Phenotypic integration of post-germination traits in Quercus suber: morphology is mediated by acorn mass and leaf...

Assessing intra-specific trait covariation across populations is essential to understand species' adaptive responses to climatic variation. However, in tree species this is understudied for early-life stages despite their greater vulnerability to environmental changes, and climatic adaptations can differ between tree ages. In this paper we study the integrated phenotype of Quercus suber during the months following germination. For this purpose we assessed the covariation of key traits involved in seedlings' water and C economies along a gradient of aridity at seed origin.

We performed a provenance trial with 157 Q. suber seedlings originating from seven different populations across the species distribution. The seedlings were germinated and grown for 4 months under common conditions. Acorn mass along with 11 above- and below-ground traits involved in water and carbon use were measured. They were organized into latent variables and their covariation with increasing aridity and temperatures was analysed using structural equation modelling. Individual traits were also analysed with linear mixed-effects models to account for maternal effects.

Seedlings from arid populations displayed a higher leaf evaporative surface coupled with greater root development. They also showed greater stomatal size and chlorophyll content, strongly linked to traits conferring drought and heat tolerance (low root-shoot ratio and high flavonoids). The development of above- and below-ground tissues responded mainly to acorn mass, whereas leaf physiology variations were associated with the populations' climate.

Surprisingly, dry-origin seedlings display a more acquisitive strategy at the whole-plant level compared with seedlings from mesic provenances. This allows greater water and carbon uptake capacities following germination, which is critical for their survival during their first summer. Leaf physiology adjustments to populations' climate contrasts with observations by other studies addressing juvenile trees, highlighting the use by Q. suber of varying adaptive strategies at different ontogenic stages.