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Differences in survival, diameter, height (site index), and stem profile among eight North Carolina half-sib families and one Mississippi–Alabama commercial check of loblolly pine (Pinus taeda L.) at three spacings over 17 years were evaluated for effects on a growth-and-yield model. Actual stand volume at age 17 was determined from a 100% measurement of all trees. This volume was compared with predicted volumes from age nine measurements using (i) the unmodified model and (ii) the model after modifications for family differences. Modifications to the model included family-specific site indices for height differences and family-specific regression functions for each of the other traits. The unmodified model resulted in an underestimate of actual stand volume by 31%. Adjustments for family differences in dominant height (site index), survival, or profile had little effect on this bias. Insertion of family-specific regressions for stem profile and site index in combination with survival-diameter density effects greatly reduced the bias and provided the best estimates of future stand volumes.

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