亚洲热带山地垂直植被带对晚第四纪气候变化的响应

过去一直认为热带低海拔地区晚更新世冰期的地表气温基本维持不变。孢粉分析、树线等研究结果证明,从南半球的新几内亚和赤道附近的苏门达腊,一直到我国的热带山地在末次盛冰期垂直植被带下降了500m～1600m不等。植被带的下降不仅仅与温度变化有关,其他的综合因素如高海拔紫外线辐射、大气CO₂浓度等都可能共同影响垂直植被带的分布。综合研究认为,整个亚洲地区年均温度平均下降约4℃的结果与热带非洲和美洲基本一致,并与近两年来热带海洋表层水温的研究结果和近期的气候模拟结果相吻合。

Response of altitudinal belts of vegetation to the late quaternary climatic changes in tropical Asia

Since 70s, most tropical paleoclimate reconstruction for the Last Glacial Maximum used the sea-surface temperatures provided by CLIMAP that suggested the modest changes (<2℃) in tropical oceans of Asia^[1,2]. According to the palynological evidence from the tropics of Asia, the altitudinal belts of vegetation and the snow line of the Last Glacial Maximum (LGM) were much lower than those of present day. In New Guinea, a depression of 1 600 m of the forest limit was repported^[19]. The montane forest zone boundaries in Sumatra region was 500 m~800 m lower than today^[7-9]. In Indochina and southeastern China, the boundary between lowland rain forest and montane rain forest was lowered by the amount ranging from 600 m to 1 000 m^[10,11]. Recently, many studies showed that altitudinal zonation changes of vegetation were partly controlled by multi-factors such as CO₂ concentration^[12] and ultraviolet-B light^[7]. In fact, the reduction of CO₂ in the atmosphere during LGM plays an role as important as temperature particularly to the forest limit depression, but modestly to the boundary between rain forests. This explains the problem that amount of altitudinal zone depression of forest limit was much greater than that between montane forests. Using a number of palynological results from tropical Asia, and considering the influence of CO₂ variation specially to the forest limit during LGM, an average 4℃ cooling over the tropics of Asia was estimated (Fig-5). This result is consistent with the paleoclimate inferred from pollen time sequence in intertropical eastern Africa (4℃±2℃)^[26] and the new paleo-temperature (SSTs) estimates from planktonic foraminiferal assemblages in South China Sea^[3] and from the Sr/Ca ratio in Barbados corals of southweastern Pacific^[4]. The recent climate simulations using an AGCM showed that maintaining ocean heat transport at a value close to present day values, but with otherwise glacial boundary conditions, lead to an enhanced cooling, particularly in the tropics (5-5℃ lower)^[25]. All the above evidences suggested that the glacial climatic changes in tropical Asia were larger than those estimated by CLIMAP^[1,2].