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Recent landscape change and landscape restoration in Northern Ireland; a tephra-dated pollen study
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Review of Palaeobotany and Palynology 103 (1998) 59–68
Recent landscape change and landscape restoration in Northern Ireland: a tephra-dated pollen study Valerie A. Hall * Institute of Irish Studies, The Queen’s University of Belfast, Belfast B79 1NN, Northern Ireland Received 30 November 1996; revised version received 25 September 1997
Abstract Tephra-dated palynological investigations of lowland peats which have accumulated over the last millennium, provide an excellent means of tracing recent landscape change. Pollen analytical investigations of lowland raised bogs in the northern and western counties of the province show past regional variations. At one extreme northern lowland site, the landscape has remained relatively unchanged over much of the last millennium, while in the western county agricultural expansion linked to the rise of the great estates in the 18th century is observed. These findings provide those concerned with conservation and landscape restoration with information on which to base woodland planting. 1998 Elsevier Science B.V. All rights reserved. Keywords: pollen; tephra; Ireland
1. Introduction This paper examines two issues of concern to conservationists and those involved in landscape tourism in Northern Ireland. In spite of over a quarter of a century of civil unrest in Northern Ireland, tourism is a major currency earner. Much of the landscape of the north of Ireland has remained undamaged by industrial expansion and is still primarily an agricultural economy. The international fame of the Irish landscape attracts tourists from all over the world. Maintaining a landscape rich in environmentally sensitive areas and sites of special scientific interest requires planning and administration at high government level. This paper looks at ways in which palynology in Ireland can provide information for conŁ Corresponding
author. E-mail: [email protected]
servationists and for landscape management linked to tourism. It will look at one of the main issues, the nature of the present landscape in the northern and western counties. These represent ecosystems which require legislation for maintenance and management. It is recognised by conservation agencies throughout the British Isles that Ireland has, in comparison to other parts of Europe, relatively low woodland cover with this situation most evident in Northern Ireland. Recent alteration of the European Union policy of setting aside under-used agricultural land now permits the planting of woodland. If woodland is to be planted for amenity value what will influence the choice of agricultural land for planting? Palynological investigations of peats and lake sediments in Ireland have yielded a wealth of information about Holocene forest history. This paper will de-
c 1998 Elsevier Science B.V. All rights reserved. 0034-6667/98/$19.00 PII: S 0 0 3 4 - 6 6 6 7 ( 9 8 ) 0 0 0 2 6 - 8
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scribe palynological investigations of the woodland history of two lowland sites and describe a novel dating technique for recent peats using geochemically confirmed historically dated Icelandic volcanic ash layers. Palynological investigations can make a major contribution to further understanding of the development and management of the present landscape, but sadly the palynologist is rarely contacted for advice. Few involved in managing areas of marginal agriculture even realise that pollen studies of past landscapes have been performed, let alone that these are valuable in environmental planning. There is, however, a fair degree of uninformed thinking influencing some tree-planting schemes launched over approximately the last 10 years. In Ireland there is an assumption that as well as improving the quality of the present landscape and all aspects of its amenity value, extensive planting of native deciduous species will go some way towards returning its modern landscape to its former forested condition. This notion of the need for repair originates from a perception that our present treeless condition is something which is relatively recent, and to be deplored. There is widespread belief that up until about 400 years ago, much of the country was heavily forested. This perception persists in spite of strong historical documentary evidence describing woodland extent and the agricultural activities of people over the last 1000 years and throughout the country (McCracken, 1958, 1971; McCracken and McCracken, 1984; Hall, 1992). The truth about the extent and nature of Ireland’s historic forests lies in bogland adjacent to places where the historical documentary evidence tells of woodland. A number of palynological studies of the Irish historic landscape are published (Mitchell, 1965; van Geel and Middeldorp, 1988; Mitchell, 1988, 1990; Hall, 1990; Smith and Goddard, 1991; Jelicic and O’Connell, 1992; Hall et al., 1993; Hall, 1994). It is therefore the palynologist, rather than the historian, who must trace the landscape’s own record, by analysis of historic lowland peats. There are still numerous places throughout rural Northern Ireland where at least remnants of uncut lowland raised bogs remain. The palynological record is secure but the timescale over which the historic peats developed
is problematic as calibrated radiocarbon dates are too imprecise to allow comparison of palynological and historical information (Pilcher, 1993). This paper utilises the newly constructed historic tephrochronology for the north of Ireland (Pilcher et al., 1996). Thin layers of Icelandic tephra or volcanic ash have been detected in the recent peats from both the sites investigated and these have been linked through their geochemistry to Icelandic eruptions of known historic date (Pilcher and Hall, 1992; Pilcher et al., 1996). Tephra-dated palynological studies of recent peats greatly refines the time-frame of this research so that comparison with the historical record for landscape change may be made.
2. Site description The location of the sites and other areas mentioned in the text is shown in Fig. 1. Garry Bog in County Antrim (55º050 N, 6º340 W, altitude 50 m, grid ref. C9429) is the largest and most northerly lowland raised bog system in Northern Ireland comprising some 12 km2 of peat. Many parts of the bog have been cut to provide fuel, with grazing land developed from subsequently drained areas. The bog’s exposed character makes it a highly sensitive recorder of past environmental change (Pilcher and Baillie, 1990). The road running through the bog is a main route to the ‘Giant’s Causeway’, a world-renowned tourist attraction. Hedges comprising primarily Crataegus and Prunus spinosa are present in the area and serve as field boundary markers rather than offer protection to grazing cattle. The area is noted for the subfossil oaks and pines which have been dredged from the bog heaped to provide wind breaks for grazing stock. Garvaghullion Bog in County Tyrone (54º390 N, 7º260 W, altitude 100 m, grid ref. H365765) is one of a number of lowland raised bogs in the valley of the Fairy Water river, County Tyrone. Until recently much of this bog remains intact, with cutting for fuel provision restricted to the bog edge. It is presently being drained for the production of horticultural peat but a remnant of uncut bog remains. Presently the land surrounding the site is used for grazing cattle. No woodland remains in the vicinity of the site. Woody species such as Crataegus, Prunus spinosa, Fraxinus and Salix are restricted to hedges.
