Are You Lichen the Bark? Lichens and Mosses

Part 3: Lichens and Mosses

By Mary Free, Certified Extension Master Gardener

Photo © Mary Free, 2019-02-05, Bon Air Park, Arlington, VA.

Lichens and Mosses on Quercus phellos (willow oak), Bon Air Park, Arlington, VA.
Photo © 2019 Mary Free

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]

Lichens and mosses grow on the base and flare of the trunk of Quercus phellos (willow oak), located along the path between the MGNV demonstration Sunny and Quarry Shade Gardens in Bon Air Park, Arlington, Virginia. © 2019 Mary Free

Lichens and mosses grow on the base and flare of the trunk of Quercus phellos, located along the path between the MGNV demonstration Sunny Garden and Quarry Shade Garden in Bon Air Park, Arlington, Virginia.
Photo © 2019 Mary Free

Lichens

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.

Foliose lichens on maple bark begin to brighten and soften after a snow melt. Mosses, such as an Orthotrichum species, also grow there. © 2019 Mary Free

Foliose lichens on maple bark begin to brighten and soften after a snow melt. Mosses, such as an Orthotrichum species, also grow there. Photo © 2019 Mary Free

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.

Both foliose and fruticose lichens have attached themselves to this birch tree at the Fort Shantok Archaeological District in Montville, Connecticut. Crustose and foliose lichens cover the boulder in the foreground. © 2019 Mary Free

Both foliose and fruticose lichens have attached themselves to this birch tree at the Fort Shantok Archaeological District in Montville, Connecticut. Crustose and foliose lichens cover the boulder in the foreground. Photo © 2019 Mary Free

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.
Notice the black cilia on the lobe margins of the larger foliose lichens, and the cup-shaped apothecia among the smaller foliose lichens found on the bark of a maple tree. © 2019 Mary Free

Notice the black cilia on the lobe margins of the larger foliose lichens, and the cup-shaped apothecia among the smaller foliose lichens found on the bark of a maple tree.  Photo © 2019 Mary Free

Apothecia look like black dots in the center of the crustose lichens on the stone ledge in the Quarry Shade Garden. © 2019 Mary Free

Apothecia look like black dots in the center of the crustose lichens on the stone ledge in the  Quarry Shade Garden. Photo © 2019 Mary Free

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.

Upon closer inspection, the colorful blotches on this boulder along Four Mile Run in Bon Air Park turn out to be foliose and crustose lichens along with some mosses growing in a crevice. © 2019 Mary Free

Upon closer inspection, the colorful blotches on this boulder along Four Mile Run in Bon Air Park turn out to be foliose and crustose lichens along with some mosses growing in a crevice.
Photo © 2019 Mary Free

Foliose lichen, Candelaria concolor (candleflame lichen), can be mistaken for crustose lichen without a magnifying lens to see its lobes and short rhizines (root-like structures) underneath. © 2019 Mary Free

Foliose lichen, Candelaria concolor (candleflame lichen), can be mistaken for crustose lichen without a magnifying lens to see its lobes and short rhizines (root-like structures) underneath.
Photo © 2019 Mary Free

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.

A variety of crustose and foliose lichens colonize the trunk and branches of a declining Amelanchier arborea in the Sunny Garden. In this case, their presence is symptomatic of an unhealthy tree. Photo © 2019 Mary Free

A variety of crustose and foliose lichens colonize the trunk and branches of a declining Amelanchier arborea in the Sunny Garden. In this case, their presence is symptomatic of an unhealthy tree.
Photo © 2019 Mary Free

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

Mosses grow on top of the crushed stone used between the lichen-covered granite slabs of this Connecticut patio. © 2019 Mary Free

Mosses grow on top of the crushed stone used between the lichen-covered granite slabs of this Connecticut patio.  Photo © 2019 Mary Free

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.

Spore-producing capsules can be seen on the sporophytes of this moss adjacent to lichens growing on a granite slab. © 2019 Mary Free

Spore-producing capsules can be seen on the sporophytes of this moss adjacent to lichens growing on a granite slab.  Photo © 2019 Mary Free

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.

Mosses, maybe Homomalium adnatum, and lichens cover this boulder surrounded by Galanthus elwesii, blooming in the Shade Garden in early February. © 2019 Mary Free

Mosses, maybe Homomalium adnatum, and lichens cover this boulder surrounded by Galanthus elwesii, blooming in the Quarry Shade Garden in early February. Photo © 2019 Mary Free

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.


Sources


Are You Lichen the Bark? is a three part series:

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