An international conservation group is warning that more than half of the European tree species that exist nowhere else in the world are threatened with extinction.

The International Union for the Conservation of Nature (IUCN) said in a new report Friday that 58 percent of Europe's 265 endemic trees face an elevated risk of disappearing from the continent.

The European Red List of Trees classified 37 percent of Europe's 454 native tree species as "threatened." Of those, 15 percent are "critically endangered," a step away from extinction, the report said.

The findings come amid heightened concern about environmental issues and extinction risks in Europe and beyond. A U.N. report on biodiversity released in May warned that extinction looms for over a million species of plants and animals.

IUCN said that "invasive and problematic" species are the top threat to European trees, with urban development and "unsustainable logging" as other factors.

The conservancy highlighted Aesculus hippocastanum, or the horse chestnut, native to southeastern Europe. The polished brown conker inside its spiked fruit "is perhaps more famous than the tree itself" because of its use in children's playground games, the report said.

The species, present in Europe since before the last Ice Age, has been threatened by defoliation because of the leaf miner moth, and a blotch caused by a fungus, as well as by human pressures. It is endangered in Bulgaria and Greece and critically endangered in Albania.

(Cover image: horse chestnut. /VCG Photo)

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