Abstract The additive contribution of fine root biomass for Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) to the stand average fine root biomass were estimated for eight conifer stands in the Pacific Northwest. Based on the Ribbens model fits to root coring data and the size and location of trees in the conifer stands, the estimated stand-level fine root biomass was 188–1157 g m−2 for P. menziesii and 24–347 g m−2 for T. heterophylla. Site differences in the stand-level fine root biomass for P. menziesii and T. heterophylla were largely explained by climate and soil nitrogen. Fine root biomass for P. menziesii was inversely related to soil nitrogen and, to a lesser extent, annual precipitation and temperature. These relationships were unchanged when data from our sites and four studies in the published literature were combined. In comparison, fine root biomass for T. heterophylla was positively related to temperature and precipitation and, to a lesser extent, inversely related to soil nitrogen. Because temperature and precipitation have opposite effects on fine root biomass for P. menziesii and T. heterophylla, these climate variables were not important predictor variables for total fine root biomass. In addition, the inverse relationship between total fine root biomass and soil nitrogen was obscured and confounded by the root–climate–species interaction.