AbstractTree biomass allocation to leaves, roots, and wood affects the residence time of carbon in forests, with potentially dramatic implications for ecosystem carbon storage. However, drivers of tree biomass allocation remain poorly quantified. Using a combination of global data sets, we tested the relative importance of climate, leaf habit, and tree mycorrhizal associations on biomass allocation. We show that trees that associate with arbuscular mycorrhizal (AM) fungi allocate roughly 4% more of their biomass to root tissue than trees that associate with ectomycorrhizal (ECM) fungi. Further, the effect of mycorrhizal association on root biomass allocation was greater than that of climate and similar in magnitude to that of leaf habit (evergreen vs. deciduous). These patterns in whole‐plant biomass allocation are likely due to differences in carbon investment toward root versus fungal tissues, where trees with AM fungi favor root production while trees with ECM fungi favor fungal tissue production. These results suggest that considering tree mycorrhizal associations could improve our understanding of ecosystem carbon storage in terrestrial biosphere models: specifically, that greater within‐tree allocation to root biomass in AM‐associated tree species may contribute to stable soil carbon pools in forests dominated by AM fungi.