Understanding physiological mechanisms of European beech dieback responses to climate using a triple isotope approach in northern Switzerland

To investigate which physiological predispositions led to the drought-induced vitality decline in beech ( Fagus sylvatica L.) following the severe 2018 drought, we studied trees with premature leaf discoloration and shedding (early-browning trees) and trees showing no symptoms (vital trees) in a forest in northern Switzerland. We analyzed annual tree-ring width (TRW) and applied a triple isotope approach (i.e., carbon (δ 13 C), oxygen (δ 18 O), and hydrogen (δ 2 H) isotopes) in tree-ring cellulose for the period 1960–2020. To retrieve tree physiological responses, we normalized the tree-ring δ values to temporal isotopic variations in CO 2 or precipitation (Δ). Δ 13 C and Δ 18 O values suggest that the early-browning trees had a more conservative water-use strategy and lower stomatal conductance than the vital trees in the initial decades of the measurement period. However, several decades before the onset of crown dieback in 2018, the early-browning trees showed a decrease in TRW and an increase in Δ 2 H, suggesting a higher use of carbon reserves for the early-browning trees. These long-term trends may be the first signs of a progressive deterioration of the physiology of the early-browning trees. During and after the 2018 drought, changes in Δ 2 H suggested high carbon investments into drought damage repair for the early-browning trees. Moreover, a higher TRW and isotope sensitivity to previous year’s summer climate in early-browning than vital trees suggests stronger negative carry-over effects. Our findings highlight that the early-browning trees may have already been weakened before the 2018 drought, eventually pushing them beyond their physiological tipping points and inducing dieback. • Application of a novel triple isotope approach to investigate beech dieback. • Early-browning trees had a conservative water-use strategy and used more C reserves. • High sensitivity of early-browning trees to previous year summer climate. • The 2018 drought acted as an inciting factor, leading weaker trees to die back.