Before we go further in this section we should probably define "root pruning" and "wrenching."

1. Root Pruning. Commonly conducted early in spring. Usually done by pulling a sharp, horizontal, thin blade (reciprocating blade) through the soil at 4-6 inches below the soil surface. This severs the lower roots cleanly, but does not disturb the soil or seedlings appreciably. Sometimes this technique is followed two-three months later by a lateral root pruning. This practice involves using a multiple disc, with the individual blades centered between the rows. The result of both the horizontal and lateral root pruning is a more compact, fibrous root system.

2. Wrenching. In contrast, wrenching is a more vigorous treatment wherein a sharp, relatively thick, blade inclined about 30° from the horizontal is drawn under the seedlings at a depth of 4-7 inches below the soils surface. The blade tends to tear, rather than cut cleanly, the root system and disturbs the soil. In species such as radiata pine, the seedlings should wilt slightly after wrenching if the treatment is to be a success.

Undercutting was generally considered harmful until about 30 years ago. Since that time it has become an increasingly frequent practice in coniferous nurseries, but there are few published data describing the effects of either bottom or side root pruning of species indigenous to the Pacific Northwest.

1. Trappe (1971) demonstrated that root pruning of Pseudotsuga menziesii var. menziesii resulted in a more compact, fibrous root system, but that similar treatment of vr. glauca plants stimulated non-branched long roots which grew rapidly downward.

2. Eis (1968) demonstrated that, for Douglas-fir, bottom root pruning in spring stimulated root growth but that lateral root pruning was most effective in late June. Before this date, most seedlings had not extended lateral roots to the distance from the stem (3.5 inches) of the pruning; and lateral pruning occurred after the period of maximum root growth.

3. Similar work by Eis and Long (1972) with Sitka spruce and western hemlock demonstrated that lateral root pruning did not affect height growth of seedlings of either species and that the optimum time (in British Columbia) for lateral root pruning is early July for hemlock and mid-July for spruce.

4. Dykstra (1974) showed that relatively shallow bottom pruning (3-4") stimulated the greatest root regeneration for first year lodgepole pine seedlings grown under controlled conditions. However, greatest root mass at time of harvest was on the non-pruned plants.

5. Edgren (1981) demonstrated that undercutting Douglas-fir seedlings of variety menziesii during May of the second growing season resulted in slightly higher survival and better growth than that of the control seedlings after three growing seasons in the field. Similar treatment of var. glauca seedlings produced no improvement in survival or growth.

Wrenching was first developed in New Zealand about 40 years ago to facilitate planting of Pinus radiata. The literature on the effects of wrenching upon seedlings is not consistent, in part at least, because timing and depth of the operation together with seed bed density all affect results and not all trials have included attention to these details. However, the following are generally considered to result from properly conducted wrenching:

• Induces a severe moisture stress.
• Promotes root growth and development of a fibrous root system.
• Reduces shoot growth, stimulates initiation of dormancy.

  In addition, Sweet and Rook (1972) note that wrenching reduces the levels of inhibitors in the root of treated Pinus radiata seedlings and Rook (1971) demonstrated that wrenching increased the levels of carbohydrate reserves in seedling root systems.

  Recent work at Oregon State University has shown that wrenching will promote early closure of seedling stomata.

  Early trials of wrenching with Douglas-fir seedlings suggested that field survival might be improved with this technique. However, later work has failed to demonstrate either improved survival or growth. It is probable that if seedling irrigation schedules are properly maintained and plants enter dormancy by mid-July, wrenching is not necessary to properly condition the seedlings.

  Top Pruning

  Current nursery practice in the Pacific Northwest has resulted in production of seedlings which demonstrate a substantial range of size within a seedbed and which are often too large for effective reforestation. Accordingly, recent research has been concerned with the techniques necessary to successfully prune back overly tall seedlings to create a more uniform seed bed population.

  Transplanting

  Transplanting of 2-0 seedlings to produce 2-1 transplants has traditionally been done in northwest nurseries after the seeds are sown in the spring. This practice has resulted in a delayed bud-break of seedlings, which were maintained in cold storage until transplanted, and relatively little shoot the year of transplanting. Recent trials have suggested that: 1) bare root seedlings transplanted in the fall may produce very vigorous transplants the following spring; and 2) seedlings grown as "plug" seedlings for the first growing season may be outplanted in the nursery in late August to produce 1-1 transplants at the end of the following year which are very comparable with bare-root 2-1 transplants.

  Fall transplanted seedlings generally resume growth early the following spring, and if dormancy is initiated not later than mid-June, could lead to well adapted stocktypes for high elevation planting immediately after initiation of the fall rains. Thus far, there has been no research to test the validity of this hypothesis.

  Study Questions

  • For low elevation seeds is the length of the stratification period more important if the seed is sown early or if it is sown late in a nursery? Why?

  • How does the date of sowing impact the seedling's annual growth cycle?

  • Give three effects of high density seed beds upon seedling quality.

  • How does wrenching differ from root pruning?

  • Why do we not practice lateral root pruning early in the spring?

  • If the nursery is irrigated properly, would you expect a great improvement in quality of wrenched seedlings? Why?

    Readings

    1. Lavender, Denis P. 1984. Plant Physiology and Nursery Environment. Chapter 14 in Duryea, M.L. and Landis, T.D. (Eds.) Forest Nursery Manual, p. 133-141.

    2. Mexal,J.G., and Landis,T.D. 1990. Target seedling concepts: height and diameter. In Target Seedling Symposium, USFS, Gen Tech Report RM-200, pp. 17-36

    Additional Information:

    1. Lavender, D.P. et al., 1990. Regenerating British Columbia's Forests, Chapters 16, 17.

    2. Duryea, M.L. 1984. Forest Nursery Manual, Chapter 15

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    Bulk lifting seedlings at the D.L.Phipps Nursery in Elkton, Oregon.