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Lichens

    After speaking with friends, neighbors and fellow gardeners, I’ve concluded that most people understand that these growths that appear on tree bark, rocks and other surfaces are called lichens (pronounced like-ens), but know little about them. This article provides a brief overview of what they are and how they impact the trees and other surfaces they grow on and the environment more broadly.

    What are lichens?

    Foliose lichen on tree branch. Photo: R Morini

     Lichens grow out of a supportive relationship between certain fungi and photosynthetic algae or sometimes cyanobacteria. Their relationship is called symbiotic or mutualistic, meaning that they work together for mutual survival and reproduction. The fungi form the outermost visual element, so are what we see most easily. They provide physical support and protection and draw moisture and minerals from the air, which they share with their algal and bacterial partners. The algae and bacteria can photosynthesize, which the fungi can’t do, providing needed nutrition to the fungi. Recent studies have located a third potential partner, yeasts, that enable the lichen to produce acids that help defend against unwanted microbes. Their supportive partnership enables lichens to survive and thrive in a range of habitats. Lichens were among the first land-dwelling organisms, suggesting that the communal action between fungi and algae were important players in life making the transition from water to land.

    Crustose lichen on tree. Photo: R Morini
    Crustose lichen on stone. Photo: R Morini

     

    While many people’s first reaction is to think that lichens are harmful to their hosts, this is not the case. As noted, lichens produce their own food and do no harm to trees. Beyond trees, their supporting hosts can include rocks and moist soil and brick faces in addition to both healthy and unhealthy trees. While lichens on stone do extract minerals, the result is to add nutrients to soil via wind and rain, adding to nearby soil health. Fungi are generally considered the dominant organism, mainly because they have the most impact on appearance.

    Published estimates indicate that the number of different known lichen species vary from about 13,000 to 30,000 with many more not yet identified. Their greatest diversity is found in extreme sites like the arctic and antarctic, deserts, grasslands and temperate and tropical forests. Most fit into one of four families based on body physical form:

    • Crustose: appear to be painted onto their support surface and can be crusty or scale-like
    • Foliose: have a leaf-like surface (photo above)
    • Fruticose: can be tube or hairlike/multi-filamentous
    • Squamulose: display small often overlapping scales.

     

    Fruticose lichen. Photo: Howard F. Schwartz, Colorado State University, Bugwood.org

     

     

    Squamulose lichen. Photo: R Morini

    Lichens can live on many different surfaces as long as the surface is stable and well lit. They survive in all climates and altitudes. They need undisturbed surfaces and clean air. They grow on mountain cliffs, boulder fields and deserts. They don’t grow in aquatic or marine environments, except on barnacles. Most hosts are rocks, bark, leaves and soil, but they can also grow on glass, metal, plastic and cloth. They grow slowly but can live for centuries if their environment is stable. They tend to grow and spread more commonly on older trees that grow more slowly than younger ones. While individual lichens are built around a single fungal type, they can contain multiple algal species.

    Reproduction

     Lichens can reproduce in several ways: sexual, asexual and vegetative. Fungi reproduce sexually and spread through spores. They don’t form a new lichen until crossing paths with the right algae or cyanobacteria. Algal and fungal cells sometimes join and reproduce asexually. Vegetative reproduction occurs when components of a lichen break away from the parent and form a new lichen body. This versatility likely accounts for lichens’ ability to thrive so abundantly and broadly, despite their slow growing characteristics.

     

    Lichens’ Benefits

     While they are quiet contributors to environmental sustainability, lichens provide a number of benefits:

    • they absorb nutrition and chemicals from the sun and air and are often analyzed to assess pollution and air quality
    • bind and help build soil
    • fix atmospheric nitrogen
    • provide winter food for caribou, other animals and invertebrates
    • some species are consumed as food and drink flavorings and additives
    • fruticose lichens are used as fiber for clothing
    • their many colors are used as fabric dies
    • they have antibiotic properties that have been used historically as wound dressings, tonics and as a laxative.
    • they are also thought by some to be a source for medication and agricultural chemical development.
    Lichens and separate fungi on dead tree branch. Photo: R Morini
    • while lichen do no harm to their hosts, abundant presence on damaged or dead wood can indicate disease issues caused by other fungi, bacteria, viruses or insects and may require corrective action by the tree owner or tree care specialist.

    Lichen also have relationships with certain animal and insect life. For example:

    Takeaways

     Lichens, who knew? They live on every land body in the world. There are thousands of different species, with a handful of structures that grow on a variety of surfaces that have access to moisture and light. They do no harm to their hosts and in fact make a range of contributions to the environment. The communal relationships that are established between fungi, algae and/or cyanobacteria and potentially yeasts, are unique and inspirational. If only humans cooperated as willingly and effectively. Beyond all this, since I started investigating them and paying attention to their widespread presence, I can’t take a walk in the woods without being mesmerized by them. I find lichens to be truly amazing organisms and hope that readers will have a similar interest and experience.

    Featured photo: Squamulose lichen. Photo: R Morini

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