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Overgrazing, cutting and trampling and their effects on the ecology of bogs

By Dr Fiona MacGowan

Introduction

Bogs in Ireland have a long history of use by people. In many cases these uses are sustainable e.g. light grazing. In the recent past, however, utilisation levels have increased beyond sustainability. Indeed levels have reached such highs that these various uses are now posing threats to the survival of bogs as a uniquely important habitat in Ireland. The main threats at the present time are (a) animal grazing, (b) peat harvesting for the domestic market and industrial purposes in energy production, (c) recreational pressures and (d) afforestation with coniferous trees. The recent escalation of bogland degradation in Ireland has been largely mediated through:
Increased sheep numbers
Peat cutting
Tourist traffic

The damage to blanket bogs has escalated in the recent past with changes to the grant system for small farmers in disadvantaged areas, with European Union grant aid encouraging farmers to increase their stock of sheep. The payment of these headage grants was the catalyst for the overgrazing problem (Cross 1992). The significant increase in sheep numbers that resulted in the west of Ireland, coincided with a sharp decline in the practice of hand-cutting peat, since convenient tractor-mounted peat cutting machinery had become readily available. In the recent past, tourism numbers have also reached all time highs, with many tourists now walking in areas of scenic beauty. Studies in Scotland by Lance et al. (1991) revealed that recent increases in the numbers of hill walkers has been involved in the escalation of trampling impacts on the upland habitats of the Cairngorm Mountains. In Ireland, areas such as the Connemara and Wicklow National Parks, have peatlands that are seriously affected by pedestrian traffic. The links between a pristine natural environment and a thriving tourist industry are clear. In an "agriculturally disadvantaged" area such as much of the west of Ireland, where there is a preponderance of blanket bogs, many family incomes are either directly or indirectly derived from tourism, be that wholly or partially.

This chapter will centre around the findings made by the author in her postgraduate studies which focussed on the overgrazing, peat-cutting and trampling problems faced by blanket bogs in the west of Ireland. Although these studies concentrated on Atlantic blanket bogs in the Connemara region, the effects of these damaging influences are similar on all bog types in Ireland and therefore the Connemara sites will act as examples of the damage which is unfortunately occurring in many parts of Ireland.

These pressures can have particularly harmful effects on the plant cover of bogs, killing off a variety of plants. A loss of plant cover, in turn, can often lead to erosion of the surface peat by wind and rain. Where peat is being cut away, the normally wet bog gradually dries out and the habitat as a whole is utterly changed.

Human influence on bog development

As highlighted in earlier chapters, many bogs in Ireland have developed to their present state partly because of human intervention in the past. This is particularly true of blanket bogs where fire has been continuously used by man as an agricultural tool on the bogs to encourage the growth of succulent heather shoots favoured by grazers. Over the centuries fire and extensive grazing have been continuous features of blanket bog areas (Moore 1979, Doyle 1982, O'Connell 1990). The influence of these activities over the centuries was borne out by research carried out by Doyle, O'Connell and Foss in 1987. They made a study of the peat islands in bog lakes where they found a more shrub-dominated vegetation than that of the adjacent bogs and this is thought to represent a possible climax vegetation of bogs in the absence of human intervention. In historical times, extraction of hand-cut peat for domestic fuel supplies was widespread, while agricultural reclamation took place, especially in areas abutting mineral soil pockets. In the last forty years afforestation with exotic conifers has been widespread, while commercial exploitation of peat as fuel for electricity generation has developed in areas in the vicinity of power plants. This chapter, however, will concentrate on the small-time private cutting of peat for domestic use as opposed to the industrial cutting of peat, which is dealt with in other chapters.

