For instance, the common use of average diameter as a surrogate for piece size is easily mis-interpreted when exploring thinning response. The immediate shift in diameter distribution (and average diameter) due to the thinning operation itself (alias "the chain-saw effect") confounds later interpretations of average diameter and thinning response. Thinning from below immediately narrows the diameter distribution and increases the average diameter. This in turn exaggerates the effect of thinning on the time required to reach a certain piece-size.

Examining the largest diameter classes via stand and stock tables, provided by some models, gives a clearer picture of thinning response than average diameter does. A series of graphs will illustrate this concept using a pre-commercial thinning example in lodgepole pine based on TIPSY output. Figure 1 illustrates a common interpretation using average diameter as a surrogate for piece size. In this case, it appears that pre-commercial thinning reduces the rotation from 120 to 60 years to obtain an average piece size of 20 cm.

Figure 1. Rotation to an average target of 20 cm

Figure 2 illustrates the so-called "chain-saw effect" on average diameter at the time of thinning.

Figure 2. Diameter distributions at age19 (just after thinning)

Figure 3 shows the thinned and unthinned diameter distributions at age 60. Note the number of small trees unique to the unthinned stand and the number of large trees unique to the thinned stand.

Figure 3. Diameter distributions at age 60

Figure 4 indicates that it only takes about 20 additional years (not 60 as in Figure1) for the unthinned stand to attain the same number of large trees as the thinned stand has at 60 years. However, the small trees in the unthinned stand pull down the average diameter, thereby exaggerating the effect of thinning on rotation length.

Figure 4. Diameter distributions at age 60 (plus unthinned at age 80).

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