- Wood stiffness is one of the most important properties of lumber and veneer. We studied wood stiffness (modulus of elasticity, MOE), wood density, microfibril angle, and knots in a 25 year-old wind pollinated progeny test (50 families, ~ 373 trees) of coastal Douglas-fir to understand the potential for genetically improving wood stiffness. We measured the stress wave MOE of standing trees (MOEST) and logs (MOEHM) using field-based tools (ST300 and HM200) that measure stress wave velocity. We then milled the logs into 2x4s to obtain direct estimates of MOE using bending tests (MOEbl) and indirect estimates using transverse vibration (MOEtv) and stress wave (MOEsw) techniques. On basal wood disks, we measured green (DENgd) and dry (DENdd) wood density; on 2x4s, we measured lumber density (DENdl), sizes of the largest edge (KNTedg) and center knots (KNTcnt), number of knots (KNTtot), and lumber grade; and on small clearwood samples, we measured dry density (DENsc), as well as MOE (MOEsc) and microfibril angle (MFAsc) using the SilviScan system. MOEbl had moderate to strong phenotypic (rp) and additive genetic (ra) correlations with MOEHM, MOEST, MOEtv, and MOEsw (rp = 0.45 to 0.91; ra = 0.57 to 1.03) suggesting that the HM200 and ST300 tools can be used to genetically improve bending stiffness. MOEbl had moderate to strong genetic correlations with DENdl and DENdd (ra = 0.37 to 0.91), and weak correlations with KNTedg and KNTtot (ra = -0.24 and 0.22). MOEbl had a strong phenotypic correlation with DENsc (rp = 0.72) and moderate negative correlation with MFAsc (rp = 0.42). Together, DENdl, MFAsc, and KNTedg explained 49% to 62% of the variance in 2x4 MOEbl, MOEtv, and MOEsw. Compared to MFAsc and KNTedg, path analysis suggested that density had the strongest direct effect on MOEbl. Nonetheless, because density is negatively correlated with growth, and because field-based stress wave tools are now available, there is no great need to measure wood density or MFA to improve wood stiffness. Because the phenotypic and genetic correlations between knot traits and bending MOE are either weak or nonsignificant, knot traits do not seem to be important to include in breeding programs for structural lumber. The STR lumber grade had a higher MOEbl and lower KNTedg than either the S1 or S2 grades.