Part 3: Lichens and Mosses
By Mary Free, Certified Extension Master Gardener
Trees sustain biodiversity. Quercus (oak) is among the trees considered a keystone species, playing a crucial role in how the ecosystem functions. Oaks provide food or shelter for over 600 different insect species, 513 lepidopteran (moths and butterflies) species, 100 vertebrate species, more than 60 bird species (Abugattas 2010), and earthworms. Crevasses in oak and other barks form important habitats and microhabitats for ferns, orchids, fungi, and epiphytes. Epiphytes include mosses, algae, and lichens, 324 species of which have been reported on oaks in the United Kingdom (Broad 1989, 17). Lichens and mosses are integral to biodiversity in North American woodlands.
[Please click on the images below to see the full-size details]
As mentioned in Part 1 of this series, a lichen is a complex life form that is a symbiotic union of two separate life forms, a fungus and an alga. A fungus is incapable of producing food on its own so it relies on other organisms for nutrition. In return for providing support, protection, water, and minerals for the alga, its alga partner provides the food, which it makes from sunlight by photosynthesis.
Lichens, like mosses, are poikilohydric, which means that they have no means to prevent desiccation. However, they are desiccation-tolerant and can withstand “repeated periods of hydration and dehydration without injury.” (Smith 1997, 191) Lichens enter a dormant phase when they are dry; their structures are brittle and their colors are pale. They rehydrate by absorbing the moisture of fog, rain, melted snow, or even water vapor during periods of high relative humidity; their structures become supple and their hues intensify, becoming brighter or darker, indicating that they are photosynthesizing.
There are three main types of lichens, which vary in color from off-whites to grays to black, and from differing tints and shades of greens to bright yellows, oranges, and reds:
- Crustose lichens, as their name suggests, are crust-like in appearance and attach themselves firmly to tree barks, rocks, soil, and rooftops.
- Foliose lichens are flat and can be leafy-like, or convoluted with ridges and bumps.
- Fruticose lichens have a hair-like appearance, similar to bushy branches.
With the naked eye one may be able to see cilia, little hairs on the margins of lichens; cephalodia, which appear as black spots and are little pockets of cyanobacteria that form on top of chlorolichen (lichen that has green algae), and apothecia, cup-shaped structures that produce spores. An inexpensive hand (magnifying) lens will reveal even more details, aid in identification, and add to the appreciation of the intricacies of lichens and mosses.
In the winter, lichens thrive on deciduous trees as the algae have more access to sunlight. Lichens do not harm the plants to which they attach themselves since they are self-sustaining and remain on the surface. The thicker and rougher the bark, the more epiphytes (lichens and mosses) it attracts. They are less likely to attach themselves to smooth bark like beech. Some thin-skinned birches rid themselves of epiphytes and keep their lenticels open by exfoliating their bark. Lichens appear on healthy trees, but they can be symptomatic of a tree under stress or in decline when other signs such as dying foliage or stems are present. Lichen mass will decrease as a tree’s health improves because expanding bark makes a less hospitable environment on which to anchor.
Some think lichens detract from the aesthetic value of a tree; instead consider the character and beauty that they add to the bark and their importance to the environment. Lichens are used as camouflage by lizards and frogs and by ruby-throated hummingbirds (to disguise the outside of their nests) and by some nuthatches (to hide the food they put into bark crevices). Some mites utilize lichens as homes. Lichens are a food source for some herbivores and for humans (not all lichens are edible so take necessary precautions). People also use lichens for dyes, in traditional medicine, and as ingredients of numerous products from deodorants to perfumes. And they are important to scientists: since lichens need clean air to survive, and start to die as the atmosphere becomes polluted, scientists test them to track pollution. So wherever you see lichens flourish, you can be assured it is a healthy sign that you are breathing clean air!
Mosses are primitive, non-vascular plants. They absorb water by osmosis and make food by photosynthesis. They do not have true roots, but rather root-like filaments called rhizoids, which absorb water and minerals, and anchor them to the surfaces of bark, stones, soil, and other substrate. Mosses do not harm the trees to which they attach themselves. They do not have flowers. Rather, sporophytes, stalk-like structures, support capsules that make spores for reproduction.
