Postglacial Environmental Transformation
POSTGLACIAL ENVIRONMENTAL TRANSFORMATION
The Holocene interglacial epoch began around 9500 b.c. with an abrupt warming of the climate across most of Europe. Although interglacial conditions were established rather quickly, it would be wrong to imagine that the natural environments of the Early Holocene were identical to those of the present day or that they have remained static since that time. For example, most regions experienced a climatic thermal optimum between 8000 and 4000 b.c., as indicated by the extension of species, such as the water chestnut and the pond tortoise, north of their present European climatic limits. In addition, several key features of the natural European landscape were not formed until some time after the start of the Holocene. In most coastal regions, for example, recognizably modern shoreline configurations were only achieved around 5000 b.c.
The repeated build up and decay of ice sheets during the Pleistocene had locked up and then released water from the hydrological cycle, causing sea levels to rise and fall. Global sea levels were lowered on average by more than 100 meters when the ice sheets and glaciers were at their peak, creating land bridges that made it possible to walk from the European mainland across to the British Isles. The configuration of the glacial coastline differed most strongly from that of the early twenty-first century in areas with shallow offshore gradients, such as the North Sea. In these areas, land was drowned by rising sea levels between the tenth and sixth millennia b.c. at a rate that must have been noticeable from one year to the next. Human populations had to relocate themselves and their economic activities landward, as is shown by the changing locations of shell middens and other Mesolithic sites related to human habitation of the coastal areas.
In Southeast Europe, the lowered sea level caused the Black Sea to be isolated from the world oceans during glacial times because the Bosphorus Straits that connect them are only about 50 meters deep in the early twenty-first century. By the Early Holocene, world sea levels rose so that they may have become higher than those in the Black Sea, and around 5500 b.c. the two became reconnected. In Noah's Flood, William B. F. Ryan and Walter Pitman have proposed that seawater poured through the Bosphorus in a flood several hundred times greater than the world's largest modern waterfall. If true, human populations around the former Black Sea coast would have found the sea advancing toward them at about a kilometer and a half every day. Their memory of this possibly catastrophic event may provide the basis for the flood legend of the Sumerian Epic of Gilgamesh, which later came to be incorporated in the story of Noah in the Old Testament of the Bible.
Rising Early Holocene sea levels led to river valleys being drowned throughout Europe's coastal zones, with the end of the Early Holocene representing the time of maximum marine incursion inland. Since then, stabilized sea levels and river-derived siltation have led to a reversal in this trend, with the land pushing seaward at the mouths of major rivers, such as the Rhône. This process has left many ancient harbor cities, particularly around the Mediterranean, stranded several miles inland from the coast during modern times. It should be noted that a different trend was experienced during the Holocene in some high-latitude regions, such as the northern part of the Baltic Sea. There the land lifted after the ice sheets melted, which forced land formations farther above the water than they had been previously.
PLANT AND ANIMAL RESOURCES
Prior to 9500 b.c., Europe north of the Mediterranean had been largely covered by tundra-steppe and boreal forest, and it had supported large herds of reindeer, wild horses, and other herbivores. However, in the subsequent two millennia, new tree species moved in, so by 7000 b.c., the dominant vegetation type had become mixed deciduous forest. With it came new woodland animals, such as red deer, aurochs (wild ox), and wild boar. As targets of human exploitation, these animals were more dispersed and less visible in the forests than had been the concentrated and easily culled fauna of the late-glacial tundra. Yet the mixed deciduous woodland contained hundreds of potentially edible plant species, ranging from hazelnuts through berries and fruit to fungi and bracken rhizomes.
Although the distribution of vegetation types had become essentially modern by 7000 b.c., their species composition continued to change. This can be seen from many pollen diagrams in which the characteristic feature is the continued arrival and rise to dominance of new woodland plant classifications. After the pioneer woods of birch and pine, the first deciduous trees to arrive in Northwest Europe were hazel and elm. Later arrivals included oak, lime, alder, and ash. Yet other trees—for example, beech—did not achieve their maximum extents until the declining stage of the Holocene, and some trees, such as spruce, may still be expanding their ranges. The Early Holocene forests formed an almost continuous blanket across most of the central and northern European lowlands during Mesolithic times. The hunter-forager communities in those areas preferred to locate their settlements in places where there were fewer trees. These included sites in forest clearings, along the seacoast, next to rivers and wetlands, and at higher elevations close to the upper limit for tree growth. A good example is the site of Star Carr in northern England, which was the scene of pioneering archaeological investigations between 1949 and 1951 by Grahame Clark. His were among the first excavations to move beyond the study of stone tools to also include an examination of site economy and environment, which are revealed by bones, seeds, and pollen grains. Star Carr comprised a platform made of birch at the swampy edge of a lake, now filled. The waterlogged conditions are responsible for the excellent preservation of organic remains at the site. Wetlands such as this were rich in natural resources, including waterfowl, fish, and edible water plants, such as cress and water lily.
