Leaf size in conifers: global associations with climate and evolutionary history

Global analyses of leaf size suggest that large leaves predominate in favourably warm and wet climates, and small leaves occur at climatic extremes. However, these patterns are dominated by data from angiosperms, and may obscure drivers of leaf size variation in older, less speciose groups, such as conifers. Here, we investigate how climate influences leaf size evolution across conifer families, focusing on three groups with distinctive leaf architectures: scale-leaved Cupressaceae, needle-leaved Pinus, and broad-leaved Podocarpus. We found moderate to strong phylogenetic signal in conifer leaf and climate niche traits, indicating deep-time co-evolutionary associations. Phylogenetic relationships explained most of the association between leaf size and climate, with minimum winter temperatures revealed as a stronger driver than dry season precipitation. We also found that longer leaves were associated with drier climates, contrary to what has been reported in angiosperms. These patterns may reflect trade-offs in leaf hydraulic architecture under contrasting selection pressures. In single-veined leaves, lateral water transport and thus leaf width is constrained, yet this narrow leaf form can be advantageous for survival under climatic extremes such as drought and freezing. In contrast, anatomical innovations (like accessory transfusion tissue in Podocarpus) allow for broader leaves which are favourable in competitive environments, while also potentially making leaves more vulnerable to climate extremes. Our results support previous evidence for phylogenetic niche conservatism in conifers, where species track their ancestral climatic preferences rather than adapting to new environments. This conservatism, likely controlled by leaf hydraulic architecture, is a major evolutionary force shaping current bioclimatic distributions and potential responses to changing climates in conifers. This study also highlights the importance of considering phylogenetic history in functional trait evolution, especially in evolutionarily conservative groups like conifers.