In this fascinating article, horticultur-

ist and science writer Laura Wyeth explores the

truly enthralling biology of lichens, simultaneously vulner-

able and cannily adaptive organ-isms. The next time a city resident

asks you whether lichens are harmful to trees, share this piece with them!

Also recommended is this blog post from Washington State Urban Forestry in which

a concerned resident sends in a branch with what turns out to be more than six kinds of harmless lichen. The post also answers the question, “What are eutrophic lichens, and

why are they found in cities?”

—Michelle Sutton, Ed.

Lichens have been studied as bioindicators of air quality for more than 100 years. A terrific 2015 presentation called Urban Lichens: Environmental Indicators from NC State researchers dives deep into studies of lichens as bioindicators of pollutants in urban settings.

More recently, researchers are finding that when toxic emissions are under control in urban areas, other factors affecting lichen abundance and diversity come into play, like temperature and humidity level. In light of that, what is the urban heat island- lichen interplay?

The theory: lichen abundance and diversity on urban trees can be an indicator of the level of urban heat island effect but also can indicate the success of urban heat island mitigation efforts, like tree planting. More research is underway.

Multiple types of lichen on one small branch. Photo by Flickr user Jim McCulloch | CC BY 2.0 2726 CityTREES

Urban Forest Ecology Lichens: Bioindicators, Chemical Factories, Hidden Marvels in Plain SightBy Horticulturist Laura Wyeth

Lichens: Bioindicators, Chemical Factories, Hidden Marvels in Plain Sight

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The natural world is packed with surprises, and when we can stop to notice the small and

seemingly mundane components of it, we are usually rewarded. Consider the lichen: sometimes mistaken for mosses or other plants, lichens are a synthesis of two distinct life forms, a mutually beneficial union between fungi and algae. In 1877, the German botanist Albert Bernhard Frank used the term symbiosis (from the Ancient Greek meaning “to live together”) to describe the lifestyle of lichen.

At the time, the idea of two unrelated species engaged in a closely-twined, communal existence was new, and quite surprising. Lichen were one of the first such unions to make themselves known to science, and with the new paradigm and a word to describe it, symbiotic relation-ships began to be recognized in all sorts of places. Since then, a good deal of attention has been paid to these small, multifarious creatures, and we now know that symbiotic relationships are quite common on our planet. Though lichen’s contributions to their environment, or to human life, may not be obvious, they play important roles and their study has given us both a better understanding of the complexities of living things and of the effects that our human activities have on the life around us.

Lichen are found pretty much everywhere. The Earth hosts over 14,000 named “species” of lichens, in a great variety of fungal/algal combinations, on mostly every corner and kind of surface available. Lichen may live happily on rocks, on trees, on other lichen, even on the backs of insects.

The union consists of a fungi, mostly from the group known as the “cup fungi” (the Ascomycetes), the ones whose fruiting bodies are generally shaped like saucers or shallow cups. The algal component is often referred to as the photobiont, due to the role that it plays in the synthesis.

Photobionts are photosynthesizers; they transform sun-light, water, and carbon dioxide into sugars and other carbohydrates to fuel both themselves and their fun-gal partners. Photobionts come from many different groups; they can be green algae (Chlorophyta), golden algae (Chrysophyta), or brown algae (Phaeophyta)—three groups of eukaryotes that house their chlorophyll inside chloroplasts. Photobionts can also come from the cyano-bacteria, a group of prokaryotes with a more ancient lin-eage and simpler structure; they lack chloroplasts and so their chlorophyll is free-floating in the cell. (Cyanobacteria were known as the blue-green algae before their lineage was understood.)

Though there are many possible combinations of fungi

Wolf lichen (Letharia vulpina) by Jason Hollinger

Laura Wyeth is a Horticulturist with 15 years’ experience in the field and a special interest in the evolutionary adaptations of weeds.

and algae, each lichen species tends to be a specific union of one kind of each. (However, “triads,” composed of one kind of algae and two kinds of fungi, have been discovered.)