Present day influences of human activities on bogs

Grazing
Grazing pressure has increased in the western blanket bog areas of counties Galway and Mayo over recent times, especially since the introduction of the EU Headage Payment scheme, which was designed "to encourage the continuation of farming in those areas of the country which have been designated as disadvantaged so as to help to maintain the level of population and to ensure the continued conservation of the countryside" (Anon. 1991). The application of this scheme has led to dramatic increases in sheep numbers with a near three-fold increase nationally since 1980. In bogland areas, most of which are clearly ecologically fragile, large increases in sheep numbers were always likely to have a profound environmental impact. As reported in several studies (Doyle & Foss 1986, Anon. 1992, Bleasdale & Sheehy Skeffington 1992, MacGowan 1993), where blanket bog has been subjected to overgrazing there is obvious, and in some places catastrophic deterioration in vegetation. A loss in plant cover leads to baring of peat surfaces and widespread erosion of peat down to the mineral soil in some places. This leads to siltation and acidification of lakes in these regions and the spawning beds of salmon and trout are then perversely affected as reported by both Huang & O'Connell (1992) and Bleasdale (1998). This is due to the peat silt cutting off the oxygen supply to the fish eggs.

The deterioration in the bog habitat has led to problems on all sides. The practice of grazing sheep on blanket bogs all year round has proved unsustainable. In an address to the IPCC Peatlands 2000 Conference in 1997, Dr Peter Foss revealed that in the ten year period 1987-1997, overgrazing damage had left 7% of Ireland's total blanket bog area severely damaged with a further 7% under the same threat. The farmers are left with eroded land that is of little use. The sheep suffer the consequences of malnourishment and problems with domestic water supplies have arisen as a result of contamination by sheep droppings. Another aspect is the destruction of the aesthetic beauty of part of the Irish landscape - the main reason behind the large numbers of tourists that arrive in Ireland every summer. The degradation of these landscapes has been noted in both the national and foreign press (Viney 1993, Murdoch 1995, Battles 1997, Nugent 1997).

As highlighted by Patricia McKenna MEP, in her speech to the 1997 IPCC Conference, the EU have been criticised for their involvement in prompting overgrazing through the provision of subsidies. It is now hoped, however, that subsidies in another form can resolve the problem. The Rural Environment Protection Scheme (REPS) was introduced by the Irish Government to combat environmental problems in farming, such as overgrazing. The scheme involves annual payments to farmers who follow environmentally friendly farming practices according to a five-year plan that includes the protection of wildlife habitats and endangered species of flora and fauna. Parts of the scheme are now being developed in order to deal with the overstocking and consequent overgrazing problem.

Small scale, private peat-cutting
Peat has been cut for domestic fuel supplies for many centuries. Traditionally the peat was cut with a winged spade known as a slane (sleán), from the face of a bank at the edge of the bog. Once cut, the wet sods were laid out on the bog surface to dry, a process that took from about seven to ten days, depending on the weather. Sods were stacked upright on their narrow ends, leaning against each other with a couple of sods laid on top, providing a structure that was open to drying winds. Once dry, the turf was removed from the bog surface and stored in clamps and later taken home. Hand-cutting of peat was confined to bog edges, with the impact on the surface vegetation resulting from the sod drying process confined to a narrow area close to the peat face. In recent years, however, tractor-based harvesting equipment has allowed accelerated exploitation of peat for domestic fuel supplies on blanket bogs especially.

On blanket bogs the hand-cutting of marginal peat banks has in most places been replaced by the mechanised production of "sausage" peat, which involves annual extraction of sub-surface peat by ploughing. The fluid peat is extruded in sausage-like ribbons onto the surface for drying. The plant cover is affected in several ways by the harvesting process:

(a) The bog is drained since water flows out of the cuts made by the machine into the bog.
(b) The "sausages" of peat are laid out side by side to dry across the entire vegetation on the surface, impacting on the plants both by breaking them and by excluding their light supply.
(c) Damage to plants is also caused by the harvesting machines as they are driven over the bogs to the production sites and by the workers trampling the surface.

Such mechanisation of the peat-harvesting process causes impacts that are quite different from those caused by traditional hand-cutting since exploitation of relatively large areas of peat are now possible. The design of sausage machines (also known as Difco machines) mounted on tractors removes the requirement of peripheral location for peat extraction. As a consequence, extensive areas are now harvested at considerable distances from the bog margins.

