How long does it take for a rock to become covered in moss?
ASK A RESEARCHER: "It's nature's battle in a nutshell," says biologist.
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How long does it actually take for moss to grow on a rock?
50 years? 80 years?
That’s the question posed by one of Science Norway’s readers, Morten Olsen Haugen.
The rock in the photo he sent is covered with lichen, moss, and heather.
We set out to find the answer.
The Ice Age as the upper limit
Rune Halvorsen, a vegetation researcher at the Natural History Museum, says it's hard to give a precise answer for the particular rock the reaser asked about.
The only thing we know is the upper limit.
"11,000 years ago everything was covered by ice," says Halvorsen.
After the last Ice Age, bare rock and stone were exposed all over the country.
Slowly, plants began to return. At least in places that were warm, damp, and sheltered enough.
Monitored six rocks
But rocks can also be stripped bare in other ways, Sigmund Hågvar points out.
He is a retired professor at the Norwegian University of Life Sciences (NMBU).
Rockslides, forestry work, or even a stumbling moose can rip away moss and other vegetation.
So how long does it take for a bare rock to become overgrown?
Hågvar has researched exactly this.
Came up with an estimate
Over 14 years, Hågvar photographed the same six rocks in Western Norway.
Together with colleague Yngvar Gauslaa, he observed how the rocks gradually became covered with algae, lichen, and two different kinds of moss.
From these observations, the researchers came up with an estimate:
About 20 years from bare rock to being covered with moss.
"But that was in Sogn in Western Norway, and the process depends on the climate. If it's very dry, it takes longer," says Hågvar.
Don't need roots
Water is crucial for anything to grow on a rock.
Unlike most plants, which draw water from the ground through their roots, the first inhabitants on a rock have to manage differently.
"For lichen and moss, what comes from above is the most important," says Rune Halvorsen.
Algae, lichen, and moss must therefore capture rain or moisture from the air in order to carry out photosynthesis.
Only then can they use the energy from sunlight to turn CO2 into nutrients.
Live in symbiosis
Lichen is also a peculiar organism.
While moss is closely related to our houseplants, lichen has more in common with the mushrooms in your fridge.
"Lichen is mostly made up of a fungus. Within the fungus, algae grow, and together they form a lichen," explains Halvorsen.
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These algae, or in some cases cyanobacteria, live in symbiosis with the fungus.
They are the ones that capture energy from the sunlight.
Could have taken hundreds or thousands of years
But what about the rock in the photo sent by Science Norway's reader?
Here, the moss has grown into such a thick layer that soil has formed underneath, according to Sigmund Hågvar.
That allows heather and other plants with roots to thrive on top.
Reaching this stage may have taken hundreds or even thousands of years, the retired researcher believes.
But whatever the exact answer may be, Hågvar says a fascinating battle has been fought on that rock.
Nature's battle on a tiny patch
On a bare rock, algae, lichen, and moss will appear in a particular order.
Biologists call this succession.
"It's nature's battle and succession in a nutshell – something you can watch unfold on a tiny patch," says Hågvar.
Exactly which species settle there, and in what order, depends on the rock's location, adds Rune Halvorsen.
Rain, wind, moisture, and warmth all play a role in which organisms can survive.
Geologists use lichen as a clock
High in the mountains, only a few organisms manage to survive.
Here, lichen dominates.
Hikers will recognise map lichen, named for its patterns that resemble geographical maps.
This type of lichen grows at a rate of about two millimetres a year.
Geologists use this knowledge.
"They can look at map lichen – the yellow-green lichen that's very common – to estimate when a rock surface first emerged from the glacier," says Halvorsen.
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Translated by Alette Bjordal Gjellesvik
Read the Norwegian version of this article on forskning.no
References:
Hågvar, S. & Gauslaa, Y. Hvor fort dekkes stein av mose og lav? (How quickly are rocks covered by moss and lichen?), Journal of the Norwegian Botanical Society, 2006.
Wikipedia: Lichenometry
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