Late-Holocene changes in vegetation composition and fire regimes in the subalpine western Nanling Mountains in subtropic China

Abstract

Understanding the responses of vegetation composition to climate, fire regimes and human disturbance is crucial to provide valuable insights to protect current and future ecosystems. However, relevant studies were poorly performed on the heavily forested montane areas in subtropic East Asia. In this study, we present pollen and charcoal records from a 95-cm long peat core in the subalpine Daping swamp in the western Nanling Mountains in subtropic China, to discuss the possible interactions among vegetation, fire, climatic change, and human activities in the late-Holocene. Our results suggest that the vegetation in the study area was composed of deciduous–evergreen mixed forests with few fire events during 3140–630 cal yr BP, and its changes were mainly controlled by climate with rare human impacts reflected by low concentrations of charcoals, low proportions of Poaceae pollen, and high AP/NAP ratios. After 630 cal yr BP, pollen data indicate an obvious shift from dense forests to more open landscape. The sharp increase of charcoal concentrations, the extremely low AP/NAP and increased Poaceae percentages suggest that this vegetation shift is not only impacted by the dry climate conditions, but also by the intensified deforestation due to enhanced human activities such as the slash-and-burn cultivation, etc. This study reveals a vital transitional timing from the natural to the superimposed anthropogenic forcing of vegetation composition and fire regimes at ~630 cal yr BP. Regional comparison of charcoal records indicates that the onset timing of intensified human disturbance in the inland montane areas is much later than that in the coastal areas in subtropic China. We infer the obstacle impacts of the montane terrain and the different responses of the dissemination of agriculture and enhanced population migration have played a crucial role in this asynchronous spreading pattern.

Keywords

Daping swamp Pollen assemblages Charcoal concentrations Climatic changes Human activities

Introduction

Vegetation changes, which can be detected by fossil pollen records, are the most direct expression of ecosystem and landscape variations (Cheng et al., 2018). As the abundant and diversified pollen grains preserve rich information on past environmental changes, pollen records from lacustrine sediments are widely used to reconstruct past vegetation succession on different timescales (Ding et al., 2022). A global network of fossil pollen sequences shows that the rates of vegetation changes in the late-Holocene exceeded those in the deglacial period for all continents, which highlights stronger impacts of human impacts than climatic change on terrestrial ecosystems (Mottl et al., 2021; Rao et al., 2022). The selection of study regions that show notable changes in vegetation, climate, and human activities is critical to the research on the impact of climate change and human activities (Liu et al., 2022).

Subtropic East Asia hosts the world’s largest subtropic forests due to the prevailing of the Asian summer monsoon (ASM) (Fang et al., 2021). In this context, global warming and human disturbance have greatly increased the risk of fires in the highly populated subtropic East Asia (Yuan et al., 2022). Satellite imagery and field observation from 2005 to 2018 infer that 84% wildfires in China appear in subtropic area (Fang et al., 2021). As such, this area is key to understand the interactions among vegetation, fire activities, and climate conditions. Significant vegetation shifts from dense forests to more open landscapes since the late-Holocene have been widely documented in subtropic China (e.g. Ma et al., 2018; Yue et al., 2015; Zhao et al., 2017; Zheng et al., 2021). This transformation of vegetation could be a result of drier climatic conditions or intensified human disturbance (Song et al., 2022). Therefore, it is important to ask if the forest changes during the late-Holocene in subtropic China resulted from climate change or human activities.

Appropriate proxies to interpret the linkages among vegetation, climate, and anthropogenic activity are needed to solve the issue. Fire is acknowledged as both a natural phenomenon and a human tool for instance to clear or exploit forest since at least the Early Neolithic (Ma et al., 2018). In addition to natural fire triggers, frequent fire regimes in late-Holocene are often associated with intensified human activities, such as the emergence of early agriculture and the expansion of land use, especially slash-and-burn agricultural and deforestation (e.g. Chen et al., 2022; Li et al., 2009b; Wang et al., 2013). Charcoal preserved in lake sediments, peats and soils provides a record of past fire occurrence (Patterson et al., 1987), as a result, it can be served as a useful proxy to retrieve information on relevant human activities in a study area (Peng et al., 2015). In combination with pollen and charcoal records, it is possible to detect the environmental changes related to fires, deforestation, and agricultural developments (Chen et al., 2014; Hawthorne and Mitchell, 2016), thus possible to disentangle the contributions of climatic variability and anthropogenic impacts to past vegetation changes.

Specific issues such as clarifying the impacts and regional characteristics of human activities in different periods are of great significance to reveal past and current man-land interactions and their possible future trajectory (Sun et al., 2022). Most of the studies on the Holocene fire history in China primarily constrained in the north and the southeast China (especially the coastal areas) (Xu et al., 2021), the case studies from the inland montane areas are relatively rare. In fact, the timing and extent of human activities since the late-Holocene were different between the inland and coastal areas in subtropic China. Plenty of records in coastal areas in subtropic China have been reported (e.g. Chen et al., 2014; Ma et al., 2018), more evidences obtained from inland montane areas are needed to clarify the critical turning point of when human activities replaced natural factors in subtropic China.

