In this thesis, we first examine the principle hypotheses behind the improved exotic growth of select timber species, and the evidence for each, with special focus on studies which examine growth between several ranges of a species. We find that literature suggests environmental factors directly tied to net primary production are often the cause of these growth differences, but that the specific causal agent involved varies by species and region. To determine the causes of improved exotic growth of Pseudotsuga menziesii (Mirb.) Franco we identified a set of seven sites in the Oregon Cascade Range and the South Island of New Zealand which were planted with seedlings originating from a shared seed source in 2004. We sampled plots at each of these sites during the 2019 dormant season, conducting tree measurements, taking cores, assessing soil conditions, and installing meteorological instruments. We constructed an environmentally sensitive model of annual stem growth increment using the cores collected from these sites, and were able to account for between and within range variation using environmental factors. Lower growing season humidity deficit was found to be the primary cause of improved productivity in New Zealand compared to Oregon.