Two models are developed to illustrate how age or diameter may control the radial increases in wood specific gravity (SG), a feature common to lowland tropical trees. In the age-dependent model, trees of the same age produce new secondary xylem of the same SG regardless of their diameters, i.e., SG is dependent on age. In the radius-dependent model, trees of the same radius produce new secondary xylem of the same SG regardless of their ages. Then, prediaions of the two models are tested on radial wood samples from the trunkwood of Joanesia princeps Vell., growing in a 17-year-old plantation in Espirito Santo, Brazil. For this cohort, tests of four prediaions supported the age-dependent model over the radius-dependent model: final specific gravity was independent of radius (smaller trees did not have smaller final spedfic gravities), the slope of the mdial increase in SG with tree radius was negatively dependent on tree radius (smaller trees had steeper slopes), the coefficient of variation (CS) of SG of the final wood was the same or less than the CV of SG of the initial wood, and the CV of SG of the final wood was much less than the CV of the tree radius. Thus, for plantation trees of the same age, the SG of wood produced is primarily a funaion of age, not radius of the tree. Forest trees show similar relationships but with the effects of age and radius confounded because tree ages are unknown.252176182