If asked to quickly jot down a list of U.S. trees, most people could probably name 20 or maybe even 30 off the top of their head.
There are actually 881 species of native trees in the 48 contiguous U.S. states — including 85 different types of oak alone. It took a team of researchers, led by Morton Arboretum, the better part of the past five years to arrive at that total, pulling data from scores of disparate sources, including historical field expedition journals.
Some of the results aligned with what researchers already know about biodiversity in the U.S. As expected, botanically rich states like Florida, California and Texas, as well as the Southeast, have the greatest number of different tree species, said Murphy Westwood, vice president of science and conservation at Morton Arboretum.
What did surprise Westwood was the extent to which the U.S. is defined by a handful of genera, with oak and hawthorn running neck in neck for the lead, far outpacing even the closest also-rans.
“I think it’s really interesting that we as a country have some really iconic, dominant tree groups,” said Westwood. “We are a country of oaks, hawthorns, willows and pines it turns out.”
If it seems surprising that prior to 2017, no one had thought to create a single inventory of the status of U.S. tree species, it’s worth noting that the gap of knowledge was global. No one really knew how many species there were in the world.
A collection of botanic gardens and botanic experts, operating under the auspices of the International Union for Conservation of Nature, launched the Global Tree Assessment in 2015, with an aim to better understand the planet’s tree diversity as well as their risks of extinction.
The U.S. project spun out of that broader effort, spearheaded by Morton Arboretum, Botanic Gardens Conservation International-US and NatureServe.
More than a simple head count (or, perhaps more accurately, trunk count), the checklist also incorporated a threat assessment for each species, a time-consuming, exhaustive process.
“We had to consult with people who knew those trees inside and out. It was a lot of literature review, it was a lot of reaching out to experts in different states,” Westwood said.
To arrive at a threat level, the research team considered several factors: How many individuals of a species are there? Where are they located? Are the populations fragmented from each other? Are they declining? Is there any conservation action underway?
“All of those things get added into this calculus to say, ‘Is it a threatened species or not?’” said Westwood.
This first-ever comprehensive, standardized checklist and threat evaluation, with Westwood as senior author, has been published in the journal “Plants, People, Planet.”
What the assessments revealed is that 11% to 16% of the nation’s native tree species are under threat of extinction, be it due to pests, climate change, habitat destruction, logging or some other hazard.
It’s not exactly a feel-good conclusion to a half-decade’s worth of work, but Westwood, senior author of the report, views the findings less as an ending and more as a beginning, in that it points the way toward where to direct conservation efforts.
“The most important thing to note is that our job is not done just because we know what our trees are and we know which ones are threatened,” said Westwood. “It’s not enough just to list these things, we then need to take action.”
BACK FROM THE BRINK
The checklist dataset has been purposefully made available as a spreadsheet so that it can be downloaded and manipulated in myriad ways by anyone interested in doing a deeper dive.
“There’s basically a million different ways this can be used,” Westwood said. “You can take a look at the list of threats and say, ‘OK, invasive pests and diseases are a big…
Read More: Chicago Researchers Spent 5 Years Creating the First-Ever Inventory of Native