Lewis bogs.
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It may seem a little strange to have a photo at the very top of this page of something which is, clearly, not a bog. However the Callanish stone circle is one of the most recognisable and iconic images of the Isle of Lewis and up until a very few years ago only the tops of the stones were visible above the peat. The visitor to Lewis will be aware that the peat dominates the landscape but it is a little less obvious that it is constantly changing and growing and also that the bogs can "breathe," an event where they can change in height by many inches in a matter of a few hours. If you study the standing stones at Callanish closely you will observe that they are sited almost directly on the bedrock and this provides a clue to the fact that when the stones were erected there was no peat on Lewis as the climate was too warm for it to grow.

About 10,000 years before the present day the climate across the world warmed by several degrees within a period of less than 100 years and the UK experienced rapid and very significant warming. The Boreal Period which lasted from about 9000 years BP (Before Present) to about 8000BP was relatively warm and dry and is the beginning of a time known as the "warm period" or the "Thermal Optimum" or the "Hypsithermal." The archaeologists who work on circles such as Callanish might refer to this period as the "climate optimum" as it provided ideal, and much warmer, living conditions for the humans on Lewis. The living was so easy that they had time to plan and erect their stone circles. The Hypsithermal ended about 2600BP as the climate cooled towards the temperatures we experience today. 

When the circle at Callanish was erected there was no peat on Lewis because it was simply too warm for it to form under the conditions on the island but, since that time, nearly 2m of peat, since removed, had grown up around the stones leaving only the tallest visible. Not only is the peat found on Lewis today a very dynamic material, on both short and long timescales, but it is also a relative newcomer to the island when compared to the Lewisian Gneiss, the rock which makes up most of the bedrock of the island, which is believed to be around 4 billion years old.

Of course there isn't a simple answer to the question "where did the peat come from?" as the processes involved are complex and not fully understood but it may be useful to consider peatlands in general and then to address the Lewis peatland.

There are actually a range of peatland types including marshes, swamps, fens and bogs. These all have quite precise definitions with a bog being defined as ombrotrophic peatland. This sounds rather complicated but simply means that the vegetation growing on the bog, and the surface peat, receive water and nutrients only from rain, snow, dust in the air, spray from the sea and so on. Basically it means all the water and plant food comes from the sky!

This is more easily understood if you consider that other types of peatland are said to be minerotrophic because they get their nutrients through the inflow of water that has filtered through mineral soil. This water will contain good levels of the nutrients necessary for plant growth as the water will have disolved cations such as calcium and potassium on its journey through the mineral soil.

The plants growing on a bog don't have the luxury of all these minerals as their home is ombrotrophic and so is isolated from groundwater and other sources of nutrients, and it is hard to imagine that they can survive on what appears to be pure air however there are a range of plants that actually live quite well under these conditions and, in an interesting twist, some of them even cause these conditions to come about.

Bryophytes are a group of plants which include mosses, liverworts and hornworts and among the mosses we find the most important building block of the bogs - sphagnum moss. If you walk the Lewis moor you will find that there are actually a wide range of different sphagnum mosses but only a subset of the total number of species actually live on Lewis as some are adapted to a range of conditions which are rare or unknown on the island.

The sphagnum moss story is a sort of "catch 22" situation in that the sphagnum mosses are one of the few types of plants that survive well, and even thrive, in ombrotrophic conditions and they also cause the ombrotrophic conditions to come about. Over many, many years dead sphagnum moss forms a layer of slowly rotting vegetation which eventually becomes what we know as peat. Because of the nature of the decomposing sphagnum material it tends to remove useful plant nutrients from circulation in the peat and it also tends to increase the amount of hydrogen ions in the peat. This increase in hydrogen ions is what causes peat, and water, to become acid and, of course, sphagnum moss thrives in these acid conditions while other plants don't do so well. The more the moss thrives the more of it there is to die and decompose to form peat and so, over time, the peat gets deeper and deeper and the conditions become more, and more, well suited to sphagnum moss.

