Lineare anthropogene Gehölz- und Saumstrukturen im Bachgau (Gmde. Großostheim, Lkrs.Aschaffenburg)

Abstract

Linear anthropogenic woody structures and their edges in the Bachgau

Hedges and their edges are subject of this doctoral thesis. Starting with a historical analysis, the question was, which types of hedges appeared when and under which circumstances in the Bachgau. Though it is commonly believed that the charcteristic type of hedges in the Bachgau, the spontaneously colonizing hedge on contour-parallel banks between two cultivated plots, always has been there, many objections have to be raised to this opinion. These so called "Terrassenhecken" are to be assessed much younger: There is sound evidence that the "Terrassenhecken" could develop not until the late 18th century, when fundamental changes came up in agriculture and law.

A comparison between air-photos from 1945 and today shows that the total length of Bachgau hedges grew at least by 4,8 km during the past 50 years, despite the fact that many original hedges had been destroyed during farmland-consolidation in the 1950/60s. This increase in length and number of hedges is attributable to shrubs taking possession of shrub-free banks as well as to planting new hedges. As a noticeable result of this recent development, hedges today can be found also in the flat northern section of the Bachgau (lower river Main plain), where - from a historical and genetical view - hedges have never occured so far.

As a basis for further conservational assessing, the vegetation of the Bachgau´s hedges and their edges has been examined in 1997 and 1998. 14 hedge- and 9 edge-societies at different sites like contour-parallel banks between two cultivated plots, defiles and gullies have been described.

The most crucial results from ecological-statistical analysis are listed below:

• hedgerow density (length per area) shows no effect on the number of plant-species in the hedge.
• hedgerow density significantly influences the share of bird-dispersed plant-species of the total number of plant-species. This applies to the Pruno-Ligustretum (r²=0,33*), the forest-like hedges (r²=0,29*) and the hedges along defiles (r²=0,44**).
• no significant correlation was found between the width of a "Terrassenhecke" and its ELLENBERG-nitrogen figure.
• the correlation between the width of the edge and the ELLENBERG-nitrogen figure of the adjacent hedge is significantly negative (r_S=-0,52**).
• the relation between the width of the edge and the share of field-weeds of the plant-covering of the edge is significantly negative (r_S=-0,72***).
• neither the number of plant-species nor SHANNON´s diversity index nor SHANNON´s evenness of edges show significant differences concerning the horizontal position of the edge towards the adjacent hedge.

Because of the edge-effect, the function as a corridor, the different dynamic and physiognomic phases and the appearance of 218 higher plant-species found, hedges and their edges are of great conservation value. Due to enduring threats to hedges from fertilizing adjacent field and applying pesticides, recommendations are made how to protect these biocenoses. It was an aim to reconcile the interests of both farmers and conservationists. Recommendations include cutting hedges - which has not appeared so far in the Bachgau - in order to prevent branches of trees from penetrating and casting a shade upon the crops as well as creating buffer-strips between field and hedge.