Estimating volume gains in genetically improved stands at rotation age is challenging because first-generation progeny tests in Douglas-fir were typically established to measure the relative growth performance of individual trees from open-pollinated parent trees. The overall goal of this dissertation research was to improve growth simulation of genetically improved Douglas-fir plantations. The key crown attributes explaining the differential growth performance of families in progeny tests were identified. Trees with relatively short branch length and steeper branch angle tend to have higher total leaf area per unit crown length. Total leaf area per unit crown length could explain one of the mechanisms of genetic gain in Douglas-fir families based on high heritability and strong positive correlation with growth performance. The attribute, total leaf area per unit crown length, could be incorporated as an explanatory variable in diameter growth rate models to at least partly represent genetic tree improvement. The current genetic-gain multipliers approach in existing growth models were tested by simulating the Douglas-fir realized gain trials. Results from these simulations found underprediction of diameter at breast height (DBH) at high-density stands. Refining genetic-gain multipliers could improve predictions of DBH, and stand volume gains, however, these updates could induce more mortality and therefore introduce significant underpredictions in trees per hectare, and stand density index gains at high density stands. Simulation results suggested that genetically improved Douglas-fir stands might have greater carrying capacity than unimproved stands. The current genetic-gain multipliers could not predict ranking changes between elite and intermediate populations during the simulations. The mechanisms that potentially influence the observed differential growth performance among genetic improvement levels and family levels in the Douglas-fir realized gain trials were identified. The better performance of genetically improved populations and superior families tended to have relatively narrow crown and a higher total leaf area. The performance difference between improved populations, elite and intermediate, could be explained by shorter total branch lengths with a higher total foliage mass and projected leaf area relative to branches in the intermediate population. These results suggested that the intermediate population consisted of families that were closer to the crop ideotype, while the elite population would be better categorized as consisting of families conforming to a competition ideotype. More crown studies in Douglas-fir realized gain trials may improve models for estimating total leaf area, branch length, or crown shape as a function of height, DBH, crown length, and genetic-improvement effects or family effects that are less expensive to measure and would facilitate incorporation of heritable morphological attributes into existing growth models.