East Asian summer monsoon and topography co-determine the Holocene migration of forest-steppe ecotone in northern China

Journal Pre-proof East Asian summer monsoon and topography co-determine the Holocene migration of forest-steppe ecotone in northern China

Ying Cheng, Hongyan Liu, Zhibao Dong, Keqin Duan, Hongya Wang, Yue Han PII:

S0921-8181(20)30025-4

DOI:

https://doi.org/10.1016/j.gloplacha.2020.103135

Reference:

GLOBAL 103135

To appear in:

Global and Planetary Change

Received date:

26 November 2019

Revised date:

4 February 2020

Accepted date:

4 February 2020

Please cite this article as: Y. Cheng, H. Liu, Z. Dong, et al., East Asian summer monsoon and topography co-determine the Holocene migration of forest-steppe ecotone in northern China, Global and Planetary Change(2020), https://doi.org/10.1016/ j.gloplacha.2020.103135

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East Asian summer monsoon and topography co-determine the Holocene migration of forest-steppe ecotone in northern China

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Ying Cheng1 , Hongyan Liu2,* , Zhibao Dong1 , Keqin Duan1 , Hongya Wang2 , Yue Han2

Peking University, Beijing, 100871, China

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College of Urban and Environmental Sciences and MOE Laboratory for Earth Surface,

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2.

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1. School of Geography and Tourism, Shaanxi Normal University, Xi’an, 710119, China

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* Corresponding author: [email protected]

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Abstract Forest-steppe ecotones are generally regarded as very sensitive to climate change. However, it is still unclear whether they can be used to track past climate changes due to the combined effects of climate forcing and topographic factors. We first explored shifts of the whole forest-steppe ecotone in northern China during the Holocene by collecting 383 topsoil pollen

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samples to establish discriminant functions representative of forest, forest-steppe ecotone, and

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temperate steppe. The discriminant functions were applied to 39 fossil pollen sites to reconstruct

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the range of the forest-steppe ecotone during the Holocene. Our results showed that the shift of

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the forest-steppe ecotone exhibited a generally consistent trend with the intensity of the EASM,

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which was characterized by southward retreat during the early Holocene from 12,000–8000 cal. yr BP, northward expansion during the middle Holocene from 8000–4000 cal. yr BP, and

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southward retreat during the late Holocene from 4000–0 cal. yr BP, suggesting a dominant role

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of precipitation provided by the EASM. However, some sites in mountainous regions still

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indicated forest group membership within the ecotone, implying possible vertical forest migration. We stress that EASM and topography co-determine the shift of the forest-steppe ecotone in northern China, and mountainous terrain differences also benefit the vertical forest migrations when threatened by the dry climate. Our study implies that future climate change may cause three-dimensional changes in vegetation, which should be considered in climate change mitigation.

Key words: Pollen, forest-steppe ecotone, East Asian summer monsoon, topography, Holocene

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1. 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

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al., 2010; Goldblum and Rigg, 2010; Liu et al., 2013). However, due to long lifespan of trees

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and limited dispersal of forest species (Matthias et al., 2015), it is difficult to detect changes in

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the latitudinal ranges of the forest-steppe ecotone in response to recent climate changes

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(Beckage et al., 2008). The range of the forest-steppe ecotone across the landscape depends on

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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

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cal. yr BP to the present), a period characterized by natural climate forcing, appears to be an ideal

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forest-steppe ecotone.

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analogue for investigating and predicting the impact of climate change on the shift of the

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

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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.,

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2016), which is a typical ecologically vulnerable transition from the continental-scale perspective.

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Therefore, it is necessary to pay close attention to the Holocene shifting pattern of the

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forest-steppe ecotone on a large spatial scale under the influence of the EASM.

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Different from other latitudinal ecotones in the world, the mountains overlay the forest-steppe

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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

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al., 2006; Yin et al., 2013). It is also very important to see whether the forest-steppe ecotone in

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northern China was three-dimensional considering the topography factor, which has been widely

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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

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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?

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In this study, we aimed to explore the following scientific questions: (1) How did the position

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of the forest-steppe ecotone evolve during the Holocene? (2) How did the shift of the

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forest-steppe ecotone respond to the EASM? (3) Was the forest-steppe ecotone migration

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three-dimensional?

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2. Study area and methods 2.1 Study area

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The forest-steppe ecotone is located at the transition between the semi-humid and semi-arid

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areas in northern China, which is a broad area with relatively clear boundaries and basically

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along the 400 mm precipitation line (Fig. 1a). In and near the modern forest-steppe ecotone, the Great Khiigan Mountains (43º– 53.50ºN, 117.33º– 126ºE, 1100–1400 m a.s.l.), Yinshan (42ºN, 106º– 116ºE, 400–2000 m a.s.l.), and Helan (38.35º–39.37ºN, 105.82º– 106.68ºE, 2000–3556 m a.s.l.) Mountains stretch from east to west, while the Taihang (34.57º–40.72ºN, 110.23º– 114.55ºE, 2000–3556 m a.s.l.), Lvliang (35º–39ºN, 110º–112ºE, 1000–2831 m a.s.l.), and Liupan Mountains (35º– 37ºN, 106º– 110ºE, average elevations 2000–2500 m a.s.l.) extend from north to south. Among them, the Yanshan Mountains (2293 m a.s.l.), which extend from east to west, are distributed north of the Taihang Mountains. Specifically, there are two 400 mm precipitation lines, which are roughly on both sides of the mountain in northern China. The

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annual average temperature in this area is approximately 0–6 °C, and the average annual precipitation is approximately 350–450 mm (Hao et al., 2016). The climate in this area is mainly controlled by the East Asian summer monsoon, which brings precipitation from June to August. The Siberian-Mongolian high-pressure system dominates cold and dry climate in winter (Hao et al., 2016).

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The forest-steppe ecotone is a transitional zone where forest meets steppe across a climatic

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gradient (Liu et al., 2010), which is the northernmost boundary of modern forest distribution

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with an arid timberline in China. We specifically defined the modern range of the forest-steppe

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ecotone by combining the newly published definition of forest-steppe (Erdős et al., 2018) and

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the 400 mm precipitation line, obtaining the area where they overlap (Fig. 1a). The continuous forest is distributed south of this ecotone, and the temperate steppe is distributed to the north.

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Specifically, Pinus, Betula, and Quercus dominate the adjacent southern continuous forest (Hao

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et al., 2016). Artemisia, Amaranthaceae, and Poaceae dominate the northern temperate steppe.

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The above species co-exist in the forest-steppe ecotone. The topsoil pollen sites used in this study are typically distributed in or near the modern forest-steppe ecotone (Fig. 1b). Among them, 97 are distributed in the forest-steppe ecotone, 102 are distributed in the forest zone, and 184 are distributed in the temperate steppe zone (Fig. 1b and Table S1). The fossil pollen sites we used are generally distributed in or near the modern forest-steppe ecotone (Fig. 1b). Among them, 14 are distributed in the modern forest-steppe ecotone, 16 are distributed in the forest zone, and 9 are distributed in the temperate steppe zone (Fig. 1b and Table S2).

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Fig. 1. The forest-steppe ecotone in northern China. a. The yellow area represents the modern range of the forest-steppe ecotone in northern China, and the associated 400 mm and 200 mm

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precipitation lines, as well as their relations with the East Asian summer monsoon. b. Digital

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elevation model image of the study area, the distribution of fossil pollen sites (black triangle),

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and topsoil pollen sites (blue circle). There are two 400 mm precipitation lines, which are roughly on both sides of the mountain where there are significant terrain differences. The two sides of the mountains are the forest-steppe ecotone.

