Abstract:
Foresters care about site productivity and stem quality in Douglas-fir plantations for many
reasons. The profitability of forest land and the economic returns on silvicultural investments
are directly related to site productivity. Thus, understanding the relationships among
Douglas-fir productivity, stem form and site characteristics is important economically. My
objective was to identify the specific site characteristics that explained variation in Douglas-fir
productivity and stem form throughout western Oregon and Washington by using progeny
tests that substantially reduced the confounding effects of genotype by environment
interaction. These assessments were undertaken to advance our understanding of near-term
climate change effects on Douglas-fir productivity and stem form. The site characteristics I
focused on include climate, soils and topography. Measures of site productivity and stem
form were explained using correlation, random forest, and linear regression analyses. The
results of these analytical methods were summarized as total importance scores. The
consistency of important site characteristics identified by the analytical methods and the
consistency of important site characteristics for explaining variability in different productivity
measures were assessed using Spearman rank correlations. The results of this study provide
new direction and insight for future research on understanding and modeling the effects of
site characteristics on tree growth and form. It may be reasonable to focus future research
on summer drought, cold season temperatures and precipitation interactions with soil
properties, particularly available water capacity when examining site productivity.
Additionally, examining genotype by environment interactions at the family level may
provide insight to the driving site characteristics for stem forking, ramicorn branching and
sinuosity.
