云南勐海地区四万年以来植被史与气候

VEGETATION AND CLIMATE HISTORY AT MENGHAI,YUNNAN DURING THE PAST 42 000 YEARS

该文研究了云南勐海地区两个湖相沉积钻孔中的孢粉植物群,恢复了当地古植被主要是以山毛榉科中的热带、亚热带属种组成的常绿阔叶季雨林和山地雨林;反映该地区距今4万年来气候变化经历了温暖干燥、凉湿和温和凉爽的波动。在凉湿阶段年均温约低于目前当地气温1.5—3.5℃,年降雨量要多于今日500mm以上。

The Manyang and Manxing lakes belong to Menghai County of Xishuangbanna Dai Au-tonomous Prefecture in southwestern Yunnan,lying between the Lancang River and Burma(22°N and 100°30′E),at an altitude of 1 200m A. S. L. and with the highest ridge at 2 000m,where the alluvial sediments of fluviolacustrine facies have been deposited since Late Ter-tiary. This area is under the tropical climate of South Asia affected by southwest monsoon,with marked dry and wet seasons,a mean annual temperature of 18. 3℃ and an annual rain-fall of 1 274mm. It is climatically temperate--cool and humid,covered by evergreen broad-leaved forest of the subtropical zone. Palynological studies of Menghai are based on the samples from Core N at ManyangLake and Core M at Manxing Lake in the Mengzhe Basin. Both cores only reveal nearly 5m oflacustrine mud sediments about 42 000 years ago. 1. Types of palaeovegetation The changes in composition and content of trees found in the pollen spectra of Menghaiindicate that the subtropical and monsoon rain forests during the past 42 000 years can beecologically divided into the following two types: (1)Evergreen broad--leaved monsoon forest REPRESENTATIVE BEDS: Manyang Core N (at 2. 5--3. 0m and 1. 5--0. 0m); Man-xing Core M(at 3. 5--5. 0m and 2. 5--0. 0m) AGE: 42 000--38 020yrs BP,29 030--20 650yrs BP and 8 270--1 660yrs BP. FEATURES OF ASSEMBLAGE: It is dominated by such evergreen angiosperms asCastanosis, Cyclobalanopsis, Lithocarpus, Ilex, Myrica, Myrtaceae, Rutaceae, Leguminosae,Magnolia and Moraceae, accompanied by some deciduous trees such as Quercus,Salix andAlnus. (2)Rain forest of mountain land REPRESENTATIVE BEDS:Manyang Core N (at 3. 5--2. 5m) AGE: 38 020--29 030yrs BP and 20 650--11 870yrs BP FEATURES OF ASSEMBLAGE: This forest is a transitional type developed under thecondition of high altitude,low air temperature and high humidity with a discontinuous andpatch distribution. As indicated by the pollen spectra of Menghai,two peaks of Dacrydium,Podocarpus and Dacrycarpus occurred 33 500 and 11 870yrs BP,and numerous elements ofevergreen or deciduous broad--leaved trees are found such as Cyclobalanopsis,Quercus,Cas-tanopsis, Myrica, Ilex, Morus, Hamameliaceae, Rhododendron, Oleaceae, Euphorbia and Legu-minosae; there are also abundant ferns and herbs. Judging from results of pollen analysis,thepollen composition of this stage is fairly similar to the rain forest of mountain land on theHainan Island. 2. Discussion on palaeoclimate The palynoflora history of Menghai during the past 42 000 years shows several climaticchanges from warm--dry,temperate--cool to cold--humid. LATE AND MIDDLE LATE PLEISTOCENE(42 000--11 870yrs BP) As indicated by palynological assemblages from the Manyang Core N (at 2. 70--2. 50m),this stage is dominated by tropical and subtropical evergreen trees such as Castanopsis,Cy-clobalanopsis, Myrtaceae, Rutaceae, Ilex and Myrica, suggesting a warm climate of tropicaland subtropical zones. Some trees are chiefly mesophyte,indicating a warm and slightly aridclimate. COLD--HUMID STAGE OF 38 020--29 000yrs BP A great number of Dacrydium pollen grains are found from Core N (at 1. 7m). Fernspores and aquatic pollen grains increase greatly,indicating heavy rainfall or high humidityand a decline in air temperature. Dacrydium is mostly distributed in the tropical mountainlands of the Southern Hemisphere, especially in the middle mountain rain forests in thecoastal belts and islands of maritime climate. Nowadays,only one species (Dacrydium pirrei Hick) grows in the middle mountain rainforest of Wuzhi and Jianfengling Mountains at an altitude of 1 000--1 500m A. S. L. on theHainan Island,where the ecological environment is characterized by high landform of greatundulation,low air temperature with little variation annually,sufficient rainfall and relativelyhigh humidity. No plants of this species are found in today's Yunnan. In this stage,the airtemperature of Menghai is estimatedly 1. 5--3. 5℃ lower than today,and the precipitation is500mm more than today. TEMPERATE--COOL STAGE OF 29 000--27