This book explores the complex relationship between genetics and the environment in Douglas-fir trees, drawing on a detailed case study in one breeding zone of western Oregon. The author's research reveals the presence of genotype-environment interaction, a phenomenon where tree growth and performance vary significantly depending on the specific plantation site. This interaction, often overlooked in previous studies, is found to be a substantial factor influencing genetic gain predictions and models for predicting tree growth under different environmental conditions. The book further investigates the potential causes of genotype-environment interaction, considering factors such as parent tree origin, within-population genetic variation, and site productivity. The author argues that this interaction is likely a result of adaptive responses to local environmental conditions. The research presented here challenges the common assumption that genotype-environment interaction is negligible and emphasizes its importance in understanding Douglas-fir growth patterns and making accurate predictions about their performance in different environments.