不同去趋势方法对树轮气候信号识别的影响

树木生长受到气候因子、随年龄增长的内在生长趋势、环境干扰和其他扰动信号的影响。目前存在不同的去趋势方法对树木年轮进行去趋势以识别树木生长中的气候信号。以往的研究多基于单个方法识别树轮气候信号，而不同去趋势方法识别的树轮气候信号可能会有一定的差别。为了对比不同去趋势方法对树轮气候信号识别的影响，我们基于国际年轮数据库网站获取中国西部地区68个点的树轮宽度数据，采用最常用的"signal-free"方法（SsfCrn）、线性和负指数函数法（std）、67%样条函数法（spline）、firedman方法、以及基于经验模式分解去趋势方法（EEMD）5种去趋势方法分别建立树轮年表，并对比分析同一地点的不同年表对气候响应的异同。结果表明：不同去趋势方法得到的年表对温度、降水以及相对湿度等气候因素的响应具有明显差异。其中，SsfCrn去趋势方法建立的年表对温度（月平均温、月最低温、月平均最低温）响应中相关最高的样点在所有样点中占比最高；EEMD去趋势方法建立的年表对降水量、相对湿度和月最高温响应中相关最高的样点在所有样点中占比最高；firedman去趋势方法建立的年表对月平均最高温响应中相关最高的样点在所有样点中占比最高。研究结果表明SsfCrn，EEMD和firedman方法在识别树轮气候信号方法具有一定的优势。在不同研究区域中，不同去趋势方法建立的年表对不同气候条件响应有差异，因此选择不同的去趋势方法识别树木生长趋势，分析哪种方法可以更好的反应气候变化对树木生长的影响显得尤为重要。     

Does signal-free detrending increase chronology coherence in large tree-ring networks?

Over the past decade, dendrochronologists have increasingly adopted the signal-free detrending (SFD) method to remove age-size trends in tree-ring measurement series, amplify the common stand-wide signal in composite chronologies, and recover medium- to low-frequency patterns that may be inadvertently removed by other detrending approaches. However, since its introduction in 2008, no systematic evaluation of the effects of SFD on tree-ring chronologies has been performed. Here we conduct the first review of SFD in dendrochronology and assess its effects when applied to large tree-ring networks. We analyzed the PAGES North America 2 K database of nearly 300 temperature-sensitive chronologies and the Missouri River database of over 350 chronologies curated for the purpose of reconstructing Missouri River streamflow. Both databases contain multiple versions of each chronology generated by different detrending methods, including those produced with (and without) the signal-free procedure applied. We evaluated (i) whether SFD increases chronology coherence at the site level by boosting the between-tree agreement, (ii) whether SFD increases coherence on a regional basis by making neighboring chronologies more similar to each other, and (iii) whether signal-free chronologies retained more medium- to low-frequency variability than their traditional counterparts. We find that, while SFD increased the strength of common signals in many instances, the effect was not universal and some sites even show a decrease in signal coherence. At regional scales, SFD increases chronology coherence in temperature-sensitive records but had no detectable effect on moisture-sensitive records. Our results demonstrate the importance of evaluating the effects of SFD prior to deploying this method for chronology development and paleoclimate reconstruction.     

Detecting long‐term growth trends using tree rings: a critical evaluation of methods

Tree‐ring analysis is often used to assess long‐term trends in tree growth. A variety of growth‐trend detection methods (GDMs) exist to disentangle age/size trends in growth from long‐term growth changes. However, these detrending methods strongly differ in approach, with possible implications for their output. Here, we critically evaluate the consistency, sensitivity, reliability and accuracy of four most widely used GDMs: conservative detrending (CD) applies mathematical functions to correct for decreasing ring widths with age; basal area correction (BAC) transforms diameter into basal area growth; regional curve standardization (RCS) detrends individual tree‐ring series using average age/size trends; and size class isolation (SCI) calculates growth trends within separate size classes. First, we evaluated whether these GDMs produce consistent results applied to an empirical tree‐ring data set of Melia azedarach, a tropical tree species from Thailand. Three GDMs yielded similar results – a growth decline over time – but the widely used CD method did not detect any change. Second, we assessed the sensitivity (probability of correct growth‐trend detection), reliability (100% minus probability of detecting false trends) and accuracy (whether the strength of imposed trends is correctly detected) of these GDMs, by applying them to simulated growth trajectories with different imposed trends: no trend, strong trends (?6% and +6% change per decade) and weak trends (?2%, +2%). All methods except CD, showed high sensitivity, reliability and accuracy to detect strong imposed trends. However, these were considerably lower in the weak or no‐trend scenarios. BAC showed good sensitivity and accuracy, but low reliability, indicating uncertainty of trend detection using this method. Our study reveals that the choice of GDM influences results of growth‐trend studies. We recommend applying multiple methods when analysing trends and encourage performing sensitivity and reliability analysis. Finally, we recommend SCI and RCS, as these methods showed highest reliability to detect long‐term growth trends.     

