Although the importance of understory plants for ecosystem function and processes has been increasingly recognized, the contribution of understory root respiration to soil respiration in forest ecosystems has seldom been studied. In this experiment, we quantified understory root respiration in two subtropical Eucalyptus plantations (2-year-old and 24-year-old). When Eucalyptus was not girdled, understory root respiration accounted for 16% and 36% of total soil respiration in the young and old plantation, respectively. However, the percentages of understory root respiration to total soil respiration were 30% and 11% in the young and old plantation when Eucalyptus was girdled, respectively. We propose that understory root respiration can be substantial in subtropical forests and should be considered in ecosystem carbon budget.