Despite the fact that the U.S. has ample resources, and a relatively well-studied flora, there is currently no complete checklist of the native trees. There is also no comprehensive, up-to-date assessment of the threats currently facing native U.S. trees, or the conservation status of individual species. In order to fill these gaps, a research project commenced in 2017, with the express aim of documenting this information. Strangely enough, the researchers had to begin their study by defining exactly what constitutes a tree!
The most complete tree check lists and data bases available at a national level all follow different definitions of a tree, and range in estimates of the number of native tree species from 748 to 1114. And none of these definitions of a tree matches that of the Botanic Gardens Conservation International initiative known as Global Tree Assessment, which aims to publish conservation assessments for all the world’s tree species.
According to the standardized Global Tree Assessment definition, a tree is a woody plant with usually a single stem, growing to a height of at least two meters; or if multi-stemmed, then at least one vertical stem five centimeters in diameter at breast height. Based on this definition, the researchers in the current study identified 881 species of native trees in the U.S.
It has taken five years of collaborative research to produce this first comprehensive checklist of trees native to the contiguous U.S. states, as well as the extent to which each species is currently threatened. The research was conducted by scientists from Botanic Gardens Conservation International-U.S. (BGCI-US), the Morton Arboretum and NatureServe, in partnership with the United States Botanic Garden (USBG) and United States Department of Agriculture (USDA) Forest Service, and the results are published in the journal Plants People Planet.
“These results lay the groundwork for U.S. 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,” said Abby Meyer, executive director of BGCI-US, a partner on the project.
Dr. Murphy Westwood is vice president of science and conservation at The Morton Arboretum and senior author of the report. She noted that 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 U.S. is imperative to protecting those species, their habitats and the countless communities they support,” said Dr. Westwood.
The research found that most native (plants that evolved in the contiguous U.S.) and endemic (plants found only in the contiguous U.S.) trees are found in the southeastern states, including California and Texas. Florida and Texas have the highest number of native tree species, with 342 and 321, respectively, while Florida and California have the highest number of threatened tree species, with 45 and 44, respectively.
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 new assessment also reveals that 11–16 percent of native tree species in the contiguous 48 U.S. states are threatened with extinction, with the most common threat being from invasive and problematic pests and diseases.
“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 Dr. Susan Pell, acting executive director of USBG. “The U.S. Botanic Garden is proud to sponsor national partnerships such as this that advance plant conservation.”
In addition, the new and up-to-date, standardized checklist of US tree species and the associated threat assessments will help inform future urban planning, land acquisition decisions, restoration efforts, forestry, scientific research and public education. The project also helps support the first objective of the Global Strategy for Plant Conservation (GSPC), that “plant diversity is well understood, documented and recognized.”
“Trees form the basis of many of the world’s terrestrial ecosystems,” said Sean T. O’Brien, Ph.D., president and CEO of NatureServe. “Understanding what trees are threatened and why is critical to informing conservation for trees and ecosystems across the nation.”
According to BGCI’s PlantSearch database of plants in botanical collections, 95 percent (849) of native U.S. tree species are represented in at least one collection, such as a botanic garden, arboretum or seed bank. Most species are actually present in dozens or even hundreds of collections, such as Franklinia alatamaha, which is extinct in the wild. However, the research revealed that there are 17 threatened tree species that are not currently conserved in any collection and thus have no insurance policy against extinction.
“Through initiatives like the Global Tree Assessment, tree research and conservation has evolved from a series of small individual efforts to a global venture grounded in collaborative, scientifically-backed strategies,” said Dr. Westwood. “The checklist is a major milestone for trees, but most importantly, our hope is that this study will inform and amplify the scope of tree conservation efforts across the country,” she added.