Bog woodland may be defined as areas in which woodland and bog co-exist in a relatively stable relationship.
Scotland's native forests have many faces. The local climate, altitude, terrain and geology all add to the variety and character of the Caledonian Forest. Native pinewoods, mountain woodland, Atlantic oakwoods and birchwoods are among the garments that once cloaked large areas of Scotland, and now remain in scattered pockets.
Bog woodlands are yet another of these garments. With their primeval feel, they may be defined as areas in which woodland and bog (or mire) co-exist in a relatively stable relationship. They highlight the way in which Nature often blends the boundaries between one habitat and another.
Bog woodland can be found in many parts of the boreal and temperate zones, for example on the west coast of Canada. Parts of Scandinavia have quite extensive areas of bog woodland, and it also occurs in Germany, Poland, Slovakia, the Czech Republic and some of the foothills of the Alps.
In contrast, it is rare in the UK: it is estimated that there are only around 1,000 hectares (2,500 acres) left, although it was once much more widespread. There are fragments in various parts of the country including some birch (Betula spp.) bog woodland in the New Forest, in Hampshire, and a number of sites in Ireland.
In Scotland, where Scots pine (Pinus sylvestris) is the main tree found in these habitats, there are some good examples in Glen Affric and near Loch Maree to the west. However, most are in the east, especially in the native pinewoods on the north side of the Cairngorms and the RSPB's Abernethy Reserve in particular has many bog woodlands.
Bog woodlands are areas of peaty ground on which the high water table and shortage of nutrients restrict tree growth. The unique character of this open habitat is defined by the scattered trees which are gnarled and stunted, with twisted branches. Some of the Scots pines can be up to 350 years old, but deceptively small in size.
Bog woodland usually forms in areas where the topography is quite varied, often a product of irregular glacial deposits resulting in diverse drainage patterns. Hollows become filled with water, allowing bogs to form. Drier ridges and mounds will support larger pines, as well as birch and rowan (Sorbus aucuparia). Scrubby willows (Salix spp.) sometimes become established in nutrient-rich flushes.
Like raised and blanket bogs, the ground is covered with layers of Sphagnum moss which have built up over centuries. The distinction from raised and blanket bogs is that when undisturbed by human activity, trees and bog habitats co-exist in a relatively stable relationship within a bog woodland.
At Abernethy, ecologists have identified different types of bog with Scots pine. The first of these are areas with a permanently high water table. There is plenty of bog vegetation, and heavy seed rain from the surrounding woodland leads to high tree seedling density, although there is also high mortality. The trees that do survive are often stunted or diseased. The second type of bog woodland still has a high water table, although it is more variable, and the scattered trees grow to a moderate height. The third type is slightly drier and has predominantly woodland vegetation, with relicts of bog vegetation.
While the bog woodland community is relatively stable (in that the bog and the woodland aspects can co-exist for many generations of trees), the community does shift between these different types for various reasons. These may include periods of drought or fire, or changes in the water table which permit tree roots to become more well established. When a tree dies, the area may shift towards wetter conditions, as less water is being drawn out of the ground by the tree.
The scattered trees in these woodlands add an extra dimension to the bog. Trees and scrub greatly increase the variety and surface area of habitat available, and afford food and cover to a host of invertebrates and birds. The shelter they offer influences the microclimate, creating conditions of relative warmth and humidity, and allowing woodland flora and fauna to thrive alongside those of bogs.
Bog woodland defies strict categories, which makes it very interesting botanically. A characteristic bog plant is Sphagnum moss. Sphagnums often form lush carpets of bright green and rich red. These mosses are able to hold a large amount of water, and have a key role in peat formation. The thick, spongy hummocks provide a varied 'microtopography'. At Pitmaduthy Moss - a bog woodland site in Easter Ross - there are 19 species of Sphagnum, including S. imbricatum and S. fuscum, which are of national importance.
The herb layer in bog woodland varies between wetter and drier areas. The wettest parts will often have luxuriant plumes of purple moor-grass (Molinia caerulea). Other mire species include hare's-tail cotton grass (Eriophorum vaginatum). In drier areas there may be the more usual woodland plants, such as ling heather(Calluna vulgaris), blaeberry (Vaccinium myrtillus) and Hylocomium moss.
A fascinating adaptation to the low nutrient levels in wet, peaty areas is demonstrated by insectivorous plants such as butterworts (Pinguicula spp.) and sundews (Drosera spp.). Butterwort has a star-shaped rosette of yellowish, sticky leaves, the margins of which roll inwards to trap and digest insects. There are three species of sundew in Britain - all have sticky hairs which trap insects, thereby supplementing the plant's diet.
