Multi-year study on the effects of elevated CO ₂ in mature oaks unravels subtle metabolic adjustments but stable biotic stress resistance

Rising atmospheric CO ₂ levels are predicted to influence forest health directly and indirectly, yet the long-term effects of elevated CO ₂ (eCO ₂ ) on mature trees in natural ecosystems remain poorly understood. Understanding how eCO ₂ affects susceptibility to biotic stress and alters leaf metabolism is critical for predicting forest responses to climate change.

We examined the effects of eCO ₂ (+150 ppm) on 180-year-old Quercus robur at the Birmingham Institute of Forest Research (BIFoR) Free Air CO ₂ Enrichment (FACE) facility. From 2016 (pre-treatment) to 2024 (year 8 of enrichment), we monitored natural powdery mildew infection and insect herbivory, alongside targeted and untargeted metabolomic profiling of leaf material collected across the growing season.

While seasonal patterns and an overall decline in PM and herbivory were observed, no consistent differences in biotic stress incidence emerged due to eCO ₂ . Metabolomic data revealed subtle but widespread shifts, especially in amino acid, CoenzymeA, and redox pathways.

These results suggest that although eCO ₂ drives extensive metabolic changes, it does not alter biotic stress resistance in mature oaks. Instead, eCO ₂ appears to promote physiological plasticity that may shape future responses to combined environmental stressors. These insights offer a valuable reference point for interpreting long-term ecosystem dynamics.