How to Rewild

Bog

Habitat Management Plan

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Bog
Habitat Guide

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A bog is a raised feature, sitting above the water table, which means its only input of moisture comes from rain (contrasted with a Fen, which sits below the water table). This creates a nutrient-poor soil, as minerals from the rocks beneath do not flow into the system. So the vegetation here is low-growing – a specialist plant community of moss, grass, stunted trees and shrubs.

The underlying peat soils form slowly, over hundreds or thousands of years, accumulating at about 1mm/year. This ecosystem is easily damaged by humans, due to its reliance on a high water table – harvesting peat for compost or fuel can start a decline that rapidly dries out the soil and kills off the sphagnum. But restoration is possible, and it is happening in many places across the UK.

Bogs have a bit of a reputation for being boring – ‘miles and miles of bugger-all’ or ‘MAMBA’ as they were historically known. But that’s because most of the action is going on at ground level – take a look down there and you’ll find glistening sundews, delicate mosses, fluffy cotton grass and amphibians hiding in secluded pools. Just be sure to wear your wellies.

Sub Habitats

Upland bog sitting above the water table, fed by rainwater only. This habitat is formed by sphagnum which sits above the land, retaining water like a sponge.

Lowland bog, sitting above the water table, typically formed in a hollow as a lake or pond surface is slowly covered by vegetation, forming a ‘lens’. The bog rests on a deepening layer of peat, which may be made up of decomposing reeds and/or sphagnum.

Value

Peatland is a low diversity habitat with a specialist community of organisms that create a unique landscape. While the biodiversity here is low, the species that do survive are often threatened or endemic to bogs (found in no other habitats). Peat forms from rotting plant matter in waterlogged, anaerobic (low oxygen) conditions, which also preserve remains (human, animal and plant) for many centuries, allowing us to use bogs as a historical and ecological record.

While some bogs may have begun life as reedbeds (particularly in the lowlands), and fen peat can be present at the base of these wetlands, once they begin raising above groundwater level, it is sphagnum that keeps them healthy. This family of mosses stores water in dead cells throughout their body – in fact, the moss is generally more dead than alive. This has led to bogs being described as a ‘sponge’ that can prevent flooding. However, some peatland researchers caution that, during winter storms, bogs are often saturated from the start, so any water falling on the system cannot be soaked up, but instead flows away downstream.

The evidence for peatlands’ power to reduce stormwater flooding is confusing and contradictory. In models, a bog is not very effective at reducing runoff, as this wet system has little capacity for any more water. Yet, according to a number of different real-world studies of river flow, compared to a degraded peatland, a healthy or restored bog regularly reduces stormwater peaks in wet periods downstream and can produce more consistent stream water during dry periods (i.e. the sponge effect is real).

Then there’s carbon – a more thorny issue! Peatbogs cover 3% of the Earth’s surface, but make up 21% of the soil carbon – laid down very slowly over long time periods. A degraded bog releases greenhouse gases into the atmosphere, so restoring these habitats is likely to help fight climate change in the short to medium term, but long term results are uncertain. The climate is changing, and the climatic zone that supports bogs is shifting northwards over time. As a result, many bogs are drying out as summer droughts and higher heat levels make conditions unsuitable for the specialist plant community which makes this habitat viable.

So, by investing ‘carbon offsets’ in peatland restoration, we are creating a system that has the potential to catastrophically fail in the future. There are certainly benefits to restoring bogs, but some in the industry have started questioning whether more southern bogs are viable in the long term. On the other hand, these wet patches in upland areas can resist the spread of wildfires, which are becoming increasingly common with summer droughts. Each wildfire releases huge amounts of carbon into the atmosphere, so peat bogs could protect the climate in another way. Again, caution is advised, as peat itself can catch alight in very dry conditions, making the emissions from a wildfire far more serious for the climate. This is a nuanced and location-specific issue which should be approached in consultation with experts.

Peatland degradation
Peat extraction is a major threat to the health of blanket bogs, as it reduces the water table height, killing off local sphagnum populations. Image: Nick D'Agorne

Protect

Peat Extraction

While bogs may be threatened by climate change, they are much more likely to be degraded by human activities. Peat extraction is bad news, as this may remove the edge of the bog, or allow a gully to form, potentially leading to system collapse. As soon as a channel appears below the surface, the water table begins to drop, but sphagnum requires very consistently damp conditions to survive. Reduce the water table depth, and the bog will dry out rapidly and die off.

Surface Erosion

Erosion in upland areas is generally caused by overgrazing of sheep, but some tracks and footpaths can also be an issue. Tracks become routes for water to flow off the land, carrying soil and peat away, particularly on degraded upland bogs. These gullies deepen over time, but they can be blocked up with either rocks, or ‘plugs’ of peat that are extracted from within the body of the bog, creating a ‘borrow pit’. Over a period of a few years, plugs raise the water table again, and slow the flow of runoff down the gully. The effect of these plugs has been found to reduce stormwater peaks downstream, as they create beaver pond-like retention pools upstream of each dam.