2.2 Topsoil and sediment pollen data collections We collected 383 topsoil pollen samples covering different vegetation zones in northern China, including forest, forest-steppe ecotone, and temperate steppe (Fig. 1b and Table S1). The topsoil pollen data used in this study was selected from the sites with minimal disturbance from human activities (Table S1). For the samples collected in our research group (Table S1), mosses and the upper part of the soil were collected carefully at each sampling site. Then pollen was extracted by treating with acid and alkali, and floated with heavy liquid using standard techniques (Moore

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et al., 1991). Finally, a minimum of 200 pollen grains were counted for each sample under an Olympus optical microscope at 400× magnification. Specifically, the species composition of the topsoil pollen, the proportion of taxa in topsoil pollen assemblages, and their relationship with actual vegetation were analysed in detail (Liu et al., 1999). We selected 39 fossil pollen sites in northern China and digitized all the pollen taxa using

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C ages for the entire record were

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basically met the following requirements: 1) at least four

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GetData Graph Digitizer v2.25 software (Fig. 1b and Table S2). The selected sequences

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measured in the original paper; 2) the temporal resolution was at least 200 years; 3) the 14

C dates

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sequences were near the modern forest-steppe ecotone in northern China. The original

with IntCal13 (Reimer et al., 2013).

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2.3 Discriminant analysis

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in the selected sequences were converted to calendar years (cal. yr BP) using Calib Rev 7.0.4

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Discriminant analysis is a method of classifying samples into groups based on calculated

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probabilities (Liu et al., 2001), which was conducted in SPSS software in our study. Discriminant analysis first attempts to derive a linear combination of variables by using the samples with known group memberships, which will ensure maximum separation between a priori sample sets and are then used to classify new samples with unknown group membership into a priori groups (Liu and Lam, 1985; Liu et al., 2010). By using this technique, we investigated the Holocene distribution of the northern Chinese forest-steppe ecotone based on topsoil pollen. Finally, we inferred the shift of the forest-steppe ecotone by comparing the palaeovegetation types among these sediment sequences. We used 10 proxies of topsoil pollen data, including Pinus, Betula, Quercus, Artemisia,

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Amaranthaceae, forest/Coniferous

Poaceae, forest

arboreal (B/C),

pollen/non-arboreal

Broadleaf

pollen

forest/Non-arboreal

(AP/NAP), pollen

Broadleaf

(B/NAP),

and

Artemisia/Amaranthaceae (Ar/Am), to establish the discriminant functions for forest, forest-steppe ecotone, and temperate steppe. Among the proxies, Pinus tabuliformis, Quercus mongolica, Betula dahurica, and Betula platyphylla are the most frequent and unique forest

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types in northern China, with the distribution corresponding to the northern edge of the summer

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monsoon (Liu et al., 1999; Shi et al., 2008; Hao et al., 2016). Pinus tabuliformis can tolerate

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cold and dry climates. Quercus mongolica generally prefers warm and humid climates, while it

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can still endure relatively cold and dry climates. Betula dahurica and Betula platyphylla

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basically live in the hardy mountainous area. Artemisia, Amaranthaceae, and Poaceae are very common, which dominate the modern temperate steppe in northern China. The difference is that

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Artemisia typically requires more moisture than Amaranthaceae during the growing season

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(Zhao et al., 2012). Moreover, we used ratios including AP/NAP, B/C, B/NAP, and Ar/Am to

3. Results

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avoid misjudgement due to the relativity of pollen percentages among sequences.

3.1 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.

Journal Pre-proof Table 1 Summary statistics of topsoil pollen samples in different vegetation zones . Pollen taxon

Forest

Forest-steppe

Mean

Standard deviations

Pinus

31.4

19.9

4.4

8.8

2.0

5.0

Betula

6.4

14.6

5.8

10.2

0.3

0.6

Standard deviations

Mean

Standard deviations

7.9

0.3

1.2

0.0

0.2

11.2 2.6

60.3 13.7

24.7 13.5

41.7 44.0

22.4 23.7

Poaceae AP/NAP

2.3 3.5

2.6 4.2

5.1 0.3

7.4 0.8

4.7 0.1

8.5 0.2

B/C

1.0

2.6

3.3

7.6

0.3

0.4

B/NAP Ar/Am

1.1

3.0

0.1

0.3

0.0

0.0

5.5

5.3

10.2

14.1

1.9

3.5

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4.5 16.5 3.4

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Artemisia Amaranthaceae

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Quercus

Mean

Steppe

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In discriminant analys is, we chose Pinus (x1), Betula (x2), Quercus (x3), Artemisia (x4),

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Amaranthaceae (x5), Poaceae (x6), AP/NAP (x7), B/C (x8), B/NAP (x9), and Ar/Am (x10) as independent variables based on their importance in reflecting different vegetation zones. Known

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vegetation zones are used as dependent variables. Table 2 gives the formulas of the discriminant

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functions for each vegetation zone. Based on the known vegetation zones, the discriminant

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functions have adjusted the relationship between topsoil pollen and actual vegetation by coefficients despite the facts that some families or genera have over-or under-representation and the possible effects of wind transported taxa (Liu et al., 1999; Sugita, 2007a, b). However, it is worth noting that the uncertainty arises from the use of topsoil to establish the discriminant function and its application to sediment records in terms of the different source areas and levels of preservation between topsoil pollen and sediment records (Jacobson and Bradshaw, 1981). Basically, the range of the relevant pollen source area increases with the increase of the sedimentary basin area, and the source area of the topsoil sample is smaller than the lake surface sediment sample (Sugita, 2007a, b). In addition, compared to the sedimentary

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environment, pollen grains can be rapidly oxidized in topsoil, thereby reducing the diversity in the sample, and only the grains with strongest or thickest exines are preserved (Jacobson and Bradshaw, 1981; Sugita, 2007a, b). Table 2 The discriminant functions for each vegetation zone. Vegetation zone

Discriminant function

Forest

f 1 = 0.413x1 + 0.475x2 + 0.473x3 + 0.317x4 + 0.293x5 + 0.396x6 + 1.113x7 + 0.022x8 -1.492 x9 + 0.033x10 - 14.978 f 2 = 0.329x1 + 0.652x2 + 0.260x3 + 0.558x4 + 0.501x5 + 0.667x6 + 1.498x7 + 0.089x8 -2.729 x9 - 0.010x10 – 25.859

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Forest-steppe

f 3 = 0.302x1 + 0.559x2 + 0.232x3 + 0.569x4 + 0.590x5 + 0.674x6 + 1.450x7 0.093x8 -2.236 x9 - 0.093x10 – 27.829

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Steppe

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By comparing the objectively predicted group membership with a priori group membership, it

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was found that the discriminant function classified 90% of the topsoil pollen samples into the

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correct vegetation zone (Table 3). Most of the misclassified samples occurred near the ecotone. Table 3 Classification results of the topsoil pollen samples. Total number of samples

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Actual group

Predicted group membership Number

Percentage

Forest Forest-steppe

102 97

99 82

97% 85%

Steppe

184

165

90%

* The average percentage of the topsoil pollen samples correctly classified is 90%. 3.2 Holocene shift of the forest-steppe ecotone Late-Glacial to Early Holocene transition : from 12,000–10,000 cal. yr BP, the forest-steppe ecotone developed in latitudes stretching between 37.56°N (constrained by Zhuyeze, Yujiagou, and Huanghua) and 40.45°N (constrained by Diaojiaohaizi, Chasuqi, Taipusi, and Haoluku), which is obviously larger than the current range of the forest-steppe ecotone (Fig. 2a and Tables

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4, S2, and S4); from 10,000–8000 cal. yr BP, the range of the forest-steppe ecotone did not change compared with the last stage (Fig. 2b and Tables 4, S2, and S4). Within the range of the forest-steppe ecotone during the early Holocene, although most of the sites showed forest-steppe type, some still featured forest, and very few featured temperate steppe without obvious southward or northward expansions (Figs. 2a and b).

