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Understory Tree Development with Repeated Stand Density Treatments in Coastal Douglas-Fir Forests of Oregon

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We studied the potential for stand density management treatments to increase growth rates of advanced regeneration in the understory of 50- to 70-year-old Douglas-fir (Pseudotsuga menziesii) forests. Study sites in Oregon included a mesic coastal site, a moderate coast range site, and a drier coast range foothills site. Commercial thinning treatments in 1974‐1984 led to the establishment of Douglas-fir and western hemlock (Tsuga heterophylla) in the understory and, in 1997, parts of these same stands were thinned for a second time to release this advanced regeneration and promote two-story structure. Both species responded positively where overstory was reduced to 30‐40 m2/ha, but western hemlock saplings grew 30‐80% more rapidly than Douglas-fir. Average annual height growth of all western hemlock saplings that were more than 1.0 m tall in 1999 was 23‐30 cm/year in twice-thinned stands but only 13 cm/year in denser, once-thinned stands. About one-half of all western hemlock saplings doubled or tripled their height in 4 years after a second overstory treatment; the largest 10% of these saplings grew 74‐93 cm/year and now represents midstory structure. The response of Douglas-fir saplings to a partial overstory removal was limited. Repeated density management treatments in these Douglas-fir forests can accelerate growth of understory saplings, resulting in the development of two or more canopy layers over time; however, repeated and/or heavy thinning will be required for Douglas-fir sapling growth. Furthermore, because of western hemlock's potential for rapid height growth, some reduction of hemlock sapling density may be needed to maintain vigorous Douglas-fir.

Keywords: gap; old-growth; sapling; second growth; seedling; thinning

Document Type: Research Article

Publication date: January 1, 2009

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  • Each regional journal of applied forestry focuses on research, practice, and techniques targeted to foresters and allied professionals in specific regions of the United States and Canada. The Western Journal of Applied Forestry covers the western United States, including Alaska, and western Canada; WJAF will also consider manuscripts reporting research in northern Mexico that has potential application in the southwestern United States.
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