中文摘要：

利用西藏中部大果圆柏的树轮宽度资料,建立了宽度年表.分析了树轮宽度指数、归一化植被指数和当雄月气象要素三者间的关系.结果表明树木的径向生长和区域归一化植被指数对气候要素的响应关系一致,与上年7-8月和当年5-6月的降水量显著正相关,与当年5-6月的温度显著负相关.由于共同受气候因素的制约,树轮差值年表与西藏中部地区7-8月平均归一化植被指数的相关系数达到0.714,二者之间的线性转换方程的方差解释量达到了51.5%,逐一剔除检验证明了方程稳定可信.1565-2006年的归一化植被指数重建序列的分析表明,近20a来的归一化植被指数是整个序列最高的时段,与西藏拉萨、林芝等地的植被指数上升的趋势一致.西藏器测降水量资料和基于树轮重建的西藏中南部降水量序列均反映了近20a来降水量增加的事实.在近20a归一化植被指数升高、植被状况好转的同时,其变异系数随之增加.极端事件出现的频率增加,导致该区域畜牧业发展的风险增大.

英文摘要：

The tree-ring width chronology of Sabina tibetica Kom.in central Tibet was established.The relationships between tree-ring width,normalized difference vegetation index（NDVI） and meteorological factors from Dangxiong were analyzed.Tree-ring width and NDVI showed similar responses to climatic factors,being positively correlated with the precipitation of July to August of the last year and May to June in the current year,and negatively correlated with the mean temperature of May to June in the current year.The correlation coefficient between tree-ring width and NDVI was 0.714,because they were both dominated by the.common climatic factor.A linear transfer function between the residual chronology and the regional NDVI of July to August was established with the explained variance being 51.1%.The leave-one-out verification method indicated the stability and reliability of the function.Low frequency analysis of the reconstructed NDVI series of 1565—2006 revealed that the recent two decades since mid-1980 s were the best period for vegetation status.This conclusion is consistent with the NDVI increase in Linzhi and Lasa,the instrumental precipitation increase and the reconstructed precipitation in south-central Tibet.Along with the increasing NDVI,the coefficient of variation also increases,which will exacerbate the risk for livestock.