June 28 2013 03:00 AM

The old ash trees under the Gateway Arch in St. Louis, Missouri, are about to die, casualties of the imminent arrival of a small green beetle. But the sapling picked to take their place, the London plane tree, is also threatened. A black fungus has infected and killed tens of thousands of plane trees across Europe, and the disease, commonly called canker stain, has roots in the Mississippi River Valley.

The tree was picked for its appearance, disease resistance and ability to survive in alkaline soils. The National Park Service says that no species is perfect. However, tree experts say the real issue isn’t the plane tree, which most agree is a decent choice. Rather, it’s the decision to plant just one species along the Arch’s walkways.

Replacing the trees will cost about $1 million, park service administrators said, part of an estimated $14 million in landscape improvements to the walkways leading to the Arch.