Effect of soil water availability on intra-annual xylem and phloem formation and non-structural carbohydrate pools in stem of Quercus pubescens

Non-structural carbohydrates (NSCs, i.e., starch and soluble sugars) are frequently quantified in the context of tree response to stressful events (e.g., drought), because they serve as a carbon reservoir for growth and respiration, as well as providing a critical osmotic function to maintain turgor and vascular transport under different environmental conditions. We investigated the impact of soil water availability on intra-annual leaf phenology, radial growth dynamics and variation in NSC amounts in the stem of pubescent oak (Quercus pubescens Willd.). from a sub-Mediterranean region. For this purpose, trees growing at two nearby plots differing in bedrock and, consequently, soil characteristics (F—eutric cambisol on eocene flysch bedrock and L—rendzic leptosol on paleogenic limestone bedrock) were sampled. Non-structural carbohydrates were analysed in outer xylem and living phloem (separately for non-collapsed and collapsed parts). Results showed that xylem and phloem increments were 41.6% and 21.2%, respectively, wider in trees from F plot due to a higher rate of cell production. In contrast, the amount of NSCs and of soluble sugars significantly differed among the tissue parts and sampling dates but not between the two plots. Starch amounts were the highest in xylem, which could be explained by the abundance of xylem parenchyma cells. Two clear seasonal peaks of the starch amount were detected in all tissues, the first in September–November, in the period of leaf colouring and falling, and the second in March–April, i.e., at the onset of cambial cell production followed by bud development. The amounts of free sugars were highest in inner phloem + cambium, at the sites of active growth. Soil water availability substantially influenced secondary growth in the stem of Q. pubescens, whereas NSC amounts seemed to be less affected. The results show how the intricate relationships between soil properties, such as water availability, and tree performance should be considered when studying the impact of stressful events on the growth and functioning of trees.