树轮记录的贺兰山区近百年来的干旱事件

利用采自东亚夏季风最北缘、地处干旱-半干旱地区的贺兰山区的树轮样芯,建立了贺兰山地区最近93年来的树轮宽度年表。与气象观测记录的相关分析结果表明,降水是限制贺兰山区树木生长的主要因素,其中5~7月份的降水与树轮宽度呈显著正相关关系,相关系数为0.522(通过95%的信度检验)。在贺兰山的树轮宽度记录中有两个主要的低生长期即20世纪20年代和70年代末到90年代末,这两个低生长期均与该区域的干旱事件相对应。通过分析还发现在干旱事件中不仅出现低降水而且同时与高气温相伴。也就是说在干旱时期内,高温和低降水的水热组合对树木影响十分显著,从而由单纯降水减少变为一种低降水高蒸发的环境,加剧了气候的干旱程度从而使树木生长出现低的生长期,形成窄轮。这种水热组合引起树轮宽度的变化对于理解过去干旱事件及其过程具有重要意义。

The drought events recorded in tree ring width in Helan Mt. over past 100 years

Tree ring cores of spruce (Picea crassifolia) were taken from Helan Mt, which is the boundary of the monsoon areas and westerly, and also the margin of the arid and semi-arid China with a climate that is transitional between temperate monsoon and continental and mountain, were used to construct a 93 year width chronology. All statistic characters indicate the chronology is stable and must record some certain environmental factors. It should be one of the important proxies to study this sensitive region of margin arid and semi-arid. In order to understand what had happened at there during past hundred years, the follow analyses were undertaken. The correlation analysis of chronology and meteorological records shows that there are good relations between the tree ring width and precipitation, especially the total precipitation of May, June and July. The positive relationship (0.522~*, over the 95% confidence lever) means that if there are good precipitation during this period trees will form wide rings, vice versa. The traditional explanation from the ecology considers that this period (May-July) is the important time for trees to form a new ring, meanwhile precipitation of study area is the key control factor of the growth of most vegetation in the study area. Good rainfall situation in this time is convenient to form a wide ring, vice versa. There are two low growth periods in tree ring record during past 93 year. After comparing the weather data quoted from weather stations and proxy data, the finding is that the low growth periods always have a combination of high temperature and low precipitation during the drought period. This situation will be a key control factor which due to the low growth period of trees. It will provide an important way to understand the drought events in history.