The discussion of the history of vegetation so far has shown two points:
• the enormous development of species taking place dependent on astronomical, tectonic and climatic events as well as on the communities in bioscenoses.
• a distinct spatial separation of the large parts of the earth's crust occurred. The formation of young mountains is almost finished. Now developments over smaller areas are started in which regional floristic, and environmental characteristics of the site play a decisive role.
The history of climate and vegetation during the Holocene in Europe has long been documented in its basic features, particularly because the availability of new methods such as pollen analysis and radiocarbon dating provided supplementary research tools. Almost all plant species found have survived to the present day.
A noticeable warming of the climate occurred about 20,000 years ago, with the retreat of glaciers into the mountains and to the north allowing warmth-demanding plants, particularly trees, to move back into the area of central Europe occupied by dwarf shrub communities and cold steppes. This did not occur continuously or from the same refuges. Climatic variations also occurred in the late and postglacial periods. They can be easily observed from the position of the timber line in the last 20,000 years, which was probably about 1200 m or even 1600 m lower in the Apennine in the last cold period compared with the present. The course of the line shows that a rapid increase in the Allerod period in the middle-late glacial was abruptly interrupted before the timber line reached its present position. This happened in the postglacial boreal. In the following Atlantic period about 6000 years ago the timberline in the Alps was about 400 m higher than today. The situation in the mountains of central Germany was similar and in Scandinavia the timberline was lower. However the forest area extended far into the present-day tundras (Fig. 4.1.6).
The movement of the most important tree species back to the earlier positions was reconstructed by comparison of pollen profiles, so-called iso-pollen maps, and occurred in different ways and at different periods, where the requirements of species for the site, the speed of migration and the position of the cold period refuges are important.
Spruce came from an easterly direction, following the "northern path" from the Urals, south of the present-day northern coasts and along northern borders of the central European mountain chain and the northern borders of the Alps. Figure 4.1.7 shows the temporal sequence of the expansion of spruce along the edges of the Alps. Fir trees, and also almost all deciduous trees (beech), originated from the Mediterranean areas and reached central Europe via the Bur-gundian Gate or the Pannonian Basin.
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