Spatial patterns of tree height variation in a series of Douglas-fir progeny trials: Implications for genetic testing

Our descriptive statistics analyses, combined with some geostatistical techniques, of these Douglas-fir progeny trials clearly demonstrated that there were large variations in tree height of age 6-12 within and among the 66 test sites. Such variations were caused by genetic and environmental factors, as well as their interactions (specifically plot-environmental interactions). When we examined the variation of a test site by arbitrarily separating related factors into family, row, column, patchiness, and plot, we found that there were about 11% of the within-site variance due to family (i.e., 22% to genetic factor), 7% to rows, 5% to columns, 12% to patchiness, 47% to within-plot, and 7% to unknown factors, but the currently applied blocking effectively removed about 5%. Gradients in row and column directions were observed in more than 44 test sites (i.e., two thirds test sites used), and the estimated slopes ranged in average from 0.33 to 1.52 cm/plot, depending on the age measured. Range values, estimated with a spherical covariance model, varied greatly over the test sites but were on average from 5.21 to 6.47 plots, indicating that the average patch size for these trials was around 18 m across. Temporal increases were large for site mean, site variance, and family variance but not much for the proportions of site variance explained by row, column, patchiness, and within-plot. When trees grew older, more significant gradients were found and larger patch sizes observed. These results have provided empirical evidence that the conventional, large complete blocking cannot accommodate the large environmental variation well on the majority of the sites examined and also shown that environments exhibited complex patterns that may not be easily modeled with simple gradients and spherical correlations. Such complexities clearly justify the need of developing and implementing small incomplete block designs that can better fragment and account for the environmental variations in the progeny trials of forest trees. The patterns of spatial variation derived from this large series of Douglas-fir progeny trials used here may not necessarily be applicable to genetic testings for other forest species. Test sites in southern British Columbia may differ from those elsewhere and the patterns of expressed microsite variation may also differ for different traits measured and for other tree species. However, some implications of these analyses and results are clear. First, ch