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Middle Holocene: from 8000–6000 cal. yr BP, the forest-steppe ecotone migrated to the north,

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with its southern boundary tending to remain stable at 39.35°N (constrained by Yujiagou,

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Sujiawan, and Jiangjunpaozi), and the northern boundary migrated northward to 41.87°N

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(constrained by Zhuyeze, Gaoximage, and Sanyi) (Fig. 2c and Tables 4, S2, and S4); from

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6000–4000 cal. yr BP, the southern and northern boundaries of the forest-steppe ecotone did not change much (Fig. 2d and Tables 4, S2, and S4). Notably, the forest-steppe ecotone featured a

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mixture of transitional vegetation types and forest during the middle Holocene, when forests

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were staggered throughout the ecotone (Figs. 2c and d).

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Late Holocene: from 4000–2000 cal. yr BP, the forest-steppe ecotone migrated to the south, its southern and northern boundary migrated to 38.55°N (constrained by Wangxianggou and Dadiwan) and 40.53°N (constrained by Gaoximage, Sanyi, and Dadiwan), respectively (Fig. 2e and Tables 4, S2, and S4); from 2000 cal. yr BP to present, the southern boundary of the forest-steppe ecotone shifted southward, reaching 38.35°N (constrained by Wangxianggou, Niangziguan, Huanghua, and Dadiwan), and the northern boundary migrated northward to 40.73°N (constrained by Anguli Nuur, Haoluku, Sanyi, and Baiyangdian) (Fig. 2f and Tables 4, S2, and S4). Most of the sites within the range of the forest-steppe ecotone showed transitional vegetation types, while some sites still featured forest and temperate steppe without obvious

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southward or northward expansions during the late Holocene (Figs. 2e and f), which is similar to

the situation during the early Holocene. Fig. 2. Shift of the forest-steppe ecotone in northern China during different periods of the Holocene. The grey dashed line indicates the modern range of the forest-steppe ecotone. The black gradient lines show the potential range of the forest-steppe ecotone during the Holocene (the lighter the color, the greater the uncertainty). The blue circles, black triangles, and pink squares indicate the sediment sequences with forest, forest-steppe, and steppe membership, respectively.

Journal Pre-proof Table 4 The average latitude changes in the southern and northern boundaries of the forest-steppe ecotone. Age (cal. yr BP)

Northern boundary (°N)

Southern boundary (°N)

2000–0 4000–2000 6000–4000

40.73 40.53 41.87

38.35 38.55 39.25

8000–6000 10,000–8000

41.87 41.16

39.35 39.18

12,000–10,000

41.16

39.18

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3.3 Quantitative relationships between the shift of the forest-steppe ecotone and the monsoon evolution as well as topography difference s

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We found a general southward shift in the forest-steppe ecotone from 12,000–8000 cal. yr BP,

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a northward shift from 8000–4000 cal. yr BP, and an obvious southward shift from 4000–0 cal.

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yr BP (Figs. 2 and 3b), which shows a pattern that generally corresponds to the postglacial

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monsoon evolution with a weak monsoon intensity from 12,000–10,000 cal. yr BP, a strong

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monsoon intensity from 10,000–6000 cal. yr BP, and a continuously weakened monsoon intensity from 6000 cal. yr BP onwards (Fig. 3a). By using the EASM index from the variability 18

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of the Sanbao cave δ O (Wang et al., 2008; Dong et al., 2010; Liu et al., 2014), a correlation analys is indicates a positive relationship between the southern and northern boundaries of the forest-steppe ecotone and the intensity of the EASM (r = 0.89, p < 0.01, and r = 0.72, p < 0.01, respectively) (Figs. 3a and b). For the fossil pollen sites that still indicated forest group membership within the forest-steppe ecotone during the different periods of the Holocene, we inferred the elevations of forest from the pollen sites (Table S3; Table S4). The elevations of these sites changed from 1011 m a.s.l. to 2430 m a.s.l., with an elevation of 1550 m a.s.l., which is much higher than the average elevation of 960 m a.s.l. of all the fossil pollen sites used in this study (Table S4). Therefore, we infer that

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the forest occurred at 1000 m a.s.l. or higher within the forest-steppe ecotone during the Holocene (Table S4). Notably, the average elevations of these sites during different periods showed a continuously increasing trend from 8000 cal. yr BP onwards (Fig. 3c), implying that the elevation pos ition suitable for forest growth has been rising, which corresponds to the weakening of the summer monsoon from the mid-late Holocene to present (r = -0.80, p < 0.01)

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(Fig. 3a).

Fig. 3 a. The EASM evolution during the Holocene, which was derived from the variability in the Sanbao cave δ 18 O (Wang et al., 2008; Dong et al., 2010; Liu et al., 2014). The more negative the value is, the stronger the intensity of the summer monsoon. b. Changes of the average latitudes of the southern and northern forest-steppe ecotone boundary during the

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Discussion

4.1 Response of the shift of the forest-steppe ecotone to the East Asian summer monsoon The discriminant analys is 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

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the weakened summer monsoon in the mid-late Holocene (Figs. 2, 3a, and b). When the summer monsoon was strong with intensified precipitation (Liu et al., 2014), it pushed the forest-steppe

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ecotone northward, and vice versa. The changes in the shift of forest-steppe ecotone and the

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intensity of the EASM were generally consistent, suggesting a dominant role of precipitation in

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determining the position of the forest-steppe ecotone in northern China. The linear correspondence between the shift of the forest-steppe ecotone and the summer monsoon

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dynamics suggests that precipitation determin es the shift of the forest-steppe ecotone in northern China, as also infered by other studies on the vegetation dynamics in monsoon-influenced

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eastern China (Zhao et al., 2009; Cheng et al., 2018). It can be seen that the monsoon has been weakening since 6000 cal. yr BP (Fig. 3a), but the forest-steppe ecotone did not retreat southward significantly during 6000–4000 cal. yr BP (Fig. 3b), implying that the forest-steppe ecotone was initially resistant when the aridity increased. During the initial stage of the monsoon weakening, it was suggested that the pine forest replaced the broadleaf forest, so the forest type changed while the position did not change significantly (e.g., Liu et al., 2010; Liu and Yin, 2013; Hao et al., 2014). Specifically, we emphasize that the obvious wet and dry alternations in the marginal area of the EASM could not only benefit the conversion between forest and steppe but also favour the transformation between the wet-prefer

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broadleaf forest and drought-tolerant pine forest. Therefore, there may be a conversion from broadleaf forest to pine forest during the gradual drying in the mid-late Holocene in northern China (e.g., Liu et al., 2010; Liu and Yin, 2013; Hao et al., 2014). However, the response pattern of the forest-steppe ecotone to climate change in northern China is very different from the rapid forest-to-prairie conversion in mid-continental North America, which is a direct response to

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rapid drying during the Holocene approximately 8 ka without a conversion or stage dominated

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by coniferous forest (Williams et al., 2009).