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Fig. 1. Location map of sites investigated and other areas referred to in the study.
In August 1992 a monolith approximately 3 m long was obtained from an excavated vertical cut face near the dome at Garry Bog. In 1992 peat cores were obtained from the dome of Garvaghullion Bog using a Russian corer (Jowsey, 1966). Palaeoenvironmental investigations of Garry Bog include Pilcher (1990), Pilcher and Baillie (1990), Brown (1990), Hall et al. (1993) and Pilcher et al. (1996). This is the first palynological investigation of the Garvaghullion Bog. The top 0.5 m of peat at each site was taken for pollen and tephra analyses.
3. Materials, methods and results 3.1. Tephra analyses Peat samples for tephra analysis were obtained by an adaptation of the sub-sampling and combustion technique described fully in Pilcher and Hall (1992). A brief description of the technique is as follows. Tephra layers were isolated from 1 cm sub-samples of the Garry peats by optical microscopy. Sub-samples 5 cm and then 1 cm thick were prepared for tephra analysis from the Garvaghullion peats. Two
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layers of tephra were detected at Garry Bog and one layer at Garvaghullion Bog. Further samples were used to prepare samples for both tephra geochemical analysis and pollen analysis. Samples for chemical characterisation of tephra were prepared by wet oxidation (Dugmore et al., 1992). Geochemical analyses of single tephra shards were performed by wavelength-dispersive electron microprobe analyses on the Jeol 733 Superprobe, in the Electron Microscope Unit, Queen’s University, Belfast. A full description of the operating conditions is given in Pilcher et al. (1996). Attribution of the tephra to its parent eruption was by reference to type material held in the Dept. of Geography, University of Edinburgh. When compared with type material held on the Edinburgh tephra database the layers at Garry Bog were confirmed as follows: – 30–32 cm Hekla 1510 AD. Analysis details are published in Pilcher et al. (1996). – 40–45 cm Oraefajokull 1362 AD. Analysis details are presented in Pilcher and Hall (1998). Geochemical analyses of the tephra from Garvaghullion Bog are held on Tephrabase HTTP:== www.geo.ed.ac.uk=tephra=basehom.html. The layer whose tephra was most abundant at 40–42 cm at Garvaghullion was confirmed as geochemically identical with that of type material from the eruption of Oraefajokull in 1362 AD. The occurrence at both sites of a layer of tephra geochemically attributed to the eruption of Oraefajokull in 1362 AD and the presence of tephra from the 1510 AD Hekla eruption at Garry Bog provided the time-frame for the palynological study (Dugmore et al., 1996). 3.2. Pollen analysis Standard techniques of pollen pretreatment were used throughout (Faegri and Iversen, 1989). Almost all pollen was well preserved. Pollen percentages were based on sums of 500 grains with counts carried out at ð640. Counts of charcoal fragments between 10 and 80 µm were expressed as a percentage of the total pollen sum. Pollen and spore nomenclature follows the recommendations of the Environmental Research Centre, University of Durham (Anon, 1992). Selected taxa only are presented in the pollen
diagrams (Figs. 2 and 3). All taxa, with the exception of Sphagnum are included in the pollen sum. Interpolated dates proposed in the pollen diagrams are based on a time–depth relationship assuming a constant accumulation rate over the period of the investigation at Garvaghullion Bog and on the accumulation rate estimated from the depths of the two tephra layers at Garry Bog. 3.3. Garvaghullion Bog 3.3.1. Lower zone (37–18 cm, 1300–1700 AD) Arboreal pollen values are highest at the base of this zone (80%) with Corylus type predominating at 50–60%. Other tree pollen values are low throughout with Quercus and Alnus only making any contribution to arboreal pollen values. Other tree taxa are represented as single grain occurrences. Gramineae and Cyperaceae are at approximately 10% with low values of Plantago lanceolata throughout. Single grain occurrences or very low percentages of a few weed taxa also occur. There is a steady rise in the values of Ericaceae from 15% to approaching 40% throughout the zone. Charcoal values are generally low. 3.3.2. Upper zone (18–0 cm, 1700–present) The most marked change in the diagram begins at approximately 18 cm with a fall in tree and shrub pollen which further diminishes and then recovers slightly at the top of the zone. Almost all of the initial drop results from a rapid reduction in Corylus type levels (over 40% to under 20%) which is simultaneous with swiftly rising Gramineae and Cyperaceae curves over the same depth. Near the top of the zone Alnus and Quercus values diminish further with increasing single grain occurrences of non-native trees such as Picea, Fagus, Acer pseudoplatanus and rising values of reintroduced Pinus. Interpolation of dates based on the lower tephra layer and assuming a constant accumulation rate would place this change in the tree pollen suite in the first decades of the 19th century. At the same depth, single grain occurrences of the hedgerow shrubs Crataegus and Prunus spinosa are observed, as is a rise in the weakly represented Cerealia and Plantago lanceolata curves. Other weed species remain poorly represented. Values along the charcoal curve fluctuate but show a general increase. Throughout the diagram there is
V.A. Hall / Review of Palaeobotany and Palynology 103 (1998) 59–68 Fig. 2. Percentage pollen diagram of selected taxa from Garvaghullion Bog, Co. Tyrone. C D less than 1%. The complete data set is logged on the European Pollen Data Base.
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Fig. 3. Percentage pollen diagram of selected taxa from Garry Bog, Co. Antrim. C D less than 1%. The complete data set is logged on the European Data Base.