On raised bogs private peat cutting mostly involves cutting with the "hopper" ­ a mechanical arm with a bucket at its end, much like those seen on JCBs. These buckets are used to cut into the margins of the bog and scoop out peat. The main impact of such machines apart from decreasing the size of the bog is that the cutting process severely dries out what remains of the bog.

Mechanised peat cutting is in some places accomplished by bank machines. These machines are specifically designed for peat cutting and are used for production of peat from banks cut in the bogs. This cutting begins at the peripheries of bogs but with the speed of the machines, the harvesting process soon moves further in to the centre of the bog.

Trampling
As of yet, the impact of trampling by tourists is a problem specific to blanket bogs in Ireland and it is not widely recognised, since most of the damage is concentrated around focal points such as the Wicklow Mountains, Glenveagh and Connemara National Parks. The problem has arisen within the past 15 years, since the number of tourists interested in outdoor activities has increased. The author as part of her doctoral studies (MacGowan 2000) carried out an in-depth study of the impacts of trampling on bog plants of the slopes of Diamond Hill in the Connemara National Park. The impacts studied here are very similar to those seen on other trampled blanket bog areas in the country and, as for areas in Glenveagh and Wicklow, hill walkers have significantly impacted on the peatlands of the Diamond Hill area, destroying the surface vegetation and promoting erosion (MacGowan & Doyle 1996, 1997 & 1998). Diamond Hill provides a focus for many walkers in the north Connemara area as it is an accessible, relatively easy climb with excellent views of the north Connemara Atlantic seaboard. As early as the 1890s, the writers Somerville and Ross (1893), on their travels through Connemara, wrote of fellow guests in their Letterfrack hotel climbing Diamond Hill. It was with the opening of the National Park in 1980, however, that visitor numbers increased substantially. The trampling impacts on the bogs have led to the formation of eroded pathways criss-crossing the landscape. The paths vary from 0.5m to 5.0m in width, getting progressively wider as walkers move out to avoid muddy path centres during wet periods.

Impacts of damage on bogland ecology

The various threats to bogs described earlier are threatening because of the specific forms of damage they cause. Presented below is a summary of the author's recent doctoral studies.

The impact of grazing on bog plants
With the increase in the numbers of grazers as described earlier, plants do not have the normal length of time to recover from grazing damage, resulting in the death of many, such as Ling heather (Calluna vulgaris) individuals, for example. This in turn leads to foraging of those species not normally preferentially grazed by sheep. In addition to the death and degradation of grassy and heather species, there is a loss of those microhabitats (subhabitats within the larger bog habitat) which favours the growth of many moss, liverwort and lichen species. With poaching (animal trampling) impacts adding to this problem, many plant species decreased in cover or died out. As a consequence, overgrazing produces areas bared of vegetation that then lose their peat cover due to the eroding effects of wind and rain.

Bogs that are overgrazed feature a decrease in vegetation cover and an associated increase in the area of bare peat. With this latter increase there is an acceleration of erosion damage leading to peat erosion by wind and rain, in some cases exposing the mineral rock underneath. This increase in bare peat allows the invasion by species alien to the blanket bog habitat. Most of these plants normally favour drier habitats, such as the heath species Bell heather (Erica cinerea). Several grasses also invade, all of which are more typically found on acid grassland habitats. Some species, such as Bulbous rush (Juncus bulbosus) are more normally associated with areas of nutrient enrichment in bog subhabitats according to Doyle (1982), conditions that were created on some bare peat areas subjected to the run-off of surface water.

Many typical bog species that remain on the overgrazed areas show obvious signs of damage. Decreases in the cover and frequency of occurrence of heather plants on overgrazed blanket bogs have been noted in many studies (Gimingham 1960, Armstong & Milne 1995, Nolan et al. 1995, Palmer 1997, MacGowan 2000). In upland habitats subject to heavy grazing, Miles (1988) noted that grass species replaced Ling heather.

