From roots to canopies: clonal size increases branch vulnerability to embolism and leaf water status in a sub-Mediterranean oak coppice

Climatic and socioeconomic changes are driving widespread tree decline in abandoned Mediterranean coppices, a process potentially modulated by their clonal structure. However, the influence of belowground clonal size on aboveground physiological performance in resprouting species remains unknown. We integrated genetic, physiological and dendrochronological approaches within a one-hectare experimental plot of Quercus pyrenaica Willd. to test the effect of clonal size on shoot vigour and physiological performance. We studied 48 vigorous and decaying shoots from 24 size-contrasted clones, characterized by clonal area, shoot number, cumulative basal and sapwood area, and leaf area index. During the prolonged 2022 summer drought, we monitored leaf water potential, water content and gas exchange across seven measurement campaigns. These measurements were complemented with analyses of leaf pressure-volume traits, leaf minimum conductance, branch vulnerability to embolism, stem non-structural carbohydrates and long-term stem radial growth. Vigorous and declining shoots did not differ in monitored leaf-level variables or leaf hydraulic traits. However, declining canopy vigour was reflected in decadal stem growth stagnation and reduced stem xylem soluble sugars after leaf flush. Increasingly negative leaf water potential during the dry season limited long-term stem growth, more markedly for declining shoots. Both leaf hydration during summer drought and leaf area per unit sapwood area increased with clonal size. Concurrently, large clones exhibited higher branch vulnerability to embolism, with no differences in leaf osmoregulatory capacity. Our findings suggest that, in this coppiced clonal system, (i) stem-level measurements better captured differences between vigorous and declining shoots than leaf-level measurements, likely due to non-uniform branch dieback, (ii) single-time-point physiological measurements can provide insight into long-term growth trends and (iii) centennial coppicing has driven clonal-size-dependent allometric adjustments that favour root water uptake while increasing embolism vulnerability. Clonal structure and size are therefore overlooked factors that should be integrated into management strategies for these ecosystems.