Wednesday, February 21, 2018

Lichen ('li-ken' or 'le-chen'?)

Lichen. In American English, pronounced “li-ken” (like “liken”). In British English, “le-chen” (like “kitchen”). Either way, the word refers to a symbiotic collective of either algae or cyanobacteria in conjunction with fungi, slowly growing on trees or rocks. Some may have leaf-like lobes while others are flat, thin, and so deeply embedded into the rock that they look more like two-dimensional circles of green/gray paint rather than any live thing. The alga or bacteria use sunlight to make nutrients for the fungus, primarily sugar, while the fungus provides minerals, shelter from the elements, and retains water captured from the environment. Even when growing on living matter, such as tree bark, lichen are not parasitically taking nutrients from the host.

Lichen growing on stones set in a stone wall.
Lichens are complex. The novel idea that what appears to be one living thing is actually a combined effort dates to microscope observations by a Swiss botanist named Simon Schwendener. He proposed this theory in 1867. Leading lichenologists (great word!) of the time were dismissive. In time, the idea became accepted science as did the name for what was going on – symbiosis. And yet there was a problem. Try as they might, scientists could separate the algae or cyanobacteria from the fungi and grow each separately, but when remixed, the result did not grow as lichen.

Only recently, and only with the help of DNA analysis, did Toby Spribille discover that two fungi species, not one, were needed to create the complex structure of lichen. Fungi of the division Ascomycetes were the known part of the partnership. What his research showed was that trace amounts of Basidiomycetes fungi were equally essential, integrated into the outer surface of the colonies. This begs the question of whether every lichen we see is the successful result of a three-way blind date, or is there a physical means of creating new colonies by all three being relocated together. Looks like the latter.

Lichens get around. Volcanic activity about 20 miles from Iceland’s south coast ended up creating an island – Surtsey – in 1967. Scientists observed biocolonization over the years. Moss and lichen were observed within a few years. Over time, birds nesting on the island transported seeds caught in their feathers and in their feces, which also added to create fertile soil (as did their carcasses when they died), but lichens and mosses still dominate much of the island to this day.   

Lichen growing on a headstone in Lower Village Cemetery, Stow, MA
Lichens are slow-growing. The crustose types (flat, on rocks) may grow less than one millimeter a year from the edges outward, so that a colony a few inches across can be decades old. Lichenometry is the science of dating when stones arrived at a location, either a landslide, or fast-moving flood, or a stone wall. Confirmation of this as a useful dating device stemmed from knowing exactly when stones presented a surface to the air, as in stone split or cut for tombstones. With that as a benchmark, dating could be determined for archeological items too recent for radio-carbon dating. For human-made structures of wood and stone, tree-ring dating for the wood and lichen growth for the stone help confirm each other as accurate yardsticks.

Glacial erratic (boulder left behind by last ice age) showing more
 than one type of lichen, some bumping into their neighbors.
While moss prefers the north side of trees (sort of), lichen is not as particular. Moss is all about moisture, so in regions with dry times of year, the north side, not subjected to direct sunlight, stays damp longer after rains or morning dew. Lichen, with its slower growing pace, is more likely to end up anywhere on a tree trunk or on rocks on the ground.

Locally, just about any walk in the woods, or for that matter, in one’s own backyard, will yield many sightings of various types of lichen. In retrospect, actions on these woodland boulders and stone walls are ferocious territorial wars, fought small, slow and in silence. Lichen versus moss. Lichen versus lichen. At times, deus et machina a gigantic snail or slug speeds over the battlefield, rasping away at everything in its path. Interestingly, these terrestrial gastropod mollusks have a somewhat inefficient digestive system, so as they move along, defecating as they go, they leave behind the beginnings of new lichen colonies, perhaps boldly growing where no lichen has grown before (cue Star Trek theme music).

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