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Journal of Geographical Sciences >

2022 , Vol. 32 >Issue 6: 1136 - 1156

DOI: https://doi.org/10.1007/s11442-022-1989-2

Research article

Anthropogenic origin of a change in the fire-climate relationship in northern China after ~2000 yr BP: Evidence from a 15,500-year black carbon record from Dali Lake

  • ZHANG Zhiping , 1 ,
  • LIU Jianbao , 1, 2, 3, * ,
  • CHEN Shengqian 2, 3 ,
  • ZHANG Shanjia 1 ,
  • JIA Xin 4 ,
  • ZHOU Aifeng 1 ,
  • ZHAO Jiaju 5 ,
  • CHEN Jie 1 ,
  • SHEN Zhongwei 1 ,
  • CHEN Fahu 1, 2, 3
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  • 1.Key Laboratory of Western China’s Environmental Systems (Ministry of Education), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China
  • 2.Group of Alpine Paleoecology and Human Adaptation (ALPHA), State Key Laboratory of Tibetan Plateau Earth System, Resources and Environment (TPESRE), Institute of Tibetan Plateau Research, CAS, Beijing 100101, China
  • 3.CAS Center for Excellence in Tibetan Plateau Earth Sciences, CAS, Beijing 100101, China
  • 4.School of Geography, Nanjing Normal University, Nanjing 210023, China
  • 5.State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS, Xi’an 710061, China
* Liu Jianbao (1985-), Professor, specialized in paleoclimatology and paleolimnology. E-mail:

Zhang Zhiping (1993-), PhD Candidate, specialized in paleoclimatology and paleolimnology. E-mail:

Received date: 2021-12-10

  Accepted date: 2022-02-14

  Online published: 2022-08-25

Supported by

National Natural Science Foundation of China(41790421)

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Abstract

There are debates regarding whether a wet and warm climate or a dry and cold climate dominated Holocene fire activity in northern China on the millennial timescale, and when human activities overtook climate change as the dominant control on fire occurrence in the region. Here we present a high-resolution fire history for the past ~15,500 years from a sediment core in Dali Lake, located in the foothills of the Greater Hinggan Mountains, one of the areas of highest fire risk in China. The results demonstrate that fire activity was rare during the last deglaciation (~15,500-11,700 yr BP), gradually increased at the beginning of the Holocene, and reached its highest level during ~9000-5000 yr BP, after which there was a decreasing trend. However, after ~2000 yr BP this decreasing trend ended, and the most prominent feature is a peak in fire activity during the Medieval Warm Period (MWP). Overall, fire activity corresponded well to changes in the East Asian summer monsoon (EASM) precipitation on the millennial timescale during ~15,500-2000 yr BP, but this relationship changed after ~2000 yr BP. We propose that fire activity in northern China on the millennial timescale during ~15,500-2000 yr BP was dominated by the biofuels reserve under the control of the EASM precipitation. In contrast, with the intensification of human activities after ~2000 yr BP, human activity caused a ~62%-73% increase in fire activity, which altered the fire-climate relationship that had previously prevailed in northern China. Our results indicate that a wet-warm climate (increased EASM intensity), rather than a dry-cold climate, was the dominant control on fire activity in northern China during 15,500-2000 yr BP on the millennial timescale, but that human activities played an important role in fire occurrence after ~2000 yr BP.

Key words： fire activity; East Asian summer monsoon; biofuels reserve; human activities; northern China

Cite this article

ZHANG Zhiping , LIU Jianbao , CHEN Shengqian , ZHANG Shanjia , JIA Xin , ZHOU Aifeng , ZHAO Jiaju , CHEN Jie , SHEN Zhongwei , CHEN Fahu . Anthropogenic origin of a change in the fire-climate relationship in northern China after ~2000 yr BP: Evidence from a 15,500-year black carbon record from Dali Lake[J]. Journal of Geographical Sciences, 2022 , 32(6) : 1136 -1156 . DOI: 10.1007/s11442-022-1989-2