As well as removing nutrients and causing generally acid conditions the peat forms a physical barrier between the growing plants on its surface and the physical rocks from which plants usually derive their nutrients and if you look closely at a peat bog you will find that the plants growing on the surface may have several meters of peat between themselves and any rocks, minerals or mineral soils which may be below them. They are isolated from minerals which might otherwise have been available for growth. This is how ombrotrophic conditions come about and on Lewis we can lay the blame firmly at the door of the bryophytes and sphagnum in particular.

Of course the problem is that if sphagnum causes peat, and peat provides conditions ideal for sphagnum then how does the sphagnum get a hold in the first place?







Dollag's Cottage @ 7 South Shawbost
The West Row at Callanish Stone Circle
To fully understand how sphagnum gets a hold and goes on to build a peat bog it is necessary to point out that not all peat is made from sphagnum. Under some circumstances grasses and sedges such as those shown in the photo of a moorland pool (above) can also form peat and, as such grasses will grow in areas that have not yet been rendered ombrotrophic so the peat they produce can be the "seed" that kicks off the sphagnum take over.

Peat formation is generally thought to happen in three main ways:

Infilling: Infilling is what you can see in the photo of the pool shown above and is the process whereby peat develops on the margins of pools and lochs.

Paludification: This refers to the development of peat over what was previously less wet mineral ground and is perhaps best illustrated by the fact that some bogs in the UK and Ireland have evidence of forests remaining in the lower layers of peat. Basically the ground becomes more wet, peat starts to form and in due course it becomes deep enough to remove access to the minerals below for the surface plants and then our friend sphagnum moves in.

Primary peat formation: This is the process whereby peat forms directly on freshly exposed mineral surfaces. For example this might occur after volcanic activity or if a glacier was to melt and expose the previously ice covered rock underneath.




One question that many people ask is “how fast does the peat grow?” and this is very difficult indeed to answer as the rate of increase in the height of a peat bog is not constant. Some researchers have found that a figure of around 2 – 3 mm per year has accurately reflected the growth in bogs they have studied but, of course, this only applies to that particular bog over that particular time scale. Others give 1mm per year as a good average figure and certainly the maximum depth of peat on Lewis would be in keeping with the 1mm per year figure. The situation is made more complex by the weight of the peat on top of a bog compressing the lower layers and decomposition of these lower layers and depending on the dynamic of this situation the bog can suffer from what a politician might call “negative growth.” There is also the further complexity of something called “bog breathing” which remains largely unexplained but is a phenomena where a bog can increase, or decrease, in height by many inches over a period of a few hours.

Given all these complexities in measuring the growth in height of a bog over time most researchers prefer to measure the weight of new material added to the bog per square meter over the period of a year. To eliminate the impact of weather the plant material is normally dried before its weight is measured and, to add to the confusion, the rate of growth is often quoted as LARCA or Long-term Apparent Rate of Carbon Accumulation. I’m sure that seemed like a good idea at the time. This figure generally comes in at an average of about 30 grams of carbon per meter squared per year though, clearly, if varies from place to place. It is considered that about 50% of the mass of peat is carbon and so this would give a total increase in mass of about 60g per meter squared per year.

So when you are standing in front of a peat bank on the Lewis moor it is reasonable to assume that it may have grown at about 2 – 3mm per year but the truth is that the rate could have varied significantly from year to year and even over the long term and so the only way to make any absolute statement is to date the peat at various heights in the peat bank and do the calculations. This process is costly and may never have been done for any peat on Lewis.