The NLM area is not only one of the important regions in the process of southward spread of rice farming from the Yangtze River Basin since the past ~5000 years (Yang et al., 2018) (Figure 1b), but also an ideal site to study the past climatic and environmental changes, which are strongly impacted by the ASM system (e.g. Ye et al., 2019; Zhong et al., 2015; Figure 1a). Previous studies on this area mainly focus on the natural processes including climate variability and ASM evolution history (e.g. Ye et al., 2019; Zhong et al., 2014, 2015, 2017, 2018), only one mercury accumulation record reveals anthropogenic signals starting from ~3400 cal yr BP (Pan et al., 2020), though it does not provide specific evidence of human activities. Robust evidences of natural and human impacts on the ecological environment, especially in the last several millennia, are still needed to elucidate the connections between early human activities and climate change (Wang et al., 2020). In this study, we present new pollen and charcoal data from a peat core in the subalpine Daping swamp in western NLM, in order to answer: (1) What factors (i.e. climate or human activities) determined the vegetation changes and fire regimes in the swamp region throughout the late-Holocene? (2) When did the anthropogenic factors start to replace natural factors as the principal driver of changes in the vegetation shifts and fire regimes in the study area?

Figure 1.

(a) Climate background of Daping swamp (the red solid circle). ISM: Indian summer monsoon; EASM: East Asian summer monsoon; EAWM: East Asian winter monsoon. (b) The geomorphology of the southeast China, the numbers represent the cores mentioned in the text (from left to right are: 1: HNDP-B (Daping swamp, 26.17°N, 110.12°E, 1620 m, this study); 2: GT-2 (Gutian, 26.09°N, 110.37°E, 1677 m) (Ma et al., 2018); 3: GY1 (Gaoyao, 22.90°N, 112.34°E, 29 m) (Ma et al., 2018); 4: SMP (Jinggang Mountains, 26.58°N, 114.08°E, 1269 m) (Huang et al., 2014); 5: HD (Huangdong, 22.95°N, 114.57°E, 7.2 m) (Chen et al., 2014); 6: DYS (Daiyunshan, 25.84°N, 118.27°E, 1579 m) (Zhao et al., 2016); 7: JSP (Jiangshan Peatland, 25.77°N, 118.11°E, 958 m) (Song et al., 2022); 8: SZY (Shuizhuyang, 26.77°N, 119.03°E, 1007 m) (Yue et al., 2012); 9: LTY (Lantianyan, 28.43°N, 119.31°E, 902 m) (Ma et al., 2018); 10: TDH18C (Lake Longgong, 27.98°N, 120.32°E, 1050 m) (Chen et al., 2022); 11: Toushe basin (23.82°N, 120.88°E, 1014 m) (Huang et al., 2020). The dotted circles indicate the major agricultural societies and the expansion of early rice farming from Yangtze Valley into southern China (modified from Chi and Hung, 2010), a: Liangzhu (Zhejiang Province)- Fangchengdui (Jiangxi)- Qujialing (Hubei)- Shijiahe (Hubei), 3500–2500 BC; b: Dingshishan- Xiaojin of Guangxi Province, after 2500 BC; c: Shixia (Guangdong)- Tanshishan (Fujian), 3000 BC; d: Late Dabenkeng (Taiwan), 2800 BC. The arrows show the expansion routes of rice cultivation. (c) Mean monthly precipitation distribution and (d) Mean annual temperature in the Daping swamp and the surrounding area, covering the period of 1901–2020, the gridded climate dataset is downloaded from https://crudata.uea.ac.uk/cru/data/hrg/ (CRU TS 4.05 at 0.5° resolution) (Harris et al., 2020). (e) Vertical vegetation belts in Nanshan Pasture (Xiao et al., 1987; Zhong et al., 2015). The left and right sides indicate the northern and southern slopes, respectively. (f) The location of the core HNDP–B (modified from Zhong et al., 2014).

Regional setting

The Daping Basin, a subalpine intermontane basin (26°10′11″N–26°10′42″N, 110°7′25″E–110°8′00″E, ~1500–2000 m a. s. l.) mainly supplied by atmospheric rainfall and several small springs (Ye et al., 2019), is located in the Bashili Grand Nanshan Mountains, western part of the NLM (Figure 1b) (Zhong et al., 2015). This region is controlled by subtropical monsoon climate, and is influenced by both the East Asian summer monsoon and Indian summer monsoon (Figure 1a). Mean annual temperature here is 10.9℃, with January and July mean temperatures of –0.5℃ and 19℃, respectively. The mean annual precipitation is ~2000 mm and the annual evaporation is ~500 mm, with 68% of the annual rainfall falling in spring and summer seasons (Pan et al., 2020) (Figure 1c and d). Typical montane climate, with relatively low temperature and evaporation as well as high rainfall and humidity, favors the development of Daping swamp in this basin. Investigations indicated that more than 1.0 m thick peat accumulated in the swamp (Sun and Zhang, 1984).

