Taxonomic diversity masks leaf vein–climate relationships: lessons from herbarium collections across a latitudinal rainfall gradient in West Africa

Cross-biome data suggest that leaf venation density increases with decreasing moisture availability. If leaf venation traits were related to climatic conditions in such a predictable manner, these traits could serve, for example, as proxies for the reconstruction of past climates from fossil leaf remnants. However, our knowledge on vein–climate relationships is still scarce and relationships are poorly understood, particularly concerning broad-scale patterns in tropical regions. In this study, based on phylogenetic generalized least squares regression, we analysed leaf venation traits together with climatic niche data from 238 herbarium specimens to infer trait–trait and trait–climate relationships along a strong north–south rainfall gradient crossing West African savannas. Variation in leaf venation traits was strong along the climate gradient, yet vein–climate relationships were weak overall. This is in contrast to our expectation from the strong adaptive forces operating in drought-adapted vegetation covered by the gradient. On the other hand, several common patterns including intervein and vein–leaf size relationships were confirmed. A key finding was that while trait–climate relationships were mostly weak, some of these relationships became stronger when breaking down the entire taxon assemblage into smaller family-level units. This suggests that vein traits underlie partially opposing constraints in different clades, a pattern that is likely to become stronger with increasing higher-rank taxonomic diversity of species assemblages.

跨生物群系的研究数据表明，叶片脉序密度随水分可利用性的降低而升高。若叶片脉序性状能够以这种可预测的方式与气候条件建立关联，那么这些性状便可作为代用指标，例如用于通过化石叶片残体重建古气候。然而，目前学界对脉序-气候关系的认知仍较为匮乏，对其内在关联的理解也十分有限，尤其是针对热带区域的大尺度格局而言。本研究基于系统发育广义最小二乘回归（phylogenetic generalized least squares regression），整合了238份标本馆标本（herbarium specimens）的叶片脉序性状与气候生态位数据，旨在解析沿横跨西非稀树草原的强烈南北降水梯度分布的性状-性状以及性状-气候关联。沿该气候梯度，叶片脉序性状的变异幅度显著，但整体而言脉序-气候的关联强度偏弱。这与我们基于该梯度覆盖的耐旱植被所经受的强适应性选择压力所做出的预期相悖。另一方面，多项共性关联模式得到了验证，包括脉间距与脉序-叶片大小的相关关系。一项核心发现为：尽管性状-气候的关联整体偏弱，但当将整个类群集合拆解为更小的科级单元时，部分关联的强度会显著提升。这表明，不同演化支（clades）内的脉序性状受到部分相互对立的约束作用，且这种模式很可能随着类群集合的高级阶元分类多样性增加而愈发显著。