Melanie Choukas-Bradley leads nature walks and tree-identification workshops throughout the D.C. area. (Judy Licht) Melanie Choukas-Bradley leads nature walks and tree-identification workshops throughout the D.C. area. (Judy Licht)

In front of the U.S. Capitol building, beside the Reflecting Pool, there’s a bur oak that’s a member of the Witness Tree Protection Program.

This tree didn’t stumble upon a hit by the elm mob. The curiously named National Park Service program identifies trees that have borne witness to history. Well over 100 years old, this bur oak was a shoo-in, says Melanie Choukas-Bradley, author of “City of Trees: The Complete Field Guide to the Trees of Washington, D.C.”

“It was witness to the history of the Capitol and the building of the National Mall,” she says. “It’s one of my favorite trees to show people, because it has these giant acorns with fuzzy caps that even adults can’t help picking up and marveling at.”

Over the past 35 years, Choukas-Bradley has introduced scores of D.C. residents to notable trees through her books and tours. On Thursday, she’s leading a free tour of native mid-Atlantic trees in the United States Botanic Garden’s Regional Garden.

“The Botanic Garden is a great place for learning about local trees and shrubs, because all the plants are labeled,” she says.

After you become an expert on native flora, you still have a lot of work to do. Thanks to D.C.’s mild climate and international citizenry, the city is home to around 350 different species of trees.

“People have come from all over the world to live here, and they have brought their favorite trees with them,” she says.

From the Tidal Basin’s graceful cherry trees to upper Northwest’s stinky ginkgoes, D.C. is overrun with foreign nationals — including a mystery tree Choukas-Bradley once found on the Capitol grounds. It was labeled “mountain maple,” but she knew it was an imposter.

“Its leaves were much too wide,” she says.

She eventually discovered the tree’s true identity. It was a Cappadocian maple, a native of Asia, rarely seen in the D.C. area.

“Sadly, it’s no longer there,” she says. “It must have been diseased or hazardous.”

Nearby, Choukas-Bradley’s favorite oak remains in good health.

“Bur oaks can live to be 600 years old, and they are so adaptable — they can tolerate floods and droughts and changes in temperature — which makes them well-suited to climate change,” she says. “This one will probably continue witnessing history for many more years to come.”

United States Botanic Garden, 100 Maryland Ave. SW; Thu., 5-7 p.m., free. 

Don't tell anyone, but this bur oak is in the Witness Tree Protection Program. (Sadie Dingfelder/Express) Don’t tell anyone, but this bur oak is in the Witness Tree Protection Program. (Sadie Dingfelder/Express)