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Our study also highlighted the influence of dry climates with very limited precipitation on the

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retreat of forest-steppe ecotones during the early and especially late Holocene. Although many

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studies with large spatial scales have pointed out the pronounced migrations of deciduous forest-boreal forest ecotones in North America, Europe, and Asia throughout the Holocene

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(Goldblum and Rigg, 2010), the forest distributions in China during the mid-Holocene or since

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the Last Glacial Maximum, and the biome migration during the Holocene (Yu et al., 1998, 2000;

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Ni et al., 2014; Cheng et al., 2018), they still lack specific consideration of the retreat of the forest-steppe ecotone forced by climate drying. Some studies have indicated that modern vegetation dynamics are closely related to dry climates, for example, drought-induced migration of the forest-steppe ecotone in New Mexico in North America in the 1950s (Allen and Breshears, 1998), and tree growth declines caused by warming-induced drying in the semi-arid forests of Inner Asia since 1994 (Liu et al., 2013). Moreover, a historical study also showed that dry climates seriously threatened terrestrial vegetation in northern China during the Holocene (Yin et al., 2013). We thus deduce that the southward retreat of the forest-steppe ecotone is a response to the southward retreat of the EASM.

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4.2 Influence of topographic factors on the shift of the forest-steppe ecotone Our pollen evidence showed that some sites located at high elevations indicate forest group membership within the forest-steppe ecotone during different periods (Fig. 2 and Table S4), implying that the shift of the forest-steppe ecotone did not completely follow the migration of EASM. The uneven response of the forest-steppe ecotone indicates the spatial changes in forests

f

might be affected by mountainous terrain differences. Topography has been suggested to have a

oo

strong impact on forest migration, as emphasized by a local case study of the forest-steppe

pr

ecotone in Anguli Nuur Lake at the southeastern margin of the Asian Gobi (Yin et al., 2013), and

e-

a regional-scale study on forest migration in northern Chinese forest-steppe ecotone (Hao et al.,

Pr

2016). Due to the overlap of mountains and the migration of forest-steppe ecotone, vertical migration of forests is suggested, different from flatlands (Cheng et al., 2018). The significant

al

terrain differences in the forest-steppe ecotone caused by the rolling mountains could provide a

rn

variety of local environments with compressed vegetation belts at different elevations, which

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may allow forest distributions when the regional climate is not suitable (Parducci et al., 2012; Yin et al., 2013; Hao et al., 2016; Cheng et al., 2018). Once the climatic conditions have ameliorated, these forests could encraoch locally (Svenning et al., 2008; Pacifici et al., 2015). We also found that the elevation position suitable for forest growth has been rising from 8000 cal. yr BP onwards, corresponding to the weakening of the EASM from the mid-late Holocene to present (Figs. 3a and c, and Tables S2 and S3). Therefore, in the case of vertical forest migration in the forest-steppe ecotone, a dry climate may first push the forest lower limit to high elevations, and as the aridity increases further, the lower part of forest is then replaced by temperate steppe (Cheng et al., 2017), which is similar to the vertical biome migration in

Journal Pre-proof

response to the dry climate (Cheng et al., 2018). Therefore, we propose that the forest-steppe ecotone in northern China was three-dimensional considering the influence of topography, which mediated the shift of the forest-steppe ecotone in the vertical direction. The shift of the forest-steppe ecotone may benefit from topographical differences with forests migrating to high altitudinal areas and changing to fragmented forest instead of continuous forests. Conclusions

f

5

oo

We found that the shift of the northern Chinese forest-steppe ecotone exhibited a generally

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consistent trend in response to the intensity of the EASM, which was characterized by southward

e-

retreat during the early Holocene from 12,000–8000 cal. yr BP, northward expansion during the

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middle Holocene from 8000–4000 cal. yr BP, and southward retreat during the late Holocene from 4000–0 cal. yr BP, suggesting a dominant role of precipitation provided by the EASM in

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determining the shift of the forest-steppe ecotone. However, some sites in mountainous regions

rn

still indicated forest group membership within the ecotone, implying possible vertical forest

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migration. We stress that EASM and topography co-determine the shift of the forest-steppe ecotone in northern China, which may benefit from mountainous terrain differences when threatened by the dry climate. Therefore, the climate indication of the forest-steppe ecotone needs to be considered from three dimensions considering the future climate change.

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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Supplementary Information for East Asian summer monsoon and topography co-determine the Holocene migration of forest-steppe ecotone in northern China Table S1 Information of the topsoil pollen samples S ite name

Latitude

Longitude

Elevation

Dominant

(°)

(°)

(m)

