Spatio-temporal dynamics of forest ecosystems revealed by the LiDAR-based characterization of medieval field systems (Vosges Mountains, France)

Relics of past agricultural practices, former field systems have strongly imprinted many modern landscapes and have thus significantly disrupted forest ecosystems over the last centuries. Former field systems in the Hautes-Vosges mountain range (north-eastern France) date primarily to the medieval period (6–15th century C.E.) and consist of parcelled or linear structures on hillslopes and valley floors. These residual features fall into three categories: ridge and furrow, terraced slopes, and stone walls. LiDAR (light detection and ranging) can detect microrelief features, such as the topographical imprints of these field systems over extended areas, and thereby establish a new temporal baseline for reconstructing forest changes over relatively long timescales, i.e., before the first historical topographic maps. Here, we digitize former field systems in the south-eastern Vosges from a high-resolution LiDAR-derived DEM to assess their spatial distribution at the mountain-range scale (1185 km²) and in relation to topography. Former field systems cover approx. 6.6 % of the study area (78.5 km²), with terraced slopes (55.5 km²) and stone walls (20.6 km²) covering a greater extent than ridge and furrow (2.4 km²). Former field systems are preferentially located on south-facing slopes above an 800 m a.s.l. threshold; this pattern indicates systematic past agricultural practices across the entire region. We then compare the LiDAR-derived spatial features with a 19th-century map of France and a modern regional land-cover database to derive the spatio-temporal trajectories of landscapes. We observe that former field systems were progressively, but unevenly, abandoned and transformed into grasslands or forests. This mid-19th century abandonment of agricultural fields and their conversion to grassland and forest is highly dependent on slope and elevation (grassland and forest: 18–19° and 610–620 m). These values differ from those associated with agricultural sites that remain under cultivation today (approx. 16° and 550 m). Finally, we demonstrate the relevance of integrating former field systems for characterizing areas of ancient forest. Less than 2 % of the area mapped as forest in the 19th century was cultivated between the 6th and 15th century. Most importantly, our approach quantifies disturbed and undisturbed ancient forest areas at the mountain-range scale. While this study opens new perspectives for accurately assessing the age of forest ecosystems, it also reveals an evolutionary pattern of land-use change in the Hautes-Vosges that is similar to that observed in other European mountainous regions.