A gentle shower fell as four people in rain gear made their way deep into a spruce-fir forest high in Great Smoky Mountains National Park. Ducking beneath bright green underbrush and stepping away from the road, a hush took over.

Just a few steps in, they came across an aging yellow birch tree covered in moss.

But it wasn’t just moss. James Hollinger, a retired computer scientist turned amateur lichen scientist, leaned closer and spotted a rare, spongy lichen that has been documented about a dozen times in the park. As far as he knows, it does not appear in any botanical guidebooks.

“So, we could, right here right now, come up with a common name for it,” Hollinger said excitedly, as fellow volunteer and lichenologist Laura Boggess unfolded her magnifying lens.…