East Asian summer monsoon and topography co-determine the Holocene migration of forest-steppe ecotone in northern China
Introduction
Global climate change has caused a dramatic change in vegetation distribution at an unprecedented rate (Allen and Breshears, 1998; Harrison and Prentice, 2003; Liu et al., 2013). The boundaries between ecosystems, such as the forest-steppe ecotone in the semi-arid region, show extremely high sensitivity to such changes (Allen and Breshears, 1998; Allen et al., 2010; Goldblum and Rigg, 2010; Liu et al., 2013). However, due to long lifespan of trees and limited dispersal of forest species (Matthias et al., 2015), it is difficult to detect changes in the latitudinal ranges of the forest-steppe ecotone in response to recent climate changes (Beckage et al., 2008). The range of the forest-steppe ecotone across the landscape depends on the climate; thus, the long-term shift of the forest-steppe ecotone in the past may provide insights into the response patterns of the forest-steppe ecotone to climate change. The Holocene (11,500 cal. yr BP to the present), a period characterized by natural climate forcing, appears to be an ideal analogue for investigating and predicting the impact of climate change on the shift of the forest-steppe ecotone.
The forest-steppe ecotone in northern China, which is located in the marginal area of the East Asian summer monsoon, is in a critical state threatened by dry climate, where precipitation depends on the strength of the EASM (Xu et al., 2017). Although forest ecosystems are expected to migrate towards higher elevations and polar regions in response to climate warming (e.g., Chen et al., 2011; Seidl et al., 2017), it is still unclear what type of relationship will be exhibited by the forest-steppe ecotone in response to EASM and how strong the relationship will be between long-term EASM forcing and the shift of the forest-steppe ecotone (Beckage et al., 2008). Furthermore, ecotone shifts should be observed on regional or continental scales to detect the vulnerability of these ecotones under dry climate condition, but many empirical studies are severely limited to local scale. For example, there is strong evidence indicating the local vegetation evolution and its relationship with the EASM in northern China (e.g., Xiao et al., 2004; Zhao et al., 2010; Xu et al., 2017), and few studies have evaluated the northern Chinese forest-steppe ecotone as a whole to determine its shifting pattern (Liu et al., 2001; Hao et al., 2016), which is a typical ecologically vulnerable transition from the continental-scale perspective. Therefore, it is necessary to pay close attention to the Holocene shifting pattern of the forest-steppe ecotone on a large spatial scale under the influence of the EASM.
Different from other latitudinal ecotones in the world, the mountains overlay the forest-steppe ecotone in northern China, where topography differences are significant. The diverse mountain environment has shaped unique topography in northern China since the Late Miocene (Zhai et al., 2006; Yin et al., 2013). It is also very important to see whether the forest-steppe ecotone in northern China was three-dimensional considering the topography factor, which has been widely considered to be very sensitive to climate change (e.g., Goldblum and Rigg, 2010; Cheng et al., 2017). Some studies have indicated the influence of topographic factors on vegetation migration in China. For example, at the continental scale, there was a study specifically indicating that the topography mediated the climate-driven Holocene biome migration in western and eastern China (Cheng et al., 2018). At the regional scale, paleoecological evidence has shown that topography strongly affects the vertical migration of forests in the forest-steppe ecotone in northern China (Hao et al., 2016). At the local scale, the resilience of a forest suggests a strong buffering effect of topography on the southeastern Inner Mongolian Plateau of China, which allowed forest ecosystems to survive during the Holocene drought events (Yin et al., 2013). Evidence based on different spatial scales together suggests that topographical factors may affect the shift of forest-steppe ecotones. However, until now, very few studies have systematically investigated whether such terrain differences exert control over the shift of forest-steppe ecotone in northern China. Can the significant terrain differences with mountain barriers promote or inhibit the shift of the forest-steppe ecotone?
In this study, we aimed to explore the following scientific questions: (1) How did the position of the forest-steppe ecotone evolve during the Holocene? (2) How did the shift of the forest-steppe ecotone respond to the EASM? (3) Was the forest-steppe ecotone migration three-dimensional?
Section snippets
Study area
The forest-steppe ecotone is located at the transition between the semi-humid and semi-arid areas in northern China, which is a broad area with relatively clear boundaries and basically along the 400 mm precipitation line (Fig. 1a). In and near the modern forest-steppe ecotone, the Great Khiigan Mountains (43°–53.50°N, 117.33°–126°E, 1100–1400 m a.s.l.), Yinshan (42°N, 106°–116°E, 400–2000 m a.s.l.), and Helan (38.35°–39.37°N, 105.82°–106.68°E, 2000–3556 m a.s.l.) Mountains stretch from east to
The relationship between topsoil pollen and modern vegetation
According to the summary statistics for the topsoil pollen samples, the percentages of each taxon in the three vegetation zones are obviously different (Table 1). In the forest zone, Pinus (31.4%), Betula (16.5%), Quercus (4.5%), and Artemisia (6.4%) pollen dominate. In the forest-steppe ecotone, Betula (5.8%), Artemisia (60.3%), Amaranthaceae (13.7%), and Poaceae (5.1%) pollen dominate. In the steppe zone, Artemisia (41.7%), Amaranthaceae (44.0%), and Poaceae (4.7%) dominate.
In discriminant
Response of the shift of the forest-steppe ecotone to the East Asian summer monsoon
The discriminant analysis in our study shows that the shift of the forest-steppe ecotone during the Holocene linearly corresponded to the evolution of the postglacial monsoon in comparison to the weakened summer monsoon in the mid-late Holocene (Fig. 2, Fig. 3a and b). When the summer monsoon was strong with intensified precipitation (Liu et al., 2014), it pushed the forest-steppe ecotone northward, and vice versa. The changes in the shift of forest-steppe ecotone and the intensity of the EASM
Conclusions
We found that the shift of the northern Chinese forest-steppe ecotone exhibited a generally consistent trend in response to the intensity of the EASM, which was characterized by southward retreat during the early Holocene from 12,000 to 8000 cal. yr BP, northward expansion during the middle Holocene from 8000 to 4000 cal. yr BP, and southward retreat during the late Holocene from 4000 to 0 cal. yr BP, suggesting a dominant role of precipitation provided by the EASM in determining the shift of
Declaration of Competing Interest
The authors declare no conflicts of interesting.
Acknowledgements
This research was funded by the National Natural Science Foundation of China (Nos. 41901092 and 41790422), and the Key Project of Ministry of Science and Technology of China (No. 2017YFA0605101). The authors declare no competing interests.
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