1 Introduction

Fire is one of the most important components of the Earth system (Bowman et al., 2009; Yue et al., 2020), and it also played a vital role in the evolution of hominids (Chazan, 2017; MacDonald et al., 2021) and it is important for the health and welfare of modern humans (Shen and Sun, 2016; Theys et al., 2020; Gatti et al., 2021). Ongoing global climatic warming and recent extensive fires are causing increasing concerns about the prospect of a ‘planet on fire’ (Liu et al., 2010; Pechony and Shindell, 2010; Abatzoglou and Williams, 2016; Nolan et al., 2020; Pang et al., 2021). In northern China, fire occurrence is expected to increase in the near future (Pechony and Shindell, 2010; Liu et al., 2012; Ji et al., 2021). Given the fragile ecosystems of the region (Li, 2008; Li et al., 2016), determining the underlying causes of fire occurrence by reconstructing its long-term fire history is therefore important for fire prevention and control, especially against the background of continued warming.
Although numerous paleo-fire reconstructions have been produced for northern China, it remains poorly understood how climate change modulated the fire occurrence in the region on the millennial timescale during the Holocene. For example, it has been suggested that wildfire in northern China was more frequent under a cold and dry climate, but it was greatly reduced under a warm and humid climate, implying an anti-phase relationship between fire activity and the EASM intensity on the millennial timescale during the Holocene (Huang et al., 2006; Jiang et al., 2008; Zhang et al., 2015; Miao et al., 2016; Tan et al., 2011, 2013, 2015, 2018). However, in a sedimentary record from Hulun Nuur Lake in the semi-arid forest-steppe ecotone in northern China, Yin et al. (2016) observed no simple inverse relationship between fire occurrence and the EASM intensity during the Holocene and suggested that vegetation was a critical factor for fire occurrence in northern Chia, via the accumulation of biofuel. Other studies have found that the Holocene fire activity in northern China was positively correlated with the EASM intensity and with tree pollen percentages, on the millennial timescale (Wang et al., 2013; Ji et al., 2021). Additionally, Pang et al. (2021) proposed that temperature, rather than the EASM precipitation, played the primary role in driving the occurrence of forest fires in northeastern China during the Holocene.
The human population of northern China increased significantly during the late Holocene, accompanied by the intensification of land use and the resulting disturbance of the Earth’s surface (Hosner et al., 2016; Ruddiman et al., 2016; Stephens et al., 2019; Chen et al., 2021; Zhang et al., 2021a). However, it remains unclear whether and when human activities overtook climate change as the dominant control of fire occurrence in the region. For example, based on the record from a sediment core from Gonghai Lake in northern China, Ji et al. (2021) proposed that the regional fire history was largely unaffected by human activity since the last deglaciation. However, Wang et al. (2013) and Mu et al. (2016) found that human activities had a significant impact on the increase in fire activity in northern China after ~2000 yr BP. Moreover, based on analyses of climate proxies from loess sediments in the Horqin Sandy Land in northern China, Mu et al. (2016) proposed that the high fire occurrence in the region during the mid-Holocene was mainly caused by the development of human civilization, while during the late Holocene both climate change and human activities were suggested to have jointly controlled the fire occurrence in the region (Mu et al., 2016). Additionally, Pang et al. (2021) found that human activities have caused a significant increase in fires in northeastern China from ~150 yr BP.
Black carbon (BC) is a carbonaceous product formed during the incomplete combustion of biomass and fossil fuels (Goldberg, 1985; Schmidt and Noack, 2000; Masiello, 2004). Due to its highly aromatic structure, and strong biological and chemical inertness, BC can be distributed globally and preserved and accumulated in geological archives, including in soils and lake and marine sediments, for thousands to millions of years (Gustafsson and Gschwend, 1998; Masiello and Druffel, 1998; Schmidt and Noack, 2000; Wang et al., 2013). Therefore, as a product of combustion processes, the BC concentration is an excellent tracer of fire activity and can be used to reconstruct fire histories on different timescales (Wolbach et al., 1985; Lim and Cachier, 1996; Schmidt and Noack, 2000; Han et al., 2018; Ji et al., 2021).
The Greater Hinggan Mountains support some of the most diverse forest ecosystems in northern Asia (Zhao et al., 2016), and they are one of the areas of highest fire risk in China (Liu et al., 2012; Tian et al., 2016), with more than 1600 forest fires reported in the region from AD 1965 to 2010 (Hu et al., 2012). Additionally, archaeological records from the region indicate that human occupation has been continuous during the Holocene (Xia et al., 2000; Han et al., 2012; Yang et al., 2012; Zhuo et al., 2013; Yang et al., 2015; Yuan et al., 2019). In this study we selected Dali Lake, a closed lake located in the foothills of the Greater Hinggan Mountains, as the research object. We used a sedimentary record of BC concentration to reconstruct the regional fire history since the last deglaciation. By comparing the fire history with records of regional climate change and human activity, we attempt to determine the factors driving fire occurrence in the region during the Holocene, in order to provide an historical reference for the formulation of regional fire prevention and control policies.

