The highest proportion of threatened trees were found on islands, where they were at high risk due to deforestation for development and agriculture, invasive species, pests and diseases.
The IUCN said climate change was increasingly threatening trees through sea level rises and more intense and frequent storms, especially in the tropics.
Habitat protection and restoration, as well as conservation in seed banks and botanic garden collections were critical to prevent extinctions, conservationists said.
Dr Grethel Aguilar, IUCN director-general, said: “Trees are essential to support life on Earth through their vital role in ecosystems, and millions of people depend upon them for their lives and livelihoods.”
Four migratory shoreline birds seen in the UK have also had their conservation status downgraded by the IUCN.