Fen Vs Bog – What’s the Difference? (The Facts)

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Fens vs Bogs – What is the Difference?

At first glance, some fens and bogs can appear quite similar, and may even contain similar plant and animal species, though by definition they are quite the opposite of one another. Some confusion is understandable, particularly when given that there are many different wetland classification systems around the world.

Even according to one of the most trusted and predominant classification systems in the world (used most commonly in the U.S.), the Cowardin System, there are dozens of different types of wetlands classified within some half a dozen categories and subclasses.

cowardin system how are wetlands classified
A basic example of how some palustrine wetlands are classified according to the Cowardin System. Courtesy of the U.S. Fish and Wildlife Service.

Our own guide to the different types of wetlands helps to break things down into digestible bits, but there is a fair amount of nuance involved in wetland classification, and fens and bogs are no exception.

Here, we will break down both how fens and bogs are alike, as well as how they are different to give you a better idea of how to identify the two and the purposes that they serve.

Similarities Between Fens and Bogs

1) Structure and Formation

sphagnum moss in a bog
Though more prevalent in bogs, thick sphagnum mosses are often found in both wetland types. Photo by Tero Laakso / CC BY-SA 2.0

According to the Cowardin System of wetland classification, there are five classes of wetlands that can be broken down into many types and subtypes. These classes are known as marine, riverine, estuarine, lacustrine, and palustrine. Both fens and bogs belong to the latter-most group of palustrine wetlands, which are those that are dominated by trees, shrubs, and/or emergent vegetation. They are also often home to many species of mosses and lichens, though this isn’t exclusive to the palustrine class, and are often rich in unique animal and plant life. Palustrine wetlands are the ones that many of us are the most familiar with, as this class also includes swamps and marshes and their sub-types, such as mangrove swamps and salt marshes.

Though fens and bogs are primarily quite different in both their structure and their formation (covered in more detail later in the article), they can be, at times, similar. For instance, fens are primarily alkaline in nature due to being fed by mineral-rich groundwater and runoff from other water sources (making them minerotrophic), but over time can build up layers of sphagnum mosses and peaty soils. Over many centuries, this will gradually reduce the flow of groundwater and can cause the fen to become more acidic, similar to a bog. In fact, some fens can actually transition into bogs over many, many years as the soil gains acidity and sphagnum mosses allow precipitation to accumulate, resulting in the wetland transitioning to being ombrotrophic rather than minerotrophic.

Additionally, fens and bogs are almost always constantly saturated, though bogs can fluctuate more and may dry out from time to time due to extended periods of drought. They can both also be technically referred to as “scrub-shrub wetlands,” as both wetland types typically contain small shrubs and somewhat scrubby, emergent vegetation like members of the Vaccinium genus and various sedges.

Bogs and fens alike occur almost exclusively in the northern hemisphere as a result of glacial formation and eventual retreat. Both can take upwards of 10,000 years to form naturally, making it all the more important that we conserve these ecosystems. Neither of them is dominated by trees but may contain a few “islands” of trees like tamaracks or cedars above the saturated water line, though these islands are much more common in bogs than in fens. Trees are more usual around the edges of bogs and fens than within them.

2) Functions and Benefits

benefits of bogs and fens
Bogs (pictured) and fens are both excellent at soaking up excess water, nutrients, AND carbon. Photo courtesy of U.S. Fish and Wildlife Service.

Both bogs and fens, like virtually all wetlands, serve the incredibly vital service of filtering water, soaking up excess nutrients and pollutants, and acting as a sort of sponge that protects surrounding environments (including our towns and cities) from flooding. They also house very unique plant and animal life and act as incredibly valuable carbon sinks that, if managed properly, can help to mitigate climate change.

