I have been intrigued by the intricate murals that result from its occupation of otherwise barren nooks and crannies on almost any surface, given long enough. Yet until recently my understanding of lichen was I realised, remote and largely ignorant. When a client enquired as to whether they should remove the lichen from their tree on the belief its presence was harmful, I advised no, realising as I spoke my belief was based on vague knowledge, anecdote and observation. The incident fomented my curiosity and led me into the realm of an ancient and curious organism whose ecological contribution may well have facilitated terrestrial colonisation of the planet, and as I discovered we have had a long held connection.
The study of lichen or lichenology is an organismal speciality of botany, a somewhat esoteric pursuit and historically largely overlooked. Even Linnaeus paid little attention to lichen, dismissively describing them as “rustici pauperrimi” or the poor peasants of vegetation, and mentioning less than half the then known lichen in his seminal work ‘Species Plantarum’. Today, lichenology is still rarefied, at least in Australia, with no more than about four or five individual professional lichenologist active at any one time, at least in the last fifty or so years. Conversely, and somewhat ironically, a search on the internet reveals lichen is attracting a burgeoning group of enthusiasts, or perhaps lichen aesthetes, as the attraction in part seems to stem from the photogenic nature of the subject.
Lichen is an ancient organism. While lichen communities have been suggested to extend into the Precambrian (prior to 600million years ago), the earliest unequivocal terrestrial lichen fossils (Winfrenatia reticulata) found in the famous Rhynie Chert deposits from Scotland date from around 410 million years ago. Lichen like marine fossils dated at around 600 million years old were discovered in 2005 in China, providing evidence of a possible ancestral lineage to the sea.
The appearance of lichen is indeed primordial and one can appreciate the early erroneous ideas concerning their nature. One idea was that they were excrementous matter produced by the earth, rocks, and trees, while some believed they were merely the result of decomposition of higher vegetation. Our true understanding of the composition of lichen can be attributed to the Swiss botanist Simon Schwendener who in 1867 in his now famous talk entitled “On the Nature of Lichen”, given before the Swiss Naturalists Club, was the first to propose the dual organism hypothesis, which unsurprisingly caused bitter controversy and was, like so many pivotal discoveries, vigorously rejected (for several decades) by many leading contemporaries.
Appearing to be a single organism, lichen is actually a composite organism, a symbiosis between a fungus (mycobiont), usually an ascomycete, sometimes a basidiomycete or deuteromycete, and one or more photosynthetic organisms (photobiont), a green algae, or a cyanobacteria (blue-green algae) and sometimes both. The fungus constitutes the majority of the composite organism’s structure and mass and provides protection, water and mineral nutrients to the photosynthetic algae or cyanobacteria in return for the carbohydrates manufactured by the photobionts. The coming together of the organisms is referred to as lichenization and together they form a structure known as a thallus.
More than just a partnership, lichen form unique structures and produce over 800 metabolites, some of which are unique to lichens. When grown separately, the fungal component is often unrecognisable relative to its lichenized form. Cyanobacteria often look the same whether free living or lichenized, whereas green algae can look very different when free living. Despite this dualism, due to the fungus being the major component, lichen are classified by systematists as members of the fungus kingdom, and consequently taxonomically identified according to the fungal partner.
Lichen morphology is grouped into four basic forms: crustose, or crust like that grows tight against the host substrate (and are the most common type of lichen), sqaumulose being scaly in form comprising numerous raised lobes growing tight to the host substrate, foliose forming flattened and leaf like structures with a defined upper and lower surface, and fruticose that develop as freestanding branching tubes either erect or pendulous with no discernable upper and lower surface. One of the most dramatic examples of the fruticose lichens is Usnea trichodeoides, a pendulous species that grows to 1m long and can be found hanging from trees in moist eucalypt forest and abutting rainforest in the eastern highlands of Queensland.
Despite this diversity, all lichen have similar internal morphology comprising an outer cortex of tightly packed fungal filaments, the photosynthetic layer in which the photobiont are held with fungal hyphae, and the medulla which comprises a loosely packed hyphae. Lichen grows on a variety of substrates both natural and synthetic, attaching themselves to a surface by root-like structures termed rhizines.
Although a symbiosis, the relationship, once famously described by the prominent lichenologist and lichen curator at University of British Columbia Herbarium Trevor Goward, as “fungi that have discovered agriculture”, often appears far from this benign description, varying between mutually beneficial to seemingly parasitic. Whatever the nature of the consortium, it appears to overwhelmingly favour the microbiont. As a whole lichens reproduce both sexually and asexually, yet there are often distinct tendencies between species to utilise one form of reproduction over another. Asexual reproduction commonly occurs via fragmentation of the thallus. Where fungal propagules are released, the fungal germinant initiates the coupling by giving rise to mycelium that ‘search ‘ for and when found ensnares the photobiont in an enduring relationship incorporating it into the lichen structure in which the photobiont can release up to 90% of their photosynthate to the mycobiont.
