Numbered metal tags are affixed to trees along 33rd Street NW, near the Lafayette recreation center. (John Kelly/The Washington Post)

I am 11 years old and in the fifth grade. While watching my brother play baseball at Lafayette Park in Chevy Chase earlier this month, I noticed that some of the trees had silver metal tags nailed onto them. I went around the park and along 33rd Street NW and made a map of all the trees with metal tags. The tags are numbered (for example, “1677”). I would like to know what the tags represent and why they are there.

— Will Lawrence, Washington

Young Will and his grandmother did quite a lot of research on their own, starting with Will’s hand-drawn map and continuing with calls to various public and private agencies that keep tabs on trees in the District. At this point, Answer Man can offer only a theory as to where the tags — circular, about the size of a quarter, and stamped with four-digit numbers — came from.

Why would someone want to tag a tree in the first place? Trees aren’t like dogs. They aren’t in danger of wandering off and getting lost.

Well, it’s easier to take care of a tree — water it, fertilize it, prune it — if you know its history. And it helps if you assign it a unique number. That was once done by hand: numbered metal tag pounded into tree; number recorded on paper. Today, arborists use a geographic information system, or GIS. That’s a computer program that links something to its location — GPS coordinates, usually — and to its various characteristics.

“A GIS specialist will tell you you can’t maintain something if you don’t know where it is,” said John Thomas, associate director of the Urban Forestry Administration, part of the District’s Department of Transportation.

“Having a location attached to an asset is highly desirable. That way, you can send a contractor or staff to make repairs. Knowing how many trees we have, we can forecast costs. Also, having them mapped and counted allows us during storm events to quickly identify what type of problems we have even before we get there.”

Arborists could do this before GIS, but it took longer to sort through reams of paper files.

The UFA is just one of several groups that plant and maintain trees in Washington — which, don’t forget, was dubbed the “City of Trees” in the late 19th century. Casey Trees, a nonprofit group, has planted nearly 18,000 trees since 2002. And a group called Restore Mass Ave has been instrumental in reinstating Embassy Row’s signature double row of lindens. More than 4,500 trees grow on the grounds of the U.S. Capitol.

Ted Bechtol, superintendent of the Capitol grounds, said a 12-digit number identifies each of his trees. More than a dozen GIS data fields capture such info as genus, species, cultivar name, size, age and health, including whether the tree has cavities. Major trees around the Capitol have 3-by-5-inch information tags attached to them so visitors can learn about them.

But even if a tree doesn’t have a tag physically attached to it, it is probably in someone’s GIS. “We have an app that we use to track our trees,” said Jessica Sanders, director of technical services and research at Casey Trees. Little plastic loops bearing QR codes are lassoed around the branches of newly-planted trees. These tags fall off eventually, but by that point the tree should be well established — both in the ground and in Casey’s GIS.

Jessica said it’s preferable not to drive a nail into a tree — it can stress the tree and invite infection — though Ted said that hasn’t been a problem with Capitol trees. The fact that the trees near the Lafayette recreation center have nailed-in metal discs suggests they were tagged some time ago, before GIS was common.

John Thomas has a hunch. He said the silver metallic tags are reminiscent of those used at one time by the National Park Service. And, in fact, until 1973, the Lafayette property was overseen by the Park Service. That year, it and other playgrounds were transferred to the District, said Jenny Anzelmo-Sarles of the Park Service’s National Capital Region.

However, Jenny said that particular type of tag didn’t come into common Park Service use until the late 1970s, after the recreation center had been transferred, and then mainly in the city’s monumental core.

So, we know why the trees were tagged, but who did it? It may have been the National Park Service. Or it may have been someone else, possibly a contractor hired at some point to survey trees in the neighborhood. If it was you, drop Answer Man a line.

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