Fig. 4.1.4. Development of the vegetation In Ice-free central Europe during the Quaternary. Simplified pollen diagram from the end of the Tertiary to the postglacial period with the percentage of a tree species of the Tertiary (e.g. of the genera Sequoia, Taxodium, Liquidambar; b thermophilic tree species (e.g. of the genera Quercus, Castanea, Corylus); c woody species of moist to wet sites (e.g. of the genera Alnus, Carya, Vitus); d conifers of different genera; e Erlcales; f grasses and herbs. (After Frenzel 1968, from Strasburger and Sltte 1998)

southern latitudes. As herbaceous plants, grasses and low-growing dwarf shrubs expand faster because of their shorter time of regeneration, there are far fewer losses of such species than of trees. In North America no mountain barriers blocked the way for warmth-loving plants that wanted to move south. According to Ellenberg (1978), a comparison of North America and Europe shows about the same number of herbaceous species and grasses, but the ratio of herbaceous species and trees is 124 to 53, respectively.

Some thermophilic species were able to occupy refuges and survived to the present, but their original closed area was separated into several disjunct areas. A disjunct distribution is shown by Rhododendron ponticum with present-day occurrence in southwest Spain, in the Pontic region and in the Lebanon, or by cedars, with some species in the North African mountains, in the Taurus and in Lebanon. Present-day growth areas of arcto-tertiary relics played an important role as refuges for forests during cold periods.

During cold periods, only very few species successfully remained in mountains, surrounded by glaciers, on the Nunatakers rising out of the ice. These are mainly mosses and lichens but also some high Alpine flowering plants. The area expansion of some species towards the south, beyond the equator, is also attributed to the changed climate during cold periods. There are several migratory paths - the East African rift valley, the American Cordillieras and the bridge southeast Asia - New Guinea - Australia, where these plants found places to grow and even today they occur in bipolar distribution regions. Examples are the genera Carex, Erica, Epilobium and Empetrum.

Frenzel's map (1968) of the vegetation in Europe during the peak of the Würm glaciation (Fig. 4.1.5) shows the shift of vegetation zones towards the south, compared to the present day. Island-like remains of oceanic summer green Mediterranean deciduous mixed forests occurred on Mediterranean islands. The Mediterranean sclerophyllic vegetation was limited to the present arid area of North Africa. Large parts of the Iberian and Balkan peninsulas were covered by forest steppes. The regions of central Europe between the edges of the ice sheet were almost tree free.

The vegetation of tundras and cold steppes of that time was reconstructed from plant remains deposited in lakes. The indicator plant of this flora is the arctic alpine Dryas octopetala which is now found on the species-rich grassland that occurs on limestone with a very shallow soil. This plant occurs together with species which occupy similar sites, e.g. Polygonum viviparum, Silene acaulis, Oxyria digyna, Tofieldia calycula-ta, as well as numerous mosses and dwarf and espalier shrubs such as Loiseleuria procumbens,

Ice shield Loess tundra

Mixed maritime forest

Ice shield Loess tundra

Tundra with trees and shrubs, and forest steppe-forest tundra

Mixed maritime forest

Dwarf shrub tundra

Sub-Arctic cold steppe (without loess)

^ Forest steppe

Fig. 4.1.5. Distribution of vegetation types in Europe during the time of maximal glaciation in the Würm glacial period. (After Frenzel 1968, from Kreeb 1983)

Salix herbacea, Empetrum nigrum and Betula nana. At locations further away from the edges of the ice sheets in eastern Europe, under continental climatic conditions, the cold steppe expanded with a rich herbaceous flora with Artemisia, Centaurea, Ephedra and Chenopodium species as well as numerous Cyperaceae and Poaceae of the genera Stipa, Festuca, Poa and Agropyron.

During the warm periods, separated by cold periods, an interglacial vegetation developed, similar to the postglacial vegetation discussed in Chapter 4.1.3. In the interglacial period a mixed forest of lime trees, elms, alder, oak and hornbeam established. This forest developed gradually from a forest in which spruce and birch dominated, and which again dominated before the onset of the Würm glaciation.

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