Erhalt und Entwicklung naturnaher Bachläufe im Wald (DBU-Projekt)

Research Results
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Main focus of the former project "Gewässerentwicklung im Wald" in 2002/03 was to evaluate water quality, zoobenthos and the fish stocks of eleven fresh water streams, ranging from small to medium size. Aim of the study was to find out the deficits of freshwater habitats in cultivated forest areas. The surveyed rivers have been chosen to represent the geographical zones of Baden-Wuerttemberg.

Water quality in all of the surveyed rivers was II (moderately polluted) or better. There is hardly any anthropogenous nutrient input into these systems, otherwise the pollutants can be broken down by natural mechanisms. All of the rivers have been changed enormously by the construction of bridges, paved and gravel roads. For that reason the structural quality of the rivers and river banks had to be rated as not natural. Drastic deficits have been found for migrating organisms. Concreted tubes and steps in the river bed disconnected the habitat continuum for many fish and crayfish species. As a result potential habitats of high quality cannot be settled and an exchange between isolated populations doesn’t take place. Results from electrofishing show that some river sections cannot be reached by freshwater animals because of artificial obstacles (e.g. from road construction) and natural cascades. The brown trout (Salmo trutta) is the most powerful swimmer and manages to overcome obstacles of 0,6 meters height. The isolation of river sections in the epirhitral (‘salmonid’) zone can have positive effects on species preservation. Urodelans (Salamandra salamandra) and the European Crayfish (Austropotamobius torrentium) were most abundant where pressure form predacious salmonids wasn't present. Small forest rivers and creeks are the last refuge for the European Crayfish, an endangered species threatened by the crayfish disease.

Some of the structures with negative effects on migration must be changed in order to provide sustainable habitat structures with good ecological value. The need for these actions ranges from “urgent” to “medium”, i.e. the necessary actions to improve the habitat structures can take place during maintenance activities along the rivers. Most of the necessary actions do not require a big budget.

The impact of spruce monocultures on the freshwater fauna has also been studied. The results agree with earlier studies where spruce monocultures did not show a strong impact on the zoobenthic community. What could be shown was a shift in the zoobenthos composition towards specialized feeding strategies. Due to the spruce cultures and their shading effect on epilithic algae, the number of grazers declined whereas the numbers of shredders and predators increased. It seems that a well structured zoobenthos community needs only punctual input of leaf material into the system (e.g. by Alnus glutinosa). At this point of our studies there is no reason for immediate actions (clear-cuts) in order to change the accompanying vegetation along river banks from needle- to leaf trees. The change towards natural riparian forests should be achieved within one or two decades.

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Heiko Rinderspacher
Abt. Landespflege

Artikel-Datum: 01.10.2003