长白山区孤山屯沼泽地13000年以来的植被和气候变化

VEGETATIONAL AND CLIMATIC CHANGES AT GUSHANTUN BOG IN JILIN, NE CHINIA SINCE 13000 Y. BP

近年来,气候异常给世界农业和人民生活带来了极大的灾害。关于造成气候异常的原因,以及今后的发展趋势等都是人们极其关心的课题。为此,有关科学家正在采用多途径探索研究这个问题。孢粉分析法,即为其重要方法之一。利用它来探讨晚更新世晚期以来的植被和气候的演变情况及其原因,可为预测气候变化的趋向提供科学上的根据。一、自然概况我国东北长白山区现有很多适宜于花粉沉积的沼泽地和火山口湖,为孢粉研究提供了天然记录。本文研究的孤山屯沼泽地,即其中之一。该沼泽地位于长白山西北坡。

The bog under study is situated at Gushantun on the northern slope of the Changbai Mountains, NE China, at 500 m A. S. L, 42°N/126°E (Text-fig. 1). Developed on a basalt plateau, it is surrounded with mixed conifer/deciduous broad-leaved forests, belonging to the Asian temperate zone. On the water-saturated Gushantun bog are growing such plants as Carex spp., Sphagnum spp. and Shrubs of Betula. Drilling cores reveal that the peat is much softer in the western part of the bog than in the eastern part. Two cores (Gu2 and Gu3), attaining a depth of 950 cm were collected from the central-western part (Text-fig. 2), providing more than 40 samples for palynological analysis and 3 samples for carbon-14 dating. The peat was continuously accumulated above a depth of 750cm, and gradually changed to muddy clay of lacustrine origin below. Four diagrams showing the pollen grains are compiled based on the palynological result from the Cu 2c ore. The diagrams indicating the concentration of the main pollen taxa are shown in text-figs. 4, 5, arid the diagrams indicating the percentage are shown in text-figs. 6, 7. Three assemblage zones can be distinctly delineated from the pollen-curve changes as shown in the diagrams, and the two subzones (a, b) established for each zone also can be distinguished, depending on some significant details. The characteristics of Zone Ⅰ, Zones Ⅱ and Ⅲ can be briefly described in ascending order as follows. Zone Ⅰ (13400—9800 y. BP) Palynologically, this zone is characterized by the overwhelming dominance of Betula among tree components with conifers in a second place; and with the herbs surpassing the trees in total amounts. Two subzones (a, b) are distinguished based on different features. Subzone Ⅰa (13400—11400 y. BP): The pollen concentration is much lower as compared with other zones and the components are largely coldtolerant elements (Text-figs. 4, 5). The vegetation reflected seems to be somewhat comparable with the tundra-type of the Later Glacial period in Europe and N. America, or the tundra zone above the forest-line in the present Changbai Mountains. Subzone Ⅰb (11400—9800 y. BP): In this subzone, Betula was absolutely dominant, with Picea/Abies occupying the second place, and herbs reaching their acme for the first time. The developmental acmes for these groups all occurred around 10300 y. BP and then decreased markedly, while various forms of broad—leaved trees and shrubs increased gradually. From the obtained data, the vegetation succession may be deduced as: Tundra?→birch forest→mixed forest of conifers and broad-leaved plants. Accordingly, the climatic changes reflected can be considered as: In Subzone la, the tundra-like vegetation indicates a low-temperature climate, with the mean annual value being possibly 8℃ lower than that of today; while towards Subzone Ⅰb, the climate seemed to have markedly ameliorated due to the flourish of many plants; however, the temperature was not very high judjing from the participation of quite rich cold-tolerant elements in the vegetation until 10 300—9800 y.BP when the temperature went up remarkably. Zone Ⅱ (9800—4000 y. BP): This zone is characterized by the absolute dominance of broad-leaved plants with only a few conifer forms; Picea/Abies did not occur in many samples. Two subzones can be recognized based on the vegetational changes in arborescent components. Subzone Ⅱa (9800—8300 y. BP) displayed a broad-leaved and deciduous forest mainly composed of Ulmus and Fraxinus which was somewhat similar to the broad-leaved forest of Ulmus propinqua and Fraxinus mandshurica now growing in the Changbai Mountains. Subzone Ⅱb is characterized by the increase of thermophilic broad-leaved plants represented by Quercus, Juglans and Ulmaceae, indicating an aspect of a broad-leaved forest. Accordingly, the climatic changes can be inferred as: During the interval of Subzone Ⅱa, the climatic condition seemed to have approached the modern level although a little drier, while towards Subzone Ⅱb, the temperature and humudity became higher, somewhat corresponding to the 'climatic optimum' or the 'Hypsitherreal' phase in Holocene. Zone Ⅲ (4000 y. BP to present) This zone is characterized by the remarkable uprise of conifers, with a conifer/ broad-leaved forest having evolved from the broad-leaved forest in Zone Ⅱ. Based on the vegetational changes in composition and frequence, two subzones (Ⅲa and Ⅲb) also can be distinguished. Subzone Ⅲa contained a spectrum of conifer/broad-leaved forest, which was quite different from that of Zone Ⅱ. But in consideration of the fact that the increase in cold-tolerant plants was not prominent, the climate seemed to have changed not very dramatically and became just a little cooler than that of the previous interval. Subzone Ⅲb is charac terized by the presence of several acmes of cold tolerant Picea/Abies and the highest content of Pinus. The abrupt increase in such conifers indicates that the climate during this period was de teriorated once again, reflecting a climatic decline event, At the end of this period, a climatic warruing was perceptible. From the above statement, the succession of vegetation development seems to show the tendency which can be expressed as: Tundra? (13400—11400 y. BP)→open birch forest to mixed conifer/broad-leaved forest (11400—9800 y. BP)→mixed deciduous broad-leaved forest (9800—4000y. BP)→mixed conifer/broad-leaved forest (4000 y. BP to present). Accordingly, the changes in the climate can be deduced as. The mean annual temperature was possibly 8℃ lower than that of the present around 13400—11400 y. BP which was followed by a gradual warming starting about 11400 y. BP, and increasing dramatically 10300, to 9800 y. BP. The climate of 9800—8300 y. BP had possibly approached that of the modern type but a little drier. The climatic optimum in Holocene was shown around 8300—4000 y. BP and then was replaced by a decline event starting about 4000 y. BP. Several notable fluctuations of climatic reversal appeared during 2300—400y. BP and then gradually changed to a warmer condition as seen in the present day. To sum up, five climatic periods can be listed below in ascending order for convenience of correlation: 1. 13000—11400 y. BP: Tundra phase, possibly indicative of a subarctic condition. 2. 11400—9800 y. BP: The temperature appeared to have rapidly risen. 3. 9800—8300 y. BP: The climate probably was not much different from that of the present time. 4. 8300—4000 y. BP: Climatic optimum with the highest warmth, which can be correlated with the Hypsithermal Period. 5. 4000—2300 y. BP: Climate gradually declined in temperature, which can be correlated with the warmth-lowering period. 6. 2300 y. BP to present: Several reversal fluctuations occurring and then changing to modern climatic condition. The climatic changes in the studied area, asa whole, seem to show the general tendency as known in the North Hemisphere, and they are especially comparable with those observed in northern Europe and with the results from the Greenland ice stone measured by oxygen isotope method(see Table Ⅱ).