The eradication of larger plants in overgrazed areas is reflected in lower frequency of occurrence of many mosses, liverworts and lichens as the sheltered conditions they find under the larger plants are also eradicated. Rawes & Hobbs (1979) found that with a high density of grazing stock, dwarf shrub cover was reduced, grassy species gained some dominance, while the abundance of lichens decreased and bare ground cover increased. Decreases in the cover of lower plants (mosses, liverworts and lichens) are recognised as a primary indicator of damage. MacGowan (2000) found that several of the grass and sedge species, such as Purple moor grass (Molinia caerulea), Many headed bogcotton (Eriophorum angustifolium) and White beaked sedge (Rhynchospora alba), occurred with the same frequency but had reduced cover, reflecting the tenacity of these species as they survive in the overgrazed environment. MacGowan also noted in the same study that the algal aggregate Zygogonium ericetorum, was a dominant feature of overgrazed areas on the Connemara lowland blanket bogs. Here the expanses of bare peat caused by overgrazing and erosion ideally suit the spread of the algae. In some of the study areas, the algal aggregate had 90% cover.

Dead plant material is a feature of all overgrazed bogs, with dead mosses and liverworts, mainly Sphagnum species, widely recorded (MacGowan 2000). Another species, the Silver haired moss (Campylopus introflexus), thrived on overgrazed sites, where dried bare peat surfaces created conditions favourable for colonisation by this species. According to Richards (1963), this moss was first recorded in Ireland only as recently as 1942 - far from its native home of New Zealand. However, it adapted so well to Irish conditions that it is now found all over the country - particularly on dried, bare surfaces such as old walls and eroded bogs.

Grazing is known to have significant effects on vegetation height, standing live and dead biomass and sward structure (Hodgson 1990). MacGowan's study (2000) found decreased vegetation cover on overgrazed sites on Connemara blanket bogs. Plants that did survive there were noted to be reduced in stature, and often confined to patches of peat in between eroding gullies. For the purposes of the study, the vegetation was divided into four components: shrubs, graminoids (grasses, sedges and rushes), herbs and lower plants. The shrubs had reduced cover, since Ling was a favoured constituent of the sheep diet. The graminoid cover was also reduced by grazing and by poaching, which had the effect of breaking up the tussock structure of many of the graminoids typical of the habitat. The herbs were a minor component within intact vegetation and in the overgrazed situation. The lower plants showed significant changes, with mosses, liverworts and lichens all reduced in cover compared to the intact sites. These plants were not grazed but their favoured microhabitats at the bases of shrubs and graminoids were in decline, leading to the loss of lower plant cover which in turn leads to increased exposure of bare peat that is then prone to erosion.

Dead plant material was an obvious feature of overgrazed sites. This was mostly composed of dead shrub branches, graminoid leaf bases and bryophyte material. Of the different plant groups, the graminoids had both the highest cover and the greatest height on a comparative scale. This reflects the plant cover composition of intact Atlantic blanket bog sites where graminoids would also generally be the tallest plants and in many cases have the highest cover.


The impact of peat cutting on bog plants
The main result of peat-cutting activity is drainage. Cutting into the centre of the bog (i.e. the sausage machine) or at the edges (i.e. the hopper or bank machine) results in water flow off the bog. In many ways water can be seen as the lifeblood of the bogland habitat ­ water dictates the very nature of the bog from the wet peat substrate to the wetland plants and animals that live in this habitat. Therefore a loss of water will result in profound changes to the habitat: the peat dries out, the water-loving plants ­ especially the peat building bog mosses (Sphagnum species) die off and even the dome that gives Raised bogs their name slumps.

On cutover bogs, surface drying results from the impact of drains that facilitate peat harvesting operations. Machine-cut sites have obvious damage incurred by machinery that passes over the bog surface and through trampling by the people who work the sites. The harvesting process also involves physical damage in the form of crushing by the wet sods of peat that are left to dry on the bog surface, preventing light from reaching the plants below and damaging the vegetation of large areas. This problem is particularly noticeable on areas cut by sausage machines, where sausage-like extrusions are lined up along the bog surface.