Known as a symbiotic “mutualism,” lichen are considered to have a gener-ally equitable relationship. The fungi, with its reinforced cell walls, multicel-lular tissues, and resistance to desicca-tion, can withstand some dry or harsh conditions, and so provide an insulated home to the more sensitive photobi-ont. Inside this shelter, the algae enjoys higher and more regulated moisture levels than outside, a temperature buffer, and screening from potentially damaging UV rays.

For its part, the photobiont whips up breakfast, lunch, and dinner out of literal thin air. A photobiont algae has membranes that are more perme-able than its free-living (non-symbi-otic) cousins, and its sugars flow into the tissues of the surrounding fungi. Many of the cyanobacteria are also capable of nitrogen fixation and pro-vide this essential nutrient for their fungal hosts.

The body of a lichen is called a thallus, and these are found in several distinct forms. Crustose lichen, as suggested, form a thin “crusty” layer on rocks and other substrates. Foliose lichen have thicker, wavier thalli that are “leafier” than the crustose. Fruticose lichen are miniature thickets of hol-low, branching fungal tubes that can grow upright or be “pendant”—i.e. hang down from trees.

Given the tough conditions they find themselves in and their limited photo-synthesis production capacity in rela-tion to body size, lichen grow very slowly. The annual radial growth of crustose and foliose species is some-where in the neighborhood of 0.5-5 mm; growth in the fruticose species averages 1-2 cm per year. The typical lichen body is something of an algae sandwich; two slices of thick, dense, protective fungal cortex, top and bot-tom, with a nice spread of green algal butter, and a spongy layer of fungal

31Lichens collage from Perlmutter and Rivas Plata, NC State presentation

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medulla in-between.

Lichen reproduction is a complicat-ed affair involving the unique sexu-al reproductive forms of the fungal component as well as an asexual dis-persal unit called a soredia, formed of an algal cell entwined by fungal hyphae.

Lichen species are typically named after their fungal partner, but given the complexities of their possible relationships, naming and identifi-cation can be difficult. In addition, there are many different relation-ships between fungi and algae that do not result in what are considered lichen. In an inverse scenario, there are fungi that live within the tis-sues of certain seaweeds. There are also, in our weird world, green algae that find a home on the surface of mushrooms and fungi that parasitize lichen.

Like many things in nature, lichen are tougher than they look. They are found in a variety of habitats all over the globe, but they thrive in extreme environments. Lichen play the long game: go slow, stay small, and occupy an unusual niche. They are most com-mon where moisture is abundant, but many lichen are highly adapted to dry environments. They are the dom-inant vegetation in the Arctic tundra because they can survive in deep cold and short day lengths as most tradi-tional plants cannot.

Though lichen are often considered to be pioneers of bare habitats, they are not necessarily catalysts for suc-cession in short timescales. Because they tend to colonize spaces that vascular plants cannot easily inhabit, there are few plants that can replace them. Their unique coalition of two distinct organisms gives lichen a flexi-bility of properties that most vascular plants lack. Lichen cannot compete with vascular plants on the plant’s favored turf, but they can live and succeed in places where the plants

32 33Trumpet lichen (Cladonia fimbriata) Photo by USFS

in our modern world. It is released by the burning of fossil-fuels, such as from coal-burning power plants, auto-mobile emissions, and some industrial processes. Sulfur dioxide combines with atmospheric moisture to produce sulfurous acid (H2SO3) or sulfuric acid (H2SO4), a compo-nent of acid rain.

When absorbed into lichen thalli by air or raindrops, these compounds are quite harmful. Sulfur dioxide’s primary injury to lichen is its ability to degrade or destroy chloro-phyll, weakening the union and often leading to death for the species that cannot avoid or tolerate exposure. The damage caused by sulfur dioxide is most severe where moisture levels are high and pH is low. Low pH caused by acidifying acid rain is an exacerbating feedback mecha-nism. Trees have different average pH levels of their bark, and pH sensitive epiphytic lichens will be injured at dif-ferent rates depending on the trees on which they grow. Lichens growing on birches and conifers, which have more acidic bark, generally decline faster than lichen growing on oaks and sycamore, with intermediate acidity, or the more alkaline elms. Sulfur dioxide can also inhibit cellular respiration for both the fungal and algal partners and can reduce both sexual and asexual reproduction rates, lead-ing to population declines.

