River dynamics and floodplain vegetation and their alterations due to human impact
In Europe the human impact on river ecosystems is very old. Already in the late days of the Stone Age the run-off of the large lowland rivers, those which have their main catchment area in the Low Montains, was influenced by the change of landscape in the catchment area (Becker 1982, Litt 1992, Schellmann 1991). A great change in the structure of lowland river plains took place when during the Roman Times agriculture increased. The sedimentation of floodplain loam in the valley of the Weser, for example, is due to the increase in farming in the broad catchment areas, with their loamcovered hills. Before that time the floodplain of the Weser was dominated by gravel soil and the character of the whole area was totally different (Strautz 1962). One assumes that already a hundred years ago in the upper reaches of the Danube the floodplains near the river had been cleared and used for agriculture (Konold 1993). Consequently the strong human impact on flora and fauna of the lowland river plains must have started long before the major civil engineering measures had taken place in the 18th century. Because of the lack of any bigger naturally conservated lowland rivers in Europe the reconstruction of their initial forms is difficult. In contrast major intrusions on the natural water and bedload regime of the alpine rivers first started in the Middle Ages. However, more serious changes in the catchment area were caused by the increasing number of settlers in the alpine valleys. Since the 19th century many alpine floodplains have been fundamentally changed by consequent civil engineering measures. Despite these intensive measures there are still some river sections in the Alps which still show natural conditions (Martinet & Dubost 1992, Müller 1991a). Alpine floodplains are particularly suitable to answer basic questions on the function of floodplain ecosystems because they are the last still almost intact floodplains in Europe. Due to different investigations on the situation of floodplain vegetation before and after civil engineering measures (e.g. Müller et al. 1992, Schauer 1984a, Seibert & Zielonkowski 1972) they offer good prerequisites to understand the changes of floodplains influenced by man. The structure of the floodplain vegetation is the result of an interaction between abiotic and biotic factors, the cycle of life of species and the historic events. This research intends to show these interactions in natural floodplain ecosystems and to describe the impact of human influence. In particular the following questions are to be answered: Natural floodplains and river dynamics - how do they interact respectively depend on each other? Which strategies have plants developed to adapt to the special site-factors in natural foodplains? Which influence does river dynamics have on the structure of floodplain vegetation? The second part of this research deals with the impact of man on floodplain ecosystems. With the help of alpine floodplains the following topics are to be emphasized: The impact of human influence on river dynamics. Longterm changes in the floodplain vegetation due to the change in river dynamics. Reasons for the change in flora and fauna caused by the influence of man. The results presented are mainly based on investigations made at the large northern alpine rivers which flow into the Danube (Müller 1995) and the largest yet existing natural alpine river landscape - the Tagliamento in the southern Alps (Müller 1993, Müller et al. 1994; comp. Figs. 1 and 2). Basic recommendations for the conservation of nature and for restoration measures in floodplains have been concluded from the results.