The seasonal rhythm of plant growth and animal movement in temperate woodland ecosystems strongly influenced the food schedules and lifestyles of Mesolithic hunter-forager groups. But people were already capable of modifying natural environments to suit their needs. For example, selective burning of vegetation is a traditional technique of environmental management that has been practiced by hunters and pastoralists for many millennia. The new vegetation growth after a fire increases grazing and browsing potential, and the number of deer or wild cattle that can be supported responds accordingly. Charcoal provides one of the best palaeoecological indications of past fire frequencies. Charcoal fragments in soil and peat profiles suggest that recurrent burning of upland vegetation took place during the Late Mesolithic in Europe. Hazel, which sprouts new growth in response to burning, is much more abundant in the early part of the Holocene than in any previous interglacial period—possibly an indirect result of Mesolithic use of fire.
THE IMPACT OF THE FIRST FARMERS
The advent of Neolithic agriculture brought greater potential for modifying natural environments and put humans into sharper conflict with nondomesticated species. In the long run, this has meant that predators, such as the wolf and the bear, are now rare across western and central Europe, whereas wild competitors, such as the aurochs, are now extinct. Decline in some nondomesticated animal populations is partly the result of hunting but more importantly due to habitat loss, given that farming requires at least partial clearance of the existing vegetation cover. Early agriculture is also associated with the first substantial human impact upon the soil, an impact all the more permanent because of agriculture's association with a settled, or sedentary, way of life.
Between 7000 and 3500 b.c., Neolithic farming spread across Europe from the Near East, primarily northwestward along the Danube-Rhine axis. Neolithic farmers appear to have initially exploited only a small portion of the total landscape, selecting those particular habitats—notably alluvial and loess soils—best suited to their needs. Sites in the western Mediterranean and parts of northern Europe (e.g., those of the Ertebo⁄lle culture in Denmark) have shown evidence of transitional economies, indicating that, in those locations, agriculture may have been gradually adopted by preexisting Mesolithic populations. Evidence for the impact of Neolithic farmers upon European wildwoods was first recognized by Johannes Iversen in the form of clearance, or landnám, phases in pollen diagrams. There are three principal landnám phases:
- an initial clearance stage, in which tree pollen declined relative to herb and grass pollen;
- a farming stage, in which grasses, including cereal-type and weedy species, reached a maximum;
- a regeneration stage, in which shrubs, such as hazel, increased before declining as more substantial trees replace them.
Clearance phases are also sometimes associated with a rise in the frequency of charcoal, suggesting that fire was employed in a "slash and burn" manner.
The effect of Neolithic clearance on the overall woodland cover was initially rather small, although more significant changes did take place in the composition of the natural vegetation. One of the species affected was the elm tree, and a sharp and usually permanent decline in the number of elm trees occurred during Neolithic times. Although the direct cause of this decline was most likely a catastrophic disease outbreak similar to the modern Dutch elm disease, the lack of subsequent recovery of the tree population is likely to have been linked to increasing human disturbance of forest ecosystems. Another group that responded to Neolithic agriculture was weeds. Species such as ribwort plantain, stinging nettle, docks, sorrels, and grasses appear with increasing regularity in post-Mesolithic pollen diagrams. These plants thrive on disturbed ground, and they exploited humans for their dispersal and have remained a familiar part of European agricultural landscapes ever since.
CULTURAL LANDSCAPE CHANGE IN BARBARIAN EUROPE
Neolithic peasant farming societies started the long process of clearing Europe's forests to make way for farms, fields, and pastures. From Julius Caesar's description in his De bello Gallico that "the population is exceedingly large, the ground thickly studded with homesteads," it certainly appears that, in France and lowland Britain, the landscapes were already largely agricultural at the time of the Roman conquest in the first century b.c. By medieval times, around a.d. 1000, the removal of the forests was almost complete. At the time of the Domesday survey of a.d. 1086, only 15 percent of England was still wooded, and more than twice that amount of land was devoted to growing crops. It is clear that the vast majority of primary forest clearance in lowland England had taken place before the Norman conquest of the eleventh century a.d.
The so-called barbarian cultures were therefore largely responsible for the transformation of Europe from a natural to a cultural landscape, although the pace and timing of this transformation varied among different regions. In some cases, significant opening of the primeval forest took place during Neolithic times; for example, land snails and pollen from buried soils and ditch fills at Avebury, Silbury Hill, and Stonehenge show that the chalk landscape of southern England had, by the second millennium b.c., already been changed from woodland to open pasture or scrub. In general, however, organized agricultural landscapes were more often created in the Bronze Age or Early Iron Age, particularly during the second and early first millennia b.c. In part of Spain, this was associated with the development of the dehesa system, which uses and conserves oak trees in an open parkland interspersed with cereal cultivation and grazing land, whereas farther north, landscape change is linked to the emergence of proto-Celtic and Celtic societies. These societies became hierarchical and tribal, with a mode of production progressively less dependent on domestic subsistence agriculture. Change was manifest in the landscape in the creation of organized arable field systems and other forms of land allotment as well as in the creation of defensive hillfort settlements. Animals were no longer raised solely for meat but also were used for plowing, transport, and wool and milk production and to provide manure to fertilize the fields.
