Biomass, water and carbon economy shifts in oak saplings across temperate and Mediterranean bioclimatic zones

Tree growth and survival depend on how effectively trees adjust functional traits to environmental variation. Saplings are especially vulnerable to water scarcity; yet their morphological and physiological responses across contrasting climates are far from being completely understood. We investigated functional responses in four-year-old Quercus pubescens Willd. and Quercus ilex L. saplings of different localities transplanted in the corresponding sites along a latitudinal gradient in Italy, spanning from Temperate to Mediterranean climates. We integrated measurements of morphology and water relations during the summer season, as well as seasonal non-structural carbohydrate partitioning across all plant organs. Mediterranean sites underwent clear trait shifts: saplings prioritised biomass allocation belowground, reflecting an avoidance strategy under drier conditions. Functional traits also diverged between the Mediterranean sites, with saplings at the southern latitude showing a lower turgor loss point and stomatal conductance under similar drought stress levels. Results indicated organ-specific seasonal differences in non-structural carbohydrates; in particular, twigs and stems appeared to be the organs primarily involved in drought responses. Lower winter non-structural carbohydrate allocation in drier sites, particularly in Q. ilex, suggests reduced accumulation relative to synthesis, potentially affecting sapling drought resilience. Taken together, these findings and the higher mortality at Mediterranean sites indicate their potential increased survival risk. Evaluating carbon metabolism and water relations of locally sourced saplings under natural conditions at the whole-tree level is therefore pivotal as an initial step to inform restoration plans in increasingly water-limited environments.