| Being one of the most important regional borderlines of China, Daxing'an Mountains constitute the transitional terrain between Mongolian Plateau and Northeast Plain of China and also the significant humidity belt between east Asian monsoon area and west semiarid area. As a common species of arid habitat, Pinus sylvestris var. mongolicas have clear treering series and thus give plenty of eco-climatic information. We took 57 cores from 26 trees by stiletto in Bizhou forestry center (51°57' N,124°35' E, Altitude:500～600m), Xinlin. Allowing for frequent fire disturbance in Xinlin, we made diagnosis of every sample for fire disturbance and measured respective treering width by Velmex. Based on the correlation analysis, we find that late spring and early fall are the two drought periods during growing season from May to September and the Humidity index(HI:April～July may explain high frequent changes of treering width. In this paper, Humidity index is defined as the ratio of standardized solar radiation to precipitation. Taking into consideration the fact that last winter's snow melts away in spring and snow water benefits the growth of earlywood, we calculated the precipitation from October to next July. HI4～7 series during the past 230 years was reconstructed in regression model. Because rainstorm precipitation cannot be fully utilized by trees on sloping fields, moisture signals were more uncertain than drought signals in the reconstructed series. In order to decrease the disturbance of moisture, we obtained the drought signals from HI4～7 series and divided them into five grades. In Xinlin, precipitation has greater influence on drought events than temperature, solar radiation and other environmental factors. Fast Fourier Transforms and morlet wavelet analysis show 21a, 7.6a, 2.6a quasi cycles in this grade series of drought signals, indicating that East Asian monsoon and the El-Nino are the primary driving forces for drought events. |
PDF Full Text Request