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a fluctuating curve for Ericaceae but when viewed overall it remains consistently at about 25–30%. 3.4. Garry Bog 3.4.1. Lower zone (40–22 cm, 1360–1680 AD) The profile is dominated by high levels of Ericaceae pollen throughout with a general trend towards rising values near the top of the diagram. Arboreal pollen values at the start of the zone are at a maximum of less than 40% with an overall drop throughout this part of the diagram. Corylus type values range from approximately 20% at the base of the zone to around 10% at the top. Quercus and Alnus values are consistently low at about 5% but slowly dwindle to little more than single grain occurrences at the top of the zone. Charcoal values fluctuate greatly between single particle occurrences in some samples to values of over 80% in others. Within this widely varying record there is a consistent record for charcoal of about 20%. The only weed taxon to make any significant contribution to the non-arboreal pollen curve is Plantago lanceolata which is consistently at values of less than 5% throughout. 3.4.2. Upper zone (22–0 cm, 1700–present) The changes in pollen percentages which mark the zone boundary are not dramatic. There is a further fall in arboreal pollen values mostly due to reductions in Corylus type values with further reductions in the already low percentages for Alnus and Quercus. Increases in the presence or values of non-native Fagus and Pinus is barely discernable. There is a rise in the curves for Gramineae, Cyperaceae and Cerealia coincident with the reduction in Corylus type values. The fall in Ericaceae values in the sample at 14 cm is not believed to represent any change in the environment; the high levels appear anomalous. As in the previous zone, charcoal values fluctuate greatly but the overall trend is towards consistently greater percentages of charcoal in the topmost samples.
4. Discussion The historically dated tephra layers show that the profiles from both sites developed from the middle
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to late 14th century until the present. It is very rarely that such a precise time-frame can be placed in a historic pollen profile, but where this is possible it allows comparison with the historic record for local landscape development. The interpretation of pollen diagrams from sites which have experienced human impact over a long time period and where there are taxa common to both the bog and its dryer surroundings present the palynologist with some difficulties when interpreting changes in one ecosystem so that they remain independent of the other. In this study this difficulty may be best seen where the expansion in values of Cyperaceae and falls in values of Corylus are most marked. It could be argued that the rise in one curve forces a fall in the other. To partly overcome this problem one might remove taxa from the pollen sum which occur only on the bog or extend this selection to taxa which are common to the bog and its surroundings, for example Gramineae and Cyperaceae. It can be argued that this makes it simpler to interpret the behaviour of the woody taxa such as Corylus growing on the dryer land surrounding the bog if the bog taxa are excluded. In this study the decision was made to include all land taxa in the pollen sum. Removal of, for example, Cyperaceae as being a component of the bog flora oversimplifies a complex situation. Much of the damp meadowland of the north of Ireland includes species of Cyperaceae which are palynologically indistinguishable from those species restricted to the bog surface. Little is recorded about the landscape of the north of Ireland during the later mediaeval period, but during the 17th century rapid and extensive woodland exploitation and ultimately extermination is recorded in the historical records (McCracken, 1971) with much emphasis placed on the wholesale removal of oak. The bulk of this written evidence for woodland depletion in the north of Ireland is centred on the Lower Bann valley, with virtually nothing recorded for the areas investigated in this work. The Garvaghullion Bog lies in the heartland of the Gaelic O’Neill clan who were masters of this area until the first decade of the 17th century. At that time eradication of woodland from an extensive part of the mid-Ulster lowlands was planned as part of the measures to subjugate the local clans people during the colonisation by incomers from England and Scotland
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known as ‘The Plantation of Ulster’. Only the pollen record can reveal if there was sufficient woodland in the O’Neill lands to warrant its extermination for military purposes. It should be noted that documentary records show that the grand plans for massive alteration of the landscape by removing woodland cover for military purposes were never carried out. A number of pollen studies of the historic past have been performed for sites throughout the north of Ireland by the author. These show a general trend to opening of a more lightly wooded landscape in the late mediaeval with an increase in the rate of woodland extermination in the 18th century. At none of the sites investigated previously have there been the high arboreal pollen values of trees such as Quercus, Alnus or Fraxinus indicative of dense or extensive woodland, but it must be stressed that none have such low arboreal pollen values as Garry Bog. Here tree pollen values are so low that the landscape surrounding the site must have been virtually treeless for most of the last 700 years. The northern lowlands were not forested but under light woodland or scrub with damp grassland and used for low intensity farming. Today much of that wooded landscape has vanished. The present landscape is the product of a long and gentle change to open conditions in the early centuries of this millennium. The pollen values for Corylus particularly show that at Garvaghullion Bog woodland was more abundant than at Garry Bog but little in the pollen values for trees like Quercus or Alnus indicates much forest, certainly nothing like the high values for Quercus recorded for the Killarney woodlands (Mitchell, 1990). In contrast to Garry Bog, at Garvaghullion there is a more marked pattern in the pollen profile, in the form of a fall in Corylus values, for the extensive and relatively swift removal of this species in the early 18th century. The general trend common to both sites is towards a rise in grassland in the early 18th century coincident with the removal or diminution of woodland. In pollen diagrams spanning much of the last millennium from sites throughout the north of Ireland, the introduction of exotic species is observed in the topmost samples. Very low values of non-native species such as Picea, Fagus, Acer pseudoplatanus and Tilia are first seen along with rising values for reintroduced Pinus. The first planting of exotic tree
