Three centuries of land cover changes in the largest French Atlantic wetland provide new insights for wetland conservation

Grassland to cropland conversion has been a major land cover change in wetlands during recent decades. Many conservation strategies have therefore focused on the protection or restoration of grasslands considered the remnant fragments of nature in wetlands before agricultural development. Through the example of the largest French Atlantic wetland (the Marais Poitevin) and using high-resolution historical records documenting landscape changes since the beginning of the 18th century, we propose the first long-term analysis of a wetland conversion at a large spatial scale. We demonstrate that, although the last land cover changes corresponded to significant grassland to cropland conversions, the mid-20th century was characterized by an exceptional grassland cover (90% of the area). We also show that current surface areas covered by grasslands are similar to those of the 18th and 19th centuries, and that grassland to cropland conversion followed a massive decline in the most natural habitats of the wetland such as marshes, salt-marshes and other marine habitats. From a long-term perspective, grassland to cropland conversion is rather a recent change and grasslands can be viewed as the result of agricultural development in this wetland rather than pristine habitats. In contrast, marshes, salt-marshes and other marine habitats can be considered the remnant fragments of nature of conservation interest, although they are neglected in current conservation plans. We discuss the problem of the time-span and the reference state to be considered when analyzing land cover changes and assessing wetland conservation issues at large spatial scales.