Carbon (C) fluxes among different components of plant growth are important to forest ecosystem C cycling and are strongly influenced by species composition and resource availability. Although mycorrhizal fungi are crucial for nutrient acquisition and can receive a large fraction of annual net primary production, most studies do not explicitly include carbon flux to mycorrhizal fungi in ecosystem C budgets. We measured annual production of plant components (foliage, wood, fine roots) and mycorrhizal fungi across temperate forest stands varying in species composition. Production of mycorrhizal fungi was estimated using both mass balance and isotopic techniques. Total plant production varied from about 600 g C m⁻² y⁻¹ in nearly pure deciduous broadleaf stands down to about 300 g C m⁻² y⁻¹ in conifer-dominated stands. In contrast, the production of mycorrhizal fungi was highest in conifer-dominated stands, varying from less than 25 g C m⁻² y⁻¹ in deciduous broadleaf stands to more than 175 g C m⁻² y⁻¹ in nearly pure conifer stands. Isotopic data indicated that both tree species composition and ecosystem nitrogen (N) availability influenced rates of fungal production. The large investment in mycorrhizal fungi in low-N, conifer-dominated stands demonstrated that a full accounting of ecosystem carbon fluxes to plant and fungal components may help resolve current discrepancies observed in broadscale forest carbon budgets, especially across forest types.