species is not well documented except in a few cases such as the Register of Trees for some areas west of the Lower Bann during the late 18th century (McCracken, 1971; McCracken and McCracken, 1984). The interpolated dates for the first occurrences of exotic tree pollen at Garvaghullion Bog indicate that here too conifers and exotic hardwoods were planted in the late eighteenth or early nineteenth centuries with some years elapsing before flowering. What historical evidence there is for the first planting of hedges in the north of Ireland has been assembled and assessed by Robinson (1977). He states that most hedges were planted some time near the end of the 18th century. The commonest thorny species planted for hedging were Crataegus and Prunus spinosa. These Rosaceous species contribute little to the pollen spectrum of recent historical farming (Hall, 1989) and at Garvaghullion their infrequent pollen first appears in peats which accumulated in the early years of the 19th century. Pinus sylvestris was an important native tree in Ireland until the mid-Holocene, after which it became virtually extinct (Bennett, 1984; Bradshaw and Browne, 1987). Today all pine is from introduced stock. Garry Bog is unique in pollen diagrams from the north of Ireland in having such scant palynological evidence for the reintroduction of pine. The few non-native trees represented in the diagram are associated with estate management practices introduced in the 18th century along with the first planting of hedges (Robinson, 1977). Low values of these hedgerow species are observed at the same time as the first occurrences of the non-native tree species but at Garry Bog these too are virtually absent. Assessing the extent and impact of farming in Irish pollen diagrams is complicated by the dual role occupied by a number of weed taxa (Hall, 1989). For example, in the diagrams presented in this paper, a continuous curve for low values of Plantago lanceolata associated with curves for Gramineae and Cyperaceae might be interpreted as evidence for grazing (Behre, 1981), but care should be exercised in attributing a purely pastoral role to this weed in Irish pollen diagrams as Plantago lanceolata is a common weed of arable systems in this country (Hall, 1989). The low values for Cerealia provide evidence for some cereal growing being part of the local agricultural economy. These findings are
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in keeping with the historical evidence for farming and diet throughout the Irish mediaeval and include the period from the early 18th century until the present when the potato became a vital arable crop. The mediaeval Irish agrarian economy was based on dairy produce and cereals (Lucas, 1989). The hiatus in cereal cultivation during the 15th century may be linked to a reduction in population following the Black Death and changes in management practices at that time. Landscape change and agricultural impact in Ireland during the Medieaval period is the subject of further research by the author. Palynological investigations of lowland sites throughout the north of Ireland show that this relatively small area has a complex regional vegetation history. Two major findings have emerged from these and previous studies. Firstly, there is little to support the belief that the north of Ireland was heavily forested until about 500 years ago, indeed the pollen evidence from the northern lowlands shows these to have been at best lightly wooded. The palynological evidence from the earlier centuries of this millennium also make it clear that the landscape of the lowlands then was very different from that of today, with the present improved and enclosed pasture land pre-dated by a landscape where damp grassland was peppered with patches of hazel scrub, not extensive forests. For the northern Garry Bog site there is good palynological evidence to suggest that the almost treeless landscape of today is no different from that of 1000 years ago. It is interesting to note that the documentary and palynological evidence for landscape change in the north of Ireland over about the last 300 years go hand in hand. It is the preceding period which has so much myth attached to it, especially concerning the density and extent of woodland.
5. Conclusions The studies described in this paper demonstrate that there is considerable variation in landscape between localities and that many cherished beliefs about the wooded landscapes of the last 1000 years have little foundation. The results of the studies presented in this paper demonstrate that there would be little justification for planting extensive tracts of
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woodland at Garry Bog or Garvaghullion Bog as part of a management system aimed at landscape reconstruction. Where then does this leave the conservationist faced with decisions on density of tree planting and the choice of suitable species? Surely it is more honest to plant for the future using the information provided by palynology rather than impose a nationwide policy based on erroneous perceptions of past landscape. We have here a body of evidence which is derived from the landscape itself, within the peats which developed as the landscape of the north of Ireland altered to a greater or lesser extent over much of the last millennium. There is no palynological precedent for assuming that we need more trees to repair the ravages of recent loss. There appears to be no recent loss of major forests but in some areas there was a major loss of lighter woodland. Let us plant trees in abundance, and plant native species, but not motivated by some sentimental notion about a densely forested past landscape. Plant trees as part of the modern cultural landscape, not as a reconstruction for which there is so little justification. Where there is a requirement for landscape reconstruction, conservationists please take note of what palynology can tell you.
References Anon, 1992. European Pollen Database. List of common nomenclatural synonyms and other conventions followed by data compliers. Environmental Research Centre, University of Durham. Behre, K.E., 1981. The interpretation of anthropogenic indicators in pollen diagrams. Pollen Spores 23, 225–245. Bennett, K.D., 1984. The post-glacial history of Pinus sylvestris in the British Isles. Quat. Sci. Rev. 3, 133–155. Bradshaw, R.H.W., Browne, P., 1987. Changing patterns in the post-glacial distribution of Pinus sylvestris in Ireland. J. Biogeogr. 14, 237–248. Brown, D.M., 1990. Studies on Pinus sylvestris L. from Garry Bog, Co. Antrim. Unpublished M.Sc. thesis, Queens University, Belfast. Dugmore, A.J., Newton, A.J., Sugden, D.E., 1992. Geochemical stability of fine-grained silicic Holocene tephra in Iceland and Scotland. J. Quat. Sci. 7, 173–266. Dugmore, A.J., Newton, A.J., Edwards, K.J., Larsen, G., Blackford, J.J., Cook, G.T., 1996. Long-distance marker horizons from small-scale eruptions: British tephra deposits from the AD 1510 eruption of Hekla, Iceland. J. Quat. Sci. 11, 511–516.
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Faegri, K., Iversen, J., 1989. Textbook of Pollen Analysis (4th ed.). John Wiley & Sons, Chicester. Hall, V.A., 1989. A study of the modern pollen rain from a reconstructed 19th century farm. Ir. Nat. J. 23, 82–92. Hall, V.A., 1990. Recent landscape history from a Co. Down lake deposit. New Phytol. 115, 377–383. Hall, V.A., 1992. The woodlands of the Lower Bann valley in the seventeenth century; the documentary evidence. Ulster Folklife 38, 1–11. Hall, V.A., 1994. Landscape development in northeast Ireland over the 1st half millennium. Rev. Palaeobot. Palynol. 82, 75– 82. Hall, V.A., Pilcher, J.R., McCormac, F.G., 1993. Tephra-dated landscape history of the north of Ireland, A.D 750–1150. Holocene 125, 193–202. Jelicic, L., O’Connell, M., 1992. History of vegetation and land use from 3200 B.P. to the present in north-west Burren, a karstic region of western Ireland. Veget. Hist. Archaeobot. 1, 119–140. Jowsey, P.C., 1966. An improved peat sampler. New Phytol. 65, 345–348. Lucas, A.T., 1989. Cattle in Ancient Ireland. Boethius, Kilkenny, 315 pp. McCracken, E., 1958. The woodlands of Donegal 1600–1840. Donegal Annu. 4, 62–64. McCracken, E., 1971. The Irish woods in Tudor times, their distribution and exploitation. David and Charles, Newton Abbot. McCracken, E., McCracken, D., 1984. A register of trees for Co. Londonderry 1768–1911. Public Records Office, N. Ireland. Mitchell, F.J.G., 1988. The vegetational history of the Killarney oakwoods. SW Ireland: evidence from fine spatial pollen analysis. J. Ecol. 76, 415–436. Mitchell, F.J.G., 1990. The impact of grazing and human disturbance on the dynamics of woodland in SW Ireland. J. Veget.
