Urban Forest Ecology Lichens: Bioindicators, Chemical Factories, Hidden Marvels in Plain Sight By Horticulturist Laura Wyeth 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 In this fascinating can be an indicator of the level of urban heat island effect article, horticultur- but also can indicate the success of urban heat island ist and science writer mitigation efforts, like tree planting. More research Laura Wyeth explores the is underway. 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. 26 CityTREES Multiple types of lichen on one small branch. Photo by Flickr user Jim McCulloch | CC BY 2.0 27 he natural world is packed with surprises, Tand 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 Wolf lichen (Letharia vulpina) happily on rocks, on trees, on other lichen, even on the by Jason Hollinger backs of insects. Laura Wyeth is a Horticulturist with The union consists of a fungi, mostly from the group 15 years’ experience in the field and known as the “cup fungi” (the Ascomycetes), the ones a special interest in the evolutionary whose fruiting bodies are generally shaped like saucers or shallow cups. The algal component is often referred adaptations of weeds. 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 28 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 Lichens collage from Perlmutter and Rivas Plata, NC State presentation 30 31 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 Trumpet lichen (Cladonia fimbriata) Photo by USFS 32 33 are at a strong disadvantage—on cold coastal jetties, on in our modern world. It is released by the burning of bare Arctic rock, in the marginal extremes. fossil-fuels, such as from coal-burning power plants, auto- mobile emissions, and some industrial processes. Sulfur Being small, slow, and physically fragile, how can they dioxide combines with atmospheric moisture to produce defend themselves? Like many creatures, lichen employ sulfurous acid (H2SO3) or sulfuric acid (H2SO4), a compo- chemical warfare. Lichen fungi produce a unique phar- nent of acid rain. macopoeia of compounds, ones that they cannot produce without their photobionts, and these chemicals provide When absorbed into lichen thalli by air or raindrops, these extra protection for the union. Some lichen manufacture compounds are quite harmful. Sulfur dioxide’s primary their own antibiotics; some produce allelopathic com- injury to lichen is its ability to degrade or destroy chloro- pounds that can inhibit the growth of soil fungi and the phyll, weakening the union and often leading to death for seeds of vascular plants. Some, such as xanthones and the species that cannot avoid or tolerate exposure. The pulvinic acid, are concentrated in the cortex of the thallus damage caused by sulfur dioxide is most severe where to offend the tastes of invertebrate browsers, and some moisture levels are high and pH is low. Low pH caused by compounds seem to act as sunscreens for the photobiont. acidifying acid rain is an exacerbating feedback mecha- nism.