MacGowan (2000) found that the combined forms of damage outlined above led to two main changes in the plant cover and composition, (a) a loss of many typical bog species and (b) increased competition from invading plants. These invader species fared better on cutover bogs than on overgrazed or trampled sites since damage to cutover sites was often of a temporary nature, confined to a finite period of peat-cutting, rather than the year in year out damage caused by overgrazing or trampling.

According to a Canadian study by Campeau & Rochefort (1996), drainage is a crucial factor in preventing reestablishment of Sphagnum mosses on cutover sites. As a consequence, abandoned cutover bogs are colonised by invading species able to capitalise on drier conditions. Typically these species are those associated with heathland or acid grasslands, such as Creeping bent (Agrostis stolonifera) and Heath rush (Juncus squarrosus), the latter was noted by Welch (1966) for its ability to colonise newly available habitats through copious seed production, good dispersal and viability.
Depending on the type of cutting, scraws of vegetation are often thrown onto the bog surface near the cut area. Scraw is the term used to describe the plants and the top layer of peat they are rooted into which is cut off the bog surface in order to facilitate cutting. Often the areas where the scraws are thrown are important for the growth of heather species and associated mosses. Following the end of peat harvesting, these scraws can be important as a seedling source for the revegetation of cutover areas.

MacGowan (2000) took species lists from both intact (control) and cutover Atlantic blanket bog sites and compared them. Many differences were noted. Bog bean (Menyanthes trifoliata), a plant typical of bog pools, and a number of mosses and liverworts were excluded from cutover bogs. For example, the mosses Hypnum cupressiforme and Plagiothecium undulatum were both absent from the cutover bogs. Normally these mosses are associated with bushes of Ling heather and their absence was a reflection of the reduction in the amount of Ling plants and the consequent eradication of microhabitat for these two moss species.

A relatively large number of alien species were recorded from the cutover sites. This reflects the drier conditions of cutover bogs as well as the low levels of grazing animals compared to the overgrazed bogs studied earlier. Both acid grassland and heathland species were found, making their presence on bog sites a strong indication of damage related to peat harvesting.

Of the native bog species that survive on the cutover bogs, often their percentage cover is very low compared to their normal covers on intact bogs. MacGowan (2000) in her study of Atlantic blanket bogs found that other species normally found on drier parts of intact bogs increased both their percentage cover and their frequency of occurrence, e.g. Devil's bit scabious (Succisa pratensis), Heath wood-rush (Luzula multiflora) and the mosses Hylocomium splendens and Rhytidiadelphus squarrosus. The silver-haired moss, which was occasionally found on the intact sites, was frequently found on cutover sites, reflecting the plant's ability to colonise bare, dried peat. Other species apparently doing better on the cutover sites were Carnation sedge (Carex panicea), Star sedge (Carex echinata) and the Many-stalked spike rush (Eleocharis multicaulis), species that according to Doyle (1982) are normally associated with areas of nutrient enrichment on intact bogs. The increased cover of these species reflects changes in the water and nutrient regime on the cutover sites.

The drying caused by drainage at peat banks allowed colonisation of cutover sites by heathland and acid grassland species. Many of these invading species were grasses, which in the absence of grazing added to the general vegetation height of these areas. Vegetation height was lower on the cutover sites but higher than on the overgrazed sites, reflecting the near absence of grazing and the heights of the invading species.

Vegetation cover in the cutover sites was half the level of that of intact sites, despite the higher level of invading species. There was a correspondingly large amount of bare ground. The amount of dead plant material on cutover sites was significantly higher than that recorded for intact Atlantic blanket bog sites surveyed in the present study. A significant amount of dead plant material was also found on cutover mountain blanket bogs in Northern Ireland (Todd et al. 1995). The vegetation components most severely damaged on Connemara cutaways were shrubs, mosses, liverworts and lichens.

