Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, United States of America.
PLoS One. 2010 Dec 22;5(12):e15161. doi: 10.1371/journal.pone.0015161.
Present-day correlations between leaf physiognomic traits (shape and size) and climate are widely used to estimate paleoclimate using fossil floras. For example, leaf-margin analysis estimates paleotemperature using the modern relation of mean annual temperature (MAT) and the site-proportion of untoothed-leaf species (NT). This uniformitarian approach should provide accurate paleoclimate reconstructions under the core assumption that leaf-trait variation principally results from adaptive environmental convergence, and because variation is thus largely independent of phylogeny it should be constant through geologic time. Although much research acknowledges and investigates possible pitfalls in paleoclimate estimation based on leaf physiognomy, the core assumption has never been explicitly tested in a phylogenetic comparative framework. Combining an extant dataset of 21 leaf traits and temperature with a phylogenetic hypothesis for 569 species-site pairs at 17 sites, we found varying amounts of non-random phylogenetic signal in all traits. Phylogenetic vs. standard regressions generally support prevailing ideas that leaf-traits are adaptively responding to temperature, but wider confidence intervals, and shifts in slope and intercept, indicate an overall reduced ability to predict climate precisely due to the non-random phylogenetic signal. Notably, the modern-day relation of proportion of untoothed taxa with mean annual temperature (NT-MAT), central in paleotemperature inference, was greatly modified and reduced, indicating that the modern correlation primarily results from biogeographic history. Importantly, some tooth traits, such as number of teeth, had similar or steeper slopes after taking phylogeny into account, suggesting that leaf teeth display a pattern of exaptive evolution in higher latitudes. This study shows that the assumption of convergence required for precise, quantitative temperature estimates using present-day leaf traits is not supported by empirical evidence, and thus we have very low confidence in previously published, numerical paleotemperature estimates. However, interpreting qualitative changes in paleotemperature remains warranted, given certain conditions such as stratigraphically closely-spaced samples with floristic continuity.
当今，人们广泛地将叶生理特征（形状和大小）与气候之间的相关性应用于使用化石植物群来估计古气候。例如，叶边缘分析使用现代年均温度（MAT）与无齿叶物种（NT）的地点比例之间的关系来估计古温度。这种均变论方法假设叶特征的变化主要是由于适应性环境趋同的结果，而且由于这种变化在很大程度上与系统发育无关，因此它应该在地质时间内保持不变，从而为准确的古气候重建提供了依据。尽管许多研究都承认并研究了基于叶生理特征的古气候估计中可能存在的陷阱，但该核心假设从未在系统发育比较框架中得到明确测试。我们结合了一个现有的 21 个叶特征和温度数据集，以及 17 个地点的 569 个物种-地点对的系统发育假设，发现所有特征都存在不同程度的非随机系统发育信号。系统发育与标准回归通常支持以下观点，即叶特征正在适应温度的变化，但更宽的置信区间以及斜率和截距的变化表明，由于非随机的系统发育信号，精确预测气候的能力总体上有所降低。值得注意的是，现代无齿类群比例与年均温度（NT-MAT）的关系，在古温度推断中至关重要，其关系发生了很大的变化和简化，表明现代相关性主要源于生物地理历史。重要的是，某些齿特征，例如齿数，在考虑系统发育后具有相似或更陡峭的斜率，这表明在高纬度地区，叶齿显示出一种适应进化的模式。这项研究表明，使用现代叶特征进行精确、定量温度估计所需的趋同假设没有得到经验证据的支持，因此我们对以前发表的数值古温度估计值的可信度非常低。但是，在某些条件下，例如地层上接近、具有连续植物群的样本，定性地解释古温度的变化仍然是合理的。