Regardless, the relationship has the mutual benefit of allowing the composite organism to inhabit a much wider range of environments than either could in isolation, so that lichen area almost ubiquitous. Worldwide there are somewhere between 14,000 and 17,000 known species of lichen; of which Australia has about 3,700 known lichen species. Their tolerance of extreme environmental conditions has resulted in their distribution in nearly all terrestrial ecosystems, and they can be found on all continents, in all climates and altitudes, from the tropics to the tundra, sea level to above the tree line and from the Antarctic to the desert, whilst within these ecosystems they inhabit many microhabitats.
Their wide distribution is due to their being able to grow on sterile media, remarkable metabolism, and adaptations. Lichens rely on atmospheric moisture for metabolic purposes. Lacking a cuticle and stomata, they are unable to regulate water loss to the surrounding environment yet are able to tolerate dehydration to low cell or tissue water content and to recover from it without physiological damage (poikilohydric). Under extreme environmental conditions lichens can lay in a ’desiccated’ dormant state for years, some crustose lichens spending most of their lives this way. Lichen can absorb water from fog or even a saturated atmosphere, and have been able to exhibit net photosynthesis while frozen at temperatures as low as -20°C.
Lichen growth rates vary considerably between species, whilst even individuals of the same species will have different rates in different locales. While some of the fruticose lichen can grow up to 90mm per year, crustose lichens would have to rank among the world’s slowest growing organisms with growth rates typically measured in micrometres. In Continental Antarctic, growth rates may be as little as 1cm per 1000 years (compared with the relatively fast growth rates found in the milder Maritime Antarctic of up to 1cm or more per 100 years!). Consequently some lichen are very long lived, the oldest apparent lichen found growing in Greenland is thought to be around 4,500 years old. Given the stable, perennial and long-lived nature of lichen, their growth rates have been used to study the age of substrates on which they are growing (lichenometry). Initially applied in the field of geology to study glacial moraines, it is also finding application in archaeology, even proving useful in the identification of remnant ancient forests.
Lacking a cuticle and stomata also make lichen efficient accumulators of heavy metals while the sensitivity of some species to atmospheric gasses make them good indicators of air quality. Lichen sensitivity to air pollution results in various responses such as altering patterns of growth, decline in diversity, absence of sensitive species, and morphological, anatomical and physiological changes. While some species are sensitive to pollutants, others are very tolerant of pollutants (Lecanora conizaeoides for example is very tolerant of Sulphur dioxide – SO2), while others thrive in some polluted environments and readily colonise habitats vacated by sensitive species. Nitrophile species for example thrive in areas receiving nitrogen inputs from automobile exhaust agricultural and industrial processes. As early as 1859 lichen were recognized as environmental indicators when Grindson in his work “The Manchester Flora” attributed the decline of “these lovers of pure air” to the influx of factory smoke. Consequently lichens are increasingly being used for biomonitoring to understand the health of our environment.
With their ability to colonise sterile substrates, lichen play an important role in the formation of soil (pedogenesis). Lichens that live on rock substrate break down the rock surface (over very long periods) both physically and chemically releasing mineral nutrients. The penetration of root like rhizines into rock fissures can exert mechanical pressure on the rock, as can the expansion and contraction of the thallus through wetting and drying, freezing and thawing. In addition, lichens produce and excrete a variety of organic acids, particularly oxalic acid that chemically breaks down the rock. Lichen trap, dust, silt and water, they accumulate biomass, and upon their death provide organic matter to soil contributing to nutrient cycling. Having solved the dilemma of being able to grow on sterile media, it is entirely plausible these efficient colonisers predate vascular plants. It has even been postulated that being among the first land colonisers and by their contribution to organic soil formation they may have played a fundamental role in the charge of terrestrial plant life that occurred during the Devonian period.
Furthermore, the cyanobacteria found in some lichen have the ability to fix atmospheric nitrogen, which eventually makes its way into the soil to become available to plants through leaching from both living and dead lichen, as well as through lichen consumption and excretion. Studies have shown their contribution to be significant inputting up to 1.5 kg/ha per annum in the arctic tundra (which constitutes around 50% of total nitrogen input), and up to 10 kg/ha per annum by epiphytic lichens in the northern New Zealand forests (although significantly greater by volume, less as total system input).