ARSTAN程序和R语言dplR扩展包进行树轮年表分析的比较研究

在树轮年代学领域,ARSTAN是去趋势处理和建立年表方面应用最为广泛的程序,而新兴的R语言dplR扩展包实现了ARSTAN的主要功能,且具有源代码公开、扩展性强等优点,是传统程序的良好补充。使用贺兰山青海云杉(Picea Crassifolia)树轮宽度数据,分析了ARSTAN和dplR进行树轮年代学分析所得结果的差异。结果显示,两种程序计算平均敏感度和一阶自相关系数的平均误差为0.005—0.008,但具有确定的转换关系;两种程序如果使用同种方法去趋势,拟合曲线的参数相近,建立标准年表的平均误差为0.002;拟合自回归模型时差异较大,其中时域上表现为差值年表起始30a内差异显著,在频域上表现为dplR的差值年表保留的低频信息较少;年表统计量计算和公共区间分析中,不同程序计算样本总体代表性和信噪比的差异较大。分析表明,两程序在拟合生长趋势和自回归模型时存在算法上的较大差异,同时年表统计量和公共区间各指标的算法也不尽一致,但存在较为确定的转换关系。对开展不同来源数据的整合分析提出了建议,应明确不同研究中树轮数据的处理过程,在条件允许时使用同一程序或算法重新处理数据,确保结果的可比性。     

海拔梯度上青海云杉径向生长与气候关系稳定性研究

以祁连山北坡海拔2 700m(分布下限)、3 000m(中部)和3 300m(分布上限)的青海云杉为研究对象,采用长短两种不同步长的样条函数去趋势对1952～1980年和1981～2009年两个时段青海云杉径向生长与气候的关系进行对比分析,以揭示青海云杉年轮中包含的不同频率生长趋势与气候之间的关系及其在海拔梯度上的差异,探讨海拔梯度上气候变化对树木径向生长的影响。结果显示:(1)随着海拔升高,限制青海云杉径向生长的气候因子从降水转变为温度,不同时期的限制因子也发生变化。(2)在海拔2 700m处,上一年12月份平均温度与青海云杉生长的关系发生了分异,小步长的样条函数得到的年表中保留了较多降水信号;海拔3 000m处青海云杉径向生长与上年9月份降水量、平均温度表现出显著正相关关系;海拔3 300m处青海云杉生长与当年夏季降水量呈显著正相关关系,而在后一时段相关关系不显著,与当年春季、夏季的平均温度相关关系不稳定,大步长的样条函数得到的年表中表现出温度与生长关系的分异,且不同海拔青海云杉径向生长的降水量制约趋势变得明显。研究表明,祁连山北坡青海云杉与气候的关系在海拔梯度上存在差异,其对气候变化的响应也存在差异,水分的限制作用随着温度的逐年升高逐渐突出;采用不同步长的样条函数去趋势可以在年表中保留不同的气候信息,从而对分异现象分析结果产生影响,选择合理步长的样条函数进行去趋势才能得到合理准确的分析结果。     