vegetation

Reference

1

40.13

119.43

645

Forest

This study

2

2

40.58

117.85

590

Forest

This study

3

3

40.13

119.43

865

Forest

This study

4

4

40.13

119.41

745

Forest

This study

5

5

40.13

119.43

445

Forest

This study

6

6

40.57

117.48

1450

Forest

This study

7

7

40.56

117.50

1260

Forest

This study

8

8

40.56

117.48

1350

Forest

This study

9

9

37.87

111.45

1820

Forest

This study

10

10

37.88

111.44

1860

Forest

This study

11

11

37.85

111.46

1660

Forest

This study

12

12

38.99

113.50

2245

Forest

This study

13

13

40.60

117.48

1790

Forest

This study

14

14

42.46

117.28

1520

Forest-steppe

This study

15

15

37.89

111.43

1980

Forest

This study

16

16

40.59

17

17

18

18

19

19

20

20

21

pr

e-

Pr

al

rn

oo

1

f

ID

1660

Forest

This study

117.23

1465

Forest-steppe

This study

37.85

111.47

1635

Forest

This study

40.56

117.48

1360

Forest

This study

40.56

117.49

1160

Forest

This study

21

37.24

113.88

960

Forest

This study

22

22

37.44

114.03

1728

Forest

This study

23

23

37.48

114.03

1564

Forest

This study

24

24

36.15

113.72

641

Forest

This study

25

25

36.17

113.75

678

Forest

This study

26

26

36.15

113.75

494

Forest

This study

27

27

40.57

117.47

1375

Forest

This study

28

28

40.56

117.49

1050

Forest

This study

29

29

37.83

114.14

680

Forest

This study

30

30

36.14

113.71

885

Forest

This study

31

31

40.59

117.47

1620

Forest

This study

32

32

37.83

114.14

500

Forest

This study

33

33

40.55

117.49

1550

Forest

This study

Jo u

117.48

42.44

Journal Pre-proof 34

36.14

113.72

743

Forest

This study

35

35

37.33

114.19

472

Forest

This study

36

36

40.51

119.30

1220

Forest

This study

37

37

40.51

119.29

920

Forest

This study

38

38

40.13

119.41

545

Forest

This study

39

39

40.52

119.29

885

Forest

This study

40

40

36.14

113.71

885

Forest

This study

41

41

38.66

105.84

2400

Steppe

This study

42

42

38.67

105.84

2305

Steppe

This study

43

43

40.56

111.51

1020

Steppe

This study

44

44

38.67

105.80

2010

Steppe

This study

45

45

37.65

107.53

1175

Steppe

This study

46

46

39.26

107.14

1360

Steppe

This study

47

47

39.11

107.95

1530

Steppe

This study

48

48

39.72

108.62

1470

Steppe

This study

49

49

39.19

105.66

1310

Steppe

This study

50

50

38.96

105.65

1490

Steppe

This study

51

51

39.43

106.78

1180

Steppe

This study

52

52

37.73

107.35

1255

Steppe

This study

53

53

37.85

106.73

1295

Steppe

This study

54

54

37.96

106.65

1270

Steppe

This study

55

55

37.71

106.89

1375

Steppe

This study

56

56

37.77

107.04

1352.5

Steppe

This study

57

57

37.51

108.87

1460

Forest-steppe

This study

58

58

40.50

107.59

1480

Steppe

This study

59

59

41.19

113

1550

Steppe

This study

60

60

37.71

107.34

1245

Steppe

This study

61

61

37.51

107.82

1300

Steppe

This study

62

62

37.41

109.26

1170

Forest-steppe

This study

63

63

38.04

114.52

2740

Forest

This study

64

64

39.75

114.82

1550

Forest

This study

65

65

37.91

111.40

1480

Forest

This study

66

66

40.60

117.90

670

Forest

This study

67

67

37.55

111.94

810

Forest

This study

68

68

39.82

114.80

1210

Forest

This study

69

69

39

112.5

1380

Forest-steppe

This study

70

70

39.06

114.74

360

Forest

This study

71

71

37.19

114.16

386

Forest

This study

72

72

37.12

114.25

228

Forest

This study

73

73

37.95

114.07

260

Forest

This study

74

74

39.41

114.76

870

Forest

This study

75

75

40.50

117.59

550

Forest

This study

76

76

37.83

114.14

590

Forest

This study

77

77

37.92

113.92

640

Forest

This study

oo

pr

e-

Pr

al

rn Jo u

f

34

Journal Pre-proof 78

41.59

115.49

1375

Forest-steppe

This study

79

79

42.54

117.22

1510

Forest-steppe

This study

80

80

42.51

117.24

1520

Forest-steppe

This study

81

81

38.68

105.81

2175

Steppe

This study

82

82

41.97

116.01

1390

Forest-steppe

This study

83

83

42.05

116.32

1280

Forest-steppe

This study

84

84

41.72

111.77

1450

Steppe

This study

85

85

39.85

109.67

1510

Steppe

This study

86

86

39.78

110.15

1530

Steppe

This study

87

87

41.34

112.86

1640

Steppe

This study

88

88

39.22

107.18

1330

Steppe

This study

89

89

39.15

108.02

1400

Steppe

This study

90

90

38.31

106.48

1160

Steppe

This study

91

91

39.67

108.63

1000

Steppe

This study

92

92

39.33

108.21

1455

Steppe

This study

93

93

38.19

106.57

1270

Steppe

This study

94

94

38.67

105.76

1830

Steppe

This study

95

95

38.35

105.86

1495

Steppe

This study

96

96

38.41

105.73

1570

Steppe

This study

97

97

38.38

105.96

1205

Steppe

This study

98

98

39.24

107.51

1400

Steppe

This study

99

99

39.18

107.93

1550

Steppe

This study

100

100

39.61

108.54

1450

Steppe

This study

101

101

38.93

105.65

1505

Steppe

This study

102

102

39.35

106.96

1270

Steppe

This study

103

103

39.36

105.85

1260

Steppe

This study

104

104

39.24

106.85

1690

Steppe

This study

105

105

38.47

105.68

1390

Steppe

This study

106

106

38.63

105.63

1400

Steppe

This study

107

107

38.37

105.96

1200

Steppe

This study

108

108

39.42

106.83

1200

Steppe

This study

109

109

42.62

116.12

1374

Forest-steppe

This study

110

110

42.37

116.07

1310

Forest-steppe

This study

111

111

42.51

116.09

1441

Forest-steppe

This study

112

112

42.38

116.20

1314

Forest-steppe

This study

113

113

42.27

116.34

1450

Forest-steppe

This study

114

114

42.29

116.45

1281

Forest-steppe

This study

115

115

42.25

116.63

1279

Forest-steppe

This study

116

116

42.19

116.69

1272

Forest-steppe

This study

117

117

42.15

116.89

1300

Forest-steppe

This study

118

118

42.17

116.99

1262

Forest-steppe

This study

119

119

42.25

117.08

1336

Forest-steppe

This study

120

120

42.50

117.29

1565

Forest-steppe

This study

121

121

42.23

116.47

1257

Forest-steppe

This study

oo

pr

e-

Pr

al

rn Jo u

f

78

Journal Pre-proof 122

42.17

116.41

1295

Forest-steppe

This study

123

123

42.09

116.46

1320

Forest-steppe

This study

124

124

41.96

116.17

1443

Forest-steppe

This study

125

125

41.83

116.09

1489

Forest-steppe

This study

126

126

41.70

116.12

1518

Forest-steppe

This study

127

127

41.74

116.01

1462

Forest-steppe

This study

128

128

41.75

115.95

1452

Forest-steppe

This study

129

129

41.52

115.31

1447

Forest-steppe

This study

130

130

41.62

115.21

1405

Steppe

This study

131

131

41.38

114.93

1378

Steppe

This study

132

132

41.33

114.57

1364

Steppe

This study

133

133

41.29

114.38

1339

Steppe

This study

134

134

41.45

114.41

1367

Steppe

This study

135

135

41.60

114.59

1427

Steppe

This study

136

136

43.57

115.04

1066

Steppe

This study

137

137

43.75

114.75

1137

Steppe

This study

138

138

43.93

114.83

