Holocene history of land cover and land use in northwestern Europe: a new approach in pollen-based reconstruction of regional landscape

Quantitative estimates of land cover changes in the Holocene have become increasingly important for a better understanding of the human impacts on the earth surface-atmosphere feedbacks in the past. The LANDCLIM project and research network (sponsored by the Swedish [VR] and Nordic Research Councils [NordForsk]) aim to estimate the Holocene changes in regional vegetation and land cover north of Alps in western Europe, and to evaluate the effects of the changes in land cover, particularly human land-use, on the regional climate through altered feedback. This report presents the preliminary results on the Holocene land-cover change in the region. The projects use the REVEALS model (Sugita 2007; the Holocene 17, 229-241), which is theoretically derived and empirically tested, to reconstruct quantitatively the regional vegetation and land cover in the spatial resolution of 1o x 1o based on fossil pollen records. The pollen-vegetation relationship is non-linear, influenced by factors such as the size of sedimentary basins and inter-taxonomic differences in pollen productivity and dispersal characteristics. The REVEALS model corrects for biases caused by these factors and provides regional vegetation and land cover in a 104 - 105 km2 scale. Our results show that, in general, the regional landscape openness changed from 10-20% to 40-70% around 2600 cal year BP in many parts of northwestern Europe. This was the time period when land use increased, particularly crop cultivation and cattle grazing. The timing of the dramatic changes was region-specific, however. Pollen percentages, a traditional indicator of land cover changes, always underestimate the unforested areas significantly and tend to overestimate the areas covered by deciduous-tree. Climate models use simplified land-surface classifications, such as grass (i.e. unforested), deciduous, and conifer covers. Therefore, the large discrepancies in past land cover between the REVEALS estimates and pollen percentages are expected to influence model outcomes of the Holocene regional climate in northern Europe, particularly after the human impacts on land cover became significant.