Mosses are important to the ecosystem. In a forest, mosses help regulate the amount of rain that reaches the floor by absorbing the moisture and slowly releasing it later. This helps reduce erosion and nutrient leaching during rain storms and provides moisture during dry days (Monthey and Dudzik 2010, 1). Mosses also provide habitat for small mammals like mice and shrews as well as salamanders, insects, arachnids, and microscopic invertebrates like tardigrades (also known as water bears and moss piglets). Tardigrades “can most easily be found living in a film of water on lichens and mosses, as well as in sand dunes, soil, sediments, and leaf litter” (Bordenstein), and are prolific—132,000 tardigrades can live on one gram of moss (Monthey and Dudzik 2010, 1).
Chickadees and winter wrens are among the birds that use mosses to build their nests. White-breasted nuthatches hide their food by covering it with lichen or moss. Some people use mosses as an alternative to lawns. Commonly, gardeners and commercial growers use dried sphagnum moss as a plant growing medium and soil conditioner and, in the form of peat, as a fuel. People also have used it for applications as diverse as building insulation and wound dressings.
As you wander the winter woods or stroll the tree-lined streets, be on the lookout for lichens and mosses. In their unassuming way, they lend an ethereal quality to the landscape, beckoning us to stop and study. And if you forgot to put your magnifying lens in your pocket, then it’s okay to take out your cell phones—the latest models have a magnifying feature. In this case, the digital world won’t be an intrusion but rather an aid to enhance your visual experience of the natural world. Just put your cell phone away when you’re done examining the intricacies of the lichens and mosses and continue on your contemplative journey.
- Abugattas, Alonso. 2010. “Top ’10’ Lists of Wildlife Plants.”http://www.pwconserve.org/plants/alonso_top10.pdf
- Beeching, Sean Q., Robert Hill, et al. 2007. “A Guide to Twelve Common & Conspicuous Lichens of Georgia’s Piedmont.” University of Georgia, Department of Lifelong Education, Administration, and Policy; the UGA Department of Geography, Center for Remote Sensing & Mapping Science; and the Oconee River Georgia Youth Science & Technology Center (GYSTC) at Northeast Georgia Regional Educational Service Agency.https://www.eealliance.org/assets/Documents/Conference_Resources/Conference_Resources2017/guide_to_12_common_lichens_of_georgia_piedmont.pdf
- Bordenstein, Sarah. “Tardigrades (Water Bears).” University of Chicago , Marine Biological Laboratory, Microbial Life Educational Resources. Retrieved January 18, 2019. https://serc.carleton.edu/microbelife/topics/tardigrade/index.html
- Broad, K. 1989. “Lichens in Southern Woodlands.” London: Her Majesty’s Stationery Office (HMSO), Forestry Commission, Handbook 4. 17. https://www.forestry.gov.uk/PDF/FCHB004.pdf/$FILE/FCHB004.pdf
- Cornell University, Cornell Lab of Ornithology. 2015. “All About Birds.” https://www.allaboutbirds.org
- Monthey, Roger and Kenneth R. Dudzik. 2010. “Learning About Mosses in Our Northeastern Woodlots: an Introduction.” Newtown Square, PA: U.S. Department of Agriculture, Northeastern Area State and Private Forestry Forest Service. https://www.fs.usda.gov/naspf/sites/default/files/naspf/pdf/moss_article_web_access.pdf
- Ohio Moss and Lichen Association. https://ohiomosslichen.org
- Smith, Stanley D., Russell K. Monson, Jay E. Anderson. 1997. “Poikilohydric Plants.” In: Physiological Ecology of North American Desert Plants, Adaptations of Desert Organisms. Berlin, Heidelberg: Springer. 191. https://link.springer.com/chapter/10.1007/978-3-642-59212-6_10
- U.S. Department of Agriculture, Forest Service Rangeland Management & Vegetation Ecology—Botany Program. “Lichens.” Retrieved January 2019. https://www.fs.fed.us/wildflowers/beauty/lichens/
Are You Lichen the Bark? is a three part series: