It's that time of year when you head back out to the yard and assess what needs to be done.
Grass needs to be mowed, flower beds replanted -- and what's that green stuff on the tree trunks?
Don't worry about it. It's lichen.
Lichens don't harm trees. But, it's also not a plant. They are a symbiotic relationship between organisms in two different kingdoms. It's this relationship that has created a secret world beneath backyard canopies.
Lichens grow best on older, larger trees with ridges that they can cling to. But they're just hanging out there. They get their nutrients from the air and from photosynthesis.
How do I know so much about lichens? My husband, Matt, is an environmental science professor at Point Park and recently finished a study on the region's lichens. (If you want to read more about it, check out opdyke-environlab.com or listen to a podcast at carnegiesciencecenter.podomatic.com.)
If you broke off an edge of a lichen and took a close look under a microscope, you'd see bits of algae that had been trapped by fungus. The fungi have discovered agriculture. The algae provide food through photosynthesis for the fungi. When you separate the lichen out, algae does just fine, but the fungus will struggle to survive.
The coolest thing about lichens are their common names. In Pittsburgh's urban forests you'll find species like fluffy dust, candleflame and British soldiers.
Other names for lichen evoke delightful descriptions such as hammered shield, mealy rosette or pompon shadow. You never know when you might discover a smooth-footed powderhorn or stubby-stalked cladonia near a Monk's-hood. All of those are visible around Pittsburgh.
Some of the most flamboyant species in the U.S. can be seen in the temperate rain forests of Oregon and Washington. One might eye wooden signs serving as anchors for lichens. They grow much more robustly there than what you see in Schenley and Frick parks.
Lichens can be found in the extreme environments on Earth and as a whole make up one of the most abundant types of biological material. From the Antarctica to the arid desert, they often are nature's pioneers and help in creating soil. Rock tripe, for example, can survive without water for more than a year. In the boreal forest, lichens can help preserve soil moisture, add organic matter and trap seeds.
While some of the variety can been seen with the naked eye, a few true beauties can only be appreciated under a microscope when a smudge of green speckled bark sharpens into an intricate system of ridges and valleys.
Very few lichens are poisonous. Matt didn't find any poisonous species around Pittsburgh during his study. But many are beneficial in different ways. One big way is food.
In West Virginia, flying squirrels get 80 percent of their spring and summer diets from lichens. Reindeer sometimes get 90 percent. It's mostly carbohydrates and no protein so they have to eat enormous quantities to sustain themselves. Moose, deer and mountain goats also will graze on it.
Even humans have used lichens as food. Horsehair lichens are found in the western part of the United States and are eaten by native populations. A taffy candy made from it has the texture and flavor of licorice. In Japan, rock tripe, also known as rock mushroom, is prepared in soup and salads. In Siberia, Sweden and Mexico lichens have been used in making alcoholic beverages.
There's also a ground lichen that grows in the Sinai wilderness called manna lichen, which may have been eaten by the Israelites during their 40 years of wandering. It develops as hard pebble-like growths that can be blown together by winds into lumps that soften and swell in morning dew like leavened bread. Desert tribes eat the lichen and feed it to their sheep and goats.
People have found other commercial uses for different species. Beard lichen have absorptive and antibiotic properties and can be found in European products. They've also served to make royal purple and yellow dyes, fine perfumes, deodorants and laxatives. They're also in model train displays, subbing for trees and bushes.
Back in nature, lichens add to the texture of a tree's ecosystem. About 50 types of birds will use a certain type of lichen as their preferred nesting material. Flying squirrels use it, too.
The larva of lacewing have a different use. They attach a piece of living lichen to their back for two reasons. First it hides them from predators, and then it helps them sneak closer to aphid colonies for their own feasting.
In Maryland, the green salamander has evolved so that its skin has similar markings as lichen to help disguise itself.
Back in Pittsburgh, it has been decades since any published scientific study has been done on epiphytic lichens. The first report identifying lichens in Western Pennsylvania was a list by Giovanni I. Giardini published in 1922. He identified 67 species in the Carnegie Museum's herbarium. The collection has since been moved to Colorado. Another survey was done by Hugh N. Mozingo in 1948. Of course when biologists started collecting and name them, it was the Wild West and not all of the identifications were accurate.
In Matt's study, he and his Point Park students surveyed at Schenley, Frick and Mingo Creek parks as well as Forbes State Forest. (The research was funded by the Pennsylvania Department of Conservation and Natural Resources, Wild Resource Conservation. Partners on the project included Pittsburgh Parks Conservancy and the New York Botanical Gardens.)
What they found was more than 40 types of lichens growing on trees in an around Pittsburgh.
However, the rural sites of Mingo and Roaring Runs averaged 32 species compared to an average of 20 species found at Schenley and Frick parks.
At the rural sites, lichens had both larger and thicker thalli, suggesting healthier specimens. Thalli are the leaf-like structures that spread along tree trunks.
Additionally, a greater number of Cladonia species had erect reproductive structures at the rural sites, which were generally lacking on the lichen at the urban sites studied.
The dominant lichens across all sites were fluffy dust and smooth-footed powderhorn. The dominance of nitrogen oxide- and sulfur dioxide-sensitive lichens at all of the sites suggests that the lichen community within the larger geographical region is influenced by nitrogen and sulfur dioxide air pollutants.
There were differences between the urban and rural parks that were likely driven by the "urban heat-island effect."
Cities are not kind to lichens. There is less habitat for them, atmospheric moisture is lower and temperatures are often warmer. The habitat that they have also may be fragmented, which means they can't disperse as widely to help with genetic diversity in an area.
Many species of lichens require pristine air to thrive; it is one of the reasons lichens are vanishing worldwide. Some of the delicate lichens missing from the Pittsburgh area today are species such as Old Man's Beard lichen. It has a very low tolerance of contaminants in the air. They do exist in other parts of Pennsylvania.
The sensitivity of lichens to air pollutants has been known for centuries. For example, Charles Darwin's father, Erasmus Darwin, noticed more than 200 years ago that lichen failed to grow near copper smelters in England. It took until the 1960s before sulfur dioxide was identified as a major factor influencing lichen growth and that they could be used in biomonitoring.
Today, air pollutants that can be detected using lichens include ammonia, fluorine, metals, radionuclides from nuclear accidents, chlorinated hydrocarbons and acid precipitation.
Lichens have a role in evaluating air quality and climate change and are important to study. Lichens accumulate atmospheric chemicals such as sulfur dioxide and lead, which can cause biochemical and structural changes. They also lack an outer cuticle that protects most vascular organisms.
But there are efforts to make these organisms more visible.
An unprecedented study released in February, which Field Museum scientists in Chicago coordinated, revealed 100 newly discovered species of fungi and lichen in one scholarly paper. The usual inattention bestowed upon new lichens is one reason for aggregating so many new ones in a single paper in the Feb. 18 issue of the journal Phytotaxa. It was a massive undertaking involving 102 lichenologists from 37 countries, with the hopes of drawing attention to huge shortfalls in what's known of the Earth's diversity.
The researchers reported that approximately 100,000 fungal species, including 17,500 lichens, have been discovered and named.
There may be a million more species waiting to be noticed.
Heidi Opdyke is a writer and editor living in McCandless ( firstname.lastname@example.org ).