Virtually all of these functions are a result of the specially adapted plant life found in them, including peat (also known as sphagnum) mosses, sedges, various wildflowers, and many other species that are well-suited for life with wet feet, and as such are able to soak up excess water and nutrients quite readily. Bogs tend to contain a greater amount of peat mosses and peaty soils than do fens, which are particularly excellent at collecting and storing carbon dioxide (and at impressive rates!) that can be used over time by organisms as needed. However, transitional fens that contain peat can also aid in this task.

3. Species That Call Them Home

willow flycatcher habitats
The petite, charismatic willow flycatcher can be found in both bogs and fens. Photo by Andrew Weitzel / CC BY-SA 2.0

Bogs and fens alike house charismatic and rare species, though those exact species do typically differ. Fens that are more acidic in nature can contain species that are also found in bogs, such as blueberry, cranberry, leatherleaf, and bog rosemary, to name a few. As there are many hundreds of species of sphagnum mosses, they can often be found in fens and bogs alike.

Both are home to a variety of herpetofauna, like frogs, turtles, and snakes, which we’ll discuss in more detail in the “Differences” section as the exact species do often differ. Cervids like white-tailed deer and moose are known to visit both wetland types to browse on the vegetation, and many passerine bird species enjoy making their homes in the low vegetation. You are likely to find warblers and wrens in both habitats, and various owls that take advantage of the open nature of these wetlands and the prevalence of small creatures within them for hunting during the darker hours.

4. Human History and Perception of Fens and Bogs

Unlike many other wetlands, such as the misunderstood swamp, fens and bogs are both viewed overall favorably by people, though we have still damaged and removed them at alarming rates. Nearly 40% of wetlands as a whole have been lost in the last 50 years worldwide. Half of the world’s wetlands are peatlands, and of those 15% have been lost through humans draining them alone. These statistics do not account for peatlands lost through climate change, pollution, and land-use changes in surrounding areas, or other human impacts. As a whole, wetlands are often viewed as “wastelands” in that they cannot be built upon or developed. This is why they are often drained and filled to make room for our ever-increasing development.

Both fens and bogs are now able to provide cultural and educational value if protected and allowed responsible and sustainable public access. Additionally, many fens and bogs provide invaluable data that could not be obtained anywhere else, with scientists focused on anything from climate change to endangered species to plant community structure utilizing them for small-scale and minimally invasive studies.

Differences Between Fens and Bogs

You’ll notice some of the same categories here as in the “Similarities” section above, as marshes and swamps hold things in common as well as in contrast within these categories.

1) Structure and Formation

difference between fens and bogs
Fens are fed by groundwater and runoff, and often have small streams running through them. Photo by Joshua Mayer / CC BY-SA 2.0

Fens rely on runoff and groundwater for their moisture, which are often mineral-rich and result in fens being primarily alkaline and nutrient-rich in nature. By contrast, bogs have little to no groundwater and instead rely on precipitation that collects atop their thick, plush peat soils and sphagnum mosses. As a result, bogs are acidic, low in nutrients, and contain very little oxygen in their water.

Bogs can be quite variable in their water levels, sometimes containing several meters and other times possessing moisture only within the sphagnum during particularly dry bouts of weather, while fens are more consistent in their water levels due to the presence of groundwater and streams that run through them. Bogs are hardy though, and bounce back quite quickly after a good rain, while a fen that loses its source of groundwater or runoff won’t likely last terribly long.

While fens are quite safe to walk on (though do watch out for the thin streams, notoriously bumpy ground, and sudden ditches), bogs require more care. Oftentimes, the mosses found in bogs can be many meters thick, giving the appearance of sturdy ground. These can be walked on, but the soft moss can give way to hidden deep water beneath. If the ground feels a bit pudding-like when you step on it, don’t try to go further.

