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The first thinning options considered are ones that allow the removal of specific trees, or trees that are contained in a class which is defined by species, species group, tree size, or mistletoe rating.

5.2.3.1 Prescription Thinning

Prescription thinning is requested with the THINPRSC keyword record. The prescription thinning option uses the prescription codes (IPRSC, see section 4.2.1) that are input with the tree records. When a prescription thinning is requested, and a prescription code is entered in the appropriate parameter field, a proportion of all trees with that prescription code will be removed. This proportion is specified by the cutting efficiency parameter. For example, a THINPRSC record with prescription code 4 and cutting efficiency 0.8 would remove 80% of the trees represented by records with IPRSC code 4. All other records would be ignored by this thinning request.

Multiple requests for the prescription thinning option are possible to remove specific trees at specific times in the projection. When a prescription thinning is requested, and a prescription code is not entered in the appropriate parameter field, all trees with a value of IPRSC that is greater than or equal to 2 will be removed. For example, records number 3, 5, 6, 12, 13, 14, 15, 21, 22, 26, and 27 in figure 4-3 were marked for removal, and a specific prescription code was not entered on the THINPRSC keyword in the example keyword record file, figure 4-1. In this instance, all the above listed records would be removed.

Cutting efficiency is not changed when processing this type of thinning request. The proportion specified either by the global value or the cutting efficiency parameter on the thinning request is removed from all records scheduled for removal.

5.2.3.2 Diameter Limit Thinning

The diameter limit thinning option is requested with the THINDBH record, and can be used to remove segments of the diameter distribution. It is possible to limit the cut to individual species or species group, and either a residual trees or basal area per acre target for that species or species group in a specified diameter range. Thinning occurs uniformly throughout the specified diameter range. If a residual target is not specified, diameter limit thinning requests use the specified, or default, cutting efficiency to determine the proportion of each tree record to remove. However, if residual trees per acre or basal area per acre is specified, the model ignores the user-specified cutting efficiency and

calculates the cutting efficiency needed to uniformly distribute the thinning throughout the diameter range and meet the specified target. The cutting efficiency used is reported

in the Activity Summary Section of the Summary Statistics Table. The THINDBH keyword is especially useful for simulating treatments such as stand cleaning and overstory removal.

Thinning requested by a THINDBH keyword record is intended to remove a specified proportion of a diameter distribution, and priority weighting factors entered on the SPECPREF, TCONDMLT, and MISTPREF keyword records have no effect on these thinning requests (see section 5.2.4.1). In addition, the residual target is for the specified species or species group, and DBH range, and not a total stand residual. However, a way to make the residual target a total stand residual will be shown in the section 6.3 of this manual.

If both a basal area and a trees per acre target are specified on the same THINDBH keyword, the trees per acre target is used. However, neither target needs to be specified. Consider, for example, the keyword sequence:

THINDBH 2010. 5. 20. 1. ALL 70. 0.

THINDBH 2010. 20. 999. 1. DF 0. 10.

THINDBH 2010. 0. 5. 0.5 b b b

In year 2010, the first THINDBH would leave 70 trees per acre of whatever species are in the stand which are greater than, or equal to, 5.0 inches in diameter and less than 20.0 inches in diameter. The second THINDBH would remove only Douglas-fir with

diameters 20.0 inches and larger, leaving 10 square feet of basal area in Douglas-fir with diameters greater than, or equal to, 20.0 inches. Species other than Douglas-fir which are 20.0 inches in diameter, or larger, would not be affected. In both of these cases, since residual targets are specified, the model ignores the specified cutting efficiency of 1.0 and calculates the cutting efficiency it needs to achieve the desired targets while spreading the harvest across the entire specified diameter class. The third THINDBH accounts for logging damage to understory trees and tells the model to remove 50 percent of all the trees less than 5.0 inches in diameter. The cutting efficiency specified on the third THINDBH request is used since a residual target was not specified.

5.2.3.3 Height Limit Thinning

The height limit thinning option is requested with the THINHT keyword record. This keyword is also useful for simulating treatments such as stand cleaning and overstory removal. The height limit thinning option is analogous to the THINDBH keyword except it can be used to remove segments of the height distribution. It is possible to limit the cut to an individual species or species group and to either a residual trees per acre or basal area per acre target for that species or species group in the specified height range. Thinning occurs uniformly throughout the specified height range. Cutting efficiency operates exactly the same as described for the THINDBH thinning request.

Thinning done with a THINHT keyword record does not use the cutting removal priority (see section 5.2.4.1). Therefore, the priority weighting factors entered on the

thinning requests. In addition, the residual target is for the specified species or species group, and height range and not a total stand residual.

5.2.3.4 Mistletoe Sanitation Thinning

The mistletoe thinning option is requested with the THINMIST keyword record. This keyword is useful for simulating mistletoe sanitation thinning. It is possible to remove all mistletoe infested trees, or limit the cut to trees with a specific mistletoe rating in the specified diameter range. Thinning occurs uniformly throughout the specified diameter range utilizing the specified cutting efficiency.

Thinning done with a THINMIST keyword record does not use the cutting removal priority (see section 5.2.4.1). Therefore, the priority weighting factors entered on the SPECPREF, TCONDMLT, and MISTPREF keyword records have no effect on this thinning request.