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 employ a novel modelling framework, multi-response phylogenetic mixed models (MRPMM), that identifies trait correlations at both phylogenetic and phylogenetically independent levels to investigate how climate influences leaf size evolution across conifers. We show overall patterns for all conifers, and focus 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. Phylogenetic relationships explained most of the association between leaf size and climate, with temperature revealed as a stronger driver than dry season precipitation, indicating deep-time co-evolutionary associations. These patterns reflect trade-offs associated with 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 tend to track their ancestral climatic preferences rather than adapting to new environments. This conservatism, likely controlled by leaf hydraulic architecture, results in strong evolutionary constraints on current bioclimatic distributions and potential responses to changing climates in conifers. This study also highlights the importance of considering phylogenetic impacts on functional trait evolution, especially in evolutionarily conservative groups like conifers.