Different responses of oxygen and hydrogen isotopes in leaf and tree-ring organic matter to lethal soil drought

Abstract

The oxygen and hydrogen isotopic composition (δ ¹⁸O, δ ²H) of plant tissues are key tools for the reconstruction of hydrological and plant physiological processes and may therefore be used to disentangle the reasons for tree mortality. However, how both elements respond to soil drought conditions before death has rarely been investigated. To test this, we performed a greenhouse study and determined predisposing fertilization and lethal soil drought effects on δ ¹⁸O and δ ²H values of organic matter in leaves and tree rings of living and dead saplings of five European tree species. For mechanistic insights, we additionally measured isotopic (i.e. δ ¹⁸O and δ ²H values of leaf and twig water), physiological (i.e. leaf water potential and gas-exchange) and metabolic traits (i.e. leaf and stem non-structural carbohydrate concentration, carbon-to-nitrogen ratios). Across all species, lethal soil drought generally caused a homogenous ²H-enrichment in leaf and tree-ring organic matter, but a low and heterogenous δ ¹⁸O response in the same tissues. Unlike δ ¹⁸O values, δ ²H values of tree-ring organic matter were correlated with those of leaf and twig water and with plant physiological traits across treatments and species. The ²H-enrichment in plant organic matter also went along with a decrease in stem starch concentrations under soil drought compared with well-watered conditions. In contrast, the predisposing fertilization had generally no significant effect on any tested isotopic, physiological and metabolic traits. We propose that the ²H-enrichment in the dead trees is related to (i) the plant water isotopic composition, (ii) metabolic processes shaping leaf non-structural carbohydrates, (iii) the use of carbon reserves for growth and (iv) species-specific physiological adjustments. The homogenous stress imprint on δ ²H but not on δ ¹⁸O suggests that the former could be used as a proxy to reconstruct soil droughts and underlying processes of tree mortality.