2 Regional setting and sampling

Dali Lake is the second largest lake in Inner Mongolia, China. It is in the foothills of the Greater Hinggan Mountains and its basin includes the Greater Hinggan Mountains to the northeast and the Otindag Sandy Land to the south (Figure 1). The modern area of Dali Lake is ~180 km2 and the elevation is 1223 m a.s.l. The lake basin is hydrologically closed and the catchment area is ~4700 km2. Dali Lake is fed by four rivers (Figure 1): two permanent rivers, the Gongger and Salin rivers, enter the lake from the northeast, and two intermittent streams, the Holai and Liangzi rivers, enter from the southwest. The Gongger River, which originates in the Greater Hinggan Mountains, is the most important source and comprises 57% of the total annual river supply, reaching 3.2×107 m3 per year (MODNNR, 2015). The modern natural vegetation of the Dali Lake catchment is temperate steppe (Wen et al., 2017). Drought tolerant shrubs and herbs grow in the Otindag Sandy Land, and grassland is developed in the northern and western hilly lands and in the eastern lacustrine plains. Mixed coniferous-broadleaved forest is distributed on the western slopes of the Greater Hinggan Mountains. In addition, given that Dali Lake lies on the northern margin of the EASM region, within the ranges of 43.2°-43.4°N and 116.5°-116.7°E, the regional climate is sensitive to changes in the EASM. According to records from the closest meteorological station (Hexigten station) to Dali Lake, the region has a mean annual temperature of 3.2℃ and mean annual precipitation of 392 mm, 66% of which consists of intense summer precipitation during June-August.
Figure 1 The modern catchment of Dali Lake (b). Inset (a) shows the modern northern limit of the Asian summer monsoon (green dashed line, Chen et al., 2008) and the location of Dali Lake and other sites mentioned in the text (Daihai Lake, Gonghai Lake, Horqin Sandy Land (WNT site)). The main trajectories of the westerlies and the EASM are indicated by arrows.
In January 2019, we collected an 1135-cm-long sediment core (DL19B) from the lake center (43.2798°N, 116.6391°E) at a water depth of ~8 m, using a Livingstone piston corer. The base of the lacustrine sequence consists of coarse sands. The core was scanned and photographed with an X-ray fluorescence core scanner (Figure 2) and then subsampled contiguously at 1-cm intervals at the Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Lanzhou University.
Figure 2 Plots of the stratigraphy and sedimentary parameters for core DL19B. From left to right: lithology photo; AMS 14C dates (circles are dates from bulk organic matter and squares are dates from terrestrial plant remains); mean grain size (μm); total organic matter content (TOC, %); carbonate content (%). Due to the large differences in the sediment accumulation rate and sedimentary parameters, core DL19B was divided into three intervals, with different reservoir ages being used to correct the radiocarbon chronology of each interval (Section 4.1).

3 Methods

3.1 AMS 14C dating

Fifteen bulk samples of bulk organic matter and two samples of terrestrial plant remains were used for radiocarbon dating in the Beta Analytic laboratory (USA). All the 14C ages were calibrated with reference to the OxCal 4.2 and IntCal 20 datasets (Table 1). Several estimates have been made of the size of the carbon reservoir in the lacustrine sediments of Dali Lake, with estimated reservoir ages of 611 years (Xiao et al., 2008, 2009; Liu et al., 2016a), 472 years (Fan et al., 2016, 2017, 2019; Wen et al., 2017), and 0 years (Liu et al., 2016b; Goldsmith et al., 2017). In this study we calculated the reservoir age in different core intervals according to the sediment accumulation rate and the 14C ages of terrestrial plant residues (Zhou et al., 2021). The corrections were then applied to establish a new chronological framework for core DL19B (see Section 4.1 for details).
Table 1 AMS 14C ages for core DL19B
Lab code Depth (cm) Material 14C age (BP) Calibrated 14C age (2σ, cal. yr BP) δ13C (‰)
Beta-543028 7 Bulk organic matter 1510 ± 30 1403 (1521-1328) -27.8
Beta-543029 99 Bulk organic matter 1880 ± 30 1819 (1885-1728) -24.5
Beta-538246 199 Bulk organic matter 2170 ± 30 2207 (2309-2065) -24.5
Beta-543030 299 Bulk organic matter 2920 ± 30 3065 (3160-2969) -24
Beta-538248 399 Bulk organic matter 3540 ± 30 3816 (3904-3716) -24.9
Beta-538249 499 Bulk organic matter 4550 ± 30 5188 (5318-5053) -25.2
Beta-538250 599 Bulk organic matter 5290 ± 30 6077 (6182-5954) -26.8
Beta-543031 649 Bulk organic matter 6720 ± 30 7587 (7656-7515) -25.1
Beta-538251 699 Bulk organic matter 8430 ± 30 9465 (9523-9422) -26.5
Beta-543032 787 Bulk organic matter 10400 ± 30 12267 (12413-12097) -27.2
Beta-538252 787 Plant residue 9950 ± 30 11359 (11600-11249) -24.1
Beta-543033 849 Bulk organic matter 10620 ± 40 12609 (12704-12534) -25.8
Beta-538253 899 Bulk organic matter 10980 ± 30 12825 (12954-12725) -27.6
Beta-538254 999 Bulk organic matter 12200 ± 30 14092 (14205-13982) -26.6
Beta-543034 1049 Bulk organic matter 12520 ± 40 14789 (15090-14435) -26.5
Beta-538255 1050 Plant residue 12380 ± 50 14441 (14764-14138) -15.7
Beta-543035 1129 Bulk organic matter 13060 ± 40 15649 (15853-15399) -25.4