acid

numbers of creatures 




an infilling pool on the Lewis moorland
moorland and infilling on the Isle of Lewis
The image above shows Loch nan Geadh and the bog surrounding it. As you can see to the left of the loch there is a much brighter area of green and this is the sphagnum which is slowly infilling the loch from the surrounding bog. The loch actually sits on a stony mineratrophic base but the sphagnum which is encroahing from the edges is slowly laying down layers of peat which will, one day, probably meet up in the middle of the loch leaving nothing but a bog where Loch nan Geadh once sat.
One question that many people ask is “how fast does the peat grow?” and this is very difficult indeed to answer as the rate of increase in the height of a peat bog is not constant. Some researchers have found that a figure of around 2 – 3 mm per year has accurately reflected the growth in bogs they have studied but, of course, this only applies to that particular bog over that particular time scale. Others give 1mm per year as a good average figure and certainly the maximum depth of peat on Lewis would be in keeping with the 1mm per year figure. The situation is made more complex by the weight of the peat on top of a bog compressing the lower layers and decomposition of these lower layers and depending on the dynamic of this situation the bog can suffer from what a politician might call “negative growth.” There is also the further complexity of something called “bog breathing” which remains largely unexplained but is a phenomena where a bog can increase, or decrease, in height by many inches over a period of a few hours.

Given all these complexities in measuring the growth in height of a bog over time most researchers prefer to measure the weight of new material added to the bog per square meter over the period of a year. To make results comperable the plant material is normally dried before its weight is measured and, to add to the confusion, the rate of growth is often quoted as LARCA or Long-term Apparent Rate of Carbon Accumulation. I’m sure that seemed like a good idea at the time. This figure generally comes in at an average of about 30 grams of carbon per meter squared per year though, clearly, if varies from place to place. It is considered that about 50% of the mass of peat is carbon and so this would give a total increase in mass of about 60g per meter squared per year.

So when you are standing in front of a peat bank on the Lewis moor it is reasonable to assume that it may have grown at about 1 – 3mm per year but the truth is that the rate could have varied significantly from year to year and even over the long term.



acid

numbers of creatures 




Isle of Lewis moorland
The image above shows a typical Lewis bog and, as you can clearly see, it isn't very flat and it isn't all the same colour when viewed up close. The actual bog is often considered in terms of a number of vertical "layers" or "levels". To the foreground left of centre in the photo above you can see a mound constructed of sphagnum and also a few other plants and the mound is considered the highest of the layers in the bog and is usually referred to as a "hummock" and may be some tens of centimeters above the watertable which means that a hummock is often home to species of sphagnum and other plants which are not usually found in other parts of the bog. The next level down from the top of the hummock is called the "lawn" and it usually occupies areas which are about 5 - 20cm above the water table and tends to be dominated by plants such as grasses which have fairly robust rooting systems. Because of the roots lawns tend to be quite firm and so footprints disappear fairly quickly. From about 5cm above to 5cm below the water table we find the area known as the "carpet" and most of the vegetation in this part of the bog will be bryophytes (i.e. mosses) with less in the way of grasses or similar vascular plants with roots. As the carpet is made up of soft mosses a footprint will tend to remain for a long time. After the carpet comes the "mud-bottom" which, as the name suggests, tend to lack vascular plants and are often full of water (you can see some mud-bottoms in the picture above) and may often be exposed bare peat which is sometimes covered with algae, hence the name. Mud-bottoms are often full of water but they do also occasionally dry out whereas the "pools" are permanently water filled holes or dips in the peat and are the lowest of the vertical layers which make up a bog.
hummocks on the Isle of Lewis moor
Rain water usually has a pH (a measure of acidity) of around 5.6 which is slightly acidic - the water takes up carbon dioxide from the air and this causes it to become a weak acid. As we have seen all of the water on a bog has fallen from the sky as there is no inflow of mineral enriched water from outside of the bog and so the water in the bog starts as a weak acid with a pH of around 5.6. Because of the actions of sphagnum, in particular, as have been detailed above the level of acidity is further increased in the bog and so we can expect the water in some of the pools on the moor to have a pH of around 4.0. With acidity the lower the number the more acid the water is so by the time the water has filtered through some layers of sphagnum and peat it has become considerably more acidic than the rainwater which falls on the moorland.

