Lifespan outperforms climate as a predictor of wood functional traits, but secondary woodiness shows no clear climatic pattern in Heliophila, a diverse clade from the Cape Floristic Region.

Abstract

Background and aims: Annuals produce little wood due to their short life cycle, while perennials can accumulate more, though not all do. Consequently, lifespan extension is a prerequisite for-but not synonymous with-secondary woodiness. Even if a shift to perenniality does not substantially increase wood production, it may still affect wood anatomy, as annuals prioritise rapid growth, whereas perennials invest in structural resilience. Heliophila, a genus of the Brassicaceae from the Cape Floristic Region, provides an excellent system to investigate drivers of secondary woodiness and the impact of lifespan shifts on wood traits due to its multiple independent lifespan transitions and occurrence of secondary woodiness.

Methods: We reconstructed evolutionary transitions between annual and perennial lifespans and between herbaceous and secondarily woody habits. Using phylogenetically informed statistics, we analysed the relationship between climate, lifespan, and nine wood anatomical traits. Lifespan-specific evolutionary optima for these traits were estimated and compared. We also tested whether secondary woodiness in Heliophila is associated with specific climatic niches.

Key results: Lifespan shifts in Heliophila are primarily driven by water availability and seasonality, with perennials evolving in wetter and less seasonal environments. Secondary woodiness may be more frequent in warmer niches, though this trend was not statistically supported, likely due to the limited number of secondarily woody species. Lifespan, not climate, better predicted wood traits: annuals had longer, thinner-walled cells, while perennials had shorter cells with thicker walls.

Conclusions: In Heliophila, a shift in climatic niche prompts a change in lifespan, followed by slower adaptations in wood anatomy. Possibly, this pattern arises because alterations in lifespan affect stem architecture, establishing a developmental framework that governs subsequent anatomical adjustments. Furthermore, although not statistically robust, increased wood production may be linked to warmer niches, potentially associated with a temperature-driven enhancement in lignin biosynthesis that reinforces stem structure.