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 scale. We conducted an analysis of fine-root diameter for 1,163 plant species to detect root diameter patterns in relation to resource availability (e.g. carbon, nitrogen and water), stress intensity (e.g. plant/soil biodiversity, soil bulk density) and temperature. First- to fourth-order root diameter showed non-linear relationships with latitude and/or mean annual temperature (except for first-order root diameter). The diameter of five root orders decreased with increasing mean annual precipitation, but increased with net primary production (NPP), which was the strongest determinant of fine-root diameter. Increasing soil biodiversity was associated with decreasing root diameter of fourth- to fifth-order roots while increased plant biodiversity was associated with decreasing diameter of first- to third-order roots. Total soil nitrogen had a positive effect on first-order root diameter, but a negative effect on fourth- and fifth-order root diameter. The patterns reversed for total soil phosphorus. First- to third-order and fifth-order root diameters increased with increasing soil bulk density. Second- to fourth-order root diameter increased with soil pH. Overall, the variables related to climate, biology and soil explained 44% to 63% of the total variance in the diameter 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 strategies for nutrient/water acquisition and transport under climate change.