经验模态分解方法对祁连圆柏年轮宽度序列标准化初探

Study of Standardization of the Qilian Juniper Ring-width Series Based on Empirical Mode Decomposition Method

利用经验模态分解(EMD)方法对祁连圆柏树轮宽度序列进行标准化处理的初步探讨,分析该方法在圆柏宽度序列生长量订正中的应用潜力。利用EMD方法对祁连山地区的65个圆柏样芯宽度序列的逐级分解,较好地展示了各样芯由高频至低频的自然变化特征,其中最低频的趋势项虽包含了线性、负指数、类样条函数等多种形式,但末端基本趋于平稳,与树木的生物学遗传特性较为吻合,基本反映了与树木遗传相关的生物学趋势。与传统线性或负指数函数方法对祁连圆柏序列的标准化过程相比,传统处理中常常出现髓心端上翘或树皮端异常增大的问题,通过新的EMD方法的订正得到了改进,进一步分析了EMD方法在祁连圆柏轮宽序列生长量订正中的优势及不足。两种方法得到的祁连圆柏年表在整个时段上高频变化极为一致,仅在接近髓心端的100 a内传统方法处理得到的指数值偏低,方差变化较大;而EMD处理得到的指数年表整个时段方差相对稳定。

Standardization of tree-ring measurements is one of the most important procedures in dendroclimatology. In this paper, applying Empirical Mode Decomposition （EMD）method to the Qilian Juniper （Sabina przewalskii Kom）ring-width samples, the potential and limitations of the EMD method are investigated. Tree-ring series are firstly decomposed by EMD to produce a number of Intrinsic Mode Functions （IMFs）in different frequency band with different physical meanings. The lowest-frequency signals mainly reflect the age-dependent growth trend, which are employed to correct the raw ring-width series to get the standardization chronology. Both the chronologies from the traditional and EMD method preserve the same high- frequency, but the different low-frequency variations in the 50-100yrs near to the pith. In this period, the variations of tree-ring index based on traditional method are large in general, while those based on EMD method are more stable. EMD method is a suitable tool for preserving more reliable climate information than traditional method, especially for the pith periods and young periods of tree-ring series, which can be applied to fit the growth trend of Qilian Juniper.