The Forest Bark Is Talking, but Are We Listening?

Let's listen to what Lichens and Moss Reveal About Ecosystem Health Written by Kanchan Jaswal and Gopakumar Sukumaran Nair

How can something as ordinary as a fallen twig change the way you see a forest?

My own curiosity started on a field trip, with a twig no wider than a pencil lying in the leaf litter. Something about it looked too busy to ignore, crowded with colour and texture that didn't belong to the twig itself. I picked it up without knowing what I was looking at. It took a while to learn that this small, overlooked thing was hosting an entire miniature world, and that was enough to send me looking for the rest of the story!

The twig that started it all, found on a field trip, its surface already a small ecosystem of its own (© Kanchan Jaswal)

Next time you're in a forest, stop and run your hand over that grey-green crust on the bark of an old tree. Sometimes rough and velvety, this is a living world that almost everyone walks past without a second glance. A lichenologist doesn't. To them, it isn't decoration; it's a display screen, quietly recording what this forest has lived through.

That small discovery reminded me of something Rachel Carson wrote in Silent Spring (1962): the species we ignore most easily are often the ones telling us the most. Her subject was birds. Ours are lichens and moss. No roots. No stomata. No waxy cuticle. No filtering system. Moss and lichens absorb whatever the air sends their way, unfiltered, uninterrupted, lifelong.  That's exactly what makes them useful.

A Living World on Bark that Look Alike, But They're Not

The green hues on bark tell more than one story.

Moss is a true plant, but a lichen isn't!

It isn’t a single organism at all; it's a fungus and an alga (sometimes a cyanobacterium) living as one, the fungus supplying structure and moisture, the partner supplying food [1], but both are poikilohydric: Drought switches them off. Rain switches them back on, no permanent damage. Just a remarkable pause and restart. That trick, first evolved over 450 million years ago, is also what makes them such patient recorders.

A fast-growing plant reacts to the whim of weather, whereas a lichen's tissue still holds a memory of years

More Than Just Growing on Bark

Mosses and lichens act like living sponges, absorbing rain and mist quickly and releasing it slowly, helping ecosystems withstand both floods and droughts [2].

In tundra and boreal forest, moss makes up half the ecosystem's above-ground biomass; Sphagnum alone locks carbon into peat for millennia, provided fire stays away, since it takes decades to recover after a burn [3].

In drylands, lichen crusts hold soil against erosion. Lichens carrying cyanobacteria fix nitrogen where little else can; other lichens slowly dissolve rock to release phosphorus [4].

They also build habitat at every scale. Microscopic water films within lichens provide a home for tardigrades and nematodes. Mites and beetle larvae feed directly on the thallus. In the Arctic, lichens such as Cladonia and Bryoria help sustain reindeer and caribou alive through long winters [5].

The story doesn’t end there. One of the strange ecological threads is that moss shelters insects, which draws insect-hunting birds into the canopy, including hummingbirds. Those same birds pollinate flowering plants elsewhere in the forest. Pull the moss out, and a link snaps in a place nobody would think to look [6].

The signs we’ve been missing out on!

Lichens and mosses don't have the luxury of filtering the air around them. Whatever the wind carries (sulphur dioxide, heavy metals, or nitrogen compounds), they absorb it directly. Over time, the evidence begins to show: chlorophyll breaks down, their tissues become scarred, and the thalli slowly die back [7].

Then I stumbled upon a fact that completely changed the way I looked at lichens. Their growth form tracks sensitivity almost like a scale. The branching fruticose lichens are the most delicate, the leafy foliose lichens are intermediate, and the flat crustose lichens shrug off far more. In a healthy stand, all three grow side by side. Under real stress, only the crusts remain [8].

Different forms of lichen. Left to right: Crustose, foliose, and fruticose lichen (© Kanchan Jaswal).

The evidence isn't just theoretical; it is already written into forests around the world.

In Poland's Białowieża Forest, Europe's last primeval woodland, lichen diversity dropped 37% in managed stands versus untouched old growth, while moss diversity barely moved [9]. The reason is bark itself: the old, stable microhabitat lichens need takes centuries to develop, and forestry removes it long before that.

Even cities carry this signature. A 2026 study across Amsterdam, Leiden, and Rotterdam found lichen and moss communities shift visibly along the urban heat island gradient, tracking heat and dryness the same way a forest lichen tracks pollution [10].

Closer to home, the same logic holds in an Indian megacity. Around Kolkata, the foliose lichen Pyxine cocoes shifted toward tougher, more pollution-tolerant crustose forms as urban air quality worsened, with scanning electron images showing soot and particulate matter physically trapped in its tissue [11].

Yet across India, most lichen and moss research, particularly in the Western Ghats and the Himalaya, still focuses on documenting what grows where, rather than asking what their presence is telling us

A slow-motion emergency

As if pollution weren't enough, climate change adds another layer to the story, i.e., displacement. Boreal lichens like Cladonia stellaris are advancing into the newly ice-free high Arctic while retreating from their warmer, southern range edge [12]. To keep pace with shifting climate zones, these organisms would need to disperse at roughly 16 km a year, far beyond what their slow, occasional sporulation can manage [13].

The habitat may arrive on schedule. The species may not.

