The “two-third spline” (2/3S) is a frequently applied method to detrend tree-ring series. It fits a spline with a 50% frequency cutoff at a frequency equal to two-thirds of each sample length in a dataset. It was introduced to ensure a minimum loss of low-frequency variance, which is resolvable during the detrending of ring-width series.

In this paper I show potential problems that arise when rusing this method. The 2/3S runs counter the strengths of using a digital filter to detrend – i.e. one is giving up full control over the frequency-removing characteristics of the growth curve and each individual time series retains a different amount of low frequency. Thus, the 2/3S is less suitable for reconstructing climate or to compare environmental impacts on tree growth between groups – both of which comprise the majority of dendrochronological analyses – as it will likely introduce a temporal frequency bias. Within a long chronology it will result in decreasing power to resolve low frequencies towards present in a living-only trees setting, especially when the youngest segment lengths are 100 years and shorter, and more generally during the period where the chronology is constructed from samples with shorter segment lengths compared to the period with longer segment lengths. The frequency bias will also significantly impact regression slopes and correlation coefficients, possibly distorting analyses investigating multiple groups with different mean segment lengths. Highlighting these potential biases, I recommend the community to not use this method on an individual basis but rather to use a fixed spline stiffness for all samples based on the n% criterion (n = 67) of e.g. the mean segment length of the entire dataset.

“三分之二的样条曲线”（2 / 3S）是使树木年轮系列趋于下降的一种常用方法。它适合于频率截止值为50％的样条曲线，其频率等于数据集中每个样本长度的三分之二。引入它是为了确保最小的低频变化损失，该损失在环宽序列的去趋势中可以解决。

在本文中，我展示了使用此方法时可能出现的潜在问题。2 / 3S与使用数字滤波器降低趋势的优势背道而驰–即，人们正在完全放弃对增长曲线的频率去除特性的控制，并且每个单独的时间序列都保留了不同数量的低频。因此，2 / 3S不太适合重建气候或比较环境对各组之间树木生长的影响-两者都包含大多数树轮年代学分析-因为它可能会引入时间频率偏差。在很长的时间范围内，它将降低解决低频问题的能力，以解决只活树木的环境中出现的低频问题，尤其是在最小的片段长度为100年或更短的情况下，并且更普遍的是，从具有较短片段长度的样本（与具有较长片段长度的样本相比）开始，按年代顺序排列。频率偏差还将显着影响回归斜率和相关系数，可能会使调查具有不同平均段长度的多个组的分析失真。强调这些潜在的偏见，我建议社区不要单独使用此方法，而应根据n％标准（n  = 67），例如整个数据集的平均段长度。