Past land-use affects woodland functioning even after complete ecosystem destruction

Following World War One (WWI), destroyed woodlands have re-established, while former croplands became afforested. We studied the long-term effects of pre-WWI land-use and WWI-associated disturbances on two functional attributes of these woodlands: productivity and canopy water content, which were estimated using the Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) from Sentinel-2 images as respective proxies. We surveyed 399 recent (i.e. established after 1918) and 240 ancient (established before ca. 1830) woodlands in North France, which were located either over the former Somme battlefield (red zone: completely destroyed in 1918) or beyond (blue zone: non-impacted by WWI), over 8 growing seasons, from 2017 to 2024. Pre-WWI land-use, WWI-associated disturbances, and covariates (woodland area, tree height, slope, soil clay content, CEC, pH) were calculated using existing databases and historical maps. A generalized linear mixed model was fitted for each response variable (NDVI and NDWI). In the blue zone, both were higher in recent than in ancient woodlands. In the red zone, only NDWI remained higher in recent woodlands, reaching the values recorded in recent woodlands of the blue zone, while NDVI values were similar to those of ancient woodlands of the blue zone. These results indicate that WWI-associated disturbances have homogenized soil properties and introduced organic matter and chemical pollutants in the soil, thereby erasing the legacy of pre-WWI land-use on productivity. However, this legacy is still reflected by the canopy water content, as a likely consequence of different plant composition between recent and recovered ancient woodlands.