Sci. 1, 245–254. Mitchell, G.F., 1965. Littleton bog, Tipperary: an Irish agricultural record. J. R. Soc. Ant. Irel. 95, 121–123. Pilcher, J.R., 1990. Ecology of subfossil oak woods on peat. In: Doyle, G. (Ed.), Ecology and Conservation of Irish Peatlands. Royal Irish Academy, Dublin, pp. 41–47. Pilcher, J.R., 1993. Radiocarbon dating and the palynologist; a realistic approach to precision and accuracy. In: Chambers, F.M. (Ed.), Climate Change and Human Impact on the Landscape. Chapman and Hall, London, pp. 23–29. Pilcher, J.R., Baillie, M.G.L., 1990. Garry Bog. In: Wilson, P. (Ed.), North Antrim and Londonderry. Irish Association for Quaternary Studies, Field Guide 13, pp. 78–84. Pilcher, J.R., Hall, V.A., 1992. Towards a tephrochronology for the Holocene of the north of Ireland. Holocene 2 (3), 255– 259. Pilcher, J.R., Hall, V.A., 1998. A Holocene tephrochronology for the British Isles: some practical difficulties. Quat. Sci. Rev., in press. Pilcher, J.R., Hall, V.A., McCormac, F.G., 1996. An outline tephrochronology for the Holocene of the north of Ireland. J. Quat. Sci. 11, 485–494. Robinson, P.S., 1977. The spread of the hedged enclosure. Ulster Folklife 23, 57–69. Smith, A.G., Goddard, I.C., 1991. A 12,500 year record of the vegetational history of Sluggan Bog, Co. Antrim, N. Ireland (incorporating a pollen zone scheme for the non-specialist). New Phytol. 118, 167–187. Van Geel, B., Middeldorp, A., 1988. Vegetational history of Carbury bog (Co. Kildare, Ireland) during the last 850 years and a test of the temperature indicator value of 2H=1H measurements of peat samples in relation to the historical sources and meteorological data. New Phytol. 109, 377–392.
Review of Palaeobotany and Palynology 103 (1998) 59–68
Recent landscape change and landscape restoration in Northern Ireland: a tephra-dated pollen study Valerie A. Hall * Institute of Irish Studies, The Queen’s University of Belfast, Belfast B79 1NN, Northern Ireland Received 30 November 1996; revised version received 25 September 1997
Abstract Tephra-dated palynological investigations of lowland peats which have accumulated over the last millennium, provide an excellent means of tracing recent landscape change. Pollen analytical investigations of lowland raised bogs in the northern and western counties of the province show past regional variations. At one extreme northern lowland site, the landscape has remained relatively unchanged over much of the last millennium, while in the western county agricultural expansion linked to the rise of the great estates in the 18th century is observed. These findings provide those concerned with conservation and landscape restoration with information on which to base woodland planting. 1998 Elsevier Science B.V. All rights reserved. Keywords: pollen; tephra; Ireland
1. Introduction This paper examines two issues of concern to conservationists and those involved in landscape tourism in Northern Ireland. In spite of over a quarter of a century of civil unrest in Northern Ireland, tourism is a major currency earner. Much of the landscape of the north of Ireland has remained undamaged by industrial expansion and is still primarily an agricultural economy. The international fame of the Irish landscape attracts tourists from all over the world. Maintaining a landscape rich in environmentally sensitive areas and sites of special scientific interest requires planning and administration at high government level. This paper looks at ways in which palynology in Ireland can provide information for conŁ Corresponding
author. E-mail: [email protected]
servationists and for landscape management linked to tourism. It will look at one of the main issues, the nature of the present landscape in the northern and western counties. These represent ecosystems which require legislation for maintenance and management. It is recognised by conservation agencies throughout the British Isles that Ireland has, in comparison to other parts of Europe, relatively low woodland cover with this situation most evident in Northern Ireland. Recent alteration of the European Union policy of setting aside under-used agricultural land now permits the planting of woodland. If woodland is to be planted for amenity value what will influence the choice of agricultural land for planting? Palynological investigations of peats and lake sediments in Ireland have yielded a wealth of information about Holocene forest history. This paper will de-
c 1998 Elsevier Science B.V. All rights reserved. 0034-6667/98/$19.00 PII: S 0 0 3 4 - 6 6 6 7 ( 9 8 ) 0 0 0 2 6 - 8
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scribe palynological investigations of the woodland history of two lowland sites and describe a novel dating technique for recent peats using geochemically confirmed historically dated Icelandic volcanic ash layers. Palynological investigations can make a major contribution to further understanding of the development and management of the present landscape, but sadly the palynologist is rarely contacted for advice. Few involved in managing areas of marginal agriculture even realise that pollen studies of past landscapes have been performed, let alone that these are valuable in environmental planning. There is, however, a fair degree of uninformed thinking influencing some tree-planting schemes launched over approximately the last 10 years. In Ireland there is an assumption that as well as improving the quality of the present landscape and all aspects of its amenity value, extensive planting of native deciduous species will go some way towards returning its modern landscape to its former forested condition. This notion of the need for repair originates from a perception that our present treeless condition is something which is relatively recent, and to be deplored. There is widespread belief that up until about 400 years ago, much of the country was heavily forested. This perception persists in spite of strong historical documentary evidence describing woodland extent and the agricultural activities of people over the last 1000 years and throughout the country (McCracken, 1958, 1971; McCracken and McCracken, 1984; Hall, 1992). The truth about the extent and nature of Ireland’s historic forests lies in bogland adjacent to places where the historical documentary evidence tells of woodland. A number of palynological studies of the Irish historic landscape are published (Mitchell, 1965; van Geel and Middeldorp, 1988; Mitchell, 1988, 1990; Hall, 1990; Smith and Goddard, 1991; Jelicic and O’Connell, 1992; Hall et al., 1993; Hall, 1994). It is therefore the palynologist, rather than the historian, who must trace the landscape’s own record, by analysis of historic lowland peats. There are still numerous places throughout rural Northern Ireland where at least remnants of uncut lowland raised bogs remain. The palynological record is secure but the timescale over which the historic peats developed
is problematic as calibrated radiocarbon dates are too imprecise to allow comparison of palynological and historical information (Pilcher, 1993). This paper utilises the newly constructed historic tephrochronology for the north of Ireland (Pilcher et al., 1996). Thin layers of Icelandic tephra or volcanic ash have been detected in the recent peats from both the sites investigated and these have been linked through their geochemistry to Icelandic eruptions of known historic date (Pilcher and Hall, 1992; Pilcher et al., 1996). Tephra-dated palynological studies of recent peats greatly refines the time-frame of this research so that comparison with the historical record for landscape change may be made.
