Measurement of wood stiffness in standing trees using acoustic velocity. Wood stiffness or the longitudinal modulus of elasticity (MOE) of standing trees was measured by acoustic velocity using an IML hammer. Average stiffness of trees in each family showed a roughly normal distribution in the Flynn (a) and Kromelite (b) trials. Ten families with the highest (HS) and lowest stiffness (LS) were selected and their stiffness variations are shown for the Flynn (c) and Kromelite (d) sites. Two individuals were further sampled in each selected family to maximize the stiffness variation within the Flynn (e) and Kromelite (f) trials. Average wood stiffness of the 10 sampled trees with HS was significantly higher than that of the 10 LS trees (P-values ≤ 0.0001) in both trials, but average diameter at breast height (DBH) was very similar.