3.2 Black carbon analysis

The BC concentrations of 350 subsamples from core DL19B were measured at the Enenta Chemical Technology Company (Guangzhou, China) using an improved chemothermal oxidation method (CTO-375) (Kuhlbusch, 1995; Gélinas et al., 2001; Gustafsson et al., 2001; Xu et al., 2018). The analytical procedure consisted of the following steps: (1) each freeze-dried subsample (~200 mg) was homogenized and pulverized into powder (< 106 μm); (2) each subsample (~150 mg) was placed in a disposable semi-permeable ceramic crucible and then treated with 1 N HCl for 12 h to remove carbonates, followed by rinsing and drying; (3) the treated subsamples were heated in an SG-GL 1100 tube furnace at 375℃ for 24 h under a continuous air flow to remove labile organic matter; (4) the BC concentrations were measured using an ELTRA CS 800 sulfur-carbon analyzer. Before analyzing subsamples, 6 carbon standard samples (914E stainless steel, Eltra Company, Germany; 0.1860% carbon) were measured for quality control. The limit of quantification (LOQ) in this study was (2.3 ± 0.3) μg/g BC. In the uppermost 3.5 m of core DL19B, spanning the past ~2000 years, BC was measured at a ~2-cm interval, producing a higher temporal resolution than in the rest of the core.

3.3 Analyses of grain size and organic matter and carbonate content

Grain size analysis was conducted at 1-cm intervals with a Malvern Mastersizer 2000 laser grain size analyzer (grain size range of 0.01-2000 μm), following standard pretreatment procedures (Lu and An, 1998): (1) ~400 mg of freeze-dried lake sediment was heated with 10 ml of 10% H2O2 to remove organic matter; (2) 10 ml of 10% HCl were added to remove carbonates; (3) 150 ml of distilled water were added to remove acidic ions; (4) after leaving for 24 h, the particles were dispersed by adding 10 ml of 0.5 N (NaPO3)6, combined with ultrasonication. A total of 1124 subsamples were analyzed.
Total organic matter and carbonate content were determined by the loss on ignition (LOI) method (Dean, 1974), using samples at ~3-cm intervals (total of 346 samples). The procedure consisted of the following steps: (1) A clean crucible was dried for 2 h at 550℃, and after cooling to room temperature its mass (M1) was measured. (2) ~100 mg of subsample was weighed and placed in a crucible, followed by oven drying for 18 h at 105℃, and the mass of the sample and crucible (M2) was recorded after cooling to room temperature. (3) The sample was then combusted at 550℃ for 4 h and the mass of the sample and crucible (M3) was recorded after cooling to room temperature. (4) The sample was then re-combusted at 950℃ for 2 h and the mass of the sample and crucible (M4) was measured after cooling to room temperature. The total organic matter content was calculated as (M2-M3)/(M2-M1), and the carbonate content as 2.273×(M3-M4)/(M2-M1).

3.4 Archaeological records

Location information for archaeological sites in the region was digitized and georeferenced. The information was obtained mainly from the Atlas of Chinese Cultural Relics (NCHA, 2003, 2013), and from the other published literature. Digitizing the location information of the archaeological sites (including caves, settlements, stone carvings, cities, kilns, tombs and temples) during different periods enabled us to compile a spatiotemporal database comprising archaeological sites within a radius of 200 km centered on Dali Lake. A total of 816 archaeological sites were obtained. We then interpolated the chronology of the archaeological sites dated to the prehistoric period to an average time window of 100 years. The chronology of archaeological sites over the past 2000 years was determined based on the succession of Chinese dynasties.

4 Results

4.1 Chronological framework of core DL19B

Based on the 14C ages of 15 samples of bulk sample organic matter from core DL19B and their corresponding core depths, we established an age-depth model for core DL19B (Figure 2). The results indicated that core DL19B could be divided into three intervals: 0-499 cm, with a sediment accumulation rate of ~1.62 mm/yr; 599-787 cm, with a rate of ~0.37 mm/yr; and 849-1129 cm, with a rate of ~1.15 mm/yr. The three intervals with different sediment accumulation rates correspond well with changes in mean grain size, organic matter content, and carbonate content (Figure 2).
The reservoir age for each of three intervals of core DL19B was calculated. The reservoir age for the top interval was obtained by fitting a linear regression line with the following equation: y = 6.0594x+1244.8, where x is the core depth (in cm) and y is the corresponding 14C age (in years). For x = 0 (i.e., the surface sediment layer), y = 1244.8 years; therefore, we assigned a reservoir age of 1250 years for the upper interval (0-499 cm). The use of linear extrapolation to determine the reservoir age of the middle and lower intervals of core DL19B would produce a large error due to the changes in the sediment accumulation rate. However, the middle and lower intervals of core DL19B contained terrestrial plant remains, which are widely regarded as unaffected by a carbon reservoir effect. Subtracting the 14C age of the terrestrial plant remains at the depth 787 cm from the 14C age of the bulk organic matter sample from the same depth produced an estimated reservoir age of 450 years for the middle interval (599-787 cm). Similarly, subtracting the 14C age of terrestrial plant remains from the 14C age of the bulk organic matter sample at a similar depth (~1050 cm) produced a reservoir age of 140 years for the lower interval.
In summary, we were able to estimate the reservoir age of different intervals of core DL19B: the reservoir age of the six bulk organic matter samples from the depth interval of 0-499 cm is ~1250 years; that of the four bulk organic matter samples from the interval of 599-787 cm is 450 years; and that of the five bulk organic matter samples from the interval of 849-1129 cm is 140 years (Figure 2). Deducting the corresponding reservoir age from the 14C age of all 15 bulk organic matter samples from the different intervals enabled us to obtain their reservoir-corrected 14C ages (Figure 3). Finally, the 14C ages of the two samples of terrestrial plant remains and the 15 reservoir-corrected 14C ages were used to provide a final age-depth model for core DL19B, which was established using the r-bacon package in “R” software (Figure 3). The resulting chronology indicated that core DL19B spans the interval from ~15,500 yr BP to the present, with a mean BC sample resolution of ~44 years.
Figure 3 Bayesian age-depth model for core DL19B, calculated using the Bacon program with the IntCal 20 dataset