2. Site description The location of the sites and other areas mentioned in the text is shown in Fig. 1. Garry Bog in County Antrim (55º050 N, 6º340 W, altitude 50 m, grid ref. C9429) is the largest and most northerly lowland raised bog system in Northern Ireland comprising some 12 km2 of peat. Many parts of the bog have been cut to provide fuel, with grazing land developed from subsequently drained areas. The bog’s exposed character makes it a highly sensitive recorder of past environmental change (Pilcher and Baillie, 1990). The road running through the bog is a main route to the ‘Giant’s Causeway’, a world-renowned tourist attraction. Hedges comprising primarily Crataegus and Prunus spinosa are present in the area and serve as field boundary markers rather than offer protection to grazing cattle. The area is noted for the subfossil oaks and pines which have been dredged from the bog heaped to provide wind breaks for grazing stock. Garvaghullion Bog in County Tyrone (54º390 N, 7º260 W, altitude 100 m, grid ref. H365765) is one of a number of lowland raised bogs in the valley of the Fairy Water river, County Tyrone. Until recently much of this bog remains intact, with cutting for fuel provision restricted to the bog edge. It is presently being drained for the production of horticultural peat but a remnant of uncut bog remains. Presently the land surrounding the site is used for grazing cattle. No woodland remains in the vicinity of the site. Woody species such as Crataegus, Prunus spinosa, Fraxinus and Salix are restricted to hedges.
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Fig. 1. Location map of sites investigated and other areas referred to in the study.
In August 1992 a monolith approximately 3 m long was obtained from an excavated vertical cut face near the dome at Garry Bog. In 1992 peat cores were obtained from the dome of Garvaghullion Bog using a Russian corer (Jowsey, 1966). Palaeoenvironmental investigations of Garry Bog include Pilcher (1990), Pilcher and Baillie (1990), Brown (1990), Hall et al. (1993) and Pilcher et al. (1996). This is the first palynological investigation of the Garvaghullion Bog. The top 0.5 m of peat at each site was taken for pollen and tephra analyses.
3. Materials, methods and results 3.1. Tephra analyses Peat samples for tephra analysis were obtained by an adaptation of the sub-sampling and combustion technique described fully in Pilcher and Hall (1992). A brief description of the technique is as follows. Tephra layers were isolated from 1 cm sub-samples of the Garry peats by optical microscopy. Sub-samples 5 cm and then 1 cm thick were prepared for tephra analysis from the Garvaghullion peats. Two
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layers of tephra were detected at Garry Bog and one layer at Garvaghullion Bog. Further samples were used to prepare samples for both tephra geochemical analysis and pollen analysis. Samples for chemical characterisation of tephra were prepared by wet oxidation (Dugmore et al., 1992). Geochemical analyses of single tephra shards were performed by wavelength-dispersive electron microprobe analyses on the Jeol 733 Superprobe, in the Electron Microscope Unit, Queen’s University, Belfast. A full description of the operating conditions is given in Pilcher et al. (1996). Attribution of the tephra to its parent eruption was by reference to type material held in the Dept. of Geography, University of Edinburgh. When compared with type material held on the Edinburgh tephra database the layers at Garry Bog were confirmed as follows: – 30–32 cm Hekla 1510 AD. Analysis details are published in Pilcher et al. (1996). – 40–45 cm Oraefajokull 1362 AD. Analysis details are presented in Pilcher and Hall (1998). Geochemical analyses of the tephra from Garvaghullion Bog are held on Tephrabase HTTP:== www.geo.ed.ac.uk=tephra=basehom.html. The layer whose tephra was most abundant at 40–42 cm at Garvaghullion was confirmed as geochemically identical with that of type material from the eruption of Oraefajokull in 1362 AD. The occurrence at both sites of a layer of tephra geochemically attributed to the eruption of Oraefajokull in 1362 AD and the presence of tephra from the 1510 AD Hekla eruption at Garry Bog provided the time-frame for the palynological study (Dugmore et al., 1996). 3.2. Pollen analysis Standard techniques of pollen pretreatment were used throughout (Faegri and Iversen, 1989). Almost all pollen was well preserved. Pollen percentages were based on sums of 500 grains with counts carried out at ð640. Counts of charcoal fragments between 10 and 80 µm were expressed as a percentage of the total pollen sum. Pollen and spore nomenclature follows the recommendations of the Environmental Research Centre, University of Durham (Anon, 1992). Selected taxa only are presented in the pollen
diagrams (Figs. 2 and 3). All taxa, with the exception of Sphagnum are included in the pollen sum. Interpolated dates proposed in the pollen diagrams are based on a time–depth relationship assuming a constant accumulation rate over the period of the investigation at Garvaghullion Bog and on the accumulation rate estimated from the depths of the two tephra layers at Garry Bog. 3.3. Garvaghullion Bog 3.3.1. Lower zone (37–18 cm, 1300–1700 AD) Arboreal pollen values are highest at the base of this zone (80%) with Corylus type predominating at 50–60%. Other tree pollen values are low throughout with Quercus and Alnus only making any contribution to arboreal pollen values. Other tree taxa are represented as single grain occurrences. Gramineae and Cyperaceae are at approximately 10% with low values of Plantago lanceolata throughout. Single grain occurrences or very low percentages of a few weed taxa also occur. There is a steady rise in the values of Ericaceae from 15% to approaching 40% throughout the zone. Charcoal values are generally low. 3.3.2. Upper zone (18–0 cm, 1700–present) The most marked change in the diagram begins at approximately 18 cm with a fall in tree and shrub pollen which further diminishes and then recovers slightly at the top of the zone. Almost all of the initial drop results from a rapid reduction in Corylus type levels (over 40% to under 20%) which is simultaneous with swiftly rising Gramineae and Cyperaceae curves over the same depth. Near the top of the zone Alnus and Quercus values diminish further with increasing single grain occurrences of non-native trees such as Picea, Fagus, Acer pseudoplatanus and rising values of reintroduced Pinus. Interpolation of dates based on the lower tephra layer and assuming a constant accumulation rate would place this change in the tree pollen suite in the first decades of the 19th century. At the same depth, single grain occurrences of the hedgerow shrubs Crataegus and Prunus spinosa are observed, as is a rise in the weakly represented Cerealia and Plantago lanceolata curves. Other weed species remain poorly represented. Values along the charcoal curve fluctuate but show a general increase. Throughout the diagram there is
