1. Bailey, J.D. and J.C. Tappeiner.
1998. Effects of thinning on structural development in 40- to 100-year-old
Douglas-fir stands in western Oregon. Forest-Ecology-and-Management 108(1/2): 99-113.
Keywords: thinning
commercial thinning
regeneration
tree morphology
Abstract: The
composition and structure of the understorey was
studied in thinned and unthinned Douglas fir/western
hemlock (Pseudotsuga menziesii/Tsuga
heterophylla) stands on 32 sites in western Oregon. These stands had regenerated naturally after timber was
harvested between 1880 and 1940; they were thinned between 1969 and 1984.
Commercially thinned stands had 8-60% of their volume removed 10-24 yr before
the study (in 1993-95). Undisturbed old-growth Douglas fir stands were present
for comparison on 20 of these paired sites. Conifer regeneration density and
frequency were consistently greater in thinned than unthinned
stands. For example, average seedling density in thinned stands (1433/ha) was
significantly greater than in unthinned stands
(233/ha), but very similar to that in old-growth stands (1010/ha). Seedling
density and frequency were strongly related to the volume removed and to stand
density index (and other measures of overstorey
density) just after thinning. In thinned stands, the density of small trees
(intermediate crown class overstorey trees and
advanced regeneration) was 159/ha, significantly greater than in unthinned stands (90/ha), but not significantly different
from that of old-growth (204/ha). The live crown ratio of these trees in
thinned stands (66%) was greater than in unthinned
(44%) and old-growth (48%) stands. Cover and stem density of shrubs was
variable in all 3 stand types. There was significantly less tall shrub cover in
unthinned stands than in either thinned or old-growth
stands, which did not differ. Thinned stands had the most low shrub cover. Salal (Gaultheria shallon) and
bracken fern (Pteridium aquilinum)
cover was greater in thinned stands than in the other stand types, but there
was no difference in sword fern (Polystichum munitum) and Oregongrape [Berberis nervosa] cover. Leaf area index in thinned stands
(6.6) was not significantly different from that in unthinned
(6.8) and old-growth stands (7.1); however, there was more leaf area in shrubs
in the thinned stands. Thinning young Douglas-fir stands will hasten the
development of multistorey stands by recruitment of
conifer regeneration in the understorey as well as by
enabling the survival of small overstorey trees and
growth of advanced understorey regeneration. Thinning
will also help develop the shrub layer by increasing tall shrub stem density
and cover of some low shrubs.
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2. Brandeis, T.J., M. Newton and E.C.
Cole. 2001. Underplanted conifer seedling survival
and growth in thinned Douglas-fir stands. Canadian-Journal-of-Forest-Research
31(2): 302-312.
Keywords: planting operations
thinning
commercial thinning
site preparation
chemical preparation
release treatments
chemical release
growth
tree/stand health
regeneration
Abstract: In a
multilevel study conducted at the Oregon State University's McDonald-Dunn
Research Forest, Oregon, USA, to determine limits to underplanted
conifer seedling growth, Douglas-fir (Pseudotsuga menziesii), grand fir (Abies grandis), western redcedar (Thuja plicata) and western
hemlock (Tsuga heterophylla)
seedlings were planted in January 1993 beneath second-growth Douglas-fir stands
that had been thinned in 1992 to basal areas ranging from 16 to 31 m2/ha. Understorey vegetation was treated with a broadcast
herbicide (glyphosate + imazapyr)
application prior to thinning, a directed release herbicide (glyphosate, plus triclopyr for
tolerant woody stems) application 2 years later, or no treatment beyond harvest
disturbance. Residual overstorey density was
negatively correlated with percent survival for all four species. Broadcast
herbicide application improved survival of grand fir and western hemlock.
Western redcedar, grand fir and western hemlock stem
volumes were inversely related to overstorey tree
density and this effect increased over time. There was a strong indication that
this was also the case for Douglas-fir. Reduction of competing understorey vegetation resulted in larger fourth-year stem
volumes in grand fir and western hemlock.
