Tracking the isotopic fingerprint of defoliation in tree rings

Understanding changes in the physiological responses of trees to disturbances, and establishing proxies to reconstruct past events, is of high importance in a changing world. Recent studies have demonstrated the potential of δ²H in tree-ring cellulose as a proxy for physiological changes in carbon utilization, reflecting shifts between the use of current assimilates and stored C sources. These findings might explain the considerable annual variations in the strength of the δ¹⁸O and δ²H (O-H) relationship despite the shared hydrological pathway, underlining the complex interaction of hydrological and physiological processes. One of the situations where there is a clear disruption of carbon assimilation and tree functioning is defoliation events. Thus, tree-ring isotopes can be utilized to test the physiological signal recorded in tree rings by quantifying changes in δ¹³C, δ²H and δ¹⁸O values, and the decoupling of the O-H relationship. Here, we investigated the isotopic fingerprint of abiotic and biotic defoliation events in tree-ring cellulose, including (i) late-spring frost on European beech near its upper elevational limit in the Swiss Jura(ii) pine processionary moth outbreaks in northern Italy, and (iii) cockchafer moth outbreaks on archaeological oak material from Central European lowlands. Across all defoliation types, a common fingerprint was identified with significantly enriched δ²H, depleted δ¹⁸O, resulting in the decoupled (negative) O-H relationship, and non-affected δ¹³C values.  As defoliation causes reduced fresh carbon assimilation, the remobilization of stored non-structural carbohydrates (NSC) is likely the fundamental process for plant growth, metabolism, and canopy re-flushing. NSC differ in their isotopic ratio compared to fresh photosynthates, by exhibiting ²H-enrichment and ¹⁸O-depletion, explaining the negative O-H relationship in tree-ring cellulose. Since defoliation has been shown to induce allocation shifts by prioritizing NSC storage over radial growth, foliage loss also leads to substantial secondary growth reductions which was observed across all defoliation types. The generally non-significant changes in δ¹³C between outbreak and non-outbreak years indicate minor impacts on leaf stomatal conductance. In conclusion, this common isotopic fingerprint provides valuable insight into past defoliation events and their reconstruction, which is particularly relevant in the context of rapid environmental change.