Here, we investigated effects of increased atmospheric CO2 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 C-14-pulse labelling and photosynthesis measurements in combination with analysis of ergosterol content in roots. The effects on delta(15)N in leaves were also studied. Ergosterol content in hair roots was positively correlated with ecosystem photosynthesis and was higher in heat- and CO2-treated plots. Leaves from CO2 Plots tended to be more depleted in N-15 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 CO2 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 CO2.