Flora, vegetation and history of the porphyry landscape between Halle and Wettin (Saxony-anhalt, Germany).
The landscape of the porphyry outcrops in the lower valley of the river Saale near Halle is a part of a Fauna-Flora-Habitat area due to its rich biotic inventory. The peculiarity of this area is the more than 200 porphyry outcrops which are embedded in an intensively used as well as at present partly unused landscape. The outcrops are differing in size and time of their origin. Geobotanically, the vegetation is characterized by an overlapping of (sub) continental, (sub)mediterranen, subaltlantic, as well as widespread Central-European species. The main areas of the occurrence of these floristic elements and their characteristic vegetation types are situated outside of central Germany. They represent outposts of an extrazonal vegetation in our region. The vegetation of the porphyry outcrops forms a mosaic of strongly contrasting units. Depending on edaphically and climatic conditions there is a change of sand and porphyry pioneer communities (Thymo-Festucetum cinereae), acidophilic dwarf-shrub heaths (Euphorbio-Callunetum), silicate grasslands (Galio-Agrostidetum (tenuis), Filipendulo vulgaris-Helictotrichetum pratensis), continental dry and semi-dry grasslands (Festuco valesiacae-Stipetum capillatae, Festuco rupicolae-Brachypodietum pinnati, Festuca rupicola-community), half ruderal grasslands (Falcario-Agropyretum repentis, Convolvulo-Agropyretum repentis, Poa angustifolia-community), meadows (Arrhenatheretum elatioris), ruderal communities (Sisymbrio-Atriplicetum oblongifoliae), shrubs and tree stands. Altogether we found 50 plant communities and more then 370 plant species on the porphyry outcrops. On the base of our floristical and phytosociogical data set we made different analyses: The island like structure of the landscape allows to check the validity of the island theory of MacArthur & Wilson (1967). It shows that the outcrops have a high degree of isolation as well as a strong relationship between area size and number of species and communities. The rare and endangered species and plant communities grow mostly on the older and large outcrops, because there is a higher heterogeneity of edaphic and microclimatic conditions. However, on the smaller and younger outcrops the amount of ruderal species and communities is very high. There is a high number of valuable plants and xerothermic communities on the larger and older outcrops. A lot of these plants belong to the relict species of the ice age, thus one can find species which have an indicator value for historical old xerothermic grasslands. The invasion of alien plant species into the natural vegetation is a worldwide phenomenon today. Reasons therefore are expanding world trade, tourism, destruction of the landscape as well as the global climatic change. It was interesting for us to find out which establishment chance do these adventive plants have (archaeophytes and neophytes) in the semi-natural grassland vegetation. The number of adventive plants increased within the last four decades. The degree of the invasion of alien plants depends on the structure of the vegetation cover as well as the degree of hemeroby or naturalness of the plant communities. The tolerance against invasive plants is as long as the communities are in equilibrium with the determining abiotic and biotic factors of the location. An indirect comparison of the vegetation with historical releveés from 1950 und 1960 shows an increase of the ruderal species, which is also depending on the vegetation structure of the different communities. Despite of some change of the spectra of plant species, the xerothermic communities are very stable. The reasons therefore are the extreme climatic and edaphic conditions. The end of land use by sheep grazing at the beginning of the 1990ies causes a high danger with respect to a total change of the vegetation structure as a consequence of succession processes. A direct comparison of the vegetation cover on the small porphyry outcrops (<500 m2) within 8 years shows a very high dynamic. The lack of habitat differentiation and the influence of the surrounding agrarian land play a very important role. Buffer zones are missing and thus the edge-effects have a very strong influence. The numbers of species as well as the change within the plant communities are strongly influenced by stochastic effects. We validated the results of our earlier findings by means of modem multivariate methods (cluster analyse, correspondence analyse). The investigation of the seed banks of the different plant communities show the following results: Both the number of species and the number of seedlings strongly varied between the plant communities and gained with higher degree of hemeroby. The seed banks showed a seasonal rhythm. The similarity between the seed bank and the vegetation cover is low (20 to 40%). Species which are characterized by a long-term persistent seed bank are most common in the seed bank whereas most of the xerothermic species are characterized by a transient seed bank. The number of species with transient seed bank type decreased in the communities with increasing degree of hemeroby, and the species with long-term persistent seed bank type gained. The reproductive potential of the seed banks of xerothermic vegetation with a higher degree of nativeness was low, and the seed bank of grassland communities on historically old sites was species-poor. The renewal and the re-establishment of the species-rich xerothermic plant communities are a long-term and difficult process on which should be paid more attention in management practice.