High-density miniaturized seed orchards of Douglas-fir

Abstract

We studied miniaturized seed orchards (MSOs) and conventional orchards of coastal Douglas-fir (Pseudotsuga menziesii var. menziesii) in a series of experiments conducted at three locations in Oregon and Washington between 2002 and 2012. In Experiment 1, cone yields were greater using stem girdles plus stem injections of GA4/7 compared to girdling alone, root pruning, and girdling plus fertilization with Ca(NO3)2. In Experiment 2, topping and pruning in the summer following flower stimulation minimized crown volume and maximized female and male flower densities. In Experiment 3, the widest spaced orchard (4 × 6 m) generally produced more flowers and cones per tree, and greater flower and cone densities compared to the narrow spaced orchards (1 × 3 and 2 × 4 m). We conclude that narrow spaced MSOs may produce more cones per hectare in the early years after establishment, but wider spaced orchards will be more productive a few years later. In general, per-hectare cone yields were similar or greater in the MSOs compared to nearby conventionally spaced orchards. Trees grafted using scions collected from juvenile trees (age 6) generally had larger crowns, more female flowers and cones per tree, and greater female flower and cone densities compared to scions collected from middle-aged (age 29) or mature (56- to 99-year-old) trees. We found no evidence that clonal rows resulted in reduced seed quality relative to nearby conventional orchards. Implications for Douglas-fir seed orchard management are discussed.