V.A. Hall / Review of Palaeobotany and Palynology 103 (1998) 59–68 Fig. 2. Percentage pollen diagram of selected taxa from Garvaghullion Bog, Co. Tyrone. C D less than 1%. The complete data set is logged on the European Pollen Data Base.
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Fig. 3. Percentage pollen diagram of selected taxa from Garry Bog, Co. Antrim. C D less than 1%. The complete data set is logged on the European Data Base.
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a fluctuating curve for Ericaceae but when viewed overall it remains consistently at about 25–30%. 3.4. Garry Bog 3.4.1. Lower zone (40–22 cm, 1360–1680 AD) The profile is dominated by high levels of Ericaceae pollen throughout with a general trend towards rising values near the top of the diagram. Arboreal pollen values at the start of the zone are at a maximum of less than 40% with an overall drop throughout this part of the diagram. Corylus type values range from approximately 20% at the base of the zone to around 10% at the top. Quercus and Alnus values are consistently low at about 5% but slowly dwindle to little more than single grain occurrences at the top of the zone. Charcoal values fluctuate greatly between single particle occurrences in some samples to values of over 80% in others. Within this widely varying record there is a consistent record for charcoal of about 20%. The only weed taxon to make any significant contribution to the non-arboreal pollen curve is Plantago lanceolata which is consistently at values of less than 5% throughout. 3.4.2. Upper zone (22–0 cm, 1700–present) The changes in pollen percentages which mark the zone boundary are not dramatic. There is a further fall in arboreal pollen values mostly due to reductions in Corylus type values with further reductions in the already low percentages for Alnus and Quercus. Increases in the presence or values of non-native Fagus and Pinus is barely discernable. There is a rise in the curves for Gramineae, Cyperaceae and Cerealia coincident with the reduction in Corylus type values. The fall in Ericaceae values in the sample at 14 cm is not believed to represent any change in the environment; the high levels appear anomalous. As in the previous zone, charcoal values fluctuate greatly but the overall trend is towards consistently greater percentages of charcoal in the topmost samples.
4. Discussion The historically dated tephra layers show that the profiles from both sites developed from the middle
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to late 14th century until the present. It is very rarely that such a precise time-frame can be placed in a historic pollen profile, but where this is possible it allows comparison with the historic record for local landscape development. The interpretation of pollen diagrams from sites which have experienced human impact over a long time period and where there are taxa common to both the bog and its dryer surroundings present the palynologist with some difficulties when interpreting changes in one ecosystem so that they remain independent of the other. In this study this difficulty may be best seen where the expansion in values of Cyperaceae and falls in values of Corylus are most marked. It could be argued that the rise in one curve forces a fall in the other. To partly overcome this problem one might remove taxa from the pollen sum which occur only on the bog or extend this selection to taxa which are common to the bog and its surroundings, for example Gramineae and Cyperaceae. It can be argued that this makes it simpler to interpret the behaviour of the woody taxa such as Corylus growing on the dryer land surrounding the bog if the bog taxa are excluded. In this study the decision was made to include all land taxa in the pollen sum. Removal of, for example, Cyperaceae as being a component of the bog flora oversimplifies a complex situation. Much of the damp meadowland of the north of Ireland includes species of Cyperaceae which are palynologically indistinguishable from those species restricted to the bog surface. Little is recorded about the landscape of the north of Ireland during the later mediaeval period, but during the 17th century rapid and extensive woodland exploitation and ultimately extermination is recorded in the historical records (McCracken, 1971) with much emphasis placed on the wholesale removal of oak. The bulk of this written evidence for woodland depletion in the north of Ireland is centred on the Lower Bann valley, with virtually nothing recorded for the areas investigated in this work. The Garvaghullion Bog lies in the heartland of the Gaelic O’Neill clan who were masters of this area until the first decade of the 17th century. At that time eradication of woodland from an extensive part of the mid-Ulster lowlands was planned as part of the measures to subjugate the local clans people during the colonisation by incomers from England and Scotland
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known as ‘The Plantation of Ulster’. Only the pollen record can reveal if there was sufficient woodland in the O’Neill lands to warrant its extermination for military purposes. It should be noted that documentary records show that the grand plans for massive alteration of the landscape by removing woodland cover for military purposes were never carried out. A number of pollen studies of the historic past have been performed for sites throughout the north of Ireland by the author. These show a general trend to opening of a more lightly wooded landscape in the late mediaeval with an increase in the rate of woodland extermination in the 18th century. At none of the sites investigated previously have there been the high arboreal pollen values of trees such as Quercus, Alnus or Fraxinus indicative of dense or extensive woodland, but it must be stressed that none have such low arboreal pollen values as Garry Bog. Here tree pollen values are so low that the landscape surrounding the site must have been virtually treeless for most of the last 700 years. The northern lowlands were not forested but under light woodland or scrub with damp grassland and used for low intensity farming. Today much of that wooded landscape has vanished. The present landscape is the product of a long and gentle change to open conditions in the early centuries of this millennium. The pollen values for Corylus particularly show that at Garvaghullion Bog woodland was more abundant than at Garry Bog but little in the pollen values for trees like Quercus or Alnus indicates much forest, certainly nothing like the high values for Quercus recorded for the Killarney woodlands (Mitchell, 1990). In contrast to Garry Bog, at Garvaghullion there is a more marked pattern in the pollen profile, in the form of a fall in Corylus values, for the extensive and relatively swift removal of this species in the early 18th century. The general trend common to both sites is towards a rise in grassland in the early 18th century coincident with the removal or diminution of woodland. In pollen diagrams spanning much of the last millennium from sites throughout the north of Ireland, the introduction of exotic species is observed in the topmost samples. Very low values of non-native species such as Picea, Fagus, Acer pseudoplatanus and Tilia are first seen along with rising values for reintroduced Pinus. The first planting of exotic tree
