Structure of plant communities and landscape patterns in alluvial meadows of two flood plains in the north-east of France

Flood frequency and agricultural pressure can effect pattern and diversity in the plant communities and the landscape of flood plain meadows. The flood plains of north-east France are valuable semi- natural ecosystems with a high diversity of plants. This study was carried out in two valleys with plant communities showing the same zonation along a moisture gradient. About 350 measurements in each valley were carried out on 50 m2 sampling sites. Two study areas were intensively measured within each of the two valleys (1300 ha in total). Hydrological, geological and human factors have determined the unique landscape pattern of each valley. Using vegetation maps (1/5000) of the two valleys, landscape structure in terms of the size, number and form of patches were compared and the characteristics of the disturbance regimes (natural and human disturbance) creating each landscape are analysed. Variations of landscape indices are discussed in relation to the increase in agricultural pressure. Using quantitative parameters of landscape ecology to analyse vegetation mosaics provides an assessment of agricultural pressure and natural constraints on the flood plain scale. Agricultural intensification led to a decrease of meadow complexity whose natural rough shapes are made straight. Moreover flooded meadows lost thus natural connectivity with ditches and river which determined biodiversity and ecological processes of flood plains.