Lichens are under no obligation to make sense to us. They number perhaps a million species globally, and while they occur absolutely everywhere – trees, rocks, dirt, dunes, ditches, swamps, ship hulls, manhole covers, brick walls and barn doors – they remain one of the most mysterious and perplexing branches on the tree of life. Speak to a lichenologist sometime and they will describe these tiny creatures with all the reverence of a cosmologist broaching dark matter.
To begin, lichens are not “species” in the traditional sense. They are “composite organisms,” the result of several unrelated species fusing together into one indivisible whole. Composing each is a fungi (mycobiont) forming the infrastructure of the lichen, and a photosynthetic partner living inside, either an algae or cyanobacteria (the photobionts). Some lichens support all three – a fungus, algae and cyanobacteria – while others support multiples of each, a glut of photobionts sheltered by as many as two separate mycobionts. Scholars threw up their hands entirely when, only a few years ago, yeast was discovered to occasionally enter the mix. Lichens, it would seem, contain multitudes.
At first glance, these various species make for strange bedfellows, but lichens, as we conceptualize them, have evolved independently at least ten times in the last 3.8 billion years. Whatever benefit brings together fungi, algae and cyanobacteria, it’s clearly worth the trouble. Perhaps the fungus is metabolizing excess sugars produced by its photosynthetic partner(s), and perhaps the photosynthetic algae or cyanobacteria are generating more sugar inside the lichen partnership than they could outside. Or perhaps this simple exchange of sugar and shelter is one benefit of dozens emerging from this “lichenized” state.
“We don’t really know what the hell’s going on,” explained Nova Scotian lichenologist Robert Cameron. In many cases, he said, we don’t even know which species of mycobiont and photobiont are forming our most common lichens, let alone how they’re interacting.
Only some fungi, algae and cyanobacteria are even capable of lichenizing, and of these fruitful few, some can survive outside a lichenized state while others simply can’t, completely and totally dependent on the partnerships they form with others. Some lichens are large and conspicuous, like Old Man’s Beard, while others can only be identified under a magnifying glass, like Calicium glaucellum. Some are widespread, like Maritime Sunburst Lichen, while others are known from only a dozen individuals, like Seirophora aurantiaca. Some are generalists, like the ubiquitous Shield Lichen, while others are hyper-specialized, like Chaenothecopsis tsugae, found only on the sap bubbles of hemlock trees. They are as diverse a community as any on Earth, and like other communities, some of its members are prepared for swings in climate and habitat, while others are entirely at the mercy of a changing world.
Macoun’s Receding Lichen Line
John Macoun (1831-1920) became Canada’s first dominion botanist in 1882, and, in the course of a storied career, collected over 100,000 floral specimens and formally identified over 1,000 species new to science, many of whom now bear his name. He was also a sucker for lichens and collected them feverishly.
Macoun’s collections give us our earliest authoritative glimpse into the past of our native lichens, and Canada has maintained a national lichenologist ever since to build upon his work, a torch now carried by Troy McMullin with the Canadian Museum of Nature. If there is any single lesson to be gained from this proud tradition of lichenology, said McMullin, it’s that Canadian lichens have lost a lot of ground.
Macoun’s Receding Lichen Line is an ongoing project of McMullin’s, and its thesis is simple – an alarming number of the lichens documented by Macoun no longer occur anywhere near where he documented them. Some have vanished from entire provinces, others from entire regions, extirpated from habitats both disturbed and pristine. A compelling example is Black Foam lichen, identified by Macoun and others in the 1700s and 1800s throughout Quebec and southern Ontario, but now it only occurs in the Maritimes, retreating east with foreboding rapidity.
Black foam lichen (Anzia colpodes) // Source: Canada.ca
Lichens, by their very nature, are among the most sensitive organisms on Earth, explained McMullin. Because they derive their moisture and nutrients directly from the air, there is no barrier between them and atmospheric contamination. As well, many species of lichen have become so absurdly specialized that even subtle shifts in habitat can result in their widespread disappearance. In the case of Black Foam, he said, Quebec and Ontario have probably gotten too dry to support this species in only the last 100 years. As the human animal continues its reinvention of the Canadian landscape, and a changing climate adjusts patterns of heat and precipitation, many of our most sensitive lichens are being pressured out of their historic range; the first refugees of a struggling biosphere, and not all of them have somewhere to go.
“The environment is changing, and lichens reflect that very quickly, more than any other organism,” said McMullin. “They’re the canaries in the coal mine because they’re so sensitive.”
