The drought events recorded in tree ring width in Helan Mt. over past 100 years
树轮记录的贺兰山区近百年来的干旱事件

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.