Impoverishment in oceanic regions

The absence of prolonged periods of frost in coastal regions results in the leaching of soils of their nutrients by high rainfall. On arable lands this is particularly severe in winter when the lack of plant cover deprives the soil of the principal means of nutrient retention. In areas near the sea drenching with salt spray can accelerate ion exchange and further deplete soils of nutrients. Nevertheless, for early farmers oceanic pastures made it possible to overwinter greater numbers of mature cattle. It is possible that it was this facility to keep some cattle outdoors in winter that promoted the development of dairy farming with cattle in Ireland in the fifth to sixth centuries AD as opposed to milking only sheep and goats (McCormick, 1995). Cattle kept mainly for dairying, rather than just for meat, give a greater yield of energy and protein per hectare and this may have further contributed to the increase in the human population in oceanic areas.

The dependence of early agriculture on animal manure for nitrogen and phosphorus frequently caused what has been described as a nutrient flow trap (Dodgshon, 1994a). Some archaeological studies suggest that as a result of favourable conditions for grass production and outdoor overwintering of livestock, the growth of the human population frequently exceeded the carrying capacity of the land (Dodgshon, 1994b). When such populations expanded, the increased pressure for more land for arable crops reduced the area of land available for pasture. In continental climates the loss of lowland pasture can be made good by clearing more forest further uphill. However, the high lapse rate in oceanic regions (0.8 to 1 °C 100 of altitude) usually limits winter pasturage to land below 150 m (Fig. 11.15). The reduction in pasture would then have caused a diminished input of nutrients from animal manure to cultivated land and a consequent lowering of crop yields.

In addition to leaching and high rainfall, waterlogging impedes mineralization and nitrogen fixation, and frequently aggravates soil impoverishment in oceanic environments. The development of ploughing technology in the Iron Age would also have increased soil leaching in oceanic areas leading eventually to the podzolization (Fig. 11.16) of many soils - a process which had already begun in many western European locations with extensive production of heathlands in the early Neolithic (Behre, 1988). A particular case of oceanic heathland development is found in the 25 km wide belt of heathlands that extends along the coast of western Norway to the Arctic Circle. Some of the oldest heaths are found in the most westerly peninsulas and islands north-west of Bergen, where they have been dated to the Neolithic (4300 BP) with later extensions around AD 0, and again during the Viking age (Kaland, 1986).

The inevitable formation of iron pans, leading to drainage impairment and the development of gley soils and then the subsequent bog growth, further

Fig. 11.15 Langdale in the Lake District showing the strict limitation of improved pasture to the valley floor (105 m above sea level) and the neglect of potential adjacent hillside grazing in a region with an oceanic environment.

Fig. 11.16 A typical podzol soil exposed by a road cutting through a Scottish moorland. Podzol comes from the Russian roots pod underneath and zola ashes, and expresses an ancient and erroneous belief that the bleached layer was due to ashes from former forest fires. The light-coloured soil is, however, produced by the leaching of minerals such as iron and alumina which then leaves a bleached zone, which is often also depleted of clay. Podzols are found predominantly under coniferous forests and on moorlands in cool regions where rainfall exceeds evaporation. The leached minerals can accumulate lower down the soil profile to form a hard, impermeable layer (an iron pan) which eventually restricts drainage.

Fig. 11.16 A typical podzol soil exposed by a road cutting through a Scottish moorland. Podzol comes from the Russian roots pod underneath and zola ashes, and expresses an ancient and erroneous belief that the bleached layer was due to ashes from former forest fires. The light-coloured soil is, however, produced by the leaching of minerals such as iron and alumina which then leaves a bleached zone, which is often also depleted of clay. Podzols are found predominantly under coniferous forests and on moorlands in cool regions where rainfall exceeds evaporation. The leached minerals can accumulate lower down the soil profile to form a hard, impermeable layer (an iron pan) which eventually restricts drainage.

decreases the inputs of nitrogen and phosphorus and thus contributes to the general nutrient impoverishment of coastal lands that results from agricultural overexploitation. In many maritime regions this entire scenario can be described as paludification (bog growth), as former mineral soils with unimpeded drainage were gradually converted to bogs causing many Neolithic and

Iron Age farms to be abandoned. Frequently, all that remains of this early farming activity on mineral soils are ancient walls, as the farmers retreated as a result of soil exhaustion and the subsequent advance of the bog.

