Abstract
Abstract. The large variety of atmospheric circulation systems affecting the eastern Asian climate is reflected by the complex Asian vegetation distribution. Particularly in the transition zones of these circulation systems, vegetation is supposed to be very sensitive to climate change. Since proxy records are scarce, hitherto a mechanistic understanding of the past spatio-temporal climate–vegetation relationship is lacking. To assess the Holocene vegetation change and to obtain an ensemble of potential mid-Holocene biome distributions for eastern Asia, we forced the diagnostic biome model BIOME4 with climate anomalies of different transient Holocene climate simulations performed in coupled atmosphere–ocean(–vegetation) models. The simulated biome changes are compared with pollen-based biome records for different key regions.In all simulations, substantial biome shifts during the last 6000 years are confined to the high northern latitudes and the monsoon–westerly wind transition zone, but the temporal evolution and amplitude of change strongly depend on the climate forcing. Large parts of the southern tundra are replaced by taiga during the mid-Holocene due to a warmer growing season and the boreal treeline in northern Asia is shifted northward by approx. 4° in the ensemble mean, ranging from 1.5 to 6° in the individual simulations, respectively. This simulated treeline shift is in agreement with pollen-based reconstructions from northern Siberia. The desert fraction in the transition zone is reduced by 21 % during the mid-Holocene compared to pre-industrial due to enhanced precipitation. The desert–steppe margin is shifted westward by 5° (1–9° in the individual simulations). The forest biomes are expanded north-westward by 2°, ranging from 0 to 4° in the single simulations. These results corroborate pollen-based reconstructions indicating an extended forest area in north-central China during the mid-Holocene. According to the model, the forest-to-non-forest and steppe-to-desert changes in the climate transition zones are spatially not uniform and not linear since the mid-Holocene.
Highlights
Eastern Asia ranges from the tropical to the polar climate zone and is affected by three major atmospheric circulation systems, i.e. the Indian monsoon system, the East Asian monsoon system and the westerly wind circulation (Fig. 1)
In the two transition zones, i.e. the temperature-limited taiga– tundra margin and the moisture-limited forest–steppe–desert transition area, vegetation is supposed to be very sensitive to climate change and strong feedbacks are expected in case of climate and vegetation shifts due to large environmental and biophysical changes (e.g. Feng et al, 2006, and references therein)
The climate change in PLASIM is too weak to induce a shift of the forest–steppe border in the southern part of the transition zone, north of 41◦ the steppe expands to the east
Summary
Eastern Asia ranges from the tropical to the polar climate zone and is affected by three major atmospheric circulation systems, i.e. the Indian monsoon system, the East Asian monsoon system and the westerly wind circulation (Fig. 1). The warm tropical climate regions and large parts of eastern China are affected by the monsoon systems leading to high annual rainfall rates that support the growing of tropical and warm temperate forests (Ramankutty and Foley, 1999). Rodwell and Hoskins, 1996; Duan and Wu, 2005) With this transition from monsoonal influenced climate to dry (westerly wind dominated) climate, a transition zone of forest to steppe and steppe to desert forms in central East Asia. In the two transition zones, i.e. the temperature-limited taiga– tundra margin and the moisture-limited forest–steppe–desert transition area, vegetation is supposed to be very sensitive to climate change and strong feedbacks are expected in case of climate and vegetation shifts due to large environmental and biophysical changes (e.g. Feng et al, 2006, and references therein)
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