Elevated atmospheric CO₂ alters root symbiont community structure in forest trees

•  Changes in below-ground ectomycorrhizal (ECM) community structure in response to elevated CO₂ and balanced nutrient addition were investigated in a 37-yr-old Picea abies forest. • Trees in whole-tree chambers were exposed to factorial combinations of ambient/elevated CO₂ (700 ppm) and fertilization (+/-). ECM fungal community structure was determined in 1997 and 2000 using a combination of morphotyping and molecular analyses. Samples were taken both from chambers and from reference trees receiving the same fertilization treatments but without chambers. • Significant effects on ECM community structure were found in response to elevated CO₂ . Neither elevated CO₂ nor fertilization altered species richness; however, there was considerable variation among samples, which may have masked treatment effects on individual species. After 3 yr, the effects of elevated CO₂ on community composition were of the same magnitude as those seen after 15 yr of fertilization treatment. • Our results show that increasing atmospheric CO₂ concentrations affect the community structure of root symbionts colonizing forest trees. The potential effects of altered ECM community structure on allocation and turnover of carbon and nutrients within forest ecosystems are discussed.