species is not well documented except in a few cases such as the Register of Trees for some areas west of the Lower Bann during the late 18th century (McCracken, 1971; McCracken and McCracken, 1984). The interpolated dates for the first occurrences of exotic tree pollen at Garvaghullion Bog indicate that here too conifers and exotic hardwoods were planted in the late eighteenth or early nineteenth centuries with some years elapsing before flowering. What historical evidence there is for the first planting of hedges in the north of Ireland has been assembled and assessed by Robinson (1977). He states that most hedges were planted some time near the end of the 18th century. The commonest thorny species planted for hedging were Crataegus and Prunus spinosa. These Rosaceous species contribute little to the pollen spectrum of recent historical farming (Hall, 1989) and at Garvaghullion their infrequent pollen first appears in peats which accumulated in the early years of the 19th century. Pinus sylvestris was an important native tree in Ireland until the mid-Holocene, after which it became virtually extinct (Bennett, 1984; Bradshaw and Browne, 1987). Today all pine is from introduced stock. Garry Bog is unique in pollen diagrams from the north of Ireland in having such scant palynological evidence for the reintroduction of pine. The few non-native trees represented in the diagram are associated with estate management practices introduced in the 18th century along with the first planting of hedges (Robinson, 1977). Low values of these hedgerow species are observed at the same time as the first occurrences of the non-native tree species but at Garry Bog these too are virtually absent. Assessing the extent and impact of farming in Irish pollen diagrams is complicated by the dual role occupied by a number of weed taxa (Hall, 1989). For example, in the diagrams presented in this paper, a continuous curve for low values of Plantago lanceolata associated with curves for Gramineae and Cyperaceae might be interpreted as evidence for grazing (Behre, 1981), but care should be exercised in attributing a purely pastoral role to this weed in Irish pollen diagrams as Plantago lanceolata is a common weed of arable systems in this country (Hall, 1989). The low values for Cerealia provide evidence for some cereal growing being part of the local agricultural economy. These findings are
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in keeping with the historical evidence for farming and diet throughout the Irish mediaeval and include the period from the early 18th century until the present when the potato became a vital arable crop. The mediaeval Irish agrarian economy was based on dairy produce and cereals (Lucas, 1989). The hiatus in cereal cultivation during the 15th century may be linked to a reduction in population following the Black Death and changes in management practices at that time. Landscape change and agricultural impact in Ireland during the Medieaval period is the subject of further research by the author. Palynological investigations of lowland sites throughout the north of Ireland show that this relatively small area has a complex regional vegetation history. Two major findings have emerged from these and previous studies. Firstly, there is little to support the belief that the north of Ireland was heavily forested until about 500 years ago, indeed the pollen evidence from the northern lowlands shows these to have been at best lightly wooded. The palynological evidence from the earlier centuries of this millennium also make it clear that the landscape of the lowlands then was very different from that of today, with the present improved and enclosed pasture land pre-dated by a landscape where damp grassland was peppered with patches of hazel scrub, not extensive forests. For the northern Garry Bog site there is good palynological evidence to suggest that the almost treeless landscape of today is no different from that of 1000 years ago. It is interesting to note that the documentary and palynological evidence for landscape change in the north of Ireland over about the last 300 years go hand in hand. It is the preceding period which has so much myth attached to it, especially concerning the density and extent of woodland.
5. Conclusions The studies described in this paper demonstrate that there is considerable variation in landscape between localities and that many cherished beliefs about the wooded landscapes of the last 1000 years have little foundation. The results of the studies presented in this paper demonstrate that there would be little justification for planting extensive tracts of
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woodland at Garry Bog or Garvaghullion Bog as part of a management system aimed at landscape reconstruction. Where then does this leave the conservationist faced with decisions on density of tree planting and the choice of suitable species? Surely it is more honest to plant for the future using the information provided by palynology rather than impose a nationwide policy based on erroneous perceptions of past landscape. We have here a body of evidence which is derived from the landscape itself, within the peats which developed as the landscape of the north of Ireland altered to a greater or lesser extent over much of the last millennium. There is no palynological precedent for assuming that we need more trees to repair the ravages of recent loss. There appears to be no recent loss of major forests but in some areas there was a major loss of lighter woodland. Let us plant trees in abundance, and plant native species, but not motivated by some sentimental notion about a densely forested past landscape. Plant trees as part of the modern cultural landscape, not as a reconstruction for which there is so little justification. Where there is a requirement for landscape reconstruction, conservationists please take note of what palynology can tell you.
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