A new study has found that up to 16% of US tree species are under threat of extinction with pests and disease not climate change as the biggest threats to survival.

Researchers have completed threat assessments for all 881 native tree species in the contiguous United States, resulting in a comprehensive checklist and synthesis that will serve as a critical baseline to guide future tree conservation efforts.

The new assessment of US trees reveals that 11-16% of tree species in the contiguous 48 US states are threatened with extinction, with the most common threat being invasive and problematic pests and diseases.

Abby Meyer, executive director of Botanic Gardens Conservation International-U.S. (BGCI-US), a partner on the project said: “These results lay the groundwork for US tree and ecosystem conservation efforts that will contribute to achieving critical international conservation goals, including the United Nations Decade for Ecosystem Restoration and the Global Tree Assessment.”

Murphy Westwood, Ph.D., vice president of science and conservation at The Morton Arboretum and senior author of the report, added much of the world’s biodiversity depends on trees, which offer food and habitat for countless plant, animal and fungal species while providing invaluable benefits to humans. “Understanding the current state of trees within the US is imperative to protecting those species, their habitats and the countless communities they support,” she said.

Researchers examined the extinction risk, patterns of geographic and taxonomic diversity and leading threats facing tree species native to the continental US. Most US species had never been assessed or were outdated on the two most widely used threat assessment platforms, the International Union for Conservation of Nature (IUCN) Red List and NatureServe.

“This assessment advances our understanding of the threats faced by America’s native trees and will help focus the conservation efforts of public gardens, federal agencies and conservation organizations,” said Susan Pell, acting executive director of USBG.

“Trees form the basis of many of the world’s terrestrial ecosystems,” said Sean O’Brien, president and CEO of NatureServe. “Understanding what trees are threatened and why is critical to informing conservation for trees and ecosystems across the nation.”

Oaks (genus Quercus) and hawthorns (genus Crataegus) dominate the tree flora of the U.S., with 85 and 84 native species, respectively. Hawthorns and oaks were also found to have the most threatened species, with 29 and 17 species, respectively.

The research found that geographically, the distribution of native (plants that evolved in the contiguous US) and endemic (plants found only in the contiguous U.S.) trees is primarily concentrated in the southeastern US, California and Texas. Florida and Texas have the highest number of native tree species, with 342 and 321, respectively. Florida and California have the highest number of threatened tree species, with 45 and 44, respectively.

According to BGCI’s PlantSearch database of plants in botanical collections, 95% (849) of native U.S. tree species are located in at least one ex-situ (outside natural locations) collection, such as a botanic garden, arboretum or seed bank. Most species are represented in dozens or even hundreds of collections, such as Franklinia alatamaha, which is extinct in the wild. However, 17 threatened tree species are not currently conserved in any ex-situ collection and thus have no insurance policy against extinction.