全新世中期7 ka前后降温事件对中国北方植被生态的影响

Mid-Holocene cold event at ca.7 ka and its impact on vegetation ecology in Northern China

大量研究表明,随着气候变暖,全球生态环境发生了巨大变化。然而,在相对温暖的气候条件下,假若发生重大降温事件,生态环境尤其是植被生态会如何响应,值得深入探讨。利用近年来发表的全新世高分辨率古气候记录以及孢粉记录的植被变化,对全新世中期7 ka(1 ka=1000 cal.a B.P.)前后的降温事件及其对植被生态的影响进行了综合分析。结果表明,7 ka前后降温事件在北半球很大范围都有记录,在季风边缘区伴随着气候的干旱,而在西风主导的区域总体表现为气候湿润。中国北方季风区山地以及高纬度地区表现为建群种的变化,如阔叶树种的减少以及针叶树种的增加;在季风边缘区总体表现为干旱气候下植被盖度的降低;而在西北内陆山地高山区域表现为林线的降低或树种的变化,低山地区森林下限表现为森林成分的增加以及耐旱树种的减少;在山前草原和盆地荒漠草原区域,植被的响应表现为植被盖度的增加和喜湿成分的增加。降温直接影响湿度较大的高山地区的森林成分和林线高度,而在低山干旱半干旱地区,降温则通过抑制蒸发、增加有效湿度影响植被盖度和组成。由于不同钻孔年代的不确定性、不同植被类型对气候变化的敏感性不同等,不同区域发生植被转型的时间不尽一致。全新世中期7 ka降温事件的触发机制有待深入研究,可能与大型火山喷发以及太阳活动变化有关,夏季太阳辐射的持续降低以及地球系统内部反馈也是共同的原因。

A number of studies have shown that the global ecological environment has experienced tremendous changes with the global warming. However, in relatively warming climatic conditions, it is still poorly understood that the vegetation ecological responses to major/abrupt cooling events. The Northern China can be roughly divided into three regions based on vegetation and climate, including the climatically westerlies dominated arid and semi-arid areas in Central Asia, the marginal areas of the Asian summer monsoon with highly variable climate and vegetation, and the vast monsoon area in Northern China with relatively high precipitation and better vegetation. We combined published pollen data and climate change records spanning the Holocene in Northern China and nearby areas to explore the major vegetation ecological responses to abrupt climate change. Based on the Holocene high-resolution paleoclimate records and pollen records published in recent years, the cooling events around 7 ka(1 ka=1000 cal.a B. P.) in the Middle Holocene and their impacts on vegetation ecology were reviewed. This review shows that the cooling events at around 7 ka are recorded in a wide area of the Northern Hemisphere, accompanied by climate drought in the edge of the Asian summer monsoon area, while, in the arid area dominated by the westerlies, the climate was generally humid. In different areas of the Northern China, the vegetation feedbacks were quite different. The major plant species changed in the marginal areas of the summer monsoon system under drier climate, which is shown as the decrease of broad-leaved tree species, increase of coniferous tree species, and/or the decrease of vegetation coverage. In the high mountains in Northwestern China, the vegetation feedback was shown as the decrease of tree-line or the change of tree species. While, in the lowland areas surrounding the mountains in Northwestern China, the forest composition increased in the steppe-forest vegetation areas, and vegetation coverage increased in the desert-steppe area with more steppe species. Low temperature directly modified forest compositions and tree-line in the relatively wet high mountains, while, it affected vegetation coverage and composition by increasing humidity/precipitation in arid and semi-arid areas via depressing evaporation. Due to both the uncertainties of chronological data of different studies and the sensitivity of different vegetation types, the timings of vegetation transformations were not consistent. The driving forces of the 7 ka cooling event in the Middle Holocene needs to be further studied, which might be related to the strongest eruptions of large volcanoes, solar activity variabilities. The continuous decrease of summer solar insolation in the Northern Hemisphere and inner feedbacks of the earth system also contributed to this cold period.