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General Characteristics

Mature grand fir (Abies grandis [Dougl. ex D. Don] Lindl.) trees are the least abundant seed producers of associated species in northern Idaho, and grand fir seeds have a low germination percentage; yet advance grand fir regeneration is common throughout the range of this species. Grand fir is rated as being tolerant of shade but is well-adapted to growing in full sunlight, producing yields almost as high as western white pine. Natural regeneration of grand fir is best under a shelterwood, and about 30 to 40 percent crown ratio is needed for adequate growth.

Release Interactions

Heart and root rots are considerations in releasing grand fir. East of the Cascade Mountains, Indian paint fungus is a major heart rot in grand fir. For example, Indian paint fungus infected 26 percent of 1,090 grand fir trees in 11 study areas in the Blue Mountains of Oregon and Washington, accounting for 71 percent of the total board foot volume decay. Armillaria mellea and Phellinus weirii (Murr.) Gilbertson are important root rot fungi that attack grand fir. Several insects attack grand fir. The worst defoliators are the western spruce budworm and the Douglas-fir tussock moth. Both insects defoliate grand fir and cause mortality if defoliation persists for several years.

Response to Release

Even though the transition from a suppressed understory tree to conditions of full sunlight is difficult, few grand fir die after overstory removal. In the study of 2,427 released grand fir and Shasta red fir (Abies magnifica var. shastensis Lemm.) in central Oregon, only 0.3 percent died during the first 5-year period since release, while 2.9 percent died during the second 5-year period. Most of the trees that died were less than 3 ft. tall. In another study of 115 released grand fir trees in central Oregon, Seidel reported 0.9 percent mortality during the first 2 years since release and an additional 6.0 percent in the next 3 years.

Growth responses of individual trees vary widely, but there are similar trends among various studies. Both height and diameter response can be immediate or be delayed up to 3 years. Delays of up to 4 years are reported under shelterwood regeneration methods. Growth after release follows a sigmoid-shaped curve in that trees respond with increasingly larger annual growth, eventually tapering off by about 10 years time after release. The sigmoid-shaped response curve is similar for both height and diameter growth.

In Oregon, important predictors of response to release include crown ratio, crown diameter, pre-release height growth, and residual basal area. Mathematical equations developed by Seidel predict height and diameter growth response of released grand fir in northeast Oregon. In northern Idaho, equations developed by Ferguson and Adams indicate several tree, site, and stand variables including pre-release height growth, time since release, tree age at release, height at release, habitat type, logging damage, slope, aspect, residual basal area, and competing regeneration-size trees, are helpful in predicting response of released grand fir.

Pre-release growth reflects the degree of suppression by the overstory, thereby negating the need to measure pre-release overstory density. Trees exhibiting better growth prior to release have larger increments following release. Larger crowns are positively correlated with larger growth responses following release. Seidel recommended a 40 to 50 percent live crown ratio at the time of release.

Poor response to release is associated with older regeneration, tall trees having low vigor (as expressed by slow pre-release growth or small crown ratios), warm/dry habitat types, southerly aspects with steep slopes, and logging-damaged trees. Seidel reported that growth of released grand fir regeneration is not as good under a shelterwood cutting as in a clearcut. But Ferguson and Adams indicated that during the first few years after release, some residual large trees or regeneration-size trees may be beneficial in moderating site conditions until regeneration adjusts to the new environment. This effect may not be beneficial if the overstory is too dense, the clumps of regeneration-size trees are too crowded, or if either of these two sources of competition is retained too long.

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