Elevated atmospheric CO2 induced reductions in litter quality can adversely affect earthworms. However, this understanding is based on laboratory rather than field research and relates to single earthworm and tree species. Here earthworm populations were investigated under Alnus glutinosa, Betula pendula, and Fagus sylvatica in a Free Air Carbon dioxide Enrichment field experiment. Litters from this experiment were also fed to Lumbricus terrestris L. at two rates with live weight change and cast properties assessed. Elevated CO2 (580ppmv) reduced litter N (-12%) with a corresponding increase in C:N ratio, especially for A. glutinosa. In the field, elevated CO2 caused a shift in overall population composition, mainly characterised by reduced anecic biomass (-25%); endogeic and epigeic species were less affected. CO2 effects on total biomass were most pronounced for A. glutinosa (e.g. field total biomass -47% vs. -11% overall). Growth of L. terrestris was lower when fed elevated CO2 litter (-18%), although increased inputs of A. glutinosa litter mitigated this effect. In mesocosms, fresh cast respiration was lower (-14%) for elevated CO2 litter, an effect more pronounced for A. glutinosa (-24%). When normalised for C content, elevated CO2 effects on cast respiration were again negative and most marked for A. glutinosa litter. Litter N concentration, and possibly ease of litter mineralisation were factors affecting litter resource quality Litter N and P concentrations varied with A. glutinosa>B. pendula>F. sylvatica; F. sylvatica had the highest cellulose content. Field earthworm biomass was higher under A. glutinosa compared with B. pendula and F. sylvatica (+17 and +70%, respectively); live weight increased with A. glutinosa litter in the feeding trial almost three times more than for B. pendula, whereas it decreased for F. sylvatica. Cast respiration was highest for A. glutinosa, intermediate for B. pendula (ca. -36%) and lowest for F. sylvatica (ca. -78%). Earthworm responses to elevated CO2 were complex, being characteristic of individual tree and earthworm species; responses were more adverse for trees with higher quality litter and for anecic earthworms.