4.2 Fire history of Dali Lake since the last deglaciation

The fire history at Dali Lake since the last deglaciation, reconstructed from the BC record of core DL19B, is shown in Figure 4a. The BC concentration ranges from 0.07 to 1.52 wt% (percentage by weight), and the variations on the millennial timescale can be divided into three stages: (1) During ~15,500-11,700 yr BP, the BC concentration remained at a low level (~0.11 wt%), demonstrating that fire activity was relatively rare during the last deglaciation in the region. However, there was a slight increase during the Bølling/Allerød (BA) interstadial (~15,000-13,000 yr BP), and a minimum during the subsequent Younger Dryas (YD) cold event (~13,000-11,700 yr BP). (2) At the beginning of the Holocene, the BC concentration gradually increased, reflecting an enhancement of fire activity. The BC concentration reached a high level during ~9000-5000 yr BP, with a mean of 0.66 wt%, indicating the highest level of fire activity during the studied interval. The BC concentration then decreased gradually after ~5000 yr BP, demonstrating a gradual reduction of fire activity. (3) During the third stage (the past ~2000 years) the mean BC concentration is 0.35 wt% and no clear trend is evident; instead, there was a peak in BC concentration during the MWP (~1000-650 yr BP). This indicates that fire activity in northern China was relatively constant during the past 2000 years but peaked during the MWP (Figure 4a).
Figure 4 Comparison of the Holocene fire history at various sites in northern China. (a) BC concentration from Dali Lake (this study). (b) BC concentration from Gonghai Lake (Ji et al., 2021). (c) Inferred fire frequency from Daihai Lake (Wang et al., 2013). (d) BC concentration in the WNT loess-soil profile, Horqin Sandy Land (Mu et al., 2016). The broken lines in (a and b) are the result of the application of a low-pass filter with a 5000 yr window.

5 Discussion

5.1 Comparison of fire history in northern China since the last deglaciation

The fire history in northern China during the Holocene, as revealed by the BC record from Dali Lake, is generally consistent with other paleo-fire records from northern China. For example, a record of wildfire activity from a sediment core from Gonghai Lake spanning the last ~14,800 years shows that wildfire activity was relatively rare during the last deglaciation, gradually increased at the beginning of the Holocene, and reached a maximum during the mid-Holocene. After ~6500 BP the wildfire activity gradually decreased, but after ~2000 yr BP there is no evidence of a decreasing trend. However, the relatively uniform level of wildfire activity during the past 2000 years was punctuated by a peak during the MWP (Ji et al., 2021, Figure 4b), which is correlative with that at Dali Lake. Wang et al. (2013) presented a Holocene fire history reconstructed from the BC content of the sediments from Daihai Lake (Figure 4c). They concluded that the fire activity increased from the beginning of the Holocene, reached a maximum during the mid-Holocene, and then decreased until ~2000 yr BP; however, after ~2000 yr BP the fire activity increased substantially. Additionally, climate proxies from loess sediments in the Horqin Sandy Land in northern China show that the fire intensity gradually increased from the beginning of the Holocene and reached a maximum during the mid-Holocene. After ~5000 yr BP the fire intensity gradually decreased but increased again after ~2000 yr BP (Mu et al., 2016, Figure 4d).
In summary, within the limits of the available chronology, the inferred fire activity at different sites in northern China gradually increased from the beginning of the Holocene, reached a maximum during the mid-Holocene, and then gradually decreased. After 2000 BP there are minor inconsistencies in inferred regional fire activity between different studies, possible because of differences in temporal resolution; however, none of the studies show a decreasing trend in fire activity. Additionally, given that the record of BC concentration from Dali Lake has a higher temporal resolution and temporal range, our results provide a potentially clearer picture of fire activity in northern China during the Holocene.

