Mosses are some of the oldest land plants. They are found all over the world, from lush tropical rainforests to the driest deserts and even the windswept hills of Antarctica.
They are everywhere; grow in cracks along roads and paths, on the trunks of trees, on rocks and buildings, and most importantly, on the ground.
But despite this ubiquity, we have a relatively poor understanding of how important they are, especially the types of moss that thrive on Earth.
New worldwide research on soil mosses published today in Nature Geoscience reveals that they play a vital role in sustaining life on our planet. Without soil mosses, Earth’s ability to produce healthy soils, provide habitats for microbes, and fight pathogens would be greatly diminished.
A worldwide survey of soil mosses
The results of the new study indicate that we probably underestimated how important soil lichens are.
Using data from 123 sites on all continents, including Antarctica, we show that the soil under mosses contains more nitrogen, phosphorus and magnesium, and greater soil enzyme activity than bare surfaces without plants.
In fact, mosses affect all major soil functions, increasing carbon sequestration, nutrient cycling, and decomposition of organic matter. These processes are critical to sustaining life on Earth.
Our modeling revealed that benthic mosses cover a huge area of the planet, about 9 million square kilometers – equivalent to the area of China. And that excludes mosses from boreal forests, which were not included in the study.
The strength of the effect that mosses have on the soil depends on their growing conditions. They have the strongest effect in low-productivity natural environments, such as deserts. They are also more important on sandy and saline soils and where rainfall is highly variable.
It is not unexpected that mosses have the strongest effects on soils where vascular plants – those that contain specialized tissues to conduct water and minerals – are scarce.
Read more: Silver moss is a rugged survivor in the urban landscape
An intimate connection
Mosses lack the plumbing that allows vascular plants to grow tall and draw water from the soil. As a result, they remain relatively short and develop an intimate connection with the upper soil layers.
Mosses are extremely absorbent and can attract dust in the air particles. Some of these particles are absorbed into the soil below. It is therefore not surprising that they have such a strong effect on the soil.
Our modeling shows that mosses around the world store 6.4 gigatons more carbon than soils without plant cover.
The loss of just 15% of the global cover of benthic lichens would be equivalent to global carbon dioxide emissions from all land use changes during a year, such as clearing and overgrazing.
Not all mosses are equal
We also found that some mosses are more effective at promoting healthy soil than others. Long-lived mosses were usually associated with more carbon and better control of soil pathogens.
The ability of mosses to provide ecosystem services and support a diverse community of microbes, fungi and invertebrates was strongest in sites with high cover of mat and sod-forming mosses such as peat mosswhich are widely distributed in boreal forests.
Soils are a huge reservoir of soil pathogens, but the soil under mosses had a lower proportion of plant pathogens. Mosses can help reduce soil pathogens. This ability may have originated when mosses evolved as land plants.
A special group in the desert
A special kind of moss flourishes in deserts. They live hard (perennial mosses) or die young (annual mosses).
Mosses in the family Pottiaceae are ideally suited to life in dry and inhospitable conditions. Many have specialized structures that allow them to survive when water is scarce. These include boat-shaped leaves with long hairy tips that help direct water to the center of the plant. Some mosses twist around their stem to reduce the area exposed to the sun and retain moisture.
Desert mosses also protect the soil from erosion, influences how much water moves through the upper layers and even changes the survival rates of plant seedlings.
Other mosses have special moisture-absorbing cells (papillae) that swell and provide a moisture reserve when it is dry.
Our global research showed that mat and sod-forming mosses such as peat moss had the strongest positive effects on the diversity of microbes, fungi and invertebrates, and on critical services such as nutrient supply. As might be expected, longer-lived mosses supported more soil carbon and had more control over plant pathogens than short-lived mosses.
Protect the mosses
Overall, our work shows that mosses influence important soil processes and function similarly to vascular plants. Their effects may not be as strong, but their total coverage means that mosses are potentially just as important when added up around the world.
But mosses are under increasing threat worldwide; disturbance by cattle, over harvestland reclamation and even changing climates are the biggest threats.
We need more recognition of the services that soil mosses provide to all life on this planet. This means raising awareness about their positive benefits, identifying and mitigating the key threats they face, and incorporating them into routine monitoring programs.
Soil mosses are everywhere, but their future is far from certain. They are likely to play an increasingly important role as vascular plants decline under predicted hotter, drier and more variable global climates.
Read more: Antarctica’s ‘moss forests’ are drying up and dying