To McMullin’s knowledge, no species of Canadian lichen has yet gone extinct, but some declines have been precipitous, especially among those species dependent on ancient and undisturbed forest types with extremely particular regimes of seasonal sunlight and moisture. These, more often than not, are the cyanolichens.
Where Moisture Reigns
When fungi and cyanobacteria lichenize, they produce “cyanolichens,” an especially picky subset of the lichen community in terms of habitat requirements and tolerance to disturbance.
The most sensitive of our cyanolichens, for instance, will only grow in truly ancient ecosystems, those which have stood, relatively undisturbed, for hundreds and sometimes thousands of years, and which maintain more or less constant humidity, sustained dryness spelling doom for many species. More so than much of the biosphere, cyanolichens are wholeheartedly dependent on the integrity of their habitat, their host tree, and their climate.
In North America, there are roughly seven cyanolichen “hotspots” depending on how you count them, each distinguished by age and perpetual humidity. These include the old growth forests of western British Columbia, The Great Smoky Mountains, the northern shore of Lake Superior, the Avalon Peninsula of Newfoundland, and, near the top of everyone’s list for its diversity and wealth of research, the temperate rainforests of Nova Scotia.
The word “rainforest” evokes visions of the tropical Amazon or Congo basins, but temperate rainforests are entirely different beasts. Because Nova Scotia hangs long and narrow between two masses of the Atlantic Ocean (the Scotian Shelf and Bay of Fundy), its coastal forests of Balsam fir, Black spruce and Red maple are almost constantly awash in oceanic fog, and escape extremes of seasonal hot and cold. The trees which compose these forests are not particularly old – strong winds consistently blow them down in small patches – but the ecosystems themselves, wherever they’ve been spared from logging and settlement, have grown and regrown largely uninterrupted since glaciation, developing the obscure characteristics on which our rarest cyanolichens depend.
“That creates a suite of unique species,” said McMullin.
These include Blue Felt, Boreal Felt, Wrinkled Shingle, Eastern Waterfan and Vole Ears lichens, all Nova Scotian cyanolichens and all sharing space under the province’s Endangered Species Act. Black Foam lichen is the province’s only listed species which is not a cyanolichen.
Blue felt lichen (Degelia plumbea) // Source: Ways of Enlichenment
Robert Cameron, Nova Scotia’s recently retired provincial lichenologist, said the sensitivity of cyanolichens sometimes defies belief. If road construction or forest harvesting occurs anywhere near them, resulting shifts in ambient humidity or sun exposure can trigger the collapse of entire populations. Cyanolichen microclimates can even be spoiled by changing wind patterns, perhaps the result of harvesting on nearby hilltops.
“We don’t know exactly what that balance is,” said Cameron, “but we do know it’s delicate.”
Nova Scotia has special management practices in place for its rarest lichens, listed and otherwise, granting several species a buffer of 100-200 metres from harvesting or development, and while these are some of the strongest lichen protection mechanisms on the continent, they occasionally miss the point.
Protecting a circle of habitat around a rare lichen might be psychologically satisfying to the human animal, but ecosystems don’t generally occur in tidy circles. If a cyanolichen exists in relation to a wetland, said Cameron, the majority of that wetland will necessarily fall outside any 100 metre buffer. Should that wetland be altered, by development, harvesting or contamination, its associated cyanolichens will no doubt suffer. This is precisely what happened in Sweden, the country’s last known population of Boreal Felt disappearing when a wetland outside its protected buffer was drained.
“We need to have a more intelligent approach to ensuring these lichens survive on the landscape,” said Cameron, “by having an ecosystem-based approach to how we manage our forests.”
But forest management is only one threat of many assaulting the cyanolichens of Nova Scotia. Several are being eaten by invasive slugs or poisoned by acid rain, and then there is climate change. Cameron has done some climate modelling and predicts that, by 2080, the band of ideal cyanolichen habitat presently running the length of Nova Scotia will have shrunk to a handful of tiny patches in Cape Breton Island, given current trends. Put another way, one of the planet’s most significant cyanolichen hotspots will disappear in the next 58 years as a result of climate change, and while cyanolichens will be the first, they won’t be the last.
“I don’t think we’ve finished knowing which species are in trouble,” said Cameron.
Boreal Felt is one of the most sensitive cyanolichens on the planet, and, consequently, is one of the best studied. Known originally from Scandinavia (only two sites remain, both in Norway), it has since been discovered in tiny populations throughout the Northern Hemisphere, specifically Russia’s Kamchatka Peninsula, Alaska’s Mount McKinley (Denali), Newfoundland’s Bay Du Nord Wilderness Reserve, and, once again, in the temperate rainforests of Nova Scotia.
