Olympic Habitat Development Study



One goal of our thinning prescription was to create a range of growing conditions within a stand. We did this by creating a thinned matrix with skips and gaps. The matrix area was lightly thinned. We established a plot 120 m by 120 m394 feet by 394 feet (that included skips and gaps) and tagged each tree and measured where it was. Foresters call this a stem-mapped plot as it allows us to produce a map that shows where each stem is located in relation to the others (and to the skips and gaps). A stem-mapped plot is time consuming to install and a plot this large (1.44 ha)3.56 acres takes a while to measure. However, it provides lots of information on how competition influences growth of individual trees (graph). Comparison of 10-year diameter growth among trees in the subcomponents of the treatments.

On average a tree in the thinned matrix grew 4.3 cm1.7 inches in 10 years while trees in skips (unthinned areas) grew only 2.3 cm0.9 inch (see Skips at Station 3). Trees in the thinned area close to skid trails (paths in the woods where logging equipment, especially the skidder, traveled to pick up cut trees) grew 5.0 cm2.0 inches - this growth was a little better than growth for trees away from any openings or edges. Some people suggested before we installed the study that trees this old (about 70 years old at the time the study started) were too old to respond to thinning but that was clearly not the case. In addition, people at some other areas have found tree growth to be less near skid trails but that was not true here (we saw no erosion or soil puddling indicating water was not draining well). Other stops on this tour will talk about the effects of skips and gaps on tree growth and regeneration.

Thinning not only improved the growth of the overstory trees, it also improved growth of midstory trees (graph).Diameter growth for midstory hemlock in skips and thinned areas before and after thinning.
Tree growth of midstory western hemlock trees declined over time in the skips but increased substantially in the thinned matrix. Data shown is from Bait, another OHDS site.
(See Comfort et al. 2010 under Info/Publications for more information on response of midstory trees.)

Another goal of the VDT (variable-density thinning) was to increase tree species diversity. Tree species not common in this stand, such as western redcedar, red alder, and bigleaf maple -- were not cut as part of thinning. However, there were not many trees of these species present prior to the thinning so it is not easy to see the difference as you walk through the stand. However, the thinning would have reduced the number of competitors around them and that should increase the odds of their long-term survival. Cascara buckthorn (Learn more about cascara on the Natural History Tour) is a fairly common small tree in the stand. It was not harvested but since most trees were quite small, it was sometimes damaged by logging equipment (the trees were small enough that they could be run over). Regeneration of cascara was enhanced by the thinning with 600 seedlings/ha250 seedlings/acre and 2300 small saplings/ha900 small saplings/acre in the thinned area (there were none in the control plot).

Regeneration of western hemlock was greatly increased by thinning with 89,000 seedlings and 19,000 saplings/ha 36,000 seedlings and 7,700 saplings/acre present 10 years after thinning. Many of these were on the edges of gaps (see Station 4 on gaps). Regeneration of Sitka spruce (Learn more about spruce on the Natural History Tour) was also enhanced in the thinned area, especially in the more open areas such as along skid trails and on the edges of gaps.

The thinning was accomplished by felling the trees using chain saws and then using a forwarder to move the trees to landings where they were sorted and then put on log trucks to head for a mill. The skid trails were designated ahead of time and provided a network of temporary paths thru the woods. These trails required clearing a path approximately 5 m16.4 feet wide to allow the equipment to pass. The equipment could reach out from the paths to lift or pull logs to each path, thus limiting the amount of the ground area where the equipment passed.

The thinning done as part of this study provided economic benefits to the local community as loggers harvested the trees and mill workers processed the logs from the thinning.


Thinned matrix areas favor tree growth. (Matrix at Clavicle)
Narrow skid trails allowed equipment to move thru the thinned matrix area. Thinned matrix areas favor tree growth. (Matrix at Clavicle) Skid trails and other open areas favor tree regeneration by increasing the amount of light available.
Thinned matrix areas favor tree growth.
(Matrix at Clavicle)