Despite being rather acidic and being viewed as a little species poor a bog can be a very interesting place and even close up there are lots of interesting things to see, even some plants that eat insects:
sundew on the Isle of Lewis
sphagnum and sundew on the Isle of Lewis
The Lewis bog is also home to a lot of interesting creatures ranging from deer and eagles to a wide range of insects. One insect that is of interest to all visitors to Scotland is the famous Scottish midge and the visitor who is keen to encounter this iconic Scottish species will be pleased to hear that the bog is estimated to support around 20 million midges per hectare. Some research in Finland established that on bog with some pine trees there were well over a million individual insects per meter squared. If you lie on your back on the Lewis bog you are probably blocking out the sun for about a million little insects below you.

Hopefully this little guide to the Lewis bog will offer you some encouragement to to appreciate just what an exciting and interesting environment it is and while you are staying at Dollag's Cottage be sure to get out and have a close look at the bog and the wilder and more remote areas of the Lewis moorland. 
Isle of Lewis moorland
Bryophytes are a group of plants which include mosses, liverworts and hornworts and among the mosses we find the most important building block of the bogs - sphagnum moss. If you walk the Lewis moor you will find that there are actually a wide range of different sphagnum mosses but only a subset of the total number of species actually live on Lewis as some are adapted to a range of conditions which are rare or unknown on the island.

The sphagnum moss story is a sort of "catch 22" situation in that the sphagnum mosses are one of the few types of plants that survive well, and even thrive, in ombrotrophic conditions and they also cause the ombrotrophic conditions to come about. Over many, many years dead sphagnum moss forms a layer of slowly rotting vegetation which eventually becomes what we know as peat. Because of the nature of the decomposing sphagnum material it tends to remove useful plant nutrients from circulation in the peat and it also tends to increase the amount of hydrogen ions in the peat. This increase in hydrogen ions is what causes peat, and water, to become acid and, of course, sphagnum moss thrives in these acid conditions while other plants don't do so well. The more the moss thrives the more of it there is to die and decompose to form peat and so, over time, the peat gets deeper and deeper and the conditions become more, and more, well suited to sphagnum moss.

As well as removing nutrients and causing generally acid conditions the peat forms a physical barrier between the growing plants on its surface and the physical rocks from which plants usually derive their nutrients and if you look closely at a peat bog you will find that the plants growing on the surface may have several meters of peat between themselves and any rocks, minerals or mineral soils which may be below them. They are isolated from minerals which might otherwise have been available for growth. This is how ombrotrophic conditions come about and on Lewis we can lay the blame firmly at the door of the bryophytes and sphagnum in particular.

Of course the problem is that if sphagnum causes peat, and peat provides conditions ideal for sphagnum then how does the sphagnum get a hold in the first place?
S. fallax, S. Palustre and, perhaps, S. Capillifolium
Fire clubmoss on the North Harris moorland
Slime mould on the Lewis moorland
Below: Fir clubmoss (Huperzia selago) on the North Harris moorland. Fir clubmoss is a very primitive plant related to ferns and it is not actually a moss in the strict sense. It is not uncommon on the Lewis and Harris moorland though it is often found as a single isolated plant and so is easily overlooked.
Below: Slime mould (probably Fuligo septica) on the Lewis moorland. This particular slime mould is most striking for its bright yellow colour, though it soon fades to a dark brown colour and becomes much less noticable.
Below: A selection of photos of (mostly) bryophytes - basically mosses - showing the interesting forms and colours of these plants which are often ignored as we tramp them underfoot while walking on the moor. It is possible that not all of these images were taken on Lewis, though they could have been, as it is hard to keep track with close up photos like these and as I photograph bryophytes in lots of places there isn't much info to establish exactly where the picture was taken.
Moorland bryophytes
Moorland bryophytes
Lichen and sphagnum
Moorland bryophytes
Moorland bryophytes
Moorland bryophytes
Moorland bryophytes
Below: Autumn colours on the Lewis moorland near to the hill called Stacaiseal.
Autumn colours on the Isle of Lewis moorland