The study region is located in a subtropic montane area, with an altitude of 1200–1904 m a.s.l. (Xiao et al., 1987). Typical vertical vegetation zonings here from top to bottom are characterized by deciduous broad-leaved forest (>1800 m), evergreen coniferous forest (1600–1800 m of southern slope), evergreen and deciduous broad-leaved mixed forest and shrubs (1400–1800 m) (Figure 1e) (Xiao et al., 1987; Zhong et al., 2015). The dominant species of evergreen broad-leaved forests are Cyclobalanopsis glauca, Lithocarpus glaber, Castanopsis egrei, C. sclerophylla, C. jucunda, Cyclobalanopsis oxyodon, Quercus engleriana, etc. (Li et al., 2021). Special conifer and deciduous trees are composed of Pinus tabuliformis var. henryi, Davidia involucrate, Tetracentron sinense, and Fagus lucida, etc. (Li et al., 2021). Original vegetation here should be dominated by subtropic evergreen and deciduous broad-leaved mixed forests and montane coppice (Ye et al., 2019); however, after long-term burning and felling by local residents, only a small area of the typical zonal vegetation currently remains (Xiao et al., 1987). The center of the swamp is occupied by herbs (such as Persicaria hydropiper, Salvia farinacea, Juncus effusus, Eragrostis ferruginea), helophytes (e.g. Phragmites australis, Herba Sphagni) and shrubs (e.g. Fargesia spathacea, Hydrangea paniculate) are partly distributed. Hills with relative height of 30–100 m around this swamp are dominated by herbs (i.e. Roegneria kamoji, Zoysia japonica, Gnaphalium affine, Poaceae, etc.), with shrubs growing in local valleys. The existing large areas of grassland are resulted from the regressive succession after the damage of forest vegetation. In addition, small areas of farming dryland are mainly planted with corns and medicinal plants (e.g. Houpoea officinalis), and fruit plants (e.g. Pyrus sp.) are grown in the surrounding areas of the swamp (Xiao et al., 1987).

Material and methods

In September 2009, a 95-cm long peat core (26°10.472′N, 110°07.223′E, designated as core HNDP-B) was drilled using an Eijkelkamp sampler in Daping swamp (Figure 1f). Age dating was conducted in the Key Laboratory of Western China’s Environmental Systems (Ministry of Education of China), Lanzhou University (Zhong et al., 2014). The bulk portion of the grain size range between 90 and 300 μm fractions of seven bulk samples in the core were collected for conventional radiocarbon, because the conventional ¹⁴C dating of this portion can provide reliable dates agreeing very well with the pollen accelerator mass spectrometry ¹⁴C ages (Zhou et al., 2004), thus the reservoir effect of peat materials could be negligible (Zhong et al., 2014). In this study, we re-calibrated the radiocarbon dates using the latest IntCal 20 curve (Stuiver et al., 2021). The age-depth model of the core was established using the rbacon package (v 2.3) in R software (v 4.0.2).

For pollen analysis, totally 31 samples taken at 2–4 cm intervals from the core was processed. Pollen grains and charcoal fragments were extracted together following standard pollen extraction techniques (HCl, KOH, HF, and acetolysis) (Faegri et al., 1989). Prior to chemical analysis, one Lycopodium tablet (9666 ± 671 grains/tablet) was added to each sample for calculating pollen concentrations (absolute abundances). For pollen identification, published pollen atlas were referred (e.g. Institute of Botany and South China Institute of Botany Academy Sinica (IBSCIBAS), 1982; Tang et al., 2016; Wang, 1995). Microscope NIKON Eclipse Ci-L was used to identify and count pollen, spores, and charcoals normally under 40× objective in the Environmental Evolution Laboratory of South China Normal University, and in some cases 100× objective was used to confirmed some small pollen types which were difficult to identify under 40×. Totally a minimum of 300 terrestrial pollen grains (excludes aquatics and unknowns) was counted for each sample in order to reduce bias of pollen percentages for key taxa.

For charcoal analysis, we counted a minimum of 300 charcoal grains for each sample. Angular and opaque dark-black fragments were identified as charcoals (Li et al., 2010). The size of charcoal particles can help distinguish local fires from regional fires (Li et al., 2010). Based on the longest dimension, we classified charcoal fragments into two categories, that is, the microscopic charcoal (⩽125 μm) and the macroscopic charcoal (>125 μm), and calculated their concentrations according to the amount of Lycopodium.

In this study, pollen, fern spores and common freshwater algae were identified and grouped. We classified the Quercus into the deciduous Quercus (hereafter named Quercus (D)) and the evergreen Quercus (hereafter named Quercus (E)) based on their characteristics of ornamentation and pollen size. What’s more, indistinguishable pollen types with similar morphology characteristics and ecological preference were classified into one category, including Lithocarpus/Castanopsis, Cyclobalanopsis/Quercus (E), Liquidambar/Altingia and Celtis/Pteroceltis. Pollen and spore percentages were calculated based on the pollen sum (trees+shrubs+upland herbs) and pollen and spore sum respectively to independently analyze the compositional changes of seed plants and ferns. The pollen diagram was produced using Tilia software. To characterize the changes in vegetation composition, we used stratigraphically constrained cluster analysis (CONISS) (Grimm, 1987) to distinguish the distinct changes in pollen assemblages based on the proportions of conifers, broadleaved trees and shrubs, and upland herbs.

Results

Lithology and chronology

The top 10 cm of the core is dominated by black peat containing modern roots, and the materials between 10 and 95 cm depth are composed of dark gray peat with herbaceous plant residues (Figure 2) (Zhong et al., 2014). The newly calibrated dating results are listed in Table 1. Here, we use the median ages to establish the age-depth model, and the core bottom age is determined to be 3140 cal yr BP. The accumulation rates display a decreasing trend since the last 3140 years (Figure 2).

