Moss

Mosses grow well in the cool, damp climate.

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Most of the plants growing in the Flow Country are mosses from the Sphagnum family: thirty-four different species grow in Scotland. They can be difficult to tell apart, but all of them have stems and branches covered in tiny leaves, no more than 2mm long, made from a single layer of cells.

Some of these cells contain chlorophyll, allowing the moss to absorb energy from sunlight. Others hold water – lots of water. Sphagnum moss can hold up to 20 times its dry weight in water. The swollen water-holding cells give the moss some support: there is no structure of tubes and fibres to hold it up, as there is in flowering plants. Spaghnum mosses are the primary source of peat formation in the Flow Country.

Peat Formation

The blanket bogs of the Flow Country began to form at the end of the last ice age, following the retreat of the glaciers. The climate following the last ice age fluctuated from being initially warm and relatively dry to a climate which became much wetter and colder. During the initial warm period the sediment left behind by the glaciers was colonised by pioneering plant species like grasses, sedges and shrubs like birch, willow and juniper. These plants spread via seeds transported from the continent to mainland Britain. Eventually forests composed of trees like alder, ash, elm, oak and pine became established in the landscape. However these forests were short lived.

This was because the climate became wetter and colder around 8,000 years ago. High rainfall leached the soil of nutrients and lead to the development of an iron pan which slowed the movement of water away from the soil. Coupled with low evaporation, due to cool temperatures, the soil became waterlogged and infertile.

The trees began to die away and were replaced by bog plants which were better adapted to the wet and nutrient poor environment. These included Sphagnum moss, cotton grasses, crossed leaved heath, deer grass, and the insectivorous sundews. Expansion of Sphagnum moss led to very wet conditions and acidified soil. As a result, bacterial breakdown of plant remains beneath the bog surface was greatly reduced leading to the formation of peat. Over thousands of years the bog expanded in both depth and area as peat accumulated and individual bogs joined up. High rainfall and low evaporation allowed peat deposits to build up on all but the steepest slopes, created the expansive blanket bog landscape that we know today.

Deep carbon storage

A healthy, growing bog has an upper layer of living plants called the acrotelm, usually no more than 40cm thick, and a lower layer of waterlogged peat called the catotelm, which can be many metres deep. It’s the catotelm that stores carbon, locked up in the dead plant material, as well as millions of litres of water that help to regulate river flows around the bog.

The carbon stored in the layers of peat is a vital defence against climate change. But peat bogs are also a fascinating record of past environments. They hold pollen grains like fossils: samples taken from different depths in the bog can give a detailed picture of what the environment was like 2,000 , 5,000 or even 10,000 years ago.

Threats to the moss

Sphagnum mosses grow in tough conditions, but they’re easily damaged. Grazing, burning or any lowering of the water table can all kill the living moss. It’s also sensitive to pollution from sulphur dioxide, from burning fossil fuels, and nitrogen, from excess fertiliser on neighbouring farmland. If the upper layer of living moss dies off, the lower layer of peat can be exposed to erosion, releasing its stored carbon into the atmosphere as the greenhouse gas carbon dioxide.

Most of the plants growing in the Flow Country are mosses from the Sphagnum family: thirty-four different species grow in Scotland. They can be difficult to tell apart, but all of them have stems and branches covered in tiny leaves, no more than 2mm long, made from a single layer of cells.

Some of these cells contain chlorophyll, allowing the moss to absorb energy from sunlight. Others hold water – lots of water. Sphagnum moss can hold up to 20 times its dry weight in water. The swollen water-holding cells give the moss some support: there is no structure of tubes and fibres to hold it up, as there is in flowering plants. Spaghnum mosses are the primary source of peat formation in the Flow Country.

 


Further reading

34 different species of Sphagnum in Scotland

Source: Atherton, I., Bosanquet, S., and Lawley, M. (2010). Mosses and Liverworts of Britain and Ireland – a field guide.  Plymouth: Latimer Trend & Co. Ltd.

 

Sphagnum can hold up to 20 times its own weight in water

Source: Ayres, P. (2013).  Wound dressing in World War I - The kindly Sphagnum Moss. Field Bryology, (110), pp. 27 -34

 

Acrotelm & Catotelm

Source: Lindsay, R., Birnie, R., Clough, J. (2014). IUCN UK Committee Peatland Programme Briefing Note No. 2. IUCN Peatland Programme.

 

Climate Change

Source: Scottish Natural Heritage (SNH)

 

Pollen macrofossils

Source: Lindsay, R., Birnie, R., Clough, J. (2014).  IUCN UK Committee Peatland Programme Briefing Note No. 1. IUCN Peatland Programme.

 

Threats to moss

Source: Lindsay, R., Birnie, R., Clough, J. (2014). IUCN UK Committee Peatland Programme Briefing Note No. 2. IUCN Peatland Programme.

 

Vegetative succession in Caithness and Sutherland following the ice age

Source: Auton, C., Merritt, J., and Goodenough, K. (2011). Moray and Caithness: A Landscape shaped by Geology. Scottish Natural Heritage.

 

Blanket bog formation

Source: Lindsay (1995). Bogs: The ecology, classification and conservation of ombrotrophic mires. Scottish Natural Heritage.

Source: A. V. Gallego-Sala , D. J. Charman , S. P. Harrison, G. Li, and I. C. Prentice (2015) Climate-driven expansion of blanket bogs