5.2 EASM intensity controlled the fire occurrence in northern China during ~15,500- 2000 yr BP

There are two prerequisites for fire occurrence under natural conditions: sufficient biofuel for combustion, and the biofuel temperature reaching the ignition point (Ji et al., 2021). In addition, anthropogenic ignitions can also increase fire activity (Wang et al., 2013; Miao et al., 2016; Mu et al., 2016; Pang et al., 2021). We now explore the factors driving the fire history in northern China from these perspectives.
During the last deglaciation (~15,500-11,700 yr BP), the temperature was low and the EASM intensity was weak (Figures 5a and 5b), and there were frequent fluctuations in the regional climate (Figure 5). These conditions were not conductive to the flourishing of the regional vegetation, and the vegetation cover was relatively sparse (Figures 5c and 5d). The inferred sparse forest cover in northern China during the last deglaciation is confirmed by records of tree pollen percentages from Gonghai Lake (Xu et al., 2017), Daihai Lake (Li et al., 2004), and broadleaf tree pollen percentages from Moon Lake in the Greater Hinggan Mountains (Wu et al., 2016). Given that fuel is a prerequisite for fire occurrence (Ji et al., 2021), and given the good relationship between fire activity and vegetation development evident in Figure 5, we propose that the insufficient biofuels reserve, due to sparse vegetation coverage, was the reason for the low level of fire activity in the region during the last deglaciation. In addition, the Greater Hinggan Mountains may have been covered by thick snow during the last deglaciation which also reduced the occurrence of fires (Zhao et al., 2009; Pang et al., 2021). Although fire activity was low during the last deglaciation, there was a slight increase during the BA interstadial and a decrease during the subsequent YD cold period, which corresponds well with the development and subsequent degradation of forest vegetation (Figure 5). This further demonstrates that the climate-limited availability of biofuels was the dominant control on fire activity in northern China during the last deglaciation.
Figure 5 Comparison of fire activity with climatic records since the last deglaciation. (a) Synthesized Northern Hemisphere (30°-90°N) temperature record during the last deglaciation (Shakun et al., 2012) and the Holocene (Marcott et al., 2013). (b) Pollen-based precipitation reconstruction from Gonghai Lake (Chen et al., 2015). (c) Tree pollen percentages from Gonghai Lake (Chen et al., 2015). (d) Tree cover reconstruction for the Dali Lake region (Han et al., 2020). (e) Fire activity reconstructed from the BC concentration of core DL19B (this study, broken line is the result of the application of a low-pass filter with a 5000 yr window). (f) Estimated Holocene anthropogenic land use of China (Klein Goldewijk et al., 2017).
The precipitation delivered by the EASM gradually increased at the beginning of the Holocene (Figure 5b), thus promoting the growth of the regional vegetation in northern China, which would in turn have increased the biofuel reserve. The temperature increase at the beginning of the Holocene (Figure 5a) would also have caused the biofuel temperature to reach the ignition point. Therefore, fire activity in northern China gradually increased at the beginning of the Holocene (Figure 5e). During ~9000-5000 yr BP, the EASM precipitation in northern China (Chen et al., 2015) and temperature in the Northern Hemisphere (Marcott et al., 2013) both reached a peak. Additionally, an analysis of the stability of the EASM indicates that the climate was more stable during the mid-Holocene (Zhang et al., 2021b). Optimum hydrothermal conditions promoted the rapid growth of vegetation and thus led to an increase in the regional biofuel level during the mid-Holocene. Therefore, against the background of a stable and strong EASM during the mid-Holocene (9000-5000 yr BP), the rapid growth of vegetation led to an increase in the biofuels reserve, which would have enhanced the fire activity (Figure 5e). Additionally, based on a pollen record from Dali Lake, Wen et al. (2017) showed that Pinus, a resin-secreting and flammable plant, dominated the region during the mid-Holocene. This may also have increased the probability of fire occurrence during this stage.
