Agricultural Margins

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A striking feature of many early human settlements is the early date at which farmers settled in areas that would have been climatically peripheral for agriculture even under climatic conditions that were warmer than at present. Mesolithic and early Neolithic sites are to be found in northern coastal sites in Scotland and northern Scandinavia that must have been subject to a substantial risk ofcrop failure. Here and elsewhere

Fig. 1.20 Early Neolithic settlement at the Scord of Brouster, Shetland. The settlement appears to have been first occupied about 4500 BP and shows evidence of cultivation of barley as well as hunting. The end of occupation approximates to the beginning of peat growth at the site around 3500 BP. The end of occupation may also have coincided with the soil profile having become podzolized and many house sites in similar marginal areas may have been abandoned at this time (Whittle et al., 1986).

Fig. 1.20 Early Neolithic settlement at the Scord of Brouster, Shetland. The settlement appears to have been first occupied about 4500 BP and shows evidence of cultivation of barley as well as hunting. The end of occupation approximates to the beginning of peat growth at the site around 3500 BP. The end of occupation may also have coincided with the soil profile having become podzolized and many house sites in similar marginal areas may have been abandoned at this time (Whittle et al., 1986).

the practice of agriculture can be viewed as an early attempt at risk reduction in securing an adequate food supply. If marginality in relation to agriculture is quantified by an assessment of risk in terms of frequency of crop failure then nowhere is this risk more easily demonstrated than in areas with limited growing seasons. In oceanic habitats in north-western Europe high lapse rates (the rate at which temperature decreases for each unit increase in height in the atmosphere) coupled with wind exposure magnify the risk of crop failure with increasing altitude. It is therefore always surprising that many early settlements were at considerable altitudes on hillsides. Up until the Bronze Age the climate was probably warmer, which increased the upland area where crops such as barley could be grown on what are now exposed hillsides with impoverished soils (Fig. 1.20). Striking examples of such Neolithic and Bronze Age settlements are still visible in the remoter regions of the British Isles such as the Orkney and Shetland archipelagos (Section 11.2).

Current interest in climatic change has prompted extensive research into future limits for specific crops in comparing where they presently reach a limit in their viability and where these limits will lie, given a specific degree of climatic warming. Areas where wheat and potato are now grown with satisfactory yields may become marginal as future productivity may be limited by water shortages. Conversely, other areas with more oceanic climates may benefit from increased temperatures and become more productive.

The development of agriculture in Asia Minor and the Levant was once thought to have been due to increased drought as the pluvial period of the Pleistocene at low latitudes gave way to more arid climates in the Fertile Crescent (Childe, 1928). However, more recent examination does not suggest that this area suffered a sudden climatic change at the Late Glacial/ Holocene transition that occurred in the North Atlantic region (Bell & Walker, 1992). The synchrony in the multiple origins of agriculture in both the New and the Old World has been discussed in relation to a number of environmental factors including increased population density, greater social complexity and climatic change. The increase in atmospheric carbon dioxide levels from 200 to 270 imol mol~1 with the retreat of the Pleistocene ice sheets has also been proposed as an important contributing factor in the development of cereal cultivation (Sage, 1995). Among other climatic factors, increased seasonality, global temperature rise, and a reduction in rainfall may all be possible contributing causes. To identify any one cause is probably an impossible task as in different regions of the world different factors may have stimulated experimentation with crop cultivation.

Unfortunately, the invention of agriculture has in many areas been a non-sustainable solution in maintaining human population growth. The occupation of the land by annual crops that complete their growth before the onset of summer drought reduces total vegetation cover. As a result, when populations increase, and grazing intensifies, desertification has all too often been an inevitable consequence of initial agricultural success. The extensive use of irrigation by one of the earliest civilizations, the Sumerians of southern Mesopotamia (third millennium BC), is thought to have led to salination and their eventual decline. Many areas which once supported flourishing civilizations with the early development of agriculture have now either been abandoned or become agriculturally impoverished. In some cases this may have been a result of climate change, but more frequently it is as a result of landscape degradation to an extent that the region has become marginal for successful crop production.

Similarly, the introduction of Neolithic agriculture to hyperoceanic regions such as Ireland, western Scotland and Iceland, resulted in excessive soil leaching, podzolization, peat growth and consequent failure of tree regeneration. Landscape deterioration by human over exploitation in a land with a highly oceanic environment has particularly been the case in Iceland. The extensive deforestation and overgrazing that occurred from when Iceland was first populated (c. AD 874, the landnam) have been suggested as the primary external force causing soil erosion and land degradation. The ability in oceanic environments to winter livestock out of doors, together with an absence of shepherding, may have contributed more to land degradation rather than absolute numbers (Simpson et al., 2001). The sum total of all these changes is environmental impoverishment, both for the agricultural community and natural ecosystems (see Chapter 10). The large-scale abandonment of the early upland sites mentioned above may have been due as much to soil impoverishment as to climatic deterioration (Fig. 1.21).

Fig. 1.21 A satellite image of Iceland — a largely treeless land as a result of over grazing. Note the marked erosion patterns around river courses in the mountain areas in the northern part of the island (see also Fig. 11.30). (Photo Dr E. G. Duncan and reproduced with permission from Group for Earth Observation.)

Fig. 1.21 A satellite image of Iceland — a largely treeless land as a result of over grazing. Note the marked erosion patterns around river courses in the mountain areas in the northern part of the island (see also Fig. 11.30). (Photo Dr E. G. Duncan and reproduced with permission from Group for Earth Observation.)

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