Urban Forest Ecology: Lichens! Bioindicators & Hidden Marvels, with Laura Wyeth

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

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. 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 an indicator of the success of urban heat island mitigation efforts, like tree planting. More research is underway.

In this fascinating article, Council member and horticulturist Laura Wyeth explores the truly enthralling biology of lichens—simultaneously vulnerable and cannily adaptive organisms. —Michelle Sutton, Ed.   

Lichens: Bioindicators, Chemical Factories, Hidden Marvels in Plain Sight
By Laura Wyeth

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 relationships 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 ever 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), though some lichen involve the “mushroom-makers” (the Basidiomycetes), and others from the so-called “imperfect fungi” (the Deuteromycetes). 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 sunlight, water, and carbon dioxide into sugars and other carbohydrates to fuel both themselves and their fungal 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 cyanobacteria, a group of prokaryotes with a more ancient lineage 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 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 generally equitable relationship. The fungi, with its reinforced cell walls, multicellular tissues, and resistance to desiccation, can withstand some dry or harsh conditions, and so provide an insulated home to the more sensitive photobiont. 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 permeable than its free-living (non-symbiotic) cousins, and its sugars flow into the tissues of the surrounding fungi. Many of the cyanobacteria are also capable of nitrogen fixation and provide 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 hollow, 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 photosynthesis production capacity in relation to body size, lichen grow very slowly. The annual radial growth of crustose and foliose species is somewhere 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 bottom, with a nice spread of green algal butter, and a spongy layer of fungal medulla in-between.

Lichen reproduction is a complicated affair involving the unique sexual reproductive forms of the fungal component as well as an asexual dispersal 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 identification can be difficult. In addition, there are many different relationships between fungi and algae that do not result in what are considered lichen. In an inverse scenario, there are fungi that live within the tissues 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 common where moisture is abundant, but many lichen are highly adapted to dry environments. They are the dominant vegetation in the Arctic tundra because they can survive in deep cold and short day lengths as most traditional plants cannot.

Though lichen are often considered to be pioneers of bare habitats, they are not necessarily catalysts for succession 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 flexibility 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 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 pharmacopoeia 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 compounds 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 compounds 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 exclusive 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 continuous 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 cities, 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 in our modern world. It is released by the burning of fossil-fuels, such as from coal-burning power plants, automobile emissions, and some industrial processes. Sulfur dioxide combines with atmospheric moisture to produce sulfurous acid (H₂SO₃) or sulfuric acid (H₂SO₄), a component 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 chlorophyll, 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 mechanism. Trees have different average pH levels of their bark, and pH sensitive epiphytic lichens will be injured at different 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, leading 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 biomonitors. 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 concentrations are high, lichen will be absent. Just outside of these centers, pollution tolerant species will be found, and further still from the centers, decreased levels of atmospheric 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 accuracy. 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, agricultural 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 ecosystem’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 partnership within, and what other secrets they may hold. ♦

Further reading

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 (see image below). The post answers questions like, “What are eutrophic lichens, and why are they found in cities?”