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Bark-Scoring Tools to Repair Tree Grafts

Keith Windell, Project Leader

Grafted seed orchards of Douglas-fir are often subject to tree losses caused by delayed graft incompatibility. When the stock and scion (the grafted branch) are incompatible, the stock and scion wall off the foreign cells with a separation zone of dead, suberized (corky) tissue. The separation zone is termed “brownline.” This response occurs along the entire interface in the bark cells of both the stock and scion. This type of graft failure is the product of the defense mechanisms of each tree. The same process enables trees to wall off invading insects and disease organisms or to cover physical wounds with an impervious or protective covering.

If a graft becomes incompatible several years after grafting, seed production will be lost and genetic variability will be decreased. A number of years will be required to replace the incompatible grafts. Improving the long-term survival and performance of established grafts is difficult.

Bark scoring is one cultural technique that has potential to reduce graft losses and improve tree vigor. Numerous vertical cuts are made across the defective union (figure 1). The primary objective of bark scoring is to improve the flow of water and nutrients (translocation) across the graft union. Bark scoring Douglas-fir grafts temporarily overcomes the incompatibility by physically removing a portion of the barrier of dead tissue separating the living stock and scion cambial cells. New cambia differentiate across the cuts in the newly formed callus tissue. (Donald Copes, PNW-RN-487, 1989).

Figure 1—This failed tree graft was scored with a chain saw.

Small gasoline chain saws are sometimes used to make the small vertical cuts required in bark scoring. Some orchard managers feel that the chain saws are hard to guide with precision and tend to butcher the trees. Chain saws are also awkward to operate and tend to throw chain oil into the cut. A small battery-operated tool called the Woodzig (figure 2) was highly recommended by some managers. Unfortunately, it is no longer available.

Figure 2—The Woodzig was a handy bark-scoring tool.
It is no longer available.

The Missoula Technology and Development Center (MTDC) was asked to identify or develop powered tools for safe, precise, and efficient bark scoring.

Project Approach—A bark-scoring tool needs to be relatively inexpensive. Many tools and attachments were considered and purchased. The tools were quickly tested at MTDC to see if they had any inherent safety risks, such as kickback. Special guards were fabricated and the tools were sent out for field evaluation.

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since January 12, 2005