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Dynamique et gestion des pelouses calcaires de Haute-Normandie

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Thierry Dutoit

Summary

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1In Upper-Normandy, chalk grasslands are semi-natural ecosystems with high biological value. Since 1950, the former biodiversity of these ecosystems is threatened by secondary plant successions which occurred in the absence of agricultural exploitation. With a multi-scale approach (landscape, communities), some mechanisms of the successions can be apprehended and the results applied to conservation management of the grasslands.

2The former agricultural practices can be considered as key factors in the determinism of the successional trajectories. In the peri-urban sites, influenced by contrasted agricultural utilisations (cultures, pastures), two types of grasslands can be recorded (short and tall) with differences in scrub colonisation. These variability in the successional dynamic can be explain by the persistent effect in former practices (ploughing), competitive or dispersal strategies of plant species and actual perturbations (grazing).

3The application of various perturbation regimes (ploughing, mowing, grazing, abandonment) on different communities of the ecosystem (phytocenosis, soil macrofauna, seedbank), allows us to a better identification of the variations in diversity between the successional stages. Compared to abandonment, perturbations have a positive effect on specific diversity, nevertheless this one is always higher in post treatment fleeting stage. These perturbations also influence the composistion of seedbanks and soil macrofauna communities, as well as former agricultural practices. Finally, the use of functional groups, based upon plant strategies or vital attributes of species, seem to be more adapted for generalising our results to other spatial scales (sites, regions) than the use of the taxonomic classification.

4Extensive grazing is favourably considered for restoring or maintaining species-rich chalk grasslands. Nevertheless, a lot of herbaceous species have very weak mineral contents (Poaceae) or are refused by the animals (Dicotyledons). In consequence, the land-manager must choice a rotational grazing system with unimproved animals which are able to find their complementary feeding in the leaves and barks of pioneer scrub species.

5The variations observed in secondary plant successions of chalk grasslands have a multi-causal origin which determinism is under the influence of factors with high inertia. Conservation management of the biodiversity of these ecosystems must be conceive in a long-term perspective where the effects of the present practices structure the future ecosystems.

6En couverture :
Claude Monet, La Seine vue des hauteurs de Chantemesle.
Rouen, Musée des Beaux-Arts.

© Presses universitaires de Rouen et du Havre, 1996

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