If gully blocking does not occur, then, in the absence of sphagnum and other bog vegetation, the surface may revert to bare peat. This dries out and erodes, losing its embedded organic carbon to the atmosphere and into surface runoff. It is also susceptible to wildfire, given the propensity of peat to burning, and the frequency of fires in upland areas. A long term study of a recovering peatland found that the resulting increase in vegetation cover reduced damaging stormwater peaks and flow volume downstream.

Grazing

Moderate to high stocking densities of livestock can not only damage bogs, but they may also be damaged by bogs, due to the risks to animals from deep pools and quaking mires. These mires are technically a Fen habitat according to UKHab, but they are commonly found in the lowest depressions of raised bogs, where peat is actively forming in a thin layer over open water.

Many peatlands are now drying up due to climate change – the symptom of this is natural succession – succession is often caused by changes to the water table. Some conservationists are fighting back against these losses by felling trees and treating cut stumps with herbicide. In some cases, they are then introducing cattle at low densities, which can control bracken, purple moor grass, low scrub and tree sapling growth. However, even moderate levels of grazing can cause long term damage to bogs; sphagnum is believed to be resilient to no more than 1 or 2 ‘tramplings’ per year. The appropriate stocking density for sheep is about 0.4/ha (depending on body size), but there is no widely-accepted stocking density for cattle. While cattle has been used to graze bogs in the past, and they are closer to our native aurochs, their bigger bodies are likely to have a larger impact on the sensitive vegetation.

Atmospheric Pollution

Bogs may be degraded by pollution falling in rainwater – in the past, this tended to be ‘acid rain’, but in more recent decades, it has been ‘atmospheric nitrogen deposition’. This enriches the nutrient-poor surface of the bog, enabling the encroachment of trees. It contributes to the increasing rate of succession on some peat bogs, especially in areas adjacent to high-intensity livestock farms, where ammonia continues to be an issue. However, total nitrogen deposition has fallen in recent decades due to stricter air pollution policy and improvements in technology which control nitrogen oxide emissions.

Tree Plantations

Tree plantations have in the past been situated on bog, due to its historic low land value. However, the trees often grew poorly on these nutrient-deficient, waterlogged soils, even after they were drained with artificial gullies (‘grips’). These plantations are now being felled in many places, and the land beneath restored – the Lost Peatlands Project is a good example of this. Pine forests in these areas are susceptible to wildfires, and conifers planted on bog make peat’s flammability even more of an issue.

Draining

In certain parts of the country, peatland has, even until recent years, been drained for use as upland pasture. This has involved the installation of ‘mole drains’ (tubes below the surface) and grips (trenches). As per the tree plantations, it may take centuries of restoration work to undo a few weeks of labour. Preventing this drainage from occuring in the first place is a high priority.

Restoring peatland
Efforts to restore bog habitats typically involve replanting of sphagnum.

Restore

No matter the state a peatland is in, it is usually possible to restore the system. Even in lowland areas, where most bogs have now been lost to agriculture, so changing back to a wild ecosystem would result in significant productivity loss, there are viable options for farmers.

Paludiculture is the growing of plants under water, and peaty soils are ideal for this practice, as they are perfectly suited to the conditions. This is an option for farmers who wish to maintain productivity, while restoring their soils and boosting the carbon potential of their land, as wet peat retains more carbon. Current options being explored for paludiculture crops are related to plant-based fibres, including native species like Reedmace. More information about British paludiculture is available from the Paludiculture Exploration Fund.

On less-degraded land, rewetting the peat is still an important part of the process – restoring the water table is step 1 in any bog restoration project. This has to happen before sphagnum is reintroduced, as the plant is so sensitive to changes in the height of the water table. As mentioned above, blocking gullies with peat plugs is an important part of this work. However, bogs are also one of the only habitats where restoration requires reducing structural diversity. If a tree plantation has been felled, the site should be ‘tracked over’ with an excavator to reduce any surface abnormalities (trunks, root plates etc), flattening the bog to return it to a more stable condition.

Once a bog area has been sufficiently rewetted to allow the reintroduction of sphagnum, then fist-sized clumps of native moss, ideally sourced locally, should be planted across the site. These grow at a very slow rate, but they can be planted fairly sparsely, and allowed to colonise the site over time, as budget and logistical constraints usually prevent widespread planting. To prepare the site in late Autumn or Winter, other vegetation, particularly Molinia (Purple Moor Grass) should be strimmed down to ground level. Then, the bottom of the moss – what look like the sphagnum’s ‘roots’ – are thrust into a hole 6 inches deep, created with a garden dibber, and the ground around the moss squashed back into place.

On bogs which have been so degraded that the surface is rapidly eroding, some stabilisation may be required before reintroducing sphagnum. The typical strategy taken here is described as the ‘Lime-Seed-Fertiliser-Mulch’ (LSFM) approach. This temporarily establishes a non-native grass community on the peat surface, which, in previous studies, was shown to raise the water table height sufficiently to begin reintroduction of bog species.

Some bogs may be suffering from encroachment of scrub or woodland habitat. However, this is likely to become more common as climate change progresses, and attempting to resist the spread could fail over longer time periods – it is working against natural processes. The Common Haircap moss poses a similar problem – it slowly pulls sphagnum out of the water, forming tussocks across the bog. While it is recommended that you avoid widespread accidental introductions, this is a native moss that is part of the succession process, and eradicating it completely could reduce biodiversity.

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