Pollen samples from peat sediments on the south bank of the Heilongjiang River in northern Northeast China (NE China) were analyzed to reconstruct the historical response of vegetation to climate change since 7800 cal yr BP. Vegetation was found to have experienced five successions from cold-temperate mixed coniferous and broadleaved forest to forest-steppe, steppe-woodland, steppe, and finally meadow-woodland. From 7800 to 7300 cal yr BP, the study area was warmer than present, and
At present, global warming and its possible ecological consequences have become the focus for governments around the world, the scientific community, and the general public [
NE China, which has typical land ecosystems (including forests, steppes, and wetland) [
In recent years, research into the Holocene environment in this area has focused more on climatic and environmental reconstruction [
However, a longer and more detailed vegetation history of NE China is still needed to verify vegetation succession interpretative studies and vegetative responses to climate change. There have until now been no other records sufficiently reliable for testing how cold-temperate vegetation on the northern margins of NE China has responded to Holocene climate change. This study is based on pollen records derived from Holocene peat sediments on the south bank of the Heilongjiang River. It aims to reconstruct a vegetation succession history, explore vegetation succession responses to climate change (especially during the Holocene Megathermal), and provide evidence for the evaluation of the possible effects of future climate change.
The northern GKR (52°32′–53°41′N, 121°15′–125°58′E) (Figure
Map of the study area. (a) Red rectangle indicates the study area. (b) Red dot shows the Beihong Section.
The Beihong Section (53°30′8.3′′N, 123°5′24.5′′E, altitude: 280 m a.s.l.), which is located in the north of Beihong Village, on the southern bank of the Heilongjiang River in Mohe County, is dominated by peat sediments from the upper reaches of the Heilongjiang River (Figure
The GKR has been directly administered by the central government since the Yuan Dynasty (1206–1368 CE) [
The Beihong Section is 260 cm thick. The sediments are mainly peat and silt. They are delineated as follows: 0–80 cm, dark brown peat layer; 80–220 cm, light gray silty clay; 220–260 cm, gray fine sandy silt.
26 samples taken at 10 cm intervals were prepared for pollen analysis, using conventional acid-alkali treatment and heavy liquid separation [
Detrended correspondence analysis (DCA) and principal component analysis (PCA) have been widely used in forest ecosystem, shrub communities, meadow grasslands, and other vegetation researches [
Macroremains of leaves and peat selected from the samples were examined to obtain an accurate age-depth framework. AMS 14C dating was conducted at the Beta Analytic Radiocarbon Dating Laboratory in America, and three AMS 14C ages at depths of 75 cm, 155 cm, and 238 cm were obtained. Radiogenic 14C ages were calibrated to calendar ages based on IntCal13 [
AMS 14C dating results from the Beihong Section.
Sample | Laboratory code | Depth (cm) | Sample type | 14C ages (yr BP) | Calibrated 14C ages | |
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BH-16 | 378769 | 75 | Macro-remains of leaves | 3970 ± 30 |
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4524–4401 |
BH-32 | 379976 | 155 | Peat | 6200 ± 30 |
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7179–7000 |
BH-48 | 376264 | 238 | Macro-remains of leaves | 6770 ± 30 | 7669–7580 |
A chronosequence for the Beihong Section was established using linear interpolation and extrapolation methods based on three samples. The age of the base (260 cm) was calculated as 7800 cal yr BP; the top consists of modern deposits. Consequently, the Beihong Section presents a historically continuous sedimentary sequence (Figure
Beihong Section and age-depth model.
53 families and genera of pollen were identified in the Beihong Section (Table
Pollen types in the Beihong Section.
Conifers |
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Broadleaved trees |
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Shrubs and herbs | Ericaceae, |
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Ferns | Polypodiaceae, Athyriaceae, |
Based on chronological changes in pollen percentages, five successions can be recognized from pollen spectra (Figure
Pollen percentages and concentrations for the Beihong Section (the gray values to the right are magnified tenfold to emphasize changes in concentration).
In zone 1 (260–190 cm, 7800–7300 cal yr BP),
In zone 2 (190–80 cm, 7300–4500 cal yr BP), spectra were dominated by Poaceae,
In zone 3 (80–60 cm, 4500–3500 cal yr BP), this period was characterized by high percentages (48%–55%) of herb pollen, principally
In zone 4 (60–40 cm, 3500–2300 cal yr BP), Herb pollen, dominated by Poaceae,
In zone 5 (40–0 cm, 2300 cal yr BP-modern), coniferous tree pollen (17%–51%), especially
Of the 26 samples taken from top to bottom of Beihong Section, 31 families and genera were analyzed using DCA. The first two ordination axes were drawn on a two-dimensional ordination map (Figure
DCA results for 26 representative plant community samples from the Beihong Section.
