Portland scientists tap the bryophyte Orthotrichum lyellii to test urban air quality.

By Kyna Rubin

For decades, mosses and lichens have been used to gauge forest health, and in Europe they have been used to measure and map urban pollution across countries. But a recent study of air quality in Portland, Oregon, is said to mark the first time that U.S. scientists have used moss to collect and map fine-grained data on toxic metals in the air of a city. “This kind of high-density sampling on a large area is unique, at least in North America,” says Bruce McCune, a professor of botany and plant pathology at Oregon State University who is not associated with the study. “It allows you to make inferences and find surprises that you wouldn’t otherwise.”

Sometimes those surprises are unpleasant. Earlier this year, harnessing the bryophyte Orthotrichum lyellii to test the air quality of communities throughout Portland, U.S. Forest Service scientists found unexpectedly high levels of cadmium, nickel, lead, and arsenic in neighborhoods surrounding two stained glass plants. The study’s results sparked a local outcry that led to political fallout and enhanced regulatory attention.

Mosses are effective air-quality monitors because they lack roots. Living on trees, rocks, and roofs, they gather nutrients and water from the air alone. Few species grow in soil, leaving them unaffected by soil chemistry. And they are highly absorptive. Natural matter such as conifer needles, tree bark, and soil have long been used as bioindicators of air quality, but moss and lichen “provide clearer, more consistent data,” says Sarah E. Jovan, a research ecologist at the Portland office of the U.S. Forest Service and one of the study’s authors. Not just any moss will do. Many species don’t tolerate pollution and therefore don’t grow in cities. Among the five or six species of moss hosted by Portland’s hardwood trees, Orthotrichum lyellii is the only type widespread enough for use in mapping.

For cash-strapped cities, using moss as a natural air quality monitor makes economic sense. The epiphyte is far cheaper than city air monitors, which also often yield only macrolevel atmospheric readings and cannot flag sources of air pollution such as cadmium that don’t disperse widely from their emission point. The Portland research demonstrates moss’s utility in cheaply gauging pollutants at multiple sites and on a microneighborhood level. “It’s a no-brainer for coastal cities like Seattle or Vancouver with good, naturally occurring moss populations,” McCune says. The researchers are not recommending that moss replace urban air monitors. But moss can be used to determine where cities should place those systems.

This winter, Jovan will begin a new study that will compare measurements taken by moss and by air-quality monitors in several sites across Portland, to see if the two measures track. Results will be available in 2018. Meanwhile, Jovan and her colleagues will be going to Cincinnati to use a lichen to map the city’s sources of metal contamination, prompted by high levels of cadmium and other metals found in the blood samples of children. The hope, Jovan says, “is that the lichen data would be useful for predicting health outcomes.”