How Portland's Unusual Moss Uncovered an Air Pollution Problem

On a drizzly March day, Sarah Jovan pauses to inspect a leafless maple growing through a hole in a downtown Portland sidewalk. She passes a slender hand over the miniature ecosystem colonizing its trunk—chartreuse savannahs that stretch between stands of thick, shaggy moss and lone shrubs of fruticose lichen. “These look stressed,” she says of the latter, noting that the pale green epiphytes seem dull and stunted compared to their brethren in the wild.