The island of Papa Stour on the west coast of Shetland (Fig. 11.17) practised transporting turf and peat from large parts of the island to a relatively limited

Fig. 11.17 The hyperoceanic island ofPapa Stour (Shetland, UK) showing a hill dyke dividing cultivated land to the right from the outfield to the left which has been completely stripped of all turf. The island has a 5000-year history ofhuman occupation (see also Fig. 11.18) with a population of over 300 in the mid nineteenth century (now approximately 16).

sheltered area for cultivation creating the plaggen soils (Dutch, layered; Section 9.8.3). However, the very extensive removal of turf over a long period has led to the eventual destruction of the grazing value of the outfields (Fig. 11.17). This stripping of the peat and turf has, however, revealed the remains of many ancient walls that provide evidence of a well-developed pastoral economy over the greater part of the island in prehistoric times (Fig. 11.18).

In Ireland there are over 50 known locations of such prehistoric farms, which became engulfed by peat through paludification. The most famous is the early Neolithic site at Ceide Fields (County Mayo) where an extensive Neolithic walled field system, now dated to before 5000 BP, was eventually buried by 4 m of peat (Mitchell & Ryan, 1998). The stumps of ancient pines (Pinus sylvestris) are found in situ at many locations within the blanket bog that covered the site. In most cases the pine roots are either on the surface of the mineral soil under the peat, or at an intermediate level in the peat itself. The age of the trees in the bog overlying Ceide Fields has therefore been of great significance for the dating of the fields, as the trees must be younger than the bog in which they are growing, which in turn must be younger than the field system beneath (Caulfield et al., 1998). The results of the dendro-chronological studies suggest that the dates for the construction and period of use of Ceide Fields and other Neolithic pre-bog field systems in North Mayo are older than was originally thought and that the initiation of blanket bog in many parts of North Mayo began more than 5000 years ago - which is also older than previously estimated. Further, the range of dates of the pine stumps indicates a synchronic event contemporary with a similar phenomenon observed in Scotland (Caulfield et al., 1998). A range of detailed palaeo-environmental analyses carried out on a series of three peat profiles from Achill Island, Co. Mayo, western Ireland, where after a period of relatively dry climate Neolithic communities expanded in the region, found evidence for an extreme climatic event, probably a storm or series of storms, around 5200-5100 BP. This event is possibly linked to human abandonment of the area, comparable to that observed at nearby Ceide Fields (Caseldine et al., 2005). Irrespective of the exact chronology at different sites, the general trend therefore in this hyperoceanic region would appear to be for paludification starting to displace agriculture about 5000 years ago.

During periods of generally lower temperatures, as in the Little Ice Age, marginal areas that relied on agriculture were particularly susceptible to sudden

Ice Age Tundra

Fig. 11.18 Prehistoric walls on the island of Papa Stour (Shetland, UK) which have been exposed by removal of peat that grew since the walls were built. Evidence from sites such as this and archaeological investigations of Neolithic/Bronze Age settlements in Scotland and Ireland (see text) strongly suggest that peat formation became extensive in these regions as a result of human activity and tree removal, illustrating the sensitivity of oceanic habitats to environmental alteration.

Fig. 11.18 Prehistoric walls on the island of Papa Stour (Shetland, UK) which have been exposed by removal of peat that grew since the walls were built. Evidence from sites such as this and archaeological investigations of Neolithic/Bronze Age settlements in Scotland and Ireland (see text) strongly suggest that peat formation became extensive in these regions as a result of human activity and tree removal, illustrating the sensitivity of oceanic habitats to environmental alteration.

climatic oscillations. An interdisciplinary study, combining the climate record from the Greenland Ice Sheet Project 2 (GISP2) with historical documents, has related the ending of the Norse Western Settlement in Greenland (mid fourteenth century) to evidence for lowered temperatures and severe weather in the North Atlantic. It appears that periods of unfavourable climatic fluctuations exposed the cultural vulnerability of the Norse farming settlers to environmental change to a greater extent than their maritime-based Inuit neighbours (Barlow et al., 1997).

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