Exactly how Boreal Felt established itself in such comically distant locations is an enduring mystery. Lichen reproduction is such a carnival (27 separate reproductive strategies have thus far been identified) that retracing the spread of any one species is more likely to induce headaches than provide answers. In the case of Boreal Felt, its mycobiont (fungus) distributes spores which must then find a suitable cyanobacteria partner inside the water sacs of a liverwort. This convoluted rendezvous must also occur inside suitable habitat and on a suitable tree.
Boreal felt lichen (Erioderma pedicellatum) // Credit: Tegan Padgett; Source: CBC News
“There isn’t a clear answer to how they got here,” said McMullin, “and I wonder about it all the time.”
The spores produced by Boreal Felt may be small enough to travel via jet stream from one continent to the next, he reasoned, then relying on luck over thousands of years to discover the right liverwort, cyanobacteria, microclimate and tree. Genetic analysis will hopefully reveal where this species originated and in what sequence it populated the globe. Perhaps Boreal Felt is endemic to Nova Scotia in every sense of the word.
Today, Nova Scotia has more lichenologists, per capita, than any other jurisdiction in North America, but in the 1970s, the research of lichens in general, and Boreal Felt in particular, was the labour of just one man – Wolfgang Maass. His stamina for fieldwork, and his intuitive understanding of lichen biology, distinguished Maass throughout a career with the National Research Council and even in retirement, publishing entire tomes on lichens well into the 2000s. He was the first to recognize the collapse of Boreal Felt.
By the year 2000, Maass had discovered this particular cyanolichen on 159 trees across Nova Scotia. By 2008, every one of his lichens were dead. We’ve since discovered more, of course, such as in 2013 when the lichenologists of Nova Scotia uncovered an additional 113 new host trees, but almost as quickly as Boreal Felt is found, it vanishes, falling victim to threats we have only just begun to quantify.
Brad Toms, a lichenologist with MTRI, said they are dying for several reasons – nearby harvesting, grazing from invasive slugs, host tree mortality and more – but many deaths still go unexplained. The best we can do, he said, is manage the threats within our control, like forestry, and leave as much Boreal Felt habitat standing as possible, so this species might weather the coming storm of climate change. Conserving habitat for Boreal Felt is not as easy as it might sound, because even in the temperate rainforests of Nova Scotia, its spread is by no means linear, and it won’t just grow on any tree.
“We’re looking in forests with hundreds and sometimes thousands of Balsam fir, and Boreal Felt might only be on one or two of them,” said Toms. “There’s probably something to those individual trees, but it’s not something we’ve really figured out yet.”
More than likely, these host trees represent the few pockets of suitable habitat remaining to Boreal Felt, maintaining just the right blend of sunlight, humility and acidity. When Maass made his original Maritime discoveries in the 1970s, 1980s and 1990s, he found Boreal Felt on the trunks of White spruce and Red maple, as well as the branches and trunks of Balsam fir. The species was also found on a few trees in New Brunswick and Maine. Today, they are found exclusively on the trunks of Balsam fir, and only in Nova Scotia.
This is a sure sign of “niche retraction,” said Toms. When their ideal habitat is plentiful, Boreal Felt will grow more freely, on several species of tree and on several parts of those trees, but when habitat becomes scarce, they withdraw to where they are most secure – the trunks of Balsam fir. Forest harvesting and invasive slugs might be contributing to their general decline across Nova Scotia, but niche retraction signals a more fundamental shift in the province’s climate, and thus its suitability for species like Boreal Felt.
As of 2020, there were 195 trees known to host Boreal Felt in Nova Scotia. MTRI is in the process of visiting those trees for an updated figure, which will hopefully be available by 2023.
Whether you call it Macoun’s Receding Lichen Line or “niche retraction,” it all amounts to the same thing. By means direct, like forestry and development, and indirect, like climate change and invasive species, we are making our home inhospitable for entire segments of the lichen community. Their composite mycobiont and photobionts are finding each other with more and more difficulty, and in habitats and microclimates less and less ideal. If this trend continues, we may witness the extinction of some Canadian lichens this century.
But it’s their sensitivity which makes them so valuable, allowing us to gauge the integrity of our wildest spaces and the extent to which we’ve altered our home, making it hotter, drier, more acidic and less diverse. As the decades pass us by, lichens will be our surest signs of success or failure, and so they must be defended as readily as any other segments of Canadian biodiversity. That will require a “lichenized” view of the world, in which we consider the consequences of our actions big and infinitesimally small, so as to accommodate every branch on the tree of life.
“Lichens can tell us so much about the ecological world,” said McMullin. “Once you get dialed into this world, once you get lichenized, you see them everywhere.”