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3. Buermeyer, K.R. and C.A. Harrington. 2002. Fate of overstory
trees and patterns of regeneration 12 years after clearcutting
with reserve trees in Southwest Washington. Western-Journal-of-Applied-Forestry 17(2): 78-85.
Keywords: thinning
regeneration
tree/stand health
Abstract: Changes
in management objectives for some forestlands in the Pacific Northwest have spurred interest in the creation of multistoried
stands and the use of natural regeneration systems, but data on such systems
are lacking. We assessed the status of the overstory
trees and the regeneration 12 yr after a clearcut
harvest with reserve trees in an even-aged, 145-yr-old Douglas-fir stand on a
moderately productive site (site class 3) in southwest Washington. The 15 ha harvest unit was superimposed over two areas
differentially thinned 15 and 34 yr before clearcutting.
The clearcut harvest retained 18 trees/ha with a mean
diameter of 63 cm. The reserved overstory trees had a
93% survival rate after 12 yr; most dead trees had been windthrown.
Diameter growth for the reserved trees averaged 3.3 cm and was greatest during
the most recent 3 yr period, which also had the highest growing-season
precipitation. In a 1 ha mapped area, there were 5,854 seedlings/ha, and more
than 99% of the regeneration was Douglas-fir. Most seedlings were less than 2 m
tall. Seedling density was somewhat clumped (value of 2.1 for Pielou's index of nonrandomness),
but 79% of randomly located 4.04 m2 (mil-acre) plots and 98% of 5x5 m grid
cells had at least one conifer seedling. There was no obvious pattern of
regeneration based on direction from the reserved trees, but both seedling
density and seedling size within the drip lines of reserved tree crowns were
less than in the rest of the area. The number of seedlings was similar on the
two halves of the plot corresponding to the original thinning blocks, but
seedling size and age differed. In the half of the study plot that had been
twice lightly thinned, only 14% of the seedlings were >0.5 m tall; however,
41% of the seedlings were >0.5 m in the block that had been thinned more
heavily. There was no difference between the thinning blocks in the ages of
seedlings <less or =>0.5 m tall (mean age of 5 yr). This example of clearcutting with reserve trees resulted in reasonable
survival of the overstory trees and adequate stocking
but slow growth rates in the naturally regenerated Douglas-fir. Heavier
thinning before harvest was associated with more advance regeneration, more shrub
cover, and less windthrow of the reserved trees than
in the more lightly thinned block. If an abundance of tree species other than
Douglas-fir was desired on this site, interplanting
would be required.
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4. Miller, M. and B. Emmingham. 2001. Can selection thinning convert even-age
Douglas-fir stands to uneven-age structures?
Western-Journal-of-Applied-Forestry 16(1): 35-43.
Keywords: thinning
commercial thinning
growth
yield
tree/stand health
regeneration
Abstract:
Uneven-age management of Douglas-fir (Pseudotsuga menziesii) stands can be used to address aesthetic,
wildlife habitat, biodiversity and sustainability concerns, but there has been
little long-term experience with this type of management. To develop timely
information on converting even-age stands to uneven-age forests, we used
retrospective stand reconstruction methods to document harvest frequency,
intensity and stand structural development at four sites in western Oregon,
USA.
We studied stands managed by selection thinning and identified strategies for
creating and managing uneven-age forests. Selection thinning
benefited mid- and understorey trees and stimulated
natural regeneration. Although stand growth was less than expected from
low thinning, growth per unit of growing stock was similar to that in unmanaged
stands. Douglas-fir often dominated natural regeneration and had satisfactory vigour at stocking levels about half that considered full
stocking for even-age management, but good growth of regeneration may require
even lower overstorey stocking. Shade-tolerant grand
fir (Abies grandis) and
western hemlock (Tsuga heterophylla),
however, were more abundant at higher stocking levels. Selection thinning of
young Douglas-fir (Pseudotsuga menziesii)
stands can sometimes be effective in promoting viable regeneration while
providing regular income and biodiversity. Because this was a retrospective
study only, further, long-term testing is necessary.
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