Meise Botanic Garden, 1860 Meise, Belgium.
Division of Ecology, Evolution, and Biodiversity Conservation, KU Leuven, 3000 Leuven, Belgium.
Ann Bot. 2024 Oct 30;134(4):683-698. doi: 10.1093/aob/mcae111.
Leaf traits are known to be strong predictors of plant performance and can be expected to (co)vary along environmental gradients. We investigated the variation, integration, environmental relationships and evolutionary history of leaf functional traits in the genus Coffea, typically a rainforest understorey shrub, across Africa. A better understanding of the adaptive processes involved in leaf trait evolution can inform the use and conservation of coffee genetic resources in a changing climate.
We used phylogenetic comparative methods to investigate the evolution of six leaf traits measured from herbarium specimens of 58 African Coffea species. We added environmental data and data on maximum plant height for each species to test trait-environment correlations in various (sub)clades, and we compared continuous trait evolution models to identify variables driving trait diversification.
Substantial leaf trait variation was detected across the genus Coffea in Africa, which was mostly interspecific. Of these traits, stomatal size and stomatal density exhibited a clear trade-off. We observed low densities of large stomata in early-branching lineages and higher densities of smaller stomata in more recent taxa, which we hypothesize to be related to declining CO2 levels since the mid-Miocene. Brownian motion evolution was rejected in favor of white noise or Ornstein-Uhlenbeck models for all traits, implying these traits are adaptively significant rather than driven by pure drift. The evolution of leaf area was likely driven by precipitation, with smaller leaves in drier climates across the genus.
Generally, Coffea leaf traits appear to be evolutionarily labile and governed by stabilizing selection, though evolutionary patterns and correlations differ depending on the traits and clades considered. Our study highlights the importance of a phylogenetic perspective when studying trait relationships across related taxa, as well as the consideration of various taxonomic ranges.
叶片特征被认为是植物表现的强有力预测因子，并且可以预期它们会沿着环境梯度发生变化。我们调查了非洲咖啡属植物叶片功能特征的变异、整合、环境关系和进化历史。更好地了解涉及叶片特征进化的适应过程可以为在气候变化下咖啡遗传资源的利用和保护提供信息。
我们使用系统发育比较方法来研究从 58 种非洲咖啡属植物的标本中测量的 6 种叶片特征的进化。我们添加了环境数据和每个物种的最大植物高度数据，以检验不同（亚）分支中的特征-环境相关性，并比较连续特征进化模型以确定驱动特征多样化的变量。
在非洲的咖啡属中检测到了大量的叶片特征变异，这些变异主要是种间的。在这些特征中，气孔大小和气孔密度表现出明显的权衡关系。我们观察到早期分支谱系中的气孔密度较低，而最近的类群中的气孔密度较高，我们假设这与从中新世中期以来 CO2 水平的下降有关。布朗运动进化被拒绝，而所有特征都倾向于白噪声或奥恩斯坦-乌伦贝克模型，这表明这些特征是适应性显著的，而不是由纯粹的漂移驱动的。叶片面积的进化可能是由降水驱动的，在整个属中，气候较干燥的地区叶片较小。
总的来说，咖啡叶片特征似乎是进化不稳定的，受稳定选择的控制，尽管进化模式和相关性因考虑的特征和分支而异。我们的研究强调了在研究相关类群的特征关系时采用系统发育观点的重要性，以及考虑各种分类范围的重要性。
