The importance of fine-root diameter for ecosystem functioning is increasingly recognized, yet much remains to be learned about the variation in fine-root diameter at large scales. We conducted an analysis of fine-root diameter for five root orders for 1163 plant species to detect patterns in relation to resource availability (e.g. carbon, nitrogen, water and net primary production (NPP)), stress intensity (e.g. plant/soil biodiversity and soil bulk density) and temperature. First- to fourth-order root diameters showed non-linear relationships with mean annual temperature (except for first-order root diameter) and/or with latitude. The diameters of the five root orders decreased with increasing mean annual precipitation, but increased with greater NPP, which was the strongest determinant of fine-root diameter. Increasing soil biodiversity was associated with decreasing diameters of fourth- and fifth-order roots, while greater plant biodiversity was associated with decreasing diameters of first- to third-order roots. Soil total nitrogen concentration had a positive effect on first-order root diameter but a negative effect on fourth- and fifth-order root diameters. The patterns reversed for soil total phosphorus concentration. First- to third-order and fifth-order root diameters increased with greater soil bulk density. Second- to fourth-order root diameters increased with higher soil pH. Overall, the variables related to climatic, biological and edaphic factors explained 44–63% of the total variance in the diameters of the different root orders. The unique patterns of plasticity observed in fine-root diameter across root orders in response to varying environmental conditions contributes to a diversification of plant strategies for nutrient/water acquisition and transport under climate change.