Overstory and understory development in thinned and underplanted Oregon Coast Range Douglas-fir stands.

Abstract

This study 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: (1) unthinned (~550 trees/ha) (2) light thin (~250 trees/ha), (3) moderate thin (~150 trees/ha), and (4) heavy thin (~75 trees/ha). Two understory treatments were implemented within each overstory treatment: (1) underplanted with Douglas-fir and western hemlock and (2) not underplanted. Overstory trees responded to thinning through increased stem growth, crown expansion, and extended longevity of lower branches. Thinned overstory canopies began to close rapidly the third year after thinning, decreasing skylight by approximately 2 percent per year. Skylight in unthinned stands increased slightly owing to senescence of lower limbs and competitive mortality from self-thinning. Eight years after thinning, skylight in lightly thinned stands was nearly the same as that of unthinned stands, and skylight in moderately thinned stands was close to values found in lightly thinned stands just after thinning. All underplanted conifers in unthinned stands died, whereas 8th-year survival in thinned stands averaged 88 percent. Thinning to low densities and underplanting has the potential to accelerate development of multilayered stands, an important characteristic of old-growth forests.

Citation

Chan, S.S.; Larson, D.J.; Maas-Hebner, K.G.; Emmingham, W.H.; Johnston, S.R.; Mikowski, D.A. 2006. Overstory and understory development in thinned and underplanted Oregon Coast Range Douglas-fir stands. Canadian Journal of Forest Research. 36: 2696-2711