Canadian journal of forest research. 2006 Oct., v. 36, no. 10, p. 2696-2711.
NALT Subjects:
Pseudotsuga menziesii coniferous forests forest thinning overstory understory forest plantations Tsuga heterophylla tree growth forest growth forest stands species diversity forest ecology shrubs stand density old-growth forests light light intensity natural regeneration vegetation canopy gaps forest stand establishment conifers shade tolerance stand management Oregon
Other Subjects:
Oregon Coast Range
Issue Date:
Oct-2006
Abstract:
Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests managed for timber in western Oregon frequently lack structure and diversity associated with old-growth forests. We examined thinning effects on overstory and understory development for 8 years after treatment. Three 30- to 33-year-old Oregon Coast Range plantations were partitioned into four overstory treatments: unthinned (approximately 550 trees/ha) and lightly (approximately 250 trees/ha), moderately (approximately 150 trees/ha), and heavily (approximately 75 trees/ha) thinned. Within each overstory treatment, two understory treatments were established: underplanted with Douglas-fir and western hemlock (Tsuga heterophylla (Raf.) Sarg.) or not underplanted. Thinning increased overstory stem growth, crown expansion, and retained crown length. Thinned overstory canopies began to close rapidly the third year after thinning, decreasing % skylight by approximately 2%/year, whereas % skylight in unthinned stands increased slightly. All seedlings planted in unthinned stands died, whereas eighth year survival in thinned stands averaged 88%. Natural regeneration densities and distributions were highly variable. Understory shrub cover was reduced by harvesting disturbance but recovered by the fifth year. Thinning increased understory plant species diversity, and no shrub species were lost. Thinning to low densities and underplanting has the potential to accelerate development of multilayered stands characteristic of old-growth Douglas-fir forests.