Table 1.

Radiocarbon ages of core HNDP-B in Daping swamp.

Lab code	Depth (cm)	¹⁴C age (yr BP)	Calibration age (cal yr BP)		Material
Lab code	Depth (cm)	¹⁴C age (yr BP)	2σ	Median	Material
LUG09-109	10–13	140 ± 40	100–280	130	TOC
LUG09-108	22–25	1470 ± 45	1290–1510	1360	TOC
LUG09-107	42–45	2300 ± 60	2150–2490	2310	TOC
LUG10-120	47–50	2410 ± 70	2340–2710	2490	TOC
LUG09-105	55–58	2610 ± 50	2500–2850	2740	TOC
LUG09-106	62–65	2640 ± 50	2550–2870	2760	TOC
LUG10-119	92–95	2820 ± 80	2770–3160	2940	TOC

Figure 2.

The lithology and age-depth frame of core HNDP-B in Daping swamp (revised after Zhong et al., 2014). The locations of red triangles in the left part represent the depth of the dating samples; the red line on the right indicates the median ages, and gray dotted lines represent the 2σ ranges. The labels next to the line are the average sedimentary rates, which were calculated based on two adjacent ages.

Pollen results

In general, pollen concentrations of the profile are significantly high (average 200,000 grains/g), containing rich plant taxa. A total of 68 pollen taxa are identified, including 40 arboreal pollen (AP) (for trees and shrubs), 24 herbs, 2 ferns, and 2 algae (Table 2). The identified AP is assigned into three plant functional types (PFTs), that is, the coniferous AP (CAP), the deciduous broad-leaved AP (DAP), and the evergreen broad-leaved AP (EAP). The pollen assemblage of the core is dominated by AP (59.9–93.4%, mean: 82.6%), while non-arboreal pollen (NAP) fluctuated from 6.6% to 40.1% (mean: 17.4%). Among AP, Pinus of conifer taxa is the dominant one (16.3–63.3%, mean: 36.5%), followed by the broadleaved plants such as Fagus (10.4–40.5%, mean: 26.5%), Liquidambar (1.7–12.7%, mean: 5.2%), Quercus (D) (0.3–14.8%, mean: 3.3%), and Rhododendron (0.3–10.7%, mean: 2.8%), as well as Cyclobalanopsis/Quercus (E) (0.3–7.6%, mean: 2.2%). The terrestrial herbs mainly include Poaceae (4.9–25.2%, mean: 10.6%) and Persicaria (Polygonum) (0–5.9%, mean: 0.7%), while the aquatic herbs are primarily Cyperaceae (0.6–19.8%, mean: 5%). Fern spores are classified into two types: the Trilete spores and the Monolete spores, which are dominated by Gleicheniaceae and Polypodiaceae, respectively. In addition, two freshwater algae, Concentricystes and Zygnema, are also identified.

Table 2.

Identified pollen taxa of core HNDP-B in Daping swamp.

Plant functional types	Pollen taxa
Conifer AP	Pinus, Tsuga
Deciduous AP	Acer, Alnus, Betula, Carpinus, Carya, Celtis/Pteroceltis, Corylus, Ebenaceae, Fagus, Fraxinus, Hamamelidaceae, Juglans, Liquidambar/Altingia, Pterocarya, Quercus (D), Rhododendron, Syringa, Tilia, Ulmus
Evergreen AP	Acanthaceae, Araliaceae, Caprifoliaceae, Cleastraceae, Cyclobalanopsis/Quercus (E), Elaeagnaceae, Euphorbiaceae, Ilex, Lithocarpus/Castanopsis, Loranthaceae, Meliaceae, Moraceae, Myricaceae, Proteaceae, Rhus, Rutaceae, Sapindaceae, Styracaceae, Symplocos
Upland herbs	Amaranthaceae/Chenopodiaceae, Apiaceae, Artemisia, Aster, Asteraceae, Balsaminaceae, Brassicaceae, Campanulaceae, Caryophyllaceae, Labiatae, Liliaceae, Onagraceae, Poaceae, Persicaria, Ranunculaceae, Rosaceae, Scrophulariaceae, Taraxacum, Thymelaeaceae, Verbenaceae
Aquatic herbs	Alismataceae, Cyperaceae, Lobelia, Typhaceae
Ferns	Monolete spores, Trilete spores
Algae	Concentricystes, Zygnema

Based on the results of CONISS analysis, the pollen combination of the core can be divided into two prominent zones and four sub-zones (Figure 3). The characteristics of the pollen zones from bottom to top in the core are described as follows.

Figure 3.

Pollen percentage spectrum of core HNDP-B in Daping swamp. The gray shadows are exaggeration of original results of the taxa. CAP: conifer arboreal pollen; DAP: deciduous broad-leaved arbor or shrub pollen; EAP: evergreen broad-leaved arbor or shrub pollen.

Zone I (95–42 cm, 3140–2230 cal yr BP)

This zone is characterized by the alternative occupation of conifer taxa and deciduous taxa with the mean values of 42.5% and 38.7% respectively, whereas the evergreen taxa and herbs only account for the mean proportions of 4.1% and 14.7% respectively in this zone. Detail observation reveals that this zone can be also divided into two subzones (Figure 3).

