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Fine roots play a critical role in terrestrial carbon and nutrient cycling. Despite this, quantitative evaluation of the fine root dynamics in plantations and natural secondary forests is lacking. This study examined the dynamics of fine root biomass and necromass of Chinese fir plantations and natural secondary forests (5–41 years old) and employed primary forests as a reference in subtropical China. In the top 60-cm soil depth, the sum of fine root biomass of Chinese fir plantations and natural secondary forests initially increased and then decreased with stand development, attaining a peak in the 27–31-year age class. The total fine root biomass of natural secondary forests was significantly higher than that of Chinese fir plantations across all age classes. The total fine root biomass of primary forests was similar to that in the 38–41-year-old natural secondary forests and Chinese fir plantations. The total fine root necromass was higher in Chinese fir plantations than in natural secondary forests across all ages except in the youngest age class. The necromass to biomass ratio was significantly higher in Chinese fir plantations. Most of the fine root biomass resided in the topsoil (0–10 cm) for all stand types and ages. The higher fine root biomass in the secondary natural forests than in the Chinese fir plantation was most apparent in the topsoil. Across all age classes, the fine root biomass was positively correlated with stand basal area, mean diameter at breast height, and shrub cover, whereas the fine root necromass and the necromass to biomass ratio tended to be more consistent across measured stand characteristics. Our results highlight profound influences of forest management on fine root biomass and necromass and suggest that the low fine root biomass of Chinese fir plantations may have a negative impact on carbon and nutrient cycling in the subtropical forests.