Competition increased fine root biomass in Chinese fir (Cunninghamia lanceolata) plantations in Subtropical China

•Correlations between different fine root groups and soil conditions representing forest biomass are studied.•The fine root biomass of all groups was significantly and negatively correlated with forest biomass.•Transport root biomass and total fine root biomass showed significant and positive correlations with understory vegetation fine root biomass.•Fine tree root biomass is significantly promoted and increased due to significant competition from understory vegetation.