After ~5000 yr BP the EASM precipitation and temperature gradually decreased, and the resulting cold and dry climate was no longer conducive to the growth of woody vegetation (Figure 5). A decrease in the surface vegetation coverage is also shown by a gradual increase in regional aeolian sand activity (Chen et al., 2021). Against this climatic background, the biofuel available for combustion in the region gradually decreased, and thus fire activity also decreased (Figure 5e). However, after ~2000 yr BP, the fire dynamics deviated from the trend of decreasing EASM precipitation and the resulting decrease in tree cover, on the millennial timescale (Figure 5). Thus, there may have been a shift from natural forcing to an anthropogenic forcing of the fire occurrence in northern China (see Section 5.2 for details).
A good correlation between fire and the vegetation growth on the millennial timescale is also observed at other sites in northern China. For example, Ji et al. (2021) reconstructed a wildfire history on the northern marginal zone of the EASM region for the last ~15,000 years and proposed that for most of the Holocene the probability of wildfire was relatively uniform, with the wildfire occurrence being dominated by the amount of biofuel. A sedimentary BC record from the Daihai region in northern China revealed an overall synchrony between the development of forest vegetation and the wildfire frequency during the Holocene (Wang et al., 2013).
Although the increased precipitation may have wettened the biofuels making ignition more difficult, in the moisture-limited arid and semi-arid region of northern China, the water content of the biofuel may not have been a limiting factor for combustion (Ji et al., 2021). In this case and given that vegetation succession in northern China is closely related to the EASM precipitation (Chen et al., 2015; Huang et al., 2018; Zhang et al., 2018), we prefer to believe that the amount of biofuel controlled by the EASM precipitation was likely to be the dominant determinant of the regional fire dynamics on the millennial timescale during the Holocene (Wang et al., 2013; Ji et al., 2021). Regarding why fires occurred frequently in northern China against the background of a cold and dry climate that proposed in previous studies, it may be that the charcoal records of previous studies do not provide a high-resolution fire history; and it may also be due to the large chronological error of paleo-fire records existing in previous studies. Furthermore, several studies have indicated that human activities played an important role in the increase of fire activity in the Chinese Loess Plateau during the mid-late Holocene (e.g., Huang et al., 2006; Tan et al., 2011, 2015, 2018). However, the archaeological evidence shows that, before ~2000 yr BP, the number of archaeological sites in the Dali Lake catchment and the surrounding area was relatively small (Figure 6); moreover, the agricultural technology in the region was less advanced compared with that of the Yangshao culture in the Yellow River Basin (Xia et al., 2000). This implies that human activities may have had less influence on the fire dynamics of the study area before 2000 yr BP.
Figure 6 Spatial distribution of archaeological sites within a radius of 200 km centered on Dali Lake. Orange dots indicate archaeological sites older than 2000 yr BP, and the green dots indicate archaeological sites younger than 2000 yr BP.
In summary, the variation of fire activity reconstructed by the BC concentration from the sediments of Dali Lake was dominated by the biofuels reserve under the control of the EASM precipitation on the millennial timescale during ~15,500-2000 yr BP. During the early-middle Holocene, when the hydrothermal conditions were at an optimum, the vegetation biomass increased rapidly and thus the biofuels reserve increased, which led to an enhancement of fire activity. Conversely, during the last deglaciation and the late Holocene, the cold and dry climate was not conducive to the growth of vegetation and the biofuels reserve for combustion was therefore decreased, which would have reduced fire activity. This also demonstrates that, on the millennial timescale, the warm and humid climate (relatively intense EASM), rather than a cold and dry climate, drove the fire occurrence in northern China during ~15,500-2000 yr BP.