Subzone I-1 (95–87 cm, 3140–3040 cal yr BP)

The lowest pollen concentration (2.2 × 10⁴ grains/g) in the whole profile appears in this subzone. The CAP dominates by Pinus (mean: 24.3%) and Tsuga (mean: 0.8%). Fagus (mean: 25.0%), Liquidambar (mean: 3.4%), Rhododendron (mean: 6.1%), Alnus (mean: 2.4%), and Quercus (D) (mean: 8.7%) are the main DAP taxa, and the latter three reach their maximum values of the core in this period. The EAP taxa are characterized by the dominance of Cyclobalanopsis/Quercus (E) (mean: 3.9%) and Ilex (mean: 2.3%), with the presence of Caprifoliaceae and Symplocos. The NAP taxa are composed by Poaceae (mean: 10.0%) and Persicaria (mean: 1.5%). Ferns spores consist of increased Monolete spores (mean: 17.9%) and Trilete spores (mean: 4.1%). Meanwhile, the concentration of Zygnema displays an evident peak of ~1800 grains/g in this subzone.

Subzone I-2 (87–42 cm, 3040–2230 cal yr BP)

Pollen data exhibit remarkable fluctuations in this subzone (Figure 3). The average pollen concentration increases to ~6.9 × 10⁴ grains/g, and the significant increase in Pinus (mean: 44.4%) leads up to an evident rise of CAP proportion (mean: 46.0%), nearly twice as much as that in the subzone I-1. The proportions of DAP, EAP and NAP display apparent declines (reaching the mean values of 36.2%, 3.4%, and 14.4%, respectively). The percentages of the dominant DAP plants such as Fagus (mean: 23.8%), Quercus (D) (mean: 2.8%), Rhododendron (mean: 2.6%), Betula (mean: 0.8%), and Alnus (mean: 0.4%) decrease significantly. The philotherms and hygrophilous Liquidambar maintains a relatively low proportion (mean: 3.5%). Cyclobalanopsis/Quercus (E) and Ilex of the EAP decrease sharply to 1.8% and 0.7% respectively. Though the concentration of NAP decreases, Poaceae (mean: 9.5%), as the dominant taxa of NAP, exhibits a slight increase, whereas Persicaria (mean: 0.8%) decrease obviously. Meanwhile, the proportion of fern spores distinctly increased from 22.0% to 33.3%.

Zone II (42–0 cm, 2230–0 cal yr BP)

The proportions of the deciduous taxa (36.5–54.8%, mean: 45.7%) and the conifer taxa (19.5–30.6%, mean: 28.9%) displays a persisting declination, the evergreen taxa fluctuate from 1.0% to 5.7% (mean: 3.9%), while the herbs (11.2–40.1%, mean: 21.5%) exhibit a gradual increase and finally exceed the deciduous taxa. Further, this zone can also be divided into two subzones.

Subzone II-1 (42–16 cm, 2230–630 cal yr BP)

The total pollen concentration increases significantly (average of ~1.5 × 10⁵ grains/g). Comparing with the subzone I-2, the proportion of CAP taxa gradually drops to a mean of 31.5%, EAP (mean: 3.8%) and NAP (mean: 15.9%) rise slightly, while DAP (mean: 48.8%) increases rapidly. Fagus of DAP reaches to its maximum (40.5%) at ~2000 cal yr BP, and then decrease gradually, along with increased Liquidambar (mean: 6.8%). NAP is characterized by relatively stable Poaceae (mean: 9.4%). Ferns decrease slightly (mean: 33.7%), as the Trilete spores drastically reduce to a mean of 4.6%, and the Monolete spores reach the highest peak (36%) of the profile at ~1500 cal yr BP. In this subzone, no aquatic herbs are found. During 2000–1700 cal yr BP, the hygrophilous algae Concentricystes rapidly increase from 1120 to 630 cal yr BP.

Subzone II-2 (16–0 cm, after 630 cal yr BP)

The total pollen concentration continues to increase dramatically in this subzone, reaching the maximum of ~1.9 × 10⁵ grains/g. Pinus of CAP declines to the minimum (23.6%); meanwhile, DAP taxa such as Fagus, Betula, Ulmus, and Rhododendron are also significantly reduced. Particularly, Liquidambar still exhibits an increasing trend, reaching the maximum of 10.2%. Both the Quercus (D) and Cyclobalanopsis/Quercus (E) display low values at 150 cal yr BP. The NAP increases to its highest content in this stage (mean: 34.9%), of which Poaceae increases gradually to a maximum of 25.2%. Fern spores reduce to about 23.6%, and both Monolete spores and Trilete spores markedly decrease. The Concentricystes sharply decrease from 260 to 150 cal yr BP and then rapidly rise to ~2320 grains/g since ~150 cal yr BP.