Many of the mammals and birds are those which we would expect to find in an old pine forest, such as the crested tit (Parus cristatus), capercaillie (Tetrao urogallus) and pine marten (Martes martes). Red deer (Cervus elaphus) wallow in peat baths to get rid of parasites and flies, and to spread their scent over their bodies. Amphibians such as the common frog (Rana temporaria) also depend on wet habitats. Pitmaduthy Moss is an important breeding area for a number of birds, including the osprey (Pandion haliaetus).
Bog woodlands are rich in invertebrates. That quintessential pinewood insect, the wood ant (Formica spp.) builds its pine-needle covered nests on the drier banks. The pools can also be valuable habitat for dragonflies and damselflies. Among these are the northern emerald (Somatochlora arctica) - which is restricted to north-west Scotland and south-west Ireland - and the exquisite, and rare, northern damselfly (Coenagrion hastulatum). Like some more common damselfly species, the male northern damselfly has a striking metallic blue body, with black markings along its back. A dragonfly known as the white-faced darter (Leucorrhinia dubia) requires acidic bog pools for breeding, bordered with rafts of Sphagnum in which their larvae grow. This species also needs scrub or woodland for roosting and feeding, making bog woodland an ideal habitat.
Another aquatic insect - a very rare predatory diving beetle known as Agabus wasastjernae - has been found at Abernethy in the pools of water that form beneath the lifted rootplates of fallen pine trees.
Bog woodland has suffered similar problems to those faced by blanket bogs, and it is easily destroyed by insensitive management. Peat cutting is one of the most direct threats to have occurred in the past. This has been traditionally practiced for centuries in Scotland and it can severely disrupt the hydrology of a site, although it has a fairly localised effect on vegetation.
From the 1960s to the early 1980s, commercial forestry practices had a huge impact, when areas of bog woodland were drained and planted with commercial plantations of Sitka spruce (Picea sitchensis), lodgepole pine (Pinus contorta) and Scots pine. Draining drastically alters the plant community, while the planted trees dry out the site further, and also shade out much of the native vegetation.
Bog woodland is even vulnerable to insensitive management on land nearby: drainage, planting of exotics (which spread their seeds and affect the hydrology), or the use of agricultural fertilisers (which can drift on to the site and artificially boost tree growth) can all adversely affect this wet, low-nutrient habitat.
Climate change poses a potential threat, although it is difficult to predict what the effects might be. An increase in rainfall would raise the water table, leading to further waterlogging, and a reversion from woodland to bog: a decrease in rainfall would allow the bogs to dry out and more trees to encroach.
Another unexpected threat has come from overzealous conservation work. While in most cases bog habitats need to be kept open to maintain their special interest, there have been cases in which trees have been completely removed from areas where true bog woodland had developed. The fact that bog woodland is hard to categorise, means that up until recently it has been largely overlooked as a habitat type and given a low conservation profile as a result. Fortunately, there are encouraging signs that the situation is changing.
A number of organisations have worked to reverse the damage to some of our remaining bog woodland habitat. The vegetation is often able to recover from disturbance if the hydrology is restored. Clearly, as with much long-term ecological restoration it is not a case of trying to achieve a fixed and static 'natural state'. The aim is really to remove the damaging anthropogenic influences and let the habitat recover in an appropriate way. Raising the water table where the site has dried out has the effect of gradually locking up the nutrients released when the disturbance took place. This insures that the all-important low-nutrient levels are reinstated.
There are a number of ways to restore a higher water level. In some cases trees which have been planted and are drying out the bog have been removed. Blocking the drainage ditches, where they have been made, is also effective (Trees for Life has been using the same technique to restore blanket bog and alder carr at Corrimony and Achnashellach respectively).
Trees for Life's work has been contributing to the conservation and restoration of areas of wooded bog. In Glen Affric for example, the 50 hectare (125 acre) exclosure we funded at Coille Ruigh includes some wet areas in which small stunted pines are regenerating amongst the sweet-smelling bog myrtle (Myrica gale). There is similar regeneration in the Ghuibhais exclosure further west, and elsewhere. The fact that it naturally develops in many areas of forest undergoing restoration lends weight to the theory that it was once much more widespread. Our work in surveying and protecting dwarf birch (Betula nana) is also important, as this species too forms part of some bog woodland areas.
In response to the potential threat of climate change, it seems that the best course of action is to restore and protect as many different types of bog and bog woodland habitat as possible. This should prevent the complete loss of this precious habitat and allow scope for natural change and adjustment.
Bog woodland habitat is increasingly being recognised and appreciated for its conservation value and natural beauty - under the EC Habitats Directive it is a priority habitat for protection. One urgent measure is to increase our knowledge of the exact distribution of Scotland's bog woodlands, to ensure that the sites can be monitored and given adequate protection. In spite of the potential threats, there have been some encouraging successes in protecting and restoring these unique and fascinating woodlands. There is an uncertain, but hopeful future for this little-known face of the Caledonian Forest.
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