According to PCA results for the principal families and genera (Figure
PCA results for principal pollen from the Beihong Section.
Stalagmite, ice core, ocean, lake sediment, and other high resolution records show that there was a significant warm period in the mid-Holocene, similar to a scenario encompassing a 1-2°C increase in average global temperatures [
Pollen analysis is the technique most widely used and pollen data are most reliable for the reconstruction of paleovegetation and paleoclimate. Understanding the relation between modern pollen and vegetation is a prerequisite for interpreting fossil pollen records correctly and improving the accuracy of past vegetation type and paleoclimate reconstruction [
The lithology changed from fine detritus gyttja at the bottom to decomposed peat in the Beihong Section (Figure
Source areas and relative pollen representation are shown to depend on basin size [
Nevertheless, studies on alluvial pollen show that the differences between alluvial pollen assemblages are the result of different pollen origins [
Surface pollen studies have established that there is a relationship among the pollen percentages as well as vegetation type and regional climate [
In addition,
Poaceae pollen is abundant in fossil records and is often used as a paleoclimatic indicator [
Based on the pollen assemblages and the modern pollen analysis, the vegetation history in Beihong area has experienced five chronosequences since 7800 cal yr BP. In 7800–7300 cal yr BP, In 7300–4500 cal yr BP, herb pollens increase (>44%). Poaceae pollen percentages range from 9% to 28%, while In 4500–3500 cal yr BP, herb pollen increases, including In 3500–2300 cal yr BP, pollen records reveal Poaceae (32%–36%), In 2300 cal yr BP-modern, pollen assemblages indicate that Poaceae, Cyperaceae, and other mesophydrophyte-dominated meadow vegetation developed, along with an expansion in isolated coniferous forests composed largely of
Climate change has an important impact on terrestrial ecosystems [
The vegetation history mainly experienced a clear decrease in
Herbaceous plants, such as Cyperaceae and Poaceae, are the most important components of wetland communities [
Consequently, DCA reflected a clear relation between different plant communities as well as between plant communities and the environment. The first axis can reflect the temperature gradients for different plant communities, with ability to resist the cold strengthening leftward along the axis; the second axis can indicate changes in humidity, with a plant community’s ability to withstand drought strengthening upward along the axis. The first principal component of PCA reflected temperatures and the second changes in humidity.
The PCA F1 scores of the taxa on the first principal component axis better reflected the pollen percentage variations with temperature; that is, higher values may represent cold and lower values warm climates (Figure
Comparison of principal pollen percentages and the PCA F1 curve from the Beihong Section with other selected proxy records. (a) Pollen index from the Beihong Section. From the right to left are the PCA F1 curve, the percentage curves of broadleaved trees,
In order to better reflect the response of vegetation to climate change, the principal pollen percentages and the PCA F1 curve from the Beihong Section were compared with high-resolution climate indices from other regions (Figure
The Beihong Section shows that the area had developed
At ~7300 cal yr BP and ~4500 cal yr BP, warmth-loving broadleaved trees clearly decreased in number in the study area, while cold-tolerant herbs increased (Figure
The cooling event at ~7300 cal yr BP, which resulted in a vegetation succession from mixed coniferous and broadleaved forest to forest-steppe, also appears in pollen records from Moon Lake in the central GKR [
Many studies show that there is a complex interaction of climatic and ecological processes in boreal permafrost formation and degradation [
Accordingly, it can be predicted that falling global temperatures and a weakening EASM gave rise to decreased rates of evaporation and an increase in effective humidity levels in the late Holocene. At the same time, an expansion in permafrost led to precipitation and runoff being unable to seep underground, causing surface ponding, marsh expansion, and peat development [
The pollen records of peat sediments from the southern bank of the Heilongjiang River in northernmost NE China show that the period 7800–7300 cal yr BP was the warmest and wettest stage of the Holocene. At that time, the study area developed
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors sincerely thank the editor and other anonymous two reviewers for their thoughtful reviews and constructive suggestions, which helped to improve the paper. This research was supported by the CAS Strategic Priority Research Program (Grant no. XDA01020304), the National Basic Research Program of China (Grant no. 2015CB953803), and the National Natural Science Foundation of China (Grant no. 41372175). The authors thank Hanbin Liu and Qiang Gao for assistance with fieldwork.