Charcoal data

The micro-charcoal and macro-charcoal concentrations (hereafter named C_micro and C_macro, respectively) display similar fluctuations since the ~3140 cal yr BP (Figure 4), suggesting a consistent occurrence of local and regional fire events. In general, the values of C_micro and C_macro are quite low between 3140 and 630 cal yr BP (mean: ~49,460 and ~1690 grains/g, respectively), and their first peak values (C_micro: ~125,000 grains/g; C_macro: ~2800 grains/g) appear during 2550–2450 cal yr BP. After 2450 cal yr BP, they increase and maintain relatively stable values in 2000–630 cal yr BP. Their second peak values occur during 2000–1700 cal yr BP. However, since ~630 cal yr BP, the C_micro and C_macro display abrupt increases, and reach their maximum values at ~65 cal yr BP (Figure 4).

Figure 4.

Variations of charcoal and pollen signals of core HNDP-B since the 3140 cal yr BP. LIA: the Little Ice Age; MWP: the Medieval Warm Period; DACP: the Dark Age Cold Period; RWP: the Roman Warm Period; IACE: the Iron Age Cold Epoch; BAO: the Bronze Age Optimum (Kobashi et al., 2011). The gray shadows of macro- and micro-charcoals are the exaggeration of the periods before ~630 cal yr BP, whose concentrations are too low compared with those after ~630 cal yr BP.

Discussion

Vegetation and climate changes since ~3140 cal yr BP

In late-Holocene, climatic changes and human disturbance are the critical factors influencing regional vegetation composition. According to the pollen assemblages shown in Figure 3, the changes in vegetation composition that related to climatic conditions and possible human impacts can be divided into four stages.

Stage 1: 3140–3040 cal yr BP (1190–1090 BC)

This stage corresponds to pollen Zone I-1 (Figure 3). The palynological assemblages during this stage suggest that the vegetation is composed of deciduous forests dominated by Fagus, Quercus (D), and Rhododendron. Significantly increased AP/NAP ratios are probably associated with enhanced vegetation coverage based on some previous investigations (e.g. Favre et al., 2008). We interpret relatively high AP/NAP in this stage to link with dense forests, and the increased proportions of the broadleaved taxa suggest a relatively warm and wet climate, which coincides with the Bronze Age Optimum (BAO, 3400–2850 yr BP) (Figure 4) (Kobashi et al., 2011).

Stage 2: 3040–2230 cal yr BP (1090–280 BC)

This stage coincides with the pollen Zone I-2 (Figure 3). The pollen spectrum in this stage is characterized by the alternative predominance of conifer taxa and deciduous taxa (Figure 4), indicating an occupation by conifer and broad-leaved mixed forests. In this stage, increasing AP/NAP can be interpreted as an expansion of conifer and deciduous forests, probably due to seasonal dry and cold climate in favor of cool-tolerant plants prevailing. In addition, in comparison with Stage 1, the apparently decreased evergreen trees suggest declined temperature and humidity. This cold and dry period agrees with the pollen-based cool and dry conditions between 3.0 and 2.1 cal kyr BP detected from the Toushe Basin in Taiwan (Huang et al., 2020), and also corresponds to the Iron Age Cold Epoch (IACE, 2850–2350 yr BP) (Kobashi et al., 2011).

Stage 3: 2230–630 cal yr BP (280 BC–AD 1320)

This stage corresponds to pollen Zone II-1 (Figure 3). The vegetation during this stage is mainly composed of deciduous forests, dominated by Fagus and Liquidambar (Figure 3), suggesting a generally cool and wet climate. Particularly, the increased evergreen taxa from 2230 to 1750 cal yr BP (Figure 4) indicates relatively warm and humid conditions, corresponding to the warm Qin-Han Dynasty (i.e. 221 BC–AD 23) (Zhu, 1972) and the warm period of AD 1–200 derived from the integrated temperature sequences (Ge et al., 2014), and coinciding with the Roman Warm Period (RWP, 2350–1650 yr BP) (Kobashi et al., 2011). In addition, forest recovery in Toushe Basin during 2100–1300 cal yr BP also implies enhanced warm and wet conditions (Huang et al., 2020).

During 1750–1150 cal yr BP (AD 200–800), the decreased evergreen trees suggest a shift toward cold and dry conditions, corresponding to the Dark Ages Cold Period (DACP, 1650–1350 yr BP) (Kobashi et al., 2011) and the cold East Han Dynasty (AD 225–289) (Zhu, 1972), as well as the two cooling periods in AD 210–350 and 420–530 respectively (Ge et al., 2014). From 1150 to 630 cal yr BP (AD 800–1320), the increased proportions of evergreen taxa reflect relatively warm and wet conditions. This period coincides with the Medieval Warm Period (MWP, 1350–650 yr BP) (Kobashi et al., 2011), and is consistent with the warm interval (AD 950–1300) detected from the integrated temperature sequences (Ge et al., 2014).

Stage 4: 630–0 cal yr BP (AD 1320–1950)

This stage corresponds to pollen Zone II-2 (Figure 3), and roughly coincides with the Little Ice Age (LIA) (Chu et al., 2002; Kobashi et al., 2011; Zeng et al., 2012). The vegetation here is covered by deciduous forests (dominated by Fagus and Liquidambar), and the understory herbs (e.g. Poaceae) grow rapidly (Figure 3). In this stage, the pollen concentrations increase rapidly, whereas AP/NAP ratio displays a decreasing trend and reaches its minimum value of the entire core, implying an evident shrinkage of forests and a sharp expansion of grassland (Figure 4). However, an observed interesting phenomenon is that the evergreen taxa exhibit a clear increase during 630–300 and 150–0 cal yr BP (Figure 4). This asynchronous variation between AP/NAP and evergreen taxa may suggest a potential switch of the forcing mechanism (i.e. the superimposed human impact) on vegetation changes (see discussions below).

