Responses of an Old Deciduous Forest Ecosystem to Elevated CO2

ABSTRACT The free‐air CO 2 enrichment (FACE) site at the Birmingham Institute of Forest Research (BIFoR) is situated in a long‐established mature, temperate oak‐dominated forest. Over the past 8 years, these tree species have been exposed to elevated CO 2 (eCO 2 target: 150 ppm above ambient). The observed eCO 2 fertilization effect is significant in the mature oak trees, with sustained increases in photosynthesis and stem dry matter production. The fine root standing crop was greater in the eCO 2 plots, and carbon exudation flux was greatly increased, stimulating soil gross nitrogen (N) mineralization, enhanced microbial activity, and enzyme functions. Nitrification was suppressed, particularly in the rhizosphere, pointing towards ecosystem N conservation strategies. Alongside the positive forest productivity response, however, comes evidence of nascent ecosystem fragility, including susceptibility to powdery mildew and insect herbivory that is unchanged in the mature trees but increased in seedlings. Changes to invertebrate food webs were observed, including flower‐pollinator phenologies and detritivore abundances. Pollen and flower mineral contents were significantly reduced under eCO 2 , and acorns contained significantly less protein but more phytic acid, suggesting a carbon‐rich, nutrient‐poor, future diet for animals and insects in natural forest ecosystems.