The status of transitions between cultivated fields and their boundaries : ecotones, ecoclines or edge effects?

Abstract Since the beginning of the 20th century, the concepts of ecotones, ecoclines and edge effects have been discussed from a theoretical point of view. However, there have been very few experimental tests of these ideas, which are sometimes radically different. This study presents data from field experimental researches and determines the status of transitions between cereal fields and grazed grasslands. Five study sites were chosen in Southern France because they were included in agri-environmental schemes aimed at conserving arable weeds and dry grassland species. In total, 128 quadrats of 1 m2 were sampled on replicated transects running through transition zones. There was no significant increase of species-richness but there were changes in the botanical composition from cereal fields to grassland. These experimental results confirmed the opinion of Van der Maarel, E. (1990. Ecotones and ecoclines are different. J. Veg. Sci. 1, 135–138) that an ecotone is poorer in species than the adjacent ecosystems, as only a few species can adapt to the typical environmental factors in this zone. The transition zones studied rather reflected an edge effect than a real ecotone following the definition of Odum, E.P. (1971. Fundamentals of Ecology, 3e ed. W.B. Saunders Company, Philadelphie). In our case, when the transition zone between the two adjacent ecosystems is managed as a “constraint ecotone” following Vanpeene-Bruhier's, S. (1998. Transformations des paysages et dynamique de la biodiversite vegetale. Les ecotones, un concept cle pour l'etude des vegetations post-culturales. L'exemple de la commune d'Aussois (Savoie). These de Doctorat de l'ENGREF; CEMAGREF de Grenoble) definition, sheep grazing allowed the weed flora to colonise grassland boundaries via the gaps created by livestock trampling. These results are then discussed for the biological conservation of threatened arable weeds in agricultural landscapes.

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