The interaction among vegetation, fire regimes, climate, and human activities

Fire-induced reduction of forest coverage will leave high concentrations of charcoal grains in lake sediments (Tong et al., 2009). It is generally believed that because the smaller and lighter micro-charcoals would be windborne over relatively longer distances thus it can be used to reflect the regional fire events; in contrast, most of the macro-charcoals would deposit within a scope of 7 km from the source region, so it can suggest the local fire events (Li et al., 2006; Li et al., 2010). The occurrence of fire events could be natural-driven or anthropogenic-driven, the way to distinguish them is to compare the charcoal results with the palynology spectrum (Li et al., 2010): if the charcoal record is consistent with the dry/wet conditions deduced pollen taxa, it can be considered that the fires are caused by climate, whereas in other cases if the crop pollen increases significantly with the peaks of charcoal concentration, it may reflect the human induced (e.g. the reclamation activities) fire events (Li et al., 2010). Here, we try to distinguish the anthropogenic fires from the natural fires based on changes in charcoal concentration, and the proportions of the arboreal pollen and the representative anthropogenic plant pollen (e.g. Poaceae) (Ding et al., 2022; Li et al., 2008).

Evidently low C_micro and C_macro from 3140 to 3040 cal yr BP (Stage 1) indicate a very low frequency of both the regional and local fire events (Figure 4). The obvious low percentages of Poaceae pollen suggests weak human impacts. In line with the interpreted warm and wet BAO period, we argue that in this period the human induced fire events are minor, and the climate conditions are not conducive to the fire occurrence.

During the cool and dry IACE period from 3040 to 2230 cal yr BP (Stage 2), though C_micro and C_macro are generally low, there is a small peak appeared in 2550–2450 cal yr BP (Figure 4), suggesting increased regional and local fires. In this period, the increased Poaceae and decreased AP/NAP likely reflect enhanced human activities as a result of population growth during the Eastern Zhou Dynasty of Chinese history (i.e. 770–221 BC) (Ma et al., 2018). However, it should be noted that the number of archeological sites constrained in the Shang-Zhou Dynasties in the study region is much fewer than that in the Lake Dongting area in northern Hunan Province (Zhang et al., 2020), implying a relatively low intensity of human activities. Thus, it is reasonable to infer that the cool and dry conditions were still the major causes favoring increased natural fire events, whereas the contribution of human activity was still fewer.

In 2230–630 cal yr BP (Stage 3), both C_micro and C_macro increase from ~2000 cal yr BP, and then maintain relatively high concentrations with two peaks appeared at ~1800 (AD 150) and ~1200 cal yr BP (AD 750) in the East Han Dynasty (i.e. AD 25–220) and the Tang Dynasty (i.e. AD 618–907) respectively (Figure 4). These two charcoal peaks roughly coincide with the boundary of RWP and DACP, and DACP and MWP respectively, likely suggesting that local and regional fires were easily outbroken in the transitional intervals of different climatic conditions. As it should be, human activities may also play a certain role in the charcoal peak at 1800 cal yr BP, because the historical data documented that the number of the households in Linling County (where our study area located in) significantly increased sevenfold from AD 2 to 140 in the East Han Dynasty due to warfare in northern China, and the residents in the mountainous area were engaged in fishing, hunting, and logging during this period (Liao, 2006). Strengthened logging activities would result in the deforestation, and enhanced agricultural activities would favor, to some extent, increased local and regional fire events.

From 630 to 0 cal yr BP (Stage 4), both C_micro and C_macro significantly increase between 630 and 300 cal yr BP, then slightly decrease until 150 cal yr BP. After 150 cal yr BP, C_micro and C_macro display rapid increase (Figure 4). Particularly, obvious peaks of C_micro and C_macro in association with the decreased broadleaved AP and AP/NAP, and increased Poaceae at ~300 cal yr BP (Figure 4), roughly correspond to the period characterized by frequent extreme droughts from AD 1601 to 1650 in the cooling Ming Dynasty (Ge et al., 2014). On the other hand, the local chronicles of Chengbu County (where the study area located in) recorded that the human slash-and-burn cultivation in the Ming Dynasty had significantly destroyed local vegetation (Wang, 1991). This reminds us that though we are not able to perfectly explain the causes of the asynchronous variations between AP/NAP and evergreen taxa after 630 cal yr BP (Figure 4), we infer that besides the cold and dry conditions in this period, significantly increased human disturbance may also play another role in the AP shrinkage and the abnormal increased fires. In particular, the sharp increased Poaceae after ~150 cal yr BP suggest dramatically increased human activities since the Qing Dynasty (i.e. AD 1644–1912). This interpretation is supported by the abrupt decreased AP and the highest concentrations of C_micro and C_macro in the core (Figure 4), which suggest significant deforestation and drastically intensified fire events.