The Bronze Age saw an important extension of settlement into many upland regions, such as the Alps. A good example of this process is provided by Dartmoor in Southwest England, where large parts of the Bronze Age landscape have been preserved. Archaeological remains include low stone walls—or reaves—that are linked to the wider system of prehistoric land boundaries that cover the whole of Dartmoor. The Bronze Age economy was based on pastoralism, and the round stone farm dwellings in this area may have only been occupied seasonally as part of a transhumant pattern of land occupation, where livestock was moved between different areas. Pollen diagrams from peat deposits and buried soils record prehistoric woodland clearance and the inadvertent creation of acid moorland with podzolic and gley soils.
Late Holocene woodland clearance often had permanent consequences for soil resources. In some regions the fertile but superficial cover of loess—a wind-blown silt that had been deposited during glacial times—was eroded to leave skeletal, calcareous soils, the eroded soil having "sludged" downhill to form extensive colluvial deposits at the bases of slopes. Some of this eroded soil material was moved into river systems, which led to the widespread accretion of fine-grained floodplain alluvium in lowland rivers of northern Europe after 1000 b.c. At Braeroddach Loch in Scotland, soil erosion and consequent influx of sediment increased in a series of steps through time, starting with the arrival of Neolithic agriculture. In this lake catchment, soil losses under agricultural land use represent a thirty-fold increase compared with that under Early Holocene forest cover. Without doubt, land degradation in Northwest Europe has been related to increasing population growth and agrarian pressure. An extreme example of irreversible environmental change is provided by the limestone plateau of the Burren in western Ireland. The Burren's thin soil cover, which had been able to support pine, yew, and birch forests during much of the Holocene, was almost totally eroded down karstic fissures during the Late Holocene. All that is left is bare limestone pavement incongruously criss-crossed by Celtic fields with no soil inside them (fig. 1).
In many areas, such as the North European plain of Germany and Poland, the post-Roman period witnessed a decline in population, and pollen diagrams show that woodland regeneration took place. Yet the basic pattern of land occupation established in the pre-Roman Iron Age was often not greatly altered. And toward the margins of permanent settlement in northern Europe, as in Scandinavia, the first millennium a.d. was a formative period of landscape change. This is well illustrated in the Ystad Project, in which archaeologists, historical geographers, and palaeoecologists worked together to establish an integrated regional history of Holocene landscape change in an area of southern Sweden. The post-Roman period also saw the introduction of some new crops, such as rye and hemp-hop.
CLIMATIC CHANGES DURING LATER PREHISTORY
Although much less marked than during the Early Holocene, the period between 4000 b.c. and a.d. 1000 nonetheless experienced some significant shifts in climate. Notable among these was a progressive cooling following the Holocene thermal optimum. A range of biotic temperature indicators, including diatom algae, cladocera (microcrustaceans), pollen, and midge larvae, have been analyzed from lake sediment cores taken in various parts of boreal and mid-latitude Europe. Some of these records show cooling to 2–3°C (4–6°F) below modern values during the later third millennium and second millennium b.c., after which the climate recovered to modern values. Another climatic deterioration from warmer and drier to cooler and wetter conditions took place at the Subboreal-Subatlantic transition, a change dated in European peat bogs to around 600 b.c. At this humification feature, known as the Grenzhorizont (boundary horizon), dark, oxidized peat, typical of slow-growing mires and often including buried tree stumps, was replaced by relatively undecomposed sphagnum peat typical of wetter, fast-growing mires. The water balance of oceanic bogs in northwestern Europe reflects both temperature and precipitation effects, but the evidence favors temperature as the main forcing factor. Periods of wetter bog surfaces most probably reflect declining summer temperatures that, in turn, impacted evapo-transpiration.
High-latitude Europe has been intensely studied in terms of Holocene climate variability. This is because it possesses many natural climate archives with high temporal resolution, such as tree rings and varved lake sediments, and also because these northern regions were relatively little affected by human landscape disturbance. Tree-ring analysis (dendroclimatology) from regions such as Scandinavia and Ireland shows several periods of narrow growth rings that are inferred to have resulted from years of unusually severe climatic conditions. One such series of years occurred in the seventeenth century b.c. and may be linked to climatic cooling following the explosive eruption of the volcanic island of Thera in the Aegean Sea, whereas another took place in the sixth century a.d. Across much of mid-latitude Europe, however, the effect of Late Holocene cooling and warming fluctuations was often disguised by increasing human disturbance of the vegetation cover.
Pollen diagrams from many areas of Barbarian Europe typically record three phases of human land-use activity between 8000 b.c. and a.d. 1000. The first was Mesolithic hunting and gathering under wildwood; the second was small-scale Neolithic-Chalcolithic "peasant" farming within secondary woodland; and the third phase was dominated by agricultural landscapes of fields and farms created under complex, stratified, Bronze Age, Iron Age, and later societies. Because clearance of the original woodland and consequent land degradation have a long antiquity in this corner of the world, European landscapes can only be understood by considering changes in prehistoric and early historic times as well as those in more recent centuries.
See alsoStar Carr (vol. 1, part 1).
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