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Environment

Outside/Inbox: Does moss get damaged when you walk on it?

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Robert Blendea
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Public Domain
Moss!

Every other Friday on Morning Edition, the Outside/In team answers a question from a listener about the natural world.

This week, we answer three questions about moss.

Question #1: Carolyn asks via Instagram: “Does moss get damaged when you walk on it? 

Editor's Note: A previous version of this story incorrectly stated the depth that moss can photosynthesize under Antarctic ice.

It’s squishy, it’s green, and it makes forests look like magical fairy hideouts. But what is moss really?

Moss is a type of plant called a bryophyte, and what distinguishes it from a lot of other plants is that it reproduces using spores, it doesn’t have flowers, and it doesn’t have a vascular system — which is basically the water transportation system of stems and trunks and branches and whatnot. For you botanical geeks out there, that means no xylem and no phloem. Instead, moss holds water kind of like a fuzzy green land sponge.

Most plants don’t love being trampled on, and moss is no exception. But how much damage walking inflicts is all in the details.

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Nat Cleavitt is a plant ecologist with Cornell University, but works primarily at the Hubbard Experimental Forest (interesting side note: that’s where acid rain was first discovered and researched).

And when I asked her Carolyn’s question, Cleavvit responded with a few follow-ups of her own.

“How much walking? How much do the walkers weigh? Are they moving slow, or are they running? Are they barefoot or are they wearing boots? The more force that you apply to the moss or lichen, the more damage you’re going to do.”

Cleavitt says that unlike grasses, which grow from the base of the blade and recover easily from being stepped on or mowed, moss has a three-dimensional growing structure that makes it more vulnerable to getting squashed.

Contrary to what you might think, that also means that big well-developed mosses (like sphagnum mosses) are more likely to be damaged by walking than smaller ones.

And the drier and more brittle the moss is, the more vulnerable it is. Water helps to shield mosses from injury.

“[Water] makes your cells a lot more elastic,” Cleavitt says. “It also fills in all the spaces and somewhat protects against the compacting.”

Luckily for moss, it’s evolved to hold water and dry out slowly, so it can retain that protective cushion for an impressive amount of time.

In summary: if you cut a new trail over a bed of moss, all those aggregated heavy footfalls are likely to have a negative impact. But if a couple people gingerly step off the trail (say, to use nature’s bathroom) and happen to walk over some wet moss… well, it’ll probably be just fine.

All things considered, moss really is pretty resilient stuff. Moss can survive being buried in snow and ice, and some species can even survive entirely underwater. In fact, there are mosses that photosynthesize two-and-half meters under the ice in Antarctica!

Lichen, which can sometimes get mistaken for moss, is much more fragile. That’s because, whereas moss is just a plant, lichen is actually made up of somewhere between two and four separate organisms all working together: fungi, cyanobacteria, algae, and yeast. Those complex interrelationships make lichen more sensitive, and pretty easily damaged.

luminous moss
Luminous Moss, Yusuke MiyaharaFollow CC2.0 via Flickr (https://bit.ly/3wNJIRY)
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A patch of Luminous Moss, otherwise known as "Goblin's Gold"

Question #2

A listener named Tricia emailed to ask “What makes the moss on trees seem to ‘light up’ on rainy days?”

Unlike species like bioluminescent algae, moss doesn’t literally glow, but this “lighting up” effect isn’t just Trish’s imagination: Rain really can make moss (and lichen) change color and get more vibrant. Basically, all that fresh moisture kind of makes them unfurl and get bigger, like wetting a dry sponge.

You can even turn it into an experiment. “Get a spray bottle and go out in the summer when they’re dry, and spritz ‘em,” Cleavitt says. “It’s just magic.”

One type of moss that looks particularly otherworldly. Schistostega pennata, also known as Luminous Moss, Dragon’s Gold, or (my favorite) Goblin’s Gold, has specialized circular cells that act as miniature lenses. These cells can turn towards a light source and reflect even the dimmest luminescence. Even though they’re not exactly glowing, Goblin’s Gold can transform into a shimmering carpet of mystical green.

Question #3: Tricia also wants to know, “Is moss on trees a good or bad sign for the tree or surrounding woods?”

Moss almost never does damage to trees, except in rare cases where it grows so thickly that the weight can snap limbs and branches.

The presence of moss is usually just a sign that you’re looking at older-growth forest with a fair amount of regular moisture, where moss has had ample time to colonize.

Most of the time, moss offers a ton of ecosystem benefits.

“They’re not just important for themselves, they’re whole worlds!” Cleavitt says. “There are so many organisms that depend on these miniature jungles.”

Moss isn’t just good for forests. Sphagnum moss ecosystems (peat bogs) are perhaps, pound for pound, the most powerful carbon sinks on the planet. A mossy peat bog can hold twice as much carbon per hectare as a redwood forest! In fact, conservationists in Scotland are fighting climate change by, ironically, cutting trees to restore old bogs drained just a few decades ago.

So if you decide to take a stroll through a peat bog, remember - be careful where you step. Those boots might be made for walking, but the moss you’re walking on isn’t.

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If you’d like to submit a question to the Outside/In team, you can record it as a voice memo on your smartphone and send it to outsidein@nhpr.org , or call the hotline, 1-844-GO-OTTER.

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