Ecologically, lichen provides a niche habitat to several small, mainly invertebrate animals. Lichen affords small animals cover from predators either directly or as camouflage. Larger animals such as birds and small mammals use lichen as nesting material. As a food source caribou, elk, deer and reindeer all eat lichen as do mountain sheep, mountain goats, prong-horned antelope, and even sheep in the Libyan Desert; their ruminant stomachs are able to break down the complex carbohydrates. Some molluscs and insects, and even some smaller mammals such as the flying squirrel and red-backed voles include lichen in their diet.
As for humans, the consumption of lichen although widespread, has never formed a major component of any culture’s diet, though has been used on a regular basis by some including Scandinavian, North American and Arctic peoples, with some lichen being used as a delicacy (Umbilicaria esculenta in Asia ) or as a desert (Cetaria islandica in Scandinavia). Lichen has negligible nutritional value and most taxa are bitter tasting, some even poisonous to humans, often requiring proper preparation in order to make it palatable, and so were generally not a preferential food source, instead more widely used as a famine food in times of scarcity.
In addition to a food source, lichen has seen many other uses over the millennia. Perhaps incidentally due to having antibacterial, antifungal and pesticide properties, it was used as packing material in Egyptian mummification to stuff the body cavity, such as in the instance of Ramesses IV, also being inserted under the skin in an attempt to give the body more ‘fleshy’ appearance. Lichen acids have long been being used as dyes by many indigenous cultures and societies; the Romans were infatuated with lichen purples. Lichen was an important source of dye in medieval Europe, and only replaced when synthetic dyes were discovered in the mid nineteenth century, though the use of lichen dyes famously endured into the early twentieth century creating the colours in the famous Harris Tweed. Today however, the only commercially important lichen dye is used to make litmus paper.
Lichen has long being used in folk and traditional medicines, and today can be found as an additive in pharmaceutical products including tinctures and salves, toothpastes, even deodorant. Lichen still has limited use in some western medicines, while renewed interest in lichen, such as the antibiotic properties of species of Usnea, has sparked research into potential new commercial applications.
Particular species of lichen were used by several cultures as a ceremonial narcotic. Wolf Lichen (Letharia vulpine) was used an effective poison in Europe, Russia, and Scandinavia to kill wolves; the same lichen was used in the Americas to poison arrowheads. Lichen has even been used to flavour beer in Siberia, and used in the distillation of brandy in Sweden.
Presently, lichens perhaps have their greatest commercial value in the cosmetics industry used by many of the world’s leading perfume houses as an essential ingredient the manufacture of perfumes. Oakmoss (Evernia prunastri), found on oak trees, and Treemoss (Pseudevernia furfuracea), which grows on conifers are commonly are used for their rich earthy aromas, while Oakmoss in particular is highly valued as a fixative agent, binding the various fragrances together, in particular they hold and boost the strength of the lighter and middle scents.
Ignoring Linnaeus dismissive term for lichen, among the more endearing aspects of our association with this curious organism is the colourful litany of common names they attract, a small sample of which include: Rock Pimples, Bear’s Hair, Earth Wrinkles, Smoky Crottles, Angels Hair, Stone Flower, Elf-ear, Rock Liquorice, Freckle Pelts, Blackened Toadskin, Dragon’s Funnel, Tar Jelly, Green Witch’s Hair, Northern Fox Hair, Speckled Greenshields, Arctic Kidneys, Yellow Speckle Belly’s, Powdered Sunshine, Bloody Hearts, Fog Fingers, Earth Flowers, Rock Mushrooms, Stonebreakers, Old Man’s Beard, Monks Hoods, Deer Snuff, Stone Grass, Frog’s Dress, and Rock Tripes.
So does lichen harm trees? Lichen is not parasitic and the research to date indicates the use of trees and other plants as a substrate on which to attach is overwhelmingly benign; there do not appear to be any adaptive strategies by lichens that cause significant harm to the host tree. Penetration of rhizines into the bark has been shown to break up the surface layers of bark, block lenticels, and cause thickening of the cork cells. It has also been demonstrated than lichen rhizines can penetrate the host bark to the depth of conducting tissues, and one Spanish study demonstrated the negative effect lichen metabolites released into the xylem have on the chlorophyll count of the Holm Oak (Quercus ilex subsp. rotundifolia). Some pendulous lichen may create shading issues for the host tree which could be problematic where the host is already severely stressed; such has been described with the heavy set of California Spanish Moss (Ramalina menziesii) on Coast Live Oak (Queues agrifolia). The evidence to support the notion that lichen threatens trees is however rare, while reported cases are generally negligible in their effect. Instead, if your trees are graced by lichen, it should be welcomed as an indicator of local environmental health and a positive sign for both you and your trees.