2) Species That Call Them Home

species found in bogs pitcher plant
The acidic composition of most bogs makes them a haven for carnivorous plants like the pitcher plant. Photo by Nicholas A. Tonelli / CC BY-SA 2.0

Due to their high nutrient content and more alkaline nature, fens are able to host quite a large array of plant and animal life. These can include sedges (the dominant plant type in fens), many frog species such as the rare pickerel frog, ground-nesting and passerine birds, rushes, unique wildflowers like orchids, and a number of endangered species – including the very misunderstood Eastern massasauga rattlesnake in the Midwestern U.S., and the Eurasian bittern in portions of Europe. Massasaugas find the sedge and moss hummocks (these appear as small mounds atop otherwise flat ground) to make very hospitable summer homes, and dine on the many small rodents, insects, and herpetofauna that can be found in fens.

Bogs, by contrast, have fewer but far more specialized species that are well-adapted to life in acidic, low-nutrient conditions. You will find no fish in a bog, as there is very little dissolved oxygen in the water, but instead, you can find acid-tolerant plants like cranberry, blueberry, leatherleaf, bog laurel, bog rosemary, and incredibly unique carnivorous plants like pitcher plants and sundews.

You are also likely to find various ants that enjoy life in the sphagnum, some salamander and snake species (though less common in bogs, the occasional massasauga may also choose to make its summer home in the pillowy sphagnum mounds), beaver seeking out the more manageable and somewhat stunted trees that can be found bordering bogs, and the occasional muskrat if enough standing water is present. The endangered and little-understood wood turtle can be found in some sphagnum bogs in the U.S. if there are adjacent woodlands and access to open and sandy soils nearby.

3) Human History and Perception of Fens and Bogs

peat houses in Iceland.
In portions of Europe, peat was used to build houses, like this farm village in Iceland dating back to the 11th century. Photo by Bernd Thaller / CC BY-SA 2.0

Though fens and bogs share the misfortune of often being drained and filled in, their history in relation to people differs somewhat.

Historically, bogs have provided us with a pretty aesthetic and tasty food for many, many generations via their green, plush mosses and rich berries. Particularly in northern Europe, peat was used, and sometimes still is, as a very reliable and dense fuel source and building material, making bogs a traditionally sought-after resource as they are often quite rich in peat. This can and did, however, result in the further degradation and demise of many bogs when not conducted in a respectful and sustainable manner. As peat is no longer a readily used fuel source, some bogs have been protected and can be viewed either from their outskirts or via minimal boardwalks to help the public be safely exposed to and educated on them.

Traditionally in the U.K., fens were also used as sources of peat fuel, though less so than bogs. More often, farmers relied on them to help provide grazing and clean water for their cattle. While this can certainly greatly damage fens, in controlled amounts grazing by deer and even the occasional cow can actually help to stimulate further plant growth. Now, fens provide rich cultural and entertainment value, as those that are protected can become designated wildlife sanctuaries and contain non-intrusive boardwalks to help the public navigate these ecosystems without damaging them (or tripping over the countless bumps and dips…).

A Final Note of Hope 

Earlier on in the article, we discussed the plight faced by wetlands; that of fens and bogs in particular. These critically important ecosystems have been lost at astonishing rates, although thankfully, these rates seem to be slowing down somewhat thanks to improved and increased education on and understanding of the importance of wetlands. We now know that peatlands like bogs and fens in particular are among the most valuable ecosystems in the world.

They are able to protect us from flooding, help to naturally filter water, and have been able to provide indigenous and modern peoples alike with historically luxurious foods like blueberries and cranberries, plus fuels like peat. Even the insects are an invaluable boon, providing food for a vast array of animals, and many of them also serve as pollinators.

As discussed above, though it cannot be stressed enough, wetlands, including bogs and fens, act as critical carbon sinks, responsible for capturing and storing over one-third of the world’s carbon. In fact, peatlands like bogs and fens are able to store two times more carbon than the entire world’s forests!

We would like to end with the note that many wetlands are still often viewed as “wastelands” or “unproductive” and are thus destroyed to ironically create land that is, by comparison to wetlands, a monoculture of unproductivity. Our hope is that these wetland articles can serve as gentle eye-openers and educate others on their incredible value and importance, uniqueness, and beauty.

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