Correlated curve trend (CCT) thinning experiment in Douglas-fir (EP 554)

Last updated on August 22, 2023


The Correlated Curve Trend experiment, was designed in South Africa and used throughout the world to evaluate the effects of different spacing and thinning strategies. The classical CCT experiment consists of 2 series of plots: a “spacing series” and a “thinning series”.  Plots in the first series are spaced before the onset of competition and represent a range of final densities. Once the target densities are achieved, the trees are left to grow without further thinning. The second series of plots is used to assess the impact of delayed thinning. These plots are established with various initial densities and are subsequently thinned at different rates and to varying degrees. In this case, thinning generally takes place after the onset of competition. Repeated measurement of the height and diameter of the trees are made for both series and the resulting growth curves are compared among plots (within and between series) to identify trends attributable to the effects of spacing and thinning. There were 3 Correlated Curve Trend experiments established on the B.C. coast: EP 418, 469 and 554.


To determine the influence and significance of growing space on the increment of trees and on the development of the stand.


This experiment has two components:

EP554-1 was established in 1959 in a 14-year-old Douglas fir plantation (1.8m x 1.8m) on the east side of Mohun Lake, Sayward Forest. EP554-01 has the same basic design as E.P. 418 and 469 with 16 thinning treatments, each applied to a single plot, except for the unthinned control, which was replicated in three plots. The plots vary in size from 0.091ha to 0.303ha. The thinning schedules and intensity are based on O'Connor's (1935) design. There are eighteen plots in five series, with a slightly different thinning regime, with respect to number of stems removed, for each plot in a series.

EP554-2, established in 1966 to add replication to the experiment, has a modified design with nine of the sixteen original treatments represented and one additional treatment (E). 40 plots representing 10 different thinning schedules (as in EP 554-1) with 4 replications (blocks) established in the same area. Treatments in Plots 1, 2, and 9 to 16 from EP554-1 were replicated and an additional 28 plots added.




  • Omule, S.A.Y. 1985. Response of coastal Douglas-fir to pre-commercial thinning on a medium site in British Columbia. B.C. Min. For., Res. Br., Victoria, B.C. Res. Note 100.