Drought-induced carbon reallocation in European beech: linking non-structural carbohydrates, xylem anatomy, and water use efficiency

Abstract

Understanding how trees balance carbon storage, hydraulic function, and growth under drought is critical for predicting forest resilience to climate change. However, little is known about the interactive roles of non-structural carbohydrates (NSC), particularly starch and soluble sugars, in coordination with wood anatomical traits and stable isotope signatures in mediating tree responses to drought stress. This study investigates the interplay between NSC, wood anatomical traits, and intrinsic water use efficiency (iWUE) in 56 European beech (Fagus sylvatica L.) trees from different crown conditions across drought and wet years in four sites in northeastern France with contrasted soil water deficit. We followed NSC (starch and soluble sugars) content in sapwood of trees each year and analyzed retrospectively tree ring width, vessel anatomy and stable isotopes (δ13C, δ18O). Results revealed that drought years significantly reduced starch content but increased soluble sugars, reflecting their role in osmotic regulation and metabolic demands. The soluble sugars to NSC ratio increased during drought, highlighting a dynamic carbon reallocation. Growth (tree basal area increment) declined in drought years, with starch accumulation in wetter years and soluble sugars prioritizing survival under stress. Soluble sugars to NSC ratio correlated positively with vessel density and theoretical specific xylem hydraulic conductivity (Kth), suggesting their involvement in hydraulic maintenance, while starch exhibited a negative relationship with Kth, indicating a trade-off between carbon storage and hydraulic efficiency. Elevated δ13C and iWUE during drought confirmed stomatal closure to conserve water, though at the cost of reduced carbon assimilation. δ18O correlated positively with soluble sugars, closely coupling carbohydrate dynamics to altered transpiration. These findings underscore how NSC dynamics, anatomical adjustments, and isotopic signals collectively mediate drought responses, offering insights into carbon-water trade-offs in temperate forests.