Comparison of modeled and reconstructed changes in forest cover through the past 8000 years: Eurasian perspective

Reproducing the tree cover changes throughout the Holocene is a challenge for land surface-atmosphere models. Here, results of a transient Holocene simulation of the coupled climate-carbon cycle model, CLIMBER2-LPJ, driven by changes in orbital forcing, are compared with pollen data and pollen-based reconstructions for several regions of Eurasia in terms of changes in tree fraction. The decline in tree fraction in the high latitudes suggested by data and model simulations is driven by a decrease in summer temperature over the Holocene. The cooler and drier trend at the eastern side of the Eurasian continent, in Mongolia and China, also led to a decrease in tree cover in both model and data. In contrast, the Holocene trend towards a cooler climate in the continental interior (Kazakhstan) is accompanied by an increase in woody cover. There a relatively small reduction in precipitation was likely compensated by lower evapotranspiration in comparison to the monsoon-affected regions. In general the model-data comparison demonstrates that climate-driven changes during the Holocene result in a non-homogeneous pattern of tree cover change across the Eurasian continent. For the Eifel region in Germany, the model suggests a relatively moist and cool climate and dense tree cover. The Holzmaar pollen record agrees with the model for the intervals 8-3 ka and 1.7-1.3 ka BP, but suggests great reduction of the tree cover 3-2 ka and after 1.3 ka BP, when highly developed settlements and agriculture spread in the region.