The impact of trampling on bog plants
On trampled sites, the main problems result from the ongoing physical impacts on sensitive plants. These problems are compounded as once plants have been trampled, trekkers continue to walk on damaged plants which provide the path of least resistance. As a result, plants are destroyed in a relatively narrow band, which then becomes obvious to walkers as a path, making regeneration of the plants almost impossible. With the prevailing wet and windy weather conditions of the coastal Connemara region, these pathways erode and in wet weather become slippery and dangerous. In these conditions, people walk on either side of the eroded paths, spreading the trampling damage effects. As a result, the paths become wider each year. This process was noted along the Pennine Way in northern England, where paths increased in width by 300% to 900% in the 10 years from 1978 to 1988, as the hill walker numbers increased (Pearce-Higgins & Yalden 1997). The average bare peat path was between 7m and 8m, indicating how widespread the damage can become. The development of such eroded paths is contrary to good management in wilderness areas since the pristine and natural character of the area is disrupted in a most visual way.

MacGowan (2000) carried out a detailed survey of trampled Atlantic blanket bog areas located on Diamond Hill in the Connemara National Park. The trampled sites were divided into three categories of damage: minimum, medium and severe. Control sites were also surveyed on Diamond Hill in areas free of trampling damage.

The vegetation composition of the areas of minimum trampling was found to be more diverse than that of the controls. A previous study of the Round-leaved sundew (Drosera rotundifolia) populations by Gibbons (1992) found that very low levels of trampling might be beneficial to some plants. This phenomenon of low levels of damage being beneficial has been reported from other habitat types. For example, species diversity of sand dunes is known to benefit from low levels of trampling (Liddle & Greig-Smith 1975, Boorman & Fuller 1977). Some of the invading species of the sites with minimum levels of trampling were typical of heathland, and acid grassland habitats, or nutrient enriched sub-habitats on intact blanket bogs. The Carnation sedge, for example, was found along pathway edges where nutrient enrichment was supplied in water passing from the intact bog surface on either side of the paths straight onto the depressed pathways. Despite the low levels of trampling in these areas, the mosses, particularly the Sphagnum species, were adversely affected, as their soft structure was physically damaged by trampling. Compared to other damaged sites, very little of the algal aggregate Zygogonium ericetorum was found on areas of minimum trampling damage.
On the more damaged areas of medium trampling, the diversity of species had decreased. There was considerable bare ground on paths, where the only remnants of vegetation were dead, black leaf bases of the Black bog rush (Schoenus nigricans). By way of contrast the path edges were often marked by healthy Black bog rush, Bog myrtle (Myrica gale) and Deer grass (Trichophorum caespitosum). Mosses, liverworts and lichens were recorded with both low frequency and cover, and were generally confined to the path edges where suitable microhabitat was available at grass and sedge tussock bases. These trampled paths act as rudimentary drainage channels, with water running over the eroded areas of bare peat.

In the areas of most severe trampling damage the vegetation cover was sparse. Where there was severe trampling damage, vegetation structure was reduced and there were few if any flowering stems produced. Tufted plants, such as Deer grass and Black bog rush, were better able to survive in severely damaged areas, than were the heathers, whose erect branches and apical growth tips were too sensitive to the trampling damage. The relatively brittle stems of the heathers may render them susceptible to trampling damage, since stem flexibility has been shown to confer resistance to trampling in grasses and herbaceous plants (Sun & Liddle 1993).

The heights of plants in the trampled areas was also and in the areas of minimum trampling, herbs and shrubs grew vigorously but some lower plants were seen to be trampled into the peat. There were significant decreases in the overall vegetation cover, but no significant differences between the heights of the trampling control sites and the sites of minimum trampling damage. Overall the low levels of damage had resulted in a low level of change.

In the areas of medium trampling damage, there were significant reductions to all categories of vegetation cover and vegetation height. This pattern was followed through with the areas of severe trampling damage having significantly lower levels of all height and cover categories.

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