad distribution across Europe during the Pliocene, even extending to Africa and the Caucasus. Climate fluctuations in the Pleistocene forced the species into mountain refugia within the Balkan Peninsula, where it survived as a Tertiary relict under stable conditions. Greece has represented the core of its distribution since at least the last glaciation, preserving ancient genetic diversity and allowing the accumulation of new variation (Walas et al., 2019).
The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2025.
Horse chestnut faces threats from ecological, climatic, and anthropogenic pressures that endanger its long-term survival. Its range is highly fragmented, with low gene flow among isolated mountain populations, raising risks of inbreeding (Walas et al., 2019). Its ability to adapt to new conditions is limited, leaving it vulnerable to environmental change, human pressure, and competition (Walas et al., 2019). Future Mediterranean climate shifts, including hotter, drier summers, further threaten populations as they have limited room to expand north; diseases such as chestnut bleeding canker are also a threat (Thomas et al., 2019; Walas et al., 2021).
Many natural populations remain unprotected. Conservation priorities should be to safeguard the genetic diversity and survival of natural horse chestnut populations, especially large and genetically distinct stands in Greece (Walas et al., 2019). In situ conservation is urgently needed, alongside protection measures to halt habitat destruction and fragmentation. The International Union for Conservation of Nature (IUCN) recommends genetic research on natural stands to inform effective strategies and address divergence between distinct population clusters (Walas et al., 2021).
The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2025.
Further reading
Ražná, K., Ďurišová, Ľ., Kučka, M., Harenčár, Ľ., Ivanišová, E., Habán, M., and Hrubík, P. 2023. A rare cultivar of horse chestnut (Aesculus hippocastanum L. ˈBaumaniiˈ) in Slovakia: morphological, molecular and antioxidant analysis. Thaiszia – Journal of Botany, 33(1): 29–44. https://doi.org/10.33542/TJB2023-1-02
Walas, Ł., Dering, M., Ganatsas, P., Pietras, M., Pers-Kamczyc, E., and Iszkuło, G. 2018. The present status and potential distribution of relict populations of Aesculus hippocastanum L. in Greece and the diverse infestation by Cameraria ohridella Deschka & Dimić. Plant Biosystems – An International Journal Dealing with all Aspects of Plant Biology, 152(5): 1048–1058. https://doi.org/10.1080/11263504.2017.1415991
References
Prada, D., Velloza, T.M., Toorop, P.E., and Pritchard, H.W. 2011. Genetic population structure in horse chestnut (Aesculus hippocastanum L.): effects of human‐mediated expansion across Europe. Plant Species Biology, 26(1): 43–50. https://doi.org/10.1111/j.1442-1984.2010.00304.x
Thomas, P.A., Alhamd, O., Iszkuło, G., Dering, M., and Mukassabi, T.A. 2019. Biological flora of the British Isles: Aesculus hippocastanum. Journal of Ecology, 107(2): 992–1030. https://doi.org/10.1111/1365-2745.13116
Walas, Ł., Ganatsas, P., Iszkuło, G., Thomas, P.A., and Dering, M. 2019. Spatial genetic structure and diversity of natural populations of Aesculus hippocastanum L. in Greece. PLoS One, 14(12): e0226225. https://doi.org/10.1371/journal.pone.0226225
Walas, L., Iszkulo, G., Barina, Z., and Dering, M. 2021. Development of microsatellite markers for horse-chestnut (Aesculus hippocastanum), their polymorphism in natural Greek populations, and cross-amplification in related species. Dendrobiology, 85: 105–116. https://doi.org/10.12657/denbio.085.010
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