Since some varieties of lichen are more sensitive to the effects of sulfur dioxide than others, these differences can offer greater precision in the use of lichens as bio-monitors. Surveys of lichen distribution in industrial or urban areas have helped to generate maps of air pollution concentration, especially when compared with historical lichen records or surveys from less polluted regions.

At the heart of a polluted area, where toxins concentra-tions are high, lichen will be absent. Just outside of these centers, pollution tolerant species will be found, and fur-ther still from the centers, decreased levels of atmospher-ic contaminants can be deduced from increased diversity of lichen species. A point scale system for sulfur dioxide contamination based on this aspect of lichen diversity, devised by Hawksworth and Rose in 1970, has been used to map pollution levels in the U.K. with high accura-cy. England, like many densely-populated, industrialized nations, has lost many of its epiphytic lichen due to acid rain and atmospheric sulfur dioxide.

Other pollutants, such as carbon monoxide, fluoride, agricul-tural chemicals such as copper-containing anti-fungal sprays, and radiation, are similarly injurious to lichens and decrease both their species populations and overall biodiversity.

These humble creatures are excellent indicators of an eco-system’s atmospheric health, and it would be wise for us to pay them more attention. Wherever you are in the world, the next time you step outside, see how long it takes before you notice one, and then another, and another over there. Consider their age, their strength, the tiny partner-ship within, and what other secrets they may hold.

are at a strong disadvantage—on cold coastal jetties, on bare Arctic rock, in the marginal extremes.

Being small, slow, and physically fragile, how can they defend themselves? Like many creatures, lichen employ chemical warfare. Lichen fungi produce a unique phar-macopoeia of compounds, ones that they cannot produce without their photobionts, and these chemicals provide extra protection for the union. Some lichen manufacture their own antibiotics; some produce allelopathic com-pounds that can inhibit the growth of soil fungi and the seeds of vascular plants. Some, such as xanthones and pulvinic acid, are concentrated in the cortex of the thallus to offend the tastes of invertebrate browsers, and some compounds seem to act as sunscreens for the photobiont.

In addition to these marvels, lichen can proudly wear the mantle of “rock-breakers.” Many, particularly the crustose lichens, can produce weathering agents such as carbonic acid, which can interact with minerals or organic com-pounds in rock and wear them away. Lichen are powerful terraformers, but in deep time, weathering rock bit by tiny bit and over centuries, building soil.

Many lichen species of all body forms, are epiphytes, or tree-dwellers. For some, the bark of trees is their exclu-sive habitat. Lichen can appear to be more abundant on dead or dying trees and are sometimes blamed for the decline of the tree, though they pose no threat to it. The hyphae of tree-dwelling lichen grow only into the outer, dead bark cells and do no damage to the trees on which they grow. Ailing or dead trees will have fewer leaves, thereby letting more light reach lichens on the trunk and branches, fueling their growth. Also, when trees are impaired, their bark grows more slowly and bark cells are sloughed off less frequently, allowing for larger, more con-tinuous lichen communities.

Though the lichen union confers several benefits on its partners, it also makes the unit vulnerable if one of the partners is seriously impaired. Lichen have no true roots; they absorb water and dissolved minerals directly through their outer cortex, right through the walls of their thalli. Because of their minimal filtration mechanisms, they are very sensitive to environmental pollution. Air pollution has heavily impacted lichen diversity, particularly in cit-ies, though the effects have been seen in all parts of the globe. Lichen tend to accumulate minerals in their tissues, rather than excreting them, which has made them useful gauges of atmospheric and rain-bound pollutants, as well as radiation.

Sulfur dioxide, in particular, has negatively affected lichen populations worldwide. Though found naturally in the atmosphere in low concentrations, sulfur dioxide is an environmental pollutant in the concentrations present

(top) Common greenshield (Flavoparmelia caperata) from kindofcurious.com

(right) Xanthoria sp. by Francis Gwyn Jones, Bugwood.org

(bottom) Jason Sharman, Vitalitree, Bugwood.org

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