1157

Steppe

This study

139

139

43.95

114.70

1128

Steppe

This study

140

140

43.90

114.68

1011

Steppe

This study

141

141

43.91

113.64

1035

Steppe

This study

142

142

43.82

113.63

1082

Steppe

This study

143

143

43.63

113.39

1043

Steppe

This study

144

144

43.47

113.15

1048

Steppe

This study

145

145

43.25

113

1038

Steppe

This study

146

146

42

112.48

1098

Steppe

This study

147

147

42.60

112.61

1163

Steppe

This study

148

148

42.85

112.58

1112

Steppe

This study

149

149

42.90

112.73

1117

Steppe

This study

150

150

43.78

113.70

1161

Steppe

This study

151

151

43.69

113.82

1148

Steppe

This study

152

152

43.51

114.17

1094

Steppe

This study

153

153

43.28

114.33

1066

Steppe

This study

154

154

43.17

114.46

1072

Steppe

This study

155

155

43.02

114.48

1113

Steppe

This study

156

156

42.96

114.34

1072

Steppe

This study

157

157

42.94

114.39

1080

Steppe

This study

158

158

42.83

114.54

1146

Steppe

This study

159

159

42.74

114.65

1159

Steppe

This study

160

160

42.19

115.41

1224

Forest-steppe

This study

161

161

42.37

115.66

1363

Forest-steppe

This study

162

162

42.58

115.42

1279

Forest-steppe

This study

163

163

42.62

115.17

1265

Steppe

This study

164

164

42.66

114.93

1186

Steppe

This study

165

165

42.64

114.81

1148

Steppe

This study

oo

pr

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Pr

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f

122

Journal Pre-proof 166

42.51

114.81

1207

Steppe

This study

167

167

42.28

114.87

1306

Steppe

This study

168

168

42.4

114.68

1249

Steppe

This study

169

169

42.43

114.48

1285

Steppe

This study

170

170

42.55

114.24

1138

Steppe

This study

171

171

42.56

114.07

1082

Steppe

This study

172

172

42.47

114.03

1190

Steppe

This study

173

173

42.40

113.98

1240

Steppe

This study

174

174

42.33

113.93

1311

Steppe

This study

175

175

42.25

114.01

1300

Steppe

This study

176

176

42.12

113.87

1461

Steppe

This study

177

177

42.02

114.23

1116

Steppe

This study

178

178

42.13

114.56

1345

Steppe

This study

179

179

42.07

114.57

1416

Steppe

This study

180

180

41.94

114.51

1490

Steppe

This study

181

181

41.84

114.69

1461

Steppe

This study

182

182

41.85

114.92

1484

Steppe

This study

183

183

42.19

115.57

1330

Forest-steppe

This study

184

184

42.11

115.50

1406

Forest-steppe

This study

185

185

41.85

115.27

1483

Forest-steppe

This study

186

186

41.75

115.17

1443

Steppe

This study

187

187

41.64

115.02

1407

Steppe

This study

188

188

41.57

111.63

1464

Steppe

This study

189

189

41.64

111.45

1464

Steppe

This study

190

190

41.75

111.32

1529

Steppe

This study

191

191

41.88

111.20

1485

Steppe

This study

192

192

42.13

111.01

1194

Steppe

This study

193

193

42.27

110.95

1179

Steppe

This study

194

194

42.65

111.95

1003

Steppe

This study

195

195

42.75

110.75

968

Steppe

This study

196

196

42.75

110.57

1024

Steppe

This study

197

197

42.66

110.47

1053

Steppe

This study

198

198

42.92

110.81

1031

Steppe

This study

199

199

43.16

111.20

1160

Steppe

This study

200

200

43.15

111.36

1053

Steppe

This study

201

201

43.25

111.47

1057

Steppe

This study

202

202

43.35

111.59

965

Steppe

This study

203

203

43.52

112.08

984

Steppe

This study

204

204

43.20

111.82

1057

Steppe

This study

205

205

42.92

111.86

1124

Steppe

This study

206

206

42.80

112.15

1160

Steppe

This study

207

207

42.76

112.43

1099

Steppe

This study

208

208

42.72

112.58

1120

Steppe

This study

209

209

43.06

112.46

1085

Steppe

This study

oo

pr

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166

Journal Pre-proof 210

43.23

112.34

1022

Steppe

This study

211

211

43.35

112.17

1033

Steppe

This study

212

212

42.48

112.50

1239

Steppe

This study

213

213

42.34

112.37

1235

Steppe

This study

214

214

42.21

112.27

1287

Steppe

This study

215

215

42.40

117.24

1560

Forest-steppe

This study

216

216

42.82

116.79

1350

Forest-steppe

This study

217

217

42.97

116.79

1350

Forest-steppe

This study

218

218

42.96

116.76

1322

Forest-steppe

This study

219

219

42.92

116.77

1320

Forest-steppe

This study

220

220

42.78

116.96

1390

Forest-steppe

This study

221

221

42.74

116.92

1390

Forest-steppe

This study

222

222

42.60

116.99

1500

Forest-steppe

This study

223

223

42.59

117.05

1470

Forest-steppe

This study

224

224

42.59

117.14

1554

Forest-steppe

This study

225

225

42.58

117.33

1570

Forest-steppe

This study

226

226

42.31

117.50

1450

Forest-steppe

This study

227

227

42.77

116.91

1280

Forest-steppe

This study

228

228

43.67

116.82

1390

Forest-steppe

This study

229

229

43.55

116.68

1210

Forest-steppe

This study

230

230

43.50

116.70

1300

Forest-steppe

This study

231

231

43.24

116.39

1300

Forest-steppe

This study

232

232

43.08

116.71

1258

Forest-steppe

This study

233

233

43.09

116.60

1400

Forest-steppe

This study

234

234

42.89

116.72

1345

Forest-steppe

This study

235

235

42.79

116.70

1400

Forest-steppe

This study

236

236

42.67

116.68

1416

Forest-steppe

This study

237

237

42.70

116.61

1395

Forest-steppe

This study

238

238

42.76

116.56

1365

Forest-steppe

This study

239

239

42.86

116.40

1350

Forest-steppe

This study

240

240

42.91

116.62

1370

Forest-steppe

This study

241

241

42.88

116.67

1410

Forest-steppe

This study

242

242

42.05

117.09

1370

Forest-steppe

This study

243

243

42.98

117.18

1130

Forest-steppe

This study

244

244

42.96

117.30

1220

Forest-steppe

This study

245

245

42.66

116.89

1415

Forest-steppe

This study

246

246

42.66

116.80

1450

Forest-steppe

This study

247

247

42.48

117.05

1475

Forest-steppe

This study

248

248

42.30

117.06

1430

Forest-steppe

This study

249

249

41.33

114.32

1357

Steppe

This study

250

250

41.22

114.32

1497

Steppe

This study

251

251

41.26

114.35

1392

Steppe

This study

252

252

41.31

114.35

1321

Steppe

This study

253

253

41.20

114.39

1435

Steppe

This study

oo

pr

e-

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f

210

Journal Pre-proof 254

41.31

114.39

1326

Steppe

This study

255

255

39.37

107.09

1408

Steppe

This study

256

256

39.26

107.14

1325

Steppe

This study

257

257

39.24

107.81

1359

Steppe

This study

258

258

39.20

108.04

1362

Steppe

This study

259

259

39.31

108.10

1399

Steppe

This study

260

260

39.73

108.63

1419

Steppe

This study

261

261

38.86

108.39

1403

Steppe

This study

262

262

38.78

108.52

1360

Steppe

This study

263

263

38.64

108.68

1367

Steppe

This study

264

264

38.53

108.81

1323

Steppe

This study

265

265

38.36

108.66

1294

Steppe

This study

266

266

38.03

108.61

1233

Steppe

This study

267

267

38.85