5.3 Effects of human activities on the fire-climate relationship in northern China after ~2000 yr BP

The trend of fire activity in northern China has remained relatively stable over the past 2000 years, against the background of a long-term decrease in the EASM precipitation and a resulting decrease in tree cover (Figure 5). We compared the fire intensity during the past 2000 years with that in the early Holocene (11,000-9000 yr BP). The result shows that intensity of fire activity during the past ~2000 years resembles or is even greater than that during the early Holocene, even though precipitation and tree cover were lower during this interval (Figure 7). This evidence indicates that, beginning at ~2000 yr BP, there was a change in the fire-climate relationship in northern China compared to the interval of ~15,500-2000 yr BP.
Figure 7 The upper part of the figure shows a schematic diagram of the local landscape during the early Holocene (11,000-9000 yr BP) and the past ~2000 years. The lower part of the figure shows a comparison of a statistical summary (boxplots) of the EASM precipitation (Chen et al., 2015), tree cover in the Dali Lake region (Han et al., 2020) and the BC concentration of core DL19B (this study) between the early Holocene and the past 2000 years.
Modern instrumental observations have revealed that ~80% of fires in eastern Inner Mongolia during the past few decades were clustered within 10 km of cultivated land, and that fires generally occurred in areas of intense human activity (Li et al., 2017). The past 2000 years were an important period in the transformation of the Earth surface system in northern China, under the influence of human activities (Chen et al., 2020a, b, 2021; Dong et al., 2020). By ~2000 yr BP the population of China was close to 60 million, and most of the population was concentrated in northern China (Zhao and Xie, 1988). The number of archaeological sites in the Dali Lake catchment and the surrounding area also increased substantially after ~2000 yr BP (Figure 6). Furthermore, based on a high-resolution Holocene record of dust storms from the sediments of an undisturbed alpine lake in northern China, Chen et al. (2021) demonstrated that human activities contributed to a shift from natural forcing to an anthropogenic forcing of the Earth surface system in northern China since ~2000 yr BP. Global anthropogenic land-use estimates for the Holocene have indicated that the area of cultivated land and pasture in China increased rapidly during the past 2000 years (Figure 5f, Klein Goldewijk et al., 2017), corresponding to a major increase in the intensity of human activity in northern China. Therefore, we propose that the use of fire in land reclamation, industrial production, cooking and domestic heating, etc., fundamentally altered the previous relationship between the EASM precipitation and fire dynamics in northern China since ~2000 yr BP. It is worth mentioning that, although the anthropogenic land use of China increased significantly after ~1000 yr BP (Figure 5f), the sedimentary BC concentration did not increase synchronously, possibly for the following two reasons. First, against the background of a long-term decrease in the EASM precipitation and a resulting decrease in tree cover over the past 1000 years, the regional woody vegetation cover was low and thus the biofuels reserve for combustion was insufficient. Second, although fire was widely utilized in the early stage for the cultivation of arable land and the expansion of living space, with the management of fire, landscape fragmentation and the decrease of the biofuels reserve connectivity caused by intensive human activities, large fires were less likely to occur.
According to a correlation analysis of the BC concentration and the EASM precipitation (Chen et al., 2015), and with tree cover in Dali Lake region (Han et al., 2020), against the background of natural environmental conditions during 14,500-2100 yr BP, we estimate that the sedimentary BC concentrations over the past ~2000 years should be 0.18% and 0.19% (Figure 8). However, the BC concentration over the past 2000 years was 0.31%, and we attribute this increase (of ~62%-73%) to intensified human activity. These anthropogenic fires reversed the decline in fire activity caused by a reduction of the biofuels reserve against the background of the EASM recession during the late Holocene. In addition, it is worth noting that the BC record from Dali Lake shows a pronounced peak in fire activity during the MWP (Figure 5e). This suggests that the increased probability of fire in northern China on the multi-centennial timescale during this interval may also have been influenced by high temperature (Ji et al., 2021).
Figure 8 Correlation analysis of the BC concentration of core DL19B with EASM precipitation (Chen et al., 2015) and tree cover in the Dali Lake region (Han et al., 2020), during 15,000-2100 yr BP. The data processing methods are summarized as follows: According to the length of the shortest time series among the records of BC concentration at Dali Lake (this study, ~15,500-0 yr BP), EASM precipitation (Chen et al., 2015, ~14,500-0 yr BP) and tree cover in the Dali Lake region (Han et al., 2020, ~19,500-0 yr BP), we interpolated the time series of each record to an average resolution of 100 years, and selected 14,500-0 yr BP as the research interval. We then used the relationship between BC concentration and EASM precipitation and tree cover during 14,500-2100 yr BP as the natural background, which enabled us to extrapolate the theoretical value of BC concentration over the past 2000 years against the natural background.
The increase in fire activity caused by intensified human activity during the late Holocene has been confirmed by many other studies of northern China (Huang et al., 2006; Wang et al., 2013; Miao et al., 2016; Mu et al., 2016; Pang et al., 2021) and elsewhere (Ning et al., 2020; Barhoumi et al., 2021). For example, Wang et al. (2013) proposed that fire activity increased significantly in the Daihai Lake region during the late Holocene, possibly linked to an increase in human activity after ~2300 yr BP. Mu et al. (2016) showed that fires linked to agriculture may have led to increased biomass burning associated with agricultural activity after ~2000 yr BP in the Horqin Sandy Land in southeastern Inner Mongolia. In addition, although the Holocene evolution of fires in southwestern China differs from that in northern China, Ning et al. (2020) suggested that the higher fire occurrence in the region over the past 2200 years was likely related to intensified human activities.
In summary, with the intensification of human activities in northern China after 2000 yr BP, the previous positive relationship between the EASM precipitation and fire activity on the millennial timescale during ~15,500-2000 yr BP broke down. This is because human activity caused a ~62%-73% increase in fire activity in northern China after ~2000 yr BP, which reversed the previous decline in fire activity caused by the reduction of the biofuels reserve against the background of the EASM recession. Thus, our results indicate that human activities may have played an important role in increasing the fire occurrence in northern China after ~2000 yr BP.

6 Conclusions

We have produced a 15,500-year fire record from the sediments of Dali Lake in northern China. Comparison of the record with the regional climatic and human history leads to the following main conclusions:
Fire activity in northern China was rare during the last deglaciation (~15,500-11,700 yr BP), gradually increased at the beginning of the Holocene, and reached its highest level during ~9000-5000 yr BP, after which there was a decreasing trend. However, after ~2000 yr BP this decreasing trend ended and the most prominent feature is a peak in fire activity during the MWP.
The fire occurrence in northern China on the millennial timescale during ~15,500-2000 yr BP was dominated by the occurrence of a warm and wet climate (caused by an intense EASM), rather than by a cold and dry climate. However, over the past ~2000 years, anthropogenic fires increased the level of fire activity by ~62%-73%, which altered the fire-climate relationship that had existed during 15,500-2000 yr BP.
Human activities may have overtaken the EASM as the dominant control on fire occurrence in northern China after ~2000 yr BP.

Acknowledgements

We thank AZARMDEL Hassan, YAN Xinwei, LI Shuai, CHEN Lin and WANG Jianghai for their assistance with field work and laboratory analysis.

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