Field survival and early height growth of Douglas-fir rooted cuttings: relationship to stem diameter and root system quality

In 1990, three studies involving 11 half-sib families of Douglas-fir rooted cuttings were established at three sites in western Washington State (USA): two with low regeneration difficulty (RD) and one with high RD. Grading and cull studies were installed on each site and a root number study on one site. The grading study compared the performance of nine classes of rooted cuttings based on stem diameter and root system quality. The cull study evaluated five different types of putative culls. The root number study determined the correlation between the number of initial roots and field performance.

In the grading study survival and height growth reflected stem diameter and relative root quality on all three sites. Mean survival by treatment was in the range 92–100%, 82–97% and 66–87% across the three sites. First year height growth varied from approximately 10 cm to 20 cm and was greatest on the low RD sites. Second year height growth was from three to four times greater than first year height growth on the low RD sites and two to three times greater on the high RD site. Plants with larger diameter had higher survival and better growth than those with smaller diameter. Within each diameter class, plants with good or fair root systems outperformed those with poor root systems. Performance of seedlings and transplants was nearly identical to that of rooted cuttings with corresponding diameter and root system quality.

In the cull study only trees with stem diameter < 4 mm were deemed true culls owing to significantly reduced survival and height growth. In the root number study, rooted cuttings generally increased in size in the nursery in proportion to root number. However, after 2 years in the field, root number was a very poor predictor of survival and height growth.