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Fig. 4.1.7, Migration of spruce (Picea abies) in postglacial times along the margins of the Alps. (After Walter 1986)

Fig. 4.1.7, Migration of spruce (Picea abies) in postglacial times along the margins of the Alps. (After Walter 1986)

Data from pollen analyses, based on the evaluation and comparison of many pollen diagrams, allows the late and postglacial history of vegetation to be divided into several sections (Figs. 4.1.8 and 4.1.9A.B).

In the oldest tundra period, also called the Dryas period after the indicator species Dryas octopetala, a continental steppe vegetation predominated with scattered tree-free tundra islands. In the following warmer Allerod epoch, sparse spruce and birch forests and willow shrubs were able to exist, but these species receded again about 1000 years later. The steppes advanced from the east under the still extremely continental conditions. Only in the early warm period (preboreal and boreal) was the temperate forest able to extend again because conditions were no longer so cold in winter; birch was replaced by hazel and other deciduous trees (first oak and elm, followed by lime and ash). Man in the middle Stone Age about 8000 years ago was surrounded by species rich forest communities dominantly hazel. Residues of tundra occupied the increasingly melting edges of the ice sheet, and steppes were pushed towards the southeast.

The warmest climatic conditions, and the corresponding development of vegetation, were experienced by Neolithic Man in the middle warm

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Fig. 4.1.8. Pollen diagram from the Luttersee (Göttingen, northern Germany, 160 m above sea level, a.s.l.), with pollen zones according to Firbas (F) and Overbeck (O) (after Walter 1986). Scale: sum of tree pollen 100%, without hazel (Corylus)

Fig. 4.1.8. Pollen diagram from the Luttersee (Göttingen, northern Germany, 160 m above sea level, a.s.l.), with pollen zones according to Firbas (F) and Overbeck (O) (after Walter 1986). Scale: sum of tree pollen 100%, without hazel (Corylus)

| Fig. 4.1.9. A Part of the almost natural "Plllermoor" (1600 m a.s.l.) which Is a protected area In the lower Pltztal (Tyrol) and B part of the same raised bog where peat was removed more than 30 years before. Pollen analysis from this bog shows the postglacial development of vegetation of the montane region In the Tyrol. (Photos K. Muller-Hohensteln)

period (Atlantic period) about 6000 to 7000 years ago. In northern Europe, mean annual temperatures, with oceanic conditions during this time of the "climatic optimum", were 2-3 °C higher than present temperatures. Mixed oak forests with deciduous trees requiring good climatic and edaphic conditions and few beech trees were dominant. In the montane areas, fir dominated in the western Alps, spruce in the eastern Alps and pines in the eastern part of central Europe on nutrient-poor sites. Only on particularly dry sites did steppe vegetation still occur with thermophilic species.

Glacial relicts from the cold periods are still maintained in present-day vegetation as islands, e.g. Betula nana in the foothills of the Alps or Aster alpinus and Pulsatilla alpina in the Harz mountains in Germany in tree-free, particularly cold sites (edges of bogs); similarly, species from the warmest periods were also able to hold their own. These xerotherm relicts, submediterranean species (Quercus pubescens, Helianthemum num-mularium) or types from the south Siberian and Pontic area (Stipa div. spec, Adonis vernalis, Li-num flavum) were able to survive after the onset of cooling in the subboreal forest, on sites hostile to growth of trees (extremely shallow sites exposed to the south) and were able to expand again onto anthropogenic tree-free areas (meadows on soils derived from limestone that are shallow, where the site is dry and comparatively warm, but nevertheless rich in species).

Beech only became dominant in the forests of central Europe in the late warm period and took over the role of hornbeam which dominated in the interglacial period and was now predominant further to the east. Beech forms, together with spruce and fir, the montane forests up to the montane belt. Even though beech has only been dominating the forests of central Europe for 30 to 50 tree generations, distinct forest communities were formed everywhere until humans intervened (Ellenberg 1978).

Species which required higher temperatures for their development migrated back into warmer parts and outcompeted the cold period vegetation, except for a few relict species. Their expansion partly followed the edges of the ice sheet and thus many closed areas became divided and disjunctions were formed. Some are concentrated on the southern European mountains (e.g. species of the genera Primula, Saxífraga, Gentiana), others form the arcto-alpine disjunctions with interspersed occurrence on the highest parts of the mountains of central Germany (e.g. Nigritella nigra coll., Gnaphalium supinum). Disjunctly distributed, also including the North American region, are Betula nana, Salix herbacea, Dryas octo-petala and the circumpolar Loiseleuria procum-bens and Eriophorum scheuchzeri.

Only in recent decades precise dates of the history and climate of the African dry areas have been collected (Pachur 1987). According to these findings, the long-established views of a synchronous sequence of cold and warm periods in moderate latitudes and humid (pluvial) and dry periods in present dry areas must be corrected. During the last cold period (evidence for a period of ca. 20,000 years before the present day), an even shift to the south must be assumed. The Sahara could have been "green" and rain forest regions became narrower, at least from the north (Lauer and Frankenberg 1979). Mountains in the Sahara possessed a strongly Mediterranean vegetation similar to that in the later humid period during the climate optimum. Pachur (1987) speaks of a semiarid climate in the east African region.

During the climate optimum, with semiarid conditions and summer rain in the south and winter rain in the north, the Sahara was reduced to a small region from both sides. Neumann (1988) assumes on the basis of charcoal analyses that the Sahelian vegetation with species such as Balanites aegyptiaca, Acacia albida and Cadaba farinosa established up to 600 km further north in sparse dry woodlands. The southwest shift of Mediterranean influences is regarded as far smaller. After the moister conditions around 8000 years ago, Neolithic Man in the Sahara experienced increasing drought from 5000 years ago until now (Fig. 4.1.10 A-C). The tropical savannah reached its actual position again 3300 years ago. The rock relief of "crying cows" of Terarart in front of a former drinking place near Djanet in southern Algeria is taken as a symbol of this drying. For the Sahara, even today, two relatively stable and interchanging vegetational states are assumed, a "green" Sahara and a "red" Sahara, which is bare of vegetation (Claussen 2001).

Quaternary climate change was also effective in the tropics of South America. In areas near the equator the forest line fell in the high glacial period and rose again in the postglacial optimum. At the edges of the tropics a horizontal shift of vegetation limits occurred (Lauer 1986).

| Fig. 4.1.10. A, B and C Rock paintings and engravings indicate that there were once more humid climatic conditions in the central Saharan mountains. Pictures of hunters (A) and large grazing herds (B) near Sefar (Tassili), southern Algeria, are nearly 8000 years old. The "crying cows from Terarart" (C) near Djanet are about 5000 years old and indicate the gradual increase in aridity. (Photos K. Muller-Hohenstein)

In the tropical lowlands of South America the expanded moist forests shrank in the high glacial period and allowed a stronger floral and faunal exchange between the dry area taxa than was previously possible. Moist forest areas became more isolated. After the favourable conditions of the climate optimum the timber line fell again, however very quickly because of human influence. Small Polylepsis stands are witness to this; today they are found far above the actual tree limit. In lowlands, moist forests expanded initially. The long uninterrupted period of development assumed in the history of rain forests cannot be confirmed for the Neotropis. Because of increasing drought in the late Holocene and again through human intervention, areas of most forests have decreased considerably.

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