Effects of stand age and inter-annual precipitation variability on fine root biomass in poplar plantations in the eastern coastal China

•Poplar and understory vegetation FRB increased with increasing stand age.•Young poplar FRB increased in the drought year, while that in old stands decreased.•Poplar FRB at all stand ages distributed to deeper soil layers in the drought year.•Understory vegetation FRB also increased in the drought year.•In the drought recovery year, understory vegetation FRB recovered, but poplar not.