Searching for ancient forests: A 2000 year history of land use in northeastern French forests deduced from the pollen compositions of closed depressions
Résumé
Evidence of the agricultural use, during Roman or Medieval times, of forested areas formerly considered to be ancient, as well as legacies of this former land use on plant biodiversity and soil properties, have encouraged the search for archives of former land use in forests. In central Lorraine (northeastern France), thousands of small closed depressions (CD) on marlstone have been inventoried in forests over the past 150 years, and we hypothesised that these CDs could be used to reconstruct patterns of land use. Closed depressions near the Seille and Sarre valleys were selected and cored for pollen and sediment analyses. Principal Components Analysis (PCA) was used to analyse variations of pollen assemblages during the last two millennia. The history of vegetation changes depicts five main phases. During the Roman period, the region appears to have been primarily covered by grasslands, with some croplands but few forests. All areas were reforested by the end of the Roman period. During the early Medieval period, croplands with grasslands developed in the region, while the late Medieval was characterised by cereal cropping, with especially intense use at sites near the Seille valley, and a lesser extension of grasslands. The present forest cover developed over the past 500 years because of the development of the salt industry in the Seille valley, which required firewood, and the general decrease of agricultural pressure over the past 150 years. Previous investigations had provided evidence of large-scale Roman field systems in the forests covering the limestone plateau and the Vosges foothills on sandstone, areas west and east of the research focus, respectively. These convergent findings suggest that that forests considered to be 'ancient' on the basis of historical documents may have been used for pasture or agriculture over extensive periods during the last 2000 years.