Abstract:

We compared vein and stomatal traits of seedlings and adults of three Mediterranean Quercus species. Previous work suggests that gas-exchange rates tend to be higher at the seedling stage than in adults. Our objective was to determine whether vein and stomatal traits vary throughout whole-plant ontogeny in parallel with the changes in gas-exchange rates. We addressed the following alternative hypotheses: hypothesis 1—seedlings show higher vein and stomatal densities than adults; and hypothesis 2—seedlings have lower investments in vascular tissues to reduce construction costs.

Ten specimens from each growth stage were randomly sampled for each species in a location in central-western Spain. We measured mean stomatal and vein traits (size and number of stomata per unit of leaf area, vein density, vein volume, vein to epidermis distance), leaf mass per area and lamina thickness.

Minor vein density and vein volume per area increased with tree age, which seems inconsistent with the ontogenetic trends in gas-exchange rates. This discrepancy is in support of our hypothesis 2, and it suggests that, at the seedling stage, reducing investments in vascular tissues in benefit of maximizing growth rates is a priority. Larger interveinal distances in seedlings were compensated by smaller vein to epidermis distances. The thin leaves of the seedlings may thus constitute as a necessary trait for achieving shorter path length distances for the transport of water to evaporation sites without the need for a strong investment in costly vascular tissues.

Key words: leaf venation, leaf life span, plant ontogenetic changes, Quercus species, stomatal traits