Using data from acorns produced by mature oak trees in the eighth year of elevated CO2 (eCO₂), we present evidence that similar effects occur in long-established forests, with negative consequences for seed quality, that impact herbivore nutrition and health. The analysis of acorns from the near-200-year-old oak trees at the Free Air Carbon dioxide (FACE) facility at the Birmingham Institute for Forest Research (BIFoR) revealed that growth under eCO₂ increased the phytate content but decreased the protein content of acorns. Additionally the analysis of protin profiles showed significant differnves in protein abundances in both typoe of samples.