1. Fine roots play a pivotal role in terrestrial carbon and nutrient cycling. However, our knowledge on drivers of fine-root biomass (FRB) and productivity (FRP) focus on functional traits, biodiversity and abiotic factors, while less attention on allometric constraints, an indispensable driver of organism carbon partitioning. 2. We measured FRB (FRP) for 24 plots using 216 soil cores (ingrowth cores) from four forest types (birch, oak, larch and pine) on a warm-temperate mountain of north China, and investigated leaf, stem and fine-root functional traits, stand factors, diversity and soil fertility. We tested the allometric relationships among FRB, FRP, aboveground (leaf) biomass and functional traits, and examined how allometry, size-dependent growth strategies, the mass-ratio and complementary effects affected FRB and FRP directly and indirectly. 3. There is stable allometric relationship between FRP and FRB at both the soil-core and plot levels, and the former supported the predicted exponent for leaves (=1) of the metabolic scaling theory. Contrary to common observations, plot-scale FRB and FRP showed negative (or non-significant) relationships with aboveground (leaves) biomass. Instead, higher aboveground biomass led to more conservative growth strategies, which led to lower FRB, and thus lower FRP due to allometric constraints. Root traits (mass-ratio effect) showed the strongest direct effect on FRB, while diversity (complementary effect) and soil fertility revealed weak effects. FRP was strongly driven by allometry (FRB) and soil nitrogen, while functional traits and diversity affected FRP via FRB instead of directly. 4. Our results do not conflict with the positive correlations of FRB (FRP) with aboveground (leaf) biomass reported by large-scale studies, but together suggest changes of growth strategies with tree size vs. climate, which may affect aboveground-root relationship simultaneously. Thus, we suggest to carefully test allometric relationships to better understand how biodiversity, traits and stand factors affect fine-root dynamics.