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Summary• Here, we investigated effects of increased atmospheric CO 2 concentration, increased temperatures, and both factors in combination on ericoid mycorrhizal colonization, mycorrhizal functioning and below-ground carbon allocation in a subarctic forest understorey, to evaluate the hypothesis that photosynthesis is a primary driver for mycorrhizal colonization.• Treatment effects on ecosystem processes were investigated using 14 C-pulse labelling and photosynthesis measurements in combination with analysis of ergosterol content in roots. The effects on δ 15 N in leaves were also studied.• Ergosterol content in hair roots was positively correlated with ecosystem photosynthesis and was higher in heat-and CO 2 -treated plots. Leaves from CO 2 plots tended to be more depleted in 15 N compared with controls both for Vaccinium myrtillus and V. vitis-idaea .• Our results suggest that changes in ecosystem photosynthesis, plant carbon (C) allocation may give rise to changing mycorrhizal colonization under elevated CO 2 and temperature. The role of mycorrhizas in ecosystem N-cycling may change on a long-term basis as inorganic N availability declines with increasing levels of atmospheric CO 2 .

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