109.15

1304

Steppe

This study

268

268

39.06

109.10

1316

Steppe

This study

269

269

39.05

109.47

1318

Forest-steppe

This study

270

270

39.09

109.63

1315

Forest-steppe

This study

271

271

39.29

109.82

1400

Forest-steppe

This study

272

272

39.06

110.15

1238

Forest-steppe

This study

273

273

39.06

110.32

1111

Forest-steppe

This study

274

274

38.81

110.36

1131

Forest-steppe

This study

275

275

38.78

110.23

1248

Forest-steppe

This study

276

276

38.67

110.04

1193

Forest-steppe

This study

277

277

38.64

110.01

1192

Forest-steppe

This study

278

278

38.53

109.86

1275

Forest-steppe

This study

279

279

38.44

109.73

1156

Forest-steppe

This study

280

280

38.03

109.65

1006

Forest-steppe

This study

281

281

38.07

109.45

994

Forest-steppe

This study

282

282

37.97

109.28

1050

Forest-steppe

This study

283

283

37.66

109.14

1130

Forest-steppe

This study

284

284

37.58

108.68

1378

Forest-steppe

This study

285

285

37.53

108.40

1383

Forest-steppe

This study

286

286

37.50

108.28

1440

Forest-steppe

This study

287

287

37.65

108.32

1365

Steppe

This study

288

288

37.73

108.26

1329

Steppe

This study

289

289

37.73

108.31

1324

Steppe

This study

290

290

37.84

108.05

1358

Steppe

This study

291

291

37.93

107.97

1377

Steppe

This study

292

292

37.99

107.85

1373

Steppe

This study

293

293

38.07

107.68

1369

Steppe

This study

294

294

38.25

107.41

1355

Steppe

This study

295

295

38.32

107.32

1493

Steppe

This study

296

296

38.40

107.26

1465

Steppe

This study

297

297

38.08

107.40

1360

Steppe

This study

oo

pr

e-

Pr

al

rn Jo u

f

254

Journal Pre-proof 298

38.01

107.47

1301

Steppe

This study

299

299

37.99

107.38

1347

Steppe

This study

300

300

37.95

107.38

1334

Steppe

This study

301

301

42.36

116.18

1375

Forest-steppe

This study

302

302

42.55

112.48

996

Steppe

This study

303

303

43.78

111.78

966

Steppe

This study

304

304

43.76

111.63

969

Steppe

This study

305

305

43.71

111.54

1001

Steppe

This study

306

306

43.77

111.83

959

Steppe

This study

307

307

43.83

111.66

981

Steppe

This study

308

308

43.79

111.77

945

Steppe

This study

309

309

43.75

111.87

975

Steppe

This study

310

310

43.82

111.61

975

Steppe

This study

311

311

43.84

111.51

931

Steppe

This study

312

312

43.89

111.41

1007

Steppe

This study

313

313

43.91

111.39

1012

Steppe

This study

314

314

43.92

111.49

1013

Steppe

This study

315

315

43.87

111.58

957

Steppe

This study

316

Sanjiaocheng

39

103.33

1712

Steppe

Chen et al., 2006

317

Zhuyeze

39.05

103.67

1790

Steppe

Li et al., 2011

318

Qingtu Lake

39.07

103.61

1745

Steppe

Zhao et al., 2008

319

Daihai

40.55

112.66

1496

Forest-steppe

Xiao et al., 2004

320

Dadiwan

36.27

106.37

1605

Forest-steppe

Xia et al., 1998

321

Guangrunpo

36.02

114.53

66

Forest

Zhang et al., 2007

322

Qigai

39.37

109.39

1087

Steppe

Sun et al., 2013

323

Tenggeernuur

40.47

110.67

1570

Steppe

Zhao et al., 2001

324

Tianchi lake

35.26

106.31

1756

Forest

Zhao et al., 2010

325

Pi1

42.44

117.27

1538

Forest-steppe

Xu et al., 2005

326

Pi2

42.40

117.30

1463

Forest-steppe

Xu et al., 2005

327

Pi3

40.13

119.43

567

Forest

Xu et al., 2005

328

Pi4

40.58

117.85

884

Forest

Xu et al., 2005

329

Pi5

40.88

111.58

1342

Steppe

Xu et al., 2005

330

Ph2

40.13

119.43

558

Forest

Xu et al., 2005

331

Ph3

40.13

119.41

492

Forest

Xu et al., 2005

332

Ph4

40.13

119.43

566

Forest

Xu et al., 2005

333

Ph6

40.57

117.48

918

Forest

Xu et al., 2005

334

Ph7

40.56

117.50

959

Forest

Xu et al., 2005

335

Ph8

40.56

117.48

911

Forest

Xu et al., 2005

336

Pa3

37.87

111.45

1689

Forest

Xu et al., 2005

337

Pa4

37.87

111.45

1616

Forest

Xu et al., 2005

338

Pa5

37.88

111.44

1559

Forest

Xu et al., 2005

339

Pa6

42.40

117.27

1492

Forest-steppe

Xu et al., 2005

340

La1

37.85

111.46

1661

Forest

Xu et al., 2005

341

La4

40.60

117.48

1040

Forest

Xu et al., 2005

oo

pr

e-

Pr

al

rn

Jo u

f

298

Journal Pre-proof La5

42.46

117.28

1488

Forest-steppe

Xu et al., 2005

343

Be1

37.89

111.43

1690

Forest

Xu et al., 2005

344

Be2

40.59

117.48

1007

Forest

Xu et al., 2005

345

Be3

42.44

117.23

1394

Forest-steppe

Xu et al., 2005

346

Qu1

37.85

111.47

1645

Forest

Xu et al., 2005

347

Qu2

40.56

117.48

911

Forest

Xu et al., 2005

348

Qu3

40.56

117.48

918

Forest

Xu et al., 2005

349

Qu4

40.56

117.49

947

Forest

Xu et al., 2005

350

Qu5

37.24

113.88

368

Forest

Xu et al., 2005

351

Qu6

37.44

114.03

467

Forest

Xu et al., 2005

352

Qu7

37.48

114.03

610

Forest

Xu et al., 2005

353

Qu8

37.48

114.03

610

Forest

Xu et al., 2005

354

Qu9

37.44

114.03

467

Forest

Xu et al., 2005

355

Qu10

36.15

113.72

1521

Forest

Xu et al., 2005

356

Qu11

36.17

113.75

1628

Forest

Xu et al., 2005

357

Qu12

36.17

113.75

1628

Forest

Xu et al., 2005

358

Qu13

36.15

113.75

1431

Forest

Xu et al., 2005

359

Ja1

40.57

117.47

936

Forest

Xu et al., 2005

360

Ja2

40.56

117.49

947

Forest

Xu et al., 2005

361

Ca1

37.83

114.14

492

Forest

Xu et al., 2005

362

Ca2

36.14

113.71

1549

Forest

Xu et al., 2005

363

Ca3

36.14

113.71

1549

Forest

Xu et al., 2005

364

Po1

40.59

117.47

1007

Forest

Xu et al., 2005

365

Po2

40.59

117.47

1007

Forest

Xu et al., 2005

366

Pt1

37.83

114.14

492

Forest

Xu et al., 2005

367

Bh1

40.55

117.49

876

Forest

Xu et al., 2005

368

Bh2

40.55

117.49

940

Forest

Xu et al., 2005

369

Bh3

36.14

113.72

1549

Forest

Xu et al., 2005

370

Bh4

37.33

114.19

144

Forest

Xu et al., 2005

371

Bh5

40.52

119.29

597

Forest

Xu et al., 2005

372

Bh6

40.51

119.30

610

Forest

Xu et al., 2005

373

Bh7

40.52

119.28

591

Forest

Xu et al., 2005

374

Bh8

40.52

119.29

591

Forest

Xu et al., 2005

375

Bh9

40.51

119.29

588

Forest

Xu et al., 2005

376

Bh10

40.13

119.41

487

Forest

Xu et al., 2005

377

Bh11

40.52

119.29

597

Forest

Xu et al., 2005

378

Bh12

36.14

113.71

1505

Forest

Xu et al., 2005

379

Pa2

38.67

105.84

1050

Steppe

Xu et al., 2005

380

Po4

40.56

111.51

1480

Steppe

Xu et al., 2005

381

Ul2

38.67

105.80

1046

Steppe

Xu et al., 2005

382

El1

37.65

107.53

1364

Steppe

Xu et al., 2005

383

El2

37.65

107.53

1364

Steppe

Xu et al., 2005

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Table S2 Information of the fossil pollen sites Latitude (°)

Longitude (°)

Elevation (m)

M AP (mm)

M AT (℃)

Age range (cal. ya BP)

Dominant modern vegetation

Reference

Qingtu Lake

39.05 39.07

103.67 103.61

1312 1310

105.3 108.4

7.7 7.7

11000–0 7200–0

Steppe Steppe

Li et al., 2011 Zhao et al., 2008

3

Gaoximage

42.95

115.37

1243

306.7

0.9

5735–0

4 5

Hamatai Salty Lake Huangjiabao

38.72

108.75

1342

333.2

6.5

7283–0

40.57

115.15

601

347.2

5.7

14000–0 (a BP)

