The age-dependence of forest biomass (

) is shown to be a power-law monomial where the power of the age (

) is theoretically estimated to be 4/5. However, in testing the 4/5 law against observations, a "rejuvenation bias" (

) should be introduced to explain the variations in growth pattern induced by delayed development or intensive thinning of the forest stand at the early stage. In other words,