不同去趋势方法的新疆东天山高低海拔雪岭云杉树轮宽度年表对气候的响应

利用新疆东天山高低海拔雪岭云杉(Picea schrenkiana Fisch.et Mey.)年轮样本,采用样条函数法、负指数函数法和区域曲线法3种去趋势方法研制树轮宽度标准化年表,通过分析不同去趋势方法的新疆东天山高低海拔云杉树轮宽度标准化年表特征、年表与气候响应的关系和年表间在不同频域互相关,及其与其它资料对比发现:(1)3种去趋势法对东天山年表质量影响较小,低海拔树轮年表含有更多可靠的气候变化信息。(2)高海拔的雪岭云杉树木径向生长与6—9月平均气温均呈显著正相关,其中标准化树轮年表与7月平均气温相关系数达0.553(P0.01,n=58),夏季温度可能是东天山高海拔雪岭云杉径向生长的主要限制因子;低海拔雪岭云杉树木径向生长与春季降水显著正相关,同时与春季平均气温显著负相关,春季高温和缺水共同作用的春旱可能是影响低海拔雪岭云杉径向生长的主导因子,且负指数函数年表对气候响应更敏感。(3)区域曲线法能够保留树轮标准化年表中更多低频信息,但优势不明显。高低海拔标准化年表在低频域相关系数较大,并和历史干旱事件有很好的对应。在东天山高低海拔雪岭云杉的去趋势方法研究中,负指数函数法比样条函数法和区域曲线法更适合。     

不同去趋势方法对树轮年表气候信号的影响——以卧龙地区为例

基于川西卧龙地区林线位置岷江冷杉(Abies faxoniana)的树轮宽度数据, 结合地面气象站和空间网格化气象数据, 对不同去趋势年表对气候条件的响应特征进行了比较分析。研究表明: 不同去趋势年表的气候响应特征具有很大的相似性, 具体表现为前一年9月份、当年3月份和7月份的气温, 以及前一年10-12月份的降水、当年4月份和7月份的降水对树木生长的正影响作用; 前一年11月份到当年3月份的相对湿度, 以及当年2-3月份和7-8月份的霜冻频率对树木生长的负影响作用。从不同去趋势方法对气候响应的异质性特征来看, 保守曲线、67%样芯长度步长、60年步长和90年步长的平滑样条函数去趋势年表间具有很强的相关性, 对气候响应也最明显, 特征基本一致。30年步长的平滑曲线去趋势年表与其他去趋势年表的相关性较弱, 与气候要素的相关性也明显较小。另外, 未经去趋势处理的树轮宽度年表统计参量数值要明显高于去趋势年表, 它与气候要素的相关特征也基本反映出川西卧龙地区树木生长对气候响应的基本特征。     

Quantification of glucosinolates in leaves of leaf rape (Brassica napus ssp. pabularia) by near-infrared spectroscopy

The potential of near-infrared spectroscopy (NIRS) for screening the total glucosinolate (t-GSL) content, and also, the aliphatic glucosinolates gluconapin (GNA), glucobrassicanapin (GBN), progoitrin (PRO), glucoalyssin (GAL), and the indole glucosinolate glucobrassicin (GBS) in the leaf rape (Brassica napus L. ssp. pabularia DC), was assessed. This crop is grown for edible leaves for both fodder and human consumption. In Galicia (northwestern Spain) it is highly appreciated for human nutrition and have the common name of "nabicol". A collection of 36 local populations of nabicol was analysed by NIRS for glucosinolate composition. The reference values for glucosinolates, as they were obtained by high performance liquid chromatography on the leaf samples, were regressed against different spectral transformations by modified partial least-squares (MPLS) regression. The coefficients of determination in cross-validation (r2) shown by the equations for t-GSL, GNA, GBN, PRO, GAL and GBS were, respectively, 0.88, 0.73, 0.81, 0.78, 0.37 and 0.41. The standard deviation to standard error of cross-validation ratio, were for these constituents, as follows: t-GSL, 2.96; GNA, 1.94; GBN, 2.31; PRO, 2.11; GAL, 1.27, and GBS, 1.29. These results show that the equations developed for total glucosinolates, as well as those for gluconapin, glucobrassicanapin and progoitrin, can be used for screening these compounds in the leaves of this species. In addition, the glucoalyssin and glucobrassicin equations obtained, can be used to identify those samples with low and high contents. From the study of the MPLS loadings of the first three terms of the different equations, it can be concluded that some major cell components as protein and cellulose, highly participated in modelling the equations for glucosinolates.