6

Diaojiaohaizi

41.12

112.57

2038

353

0

13281–0

7

Anguli Nuur

41.33

114.36

1311

369.5

2.6

11306–0

8

Qiguoshan

42.28

118.97

565

370.9

4.6

9

Chasuqi

40.67

111.13

1011

372.6

10

Taipusi

41.98

115.18

1491

382.4

11

Bayanchagan

12

Haoluku

41.65 42.96

115.21 116.76

1370 1309

385.4 386.4

13

Daihai

40.55

112.66

1290

386.5

Liuzhouwan Xipu

42.71

116.76

1391

40.12

114.22

920

16

Yujiagou

40.15

114.48

17

Shandian River

42.22

116.62

18

S anyi

43.62

117.38

19

Xiaoniuchang

42.62

116.82

20

S ujiawan

35.54

104.52

21

Jiangjunpaozi

22

Charisu

42.37 42.95

23

Wangxianggou

24 25

Gonghai Lake Niangziguan

26

M aili

ID

Site name

1

Zhuyeze

2

14 15

Li et al., 2003

Steppe

Wang et al., 1996

Forest

Sun et al., 2001

Steppe

Yang, 2001

Steppe

Yin et al., 2013

8400–0 (a BP) 10136–0

Forest-steppe

Xu et al., 2002

Steppe

Wang et al., 1999

10758–0 12346–0 12023–0

Forest-steppe

Huang et al., 2005

1.6 -0.7

Steppe Forest-steppe

Jiang et al., 2006 Liu et al., 2001

-p

e r P 6.4 1.6

ro

5.1

14000–0 (a BP)

Steppe

Li et al., 2004

389.7

-0.2

16000–0

Forest-steppe

Liu et al., 2001

391

6.6

16000–0 (a BP)

Forest-steppe

Wang et al., 2003

393

6.4

15000–0

Forest-steppe

Xia et al., 2001

rn

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394.8

0.4

9100–0

Forest-steppe

Wang et al., 2006

1473

396.9

-1.7

Wang et al., 2005

1411

402.1

-0.3

19742–0 15731–0

Forest-steppe Forest-steppe

Liu et al., 2001

1958

415.8

5.9

15100–0

Forest-steppe

Feng et al., 2006

117.47 122.35

1567 250

430.5 431.3

-0.2 6.7

14150–0 4970–0

Forest-steppe Forest-steppe

Liu et al., 2001 Li et al., 2003

42.07

119.92

735

441.3

4.1

5370–0

Forest-steppe

Li et al., 2006

38.90

112.23

1860

468

4-8

14700–0

Forest

Xu et al., 2016

37.95

113.88

794

504.2

11.8

15000–0

Forest

Yang et al., 1999

42.87

122.88

162

516.3

6.9

3800–0

Forest-steppe

Ren, 1999

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Huanghua

38.35

117.35

7

525.3

12.1

28954–0

Forest

Guang et al., 2000

Dadiwan Baiyangdian

35.02

105.90

1459

540.8

8.2

11190–0 (a BP)

Forest

An et al., 2003

38.40

115.98

14

551.7

12.4

13233–0

Forest

Xu et al., 1988

30

Jingbo Lake

43.92

128.83

348

557.6

2.9

9560–0

Forest

Li, C. et al., 2011

31

Chadianpo

36.10

114.40

68

585

13.7

9560–0

Forest

Zhang et al., 2007

32

Wangjiadian

36.10

114.40

64

585

13.7

12000–0

Forest

33

Liupan M ountain

35.26

106.31

2430

615

3.4

6200–0

Forest

Zhao et al., 2010

34

Cangumiao

39.96

118.61

73

638.3

10.1

5259–0

Forest

Xu et al., 2004

35

M aohebei

36

Yanshengang

39.50 39.87

119.17 118.87

1 58

647 656.9

10.7 10.5

7151–0 15000–0

f o

Cao et al., 2010

Forest Forest

Li and Liang, 1985 Kong et al., 2000

37

Jinchuan

42.35

126.38

620

762.4

3.2

10800–0

Forest

Jiang et al., 2008

38 39

Changbai M ountain Xiaolongwan

42.28

126.60

797

775

2.5

11650–0

Forest

Stebich et al., 2015

42.30

126.36

655

760

5350–0 (a BP)

Forest

Xu et al., 2014

27 28 29

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2000–0 4000–2000 6000–4000

1894 1587 1452

8000–6000 10,000–8000

1438 1331

12,000–10,000

1331

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Age (cal. yr BP)

Table S4 Predicted vegetation membership based on the fossil pollen sites during the Holocene Longitu

Elevatio

(°)

de (°)

n (m)

Age (cal. yr BP)

2000–0

4000–

6000–

8000–

10,000–

12,000–

2000

4000

6000

8000

10,000

3

2

2

2

2

3

3

2

0

0

2

2

2

0

0

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Latitude

39.05

103.67

1312

3

2

39.07

103.61

1310

3

3

42.95

115.37

1243

3

4

38.72

108.75

1342

1

1

1

1

0

0

5

40.57

115.15

601

2

2

0

0

0

0

6

41.12

112.57

2038

1

1

1

1

2

2

7

41.33

114.36

1311

2

1

1

1

1

1

8

42.28

118.97

565

2

2

2

1

1

1

9

40.67

111.13

10

41.98

115.18

11

41.65

12

42.96

13

40.55

14

42.71

15

1011

1

1

1

1

2

2

1491

2

2

2

2

2

2

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ID

115.21

1370

3

3

2

2

2

2

116.76

1309

2

2

2

2

2

2

112.66

1290

1

1

1

1

1

1

116.76

1391

1

1

1

1

1

1

40.12

114.22

920

2

2

0

0

0

0

16

40.15

114.48

855

2

2

2

2

2

2

17

42.22

116.62

1261

3

1

1

1

2

2

18

43.62

117.38

1473

2

2

2

2

3

3

19

42.62

116.82

1411

2

2

2

2

2

2

20

35.54

104.52

1958

1

1

2

2

1

1

21

42.37

117.47

1567

0

1

1

2

2

2

22

42.95

122.35

250

1

1

1

0

0

0

23

42.07

119.92

735

2

2

2

0

0

0

24

38.9

112.23

1860

1

1

1

1

1

1

25

37.95

113.88

794

2

3

1

1

1

1

26

42.87

122.88

162

1

1

0

0

0

0

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117.35

7

2

1

1

1

2

2

28

35.02

105.9

1459

2

2

1

1

1

1

29

38.4

115.98

14

1

1

1

1

1

1

30

43.92

128.83

348

1

1

1

1

1

1

31

36.1

114.4

68

1

1

1

1

0

0

32

36.1

114.4

64

3

1

1

1

1

1

33

35.26

106.31

2430

1

1

1

1

0

0

34

39.96

118.61

73

1

1

1

0

0

0

35

39.5

119.17

1

1

1

1

1

1

1

36

39.87

118.87

58

3

1

1

1

1

1

37

42.35

126.38

620

1

1

1

1

1

1

38

42.28

126.6

797

1

1

1

1

1

1

39

42.3

126.36

655

1

1

1

0

0

0

f

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*The number 1 represents the forest, 2 represents the forest-steppe ecotone, 3 represents the steppe, as well as 0 indicates the vacancy value

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*Bold, black, and italic number 1 represents the forest group membership within the forest –steppe ecotone during different periods of the Holocene.

*The mean elevation of all the fossil pollen sites in this study is 960 m.

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ecotone is 1550 m.

Journal Pre-proof Highlights We stress that East Asian summer monsoon and topography co-determine the shift of the forest-steppe ecotone in northern China, and mountainous terrain differences also benefit the vertical forest migrations when threatened by the dry climate. The climate indication of the

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forest-steppe ecotone needs to be considered from three dimensions.

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The authors declare no conflicts of interesting.