Some warnings don't roar. They grow quietly on bark, waiting for someone to notice. Will we?

Cryptogamic growth on tree bark (© Stephanie Moore)

Why This Quiet Story Matters Now

Silent Spring made me wonder what we choose to notice. But it left me with an even deeper question: Are we listening to what the forest bark is telling us?

 The trees still stand, and the canopy still whispers, yet when the lichens fade, one of the forest's oldest languages falls silent.

So, press your hand against that old bark again. A rich mix of crusty, leafy, and branching lichens growing together is a good sign. A single thin crust clinging on, and nothing else, means the forest is under real strain. That's the whole reading, and it might be one of the quietest, cheapest tools we have for keeping watch over the forests still left to us.

Every patch of lichen or moss on bark is proof the forest is still speaking. As long as we keep learning its language, we're not too late to answer

References

[1] British Lichen Society. What is a lichen? British Lichen Society. Available at: britishlichensociety.org.uk.

[2] Aartsma, P., Asplund, J., Odland, A., Reinhardt, S., and Renssen, H. 2021. Microclimatic comparison of lichen heaths and shrubs: shrubification generates atmospheric heating but subsurface cooling during the growing season. Biogeosciences 18(5): 1577–1599.

[3] Stanton, D.E., Ormond, A., Koch, N.M., and Colesie, C. 2023. Lichen ecophysiology in a changing climate. Am. J. Bot. 110(2): 1–14.

[4] Porada, P., Bader, M.Y., Berdugo, M.B., Colesie, C., Ellis, C.J., Giordani, P., Herzschuh, U., Ma, Y., Launiainen, S., Nascimbene, J., and Petersen, I. 2023. A research agenda for nonvascular photoautotrophs under climate change. New Phytol. 237(5): 1495–1504.

[5] Palmroos, I., Norros, V., Keski-Saari, S., Mayra, J., Tanhuanpaa, T., Kivinen, S., Pykala, J., Kullberg, P., Kumpula, T., and Vihervaara, P. 2023. Remote sensing in mapping biodiversity — a case study of epiphytic lichen communities. For. Ecol. Manag. 538: 1–10.

[6] Fonturbel, F.E., Osorio, F., Riffo-Donoso, V., Carvallo, G.O., and Rydin, H. 2021. Cryptic interactions revisited from ecological networks: Mosses as a key link between trees and hummingbirds. Funct. Ecol. 35(1): 226–238.

[7] Dhaouadi, S., Khalloufi, N., Ayati, K., Ayeb, N., and Béjaoui, M. 2022. Use of lichen species for air pollution biomonitoring: Case of Dar-Chichou forest (Cap-Bon, North-East Tunisia). Environ. Sustain. Indic. 16(1): 1–9.

[8] Thakur, M., Bhardwaj, S., Kumar, V., and Rodrigo-Comino, J. 2024. Lichens as effective bioindicators for monitoring environmental changes: a comprehensive review. Total Environ. Adv. 9(1): 1–9.

[9] Czerepko, J., Gawrys, R., Szymczyk, R., Pisarek, W., Janek, M., Haidt, A., Kowalewska, A., Piegdon, A., Stebel, A., Kukwa, M., and Cacciatori, C. 2021. How sensitive are epiphytic and epixylic cryptogams as indicators of forest naturalness? Testing bryophyte and lichen predictive power in stands under different management regimes in the Białowieża forest. Ecol. Indic. 125: 1–19.

[10] Claerhout, T., Sparrius, L., Keßler, P., and Stech, M. 2026. Urban heat islands shape epiphytic communities of lichens and bryophytes. Urban Ecosyst. 29(2).

[11] Banerjee, S., Ram, S.S., Mukhopadhyay, A., Jana, N., Sudarshan, M., and Chakraborty, A. 2023. Potential of epiphytic lichen Pyxine cocoes, as an indicator of air pollution in Kolkata, India. Proc. Natl. Acad. Sci. India Sect. B Biol. Sci. 93(1): 165–180.

[12] Mallen-Cooper, M., Rodríguez-Caballero, E., Eldridge, D.J., Weber, B., Büdel, B., Höhne, H., and Cornwell, W.K. 2023. Towards an understanding of future range shifts in lichens and mosses under climate change. J. Biogeogr. 50(2): 406–417.

[13] Warren, S.D., St. Clair, L.L., Stark, L.R., Lewis, L.A., Pombubpa, N., Kurbessoian, T., Stajich, J.E., and Aanderud, Z.T. 2019. Reproduction and dispersal of biological soil crust organisms. Front. Ecol. Evol. 7(1): 344–351.

About the Authors

Ms. Kanchan Jaswal is a PhD Scholar in the Department of Forest Resource Management at Kerala Agricultural University, India. Her research focuses on forest ecology, wildfire science, and the application of remote sensing and GIS for forest ecosystem assessment and fire risk mapping in the Western Ghats.

Dr. Gopakumar Sukumaran Nair is a Professor in the Department of Forest Resource Management at Kerala Agricultural University, India. His research interests include forest ecology, forest management, biodiversity conservation, and sustainable natural resource management, with a strong emphasis on advancing research and mentoring early-career scientists.