As discussed above, we determine that the obviously intensified human disturbance in the study region in the past 3140 years started from ~630 cal yr BP. It is noteworthy that the onset timing of the significant human disturbance in the late-Holocene presents a regional inconsistence across the coastal and the inland montane areas in subtropic China (Figure 5). The charcoal and pollen data from core JSP in the coastal area of Fujian Province indicated an earlier presence of strengthened human activities at ~2.8 cal kyr BP (Figure 5c) (Song et al., 2022). A dataset covering coastal areas of southeast China proclaimed an expansion of rice cultivation at ~2.0 cal kyr BP, leading to a widespread deforestation and biodiversity changes in the tropical and subtropic forests (Zheng et al., 2021). In addition, pollen and charcoal results from the northern part of the South China Sea also revealed a tremendous human-induced decrease in forest coverage at ~2.0–1.0 cal kyr BP (Cheng et al., 2018). However, charcoal and pollen records from Shuizhuyang peat bog in Jiufeng Mountains in Fujian Province (Figure 5b) and Gutian wetland in western NLM in Guangxi Province (Figure 5f) revealed that the evidently enhanced fire intensity started from ~1.0 cal kyr BP, as a result of extensive development of agriculture caused by rapid immigrations in the Tang and the Song Dynasties (i.e. 1294–671 yr BP) (Ma et al., 2018). Similarly, pollen data from Daiyun Mountains in Fujian Province revealed a clear signal of human impacts at ~1.0 cal kyr BP, attributed to local porcelain industry and population blossom (Figure 5d) (Zhao et al., 2016). Moreover, a record from Mount Qiyun in Fujian Province suggested that at 770 cal yr BP, a wave of deforestation caused by abrupt expansion of cereal cultivation resulted in an evident decline of evergreen broadleaved trees and a sharp increase of fire frequency (Chen et al., 2022). Therefore, we infer that in the late-Holocene significantly intensified human activities in the coastal area in subtropic China approximately occurred at ~2.8–2.0 cal kyr BP, and then spread to the inland montane areas from the regions of Daiyun and Qiyun mountains in Fujian Province and the Gutian wetland (1000–770 cal yr BP), and then to the Daping region in the western NLM (630 cal yr BP). This spreading pattern substantially reflects the obstacle influence of the montane terrain and the impacts of the enhanced population migration (Figure 5a).

Figure 5.

Regional comparison of different charcoal records in subtropic China, whose locations are signed in Figure 1b: (a) population of China (Li et al., 2009a); (b) micro-charcoal flux of SZY (Ma et al., 2018); (c) micro-charcoal concentrations of JSP (Song et al., 2022); (d) charcoal accumulation rates of DYS-2 (Zhao et al., 2016); (e) micro-charcoal concentration of JMP (Huang et al., 2014); (f) micro-charcoal flux of GT-2 (Ma et al., 2018); (g) micro-charcoal concentrations of HNDP-B (this study). Yellow bars indicate the intensified human disturbance in the area suggested by abrupt increased of charcoal records.

Conclusions

The vegetation, fire regimes, climate and human activities since the ~3140 cal yr BP are investigated based on the pollen and charcoal data from core HNDP-B in Daping swamp in the western NLM of subtropic China. The results indicate that in the past 3140 years the vegetation in the study region is dominated by deciduous–evergreen mixed forests, and the forests become more open resulted from the expansion of herbs after ~630 cal yr BP. Generally, relatively low concentrations of the micro-charcoal (C_micro) and the macro-charcoal (C_macro) between 3140 and 630 cal yr BP suggest low occurrence of regional and local fires. Two small peaks of C_micro and C_macro appeared at ~1800 and ~1200 cal yr BP coincide with the boundaries of the Roman warming and the Dark Age periods, and the Dark Age and the Medieval warming periods, likely implying that the fires were more easily outbroken during the transitional intervals between different climatic periods. After ~630 cal yr BP, abruptly evident increases of C_micro, C_macro, and increased proportion of Poaceae pollen, as well as decreased AP/NAP ratios, suggest intensified human-induced (e.g. the stronger slash-and-burn cultivation as recorded in the local chronicles) deforestation and anthropogenic fire events. We consider that the 630 cal yr BP is likely a transitional boundary of remarkable changes of vegetation composition and fire regime in the study area, which marks the onset of strong impacts of anthropogenic activities. Regional comparison of charcoal records indicates that the onset timing of intensified human disturbance in the inland montane areas was much later than that of the coastal areas in subtropic China. This inconsistent pattern in regional scale may be caused by the obstacle effects of the montane terrain and the impacts of the enhanced population migration in Chinese history.

Footnotes

Acknowledgements

We thank Jibin Xue, Jun Ouyang, Zhanghong Peng, and Jinxuan Liu for their help in field investigation. We sincerely thank Prof. Qinghai Xu, Drs. Shengrui Zhang, and Manyue Li from Hebei Normal University, Prof. Limi Mao from Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (CAS), Prof. Chuanxiu Luo from South China Sea Institute of Oceanology, CAS for their guidance and discussions on identification for pollen types and/or manuscript improvements. Handling editor Dr. Zicheng Yu and the two anonymous reviewers are deeply thanked for their constructive comments which were helpful and useful to improve our paper significantly.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was jointly supported by the National Natural Science Foundation of China (grant nos. 41971101, 41571187, and 41071137), and the Innovation Project of Graduate School of South China Normal University (2019LKXM022).

ORCID iD

Mingying Quan

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