Specifying Thinning Method and Target Density

The keywords used to specify a density target, within a specified diameter and/or height class, also indicate whether thinnings are to be from above or from below. These

keywords are defined as follows:

THINBTA Thin from below to a trees per acre target. THINATA Thin from above to a trees per acre target.

THINBBA Thin from below to a basal area per acre target (square feet). THINABA Thin from above to a basal area per acre target (square feet). THINSDI Thin to a Stand Density Index target.

THINCC Thin to a Percent Canopy Cover target.

THINRDEN Thin to a Relative Density target (Curtis 1982).

THINRDSL Thin to a Relative Density target (Marquis and Ernst 1992).

5.2.4.2.1 Thinning From Above or Below to a Trees per Acre or Basal Area Target

The THINBTA, THINATA, THINBBA, and THINABA keywords all require the same parameters specifying the year, target density, cutting efficiency, and class boundaries. Target density pertains only to the defined size class and does not reflect residual stand conditions. FVS does not dynamically adjust the cutting efficiency for thinning requests specified with these keywords, and each tree record is considered for thinning only once

per thinning request. As a result, if the cutting efficiency is set too low, the model may not be able to attain the residual target specified on the thinning request. In this case, the thinning request will be completed, but the residual condition will exceed the specified target. If the initial condition is already less than the target specified, the thinning will be cancelled. Users should look at the “Activity Summary” portion of the output to see which thinning requests were simulated and which ones (if any) were cancelled. For the thinning requests that were simulated, users should also check the Summary Statistics table or the Tree and Stand Attributes table to assure the effect was what they intended.

5.2.4.2.2 Thinning to a Stand Density Index Target

The THINSDI keyword is more complex than those discussed previously, but it is more versatile as well. When the appropriate method of calculation is used, Stand Density Index (Reineke 1933, Stage 1968, Shaw 2000) is a measure of stocking which describes density and is also sensitive to the diameter class distribution of the stand. In this respect, it goes beyond simple measures such as basal area to better represent irregularly

structured stands. In other words, stand density index can distinguish between stands with similar basal areas but different diameter distributions.

THINSDI schedules a thinning from a specified diameter range for any species, species group, or all species to a stand density index (SDI) target. This keyword contains a cutting control parameter. Depending on the value of that parameter, this keyword can operate like a THINDBH, or like the four density target thinning requests described above.

If the specified target is nonzero and the cutting control flag is zero, thinning occurs uniformly throughout the specified diameter range. In this case, the specified cutting efficiency is ignored and the model computes the cutting efficiency it needs to spread the thinning through the diameter class and meet the specified target. The cutting priority algorithm is not used when the cutting control flag is set to zero. In this case, the keyword acts like the THINDBH keyword.

If the specified target is nonzero and the cutting control flag is nonzero, then thinning will be from below or above in the specified diameter class to the residual SDI target. In this case, both the specified cutting efficiency and the cutting priority algorithm are used. This is similar to the density target thinning requests described above. However, if the cutting efficiency is set so low that the target cannot be reached, the model recalculates the cutting efficiency to achieve the target. This behavior is different from that of the other density target thinning requests.

The cutting efficiency used is reported in the Activity Summary Section of the Summary Statistics Table.

5.2.4.2.3 Thinning to a Percent Canopy Cover Target

The THINCC keyword record schedules a thinning from a specified diameter range for any species, species group, or all species to a percent canopy cover target. This keyword

contains a cutting control parameter. Depending on the value of that parameter, this keyword can operate like a THINDBH, or like the four density target thinning requests described in section 5.2.4.2.1.

If the specified target is nonzero and the cutting control flag is zero, thinning occurs uniformly throughout the specified diameter range. In this case, the specified cutting efficiency is ignored and the model computes the cutting efficiency it needs to spread the thinning through the diameter class and meet the specified target. The cutting priority algorithm is not used when the cutting control flag is set to zero. In this case, the keyword acts like the THINDBH keyword.

If the specified target is nonzero and the cutting control flag is nonzero, then thinning will be from below or above in the specified diameter class to the residual percent canopy cover target. In this case, both the specified cutting efficiency and the cutting priority algorithm are used. This is similar to the density target thinning requests described above. However, if the cutting efficiency is set so low that the target cannot be reached, the model recalculates the cutting efficiency to achieve the target. This behavior is different from that of the other density target thinning requests discussed in section 5.2.4.2.1. The cutting efficiency used is reported in the Activity Summary Section of the Summary Statistics Table.

Percent canopy cover is corrected for overlap in FVS. However no attempt is made to reconcile crown overlap between multiple THINCC thinning requests in the same cycle. The residual percent canopy cover on a THINCC thinning request pertains only to the specified species or species group, and diameter range, and does not represent the total stand residual unless the request includes all species and all diameters in the stand. For example, if a user scheduled two THINCC thinning requests for the same cycle, one leaving 20 percent canopy cover in trees less than 10.0 inches in diameter, and one leaving 30 percent canopy cover in trees greater than or equal to 10 inches in diameter, then the targets for each of these requests would be met exactly. However, the total stand percent canopy cover would not be 50 percent because some of the crowns in the upper diameter class overlap some of the crowns in the lower diameter class.

5.2.4.2.4 Thinning to a Relative Density Target

The THINRDEN and THINRDSL keyword records schedule a thinning from a specified diameter range for any species, species group, or all species to a relative density target. These keywords contain a cutting control parameter. Depending on the value of that parameter, these keywords can operate like a THINDBH, or like the four density target thinning requests described in section 5.2.4.2.1.

If the specified target is nonzero and the cutting control flag is zero, thinning occurs uniformly throughout the specified diameter range. In this case, the specified cutting efficiency is ignored and the model computes the cutting efficiency it needs to spread the thinning through the diameter class and meet the specified target. The cutting priority

algorithm is not used when the cutting control flag is set to zero. In this case, the keyword acts like the THINDBH keyword.

If the specified target is nonzero and the cutting control flag is nonzero, then thinning will be from below or above in the specified diameter class to the residual relative density target. In this case, both the specified cutting efficiency and the cutting priority algorithm are used. This is similar to the density target thinning requests described above. However, if the cutting efficiency is set so low that the target cannot be reached, the model

recalculates the cutting efficiency to achieve the target. This behavior is different from that of the other density target thinning requests discussed in section 5.2.4.2.1.

The cutting efficiency used is reported in the Activity Summary Section of the Summary Statistics Table.

The THINRDEN keyword thins to a relative density target as defined by Curtis (1982). Equation {5.2.4.2.4.1) is used to calculate the relative density.

{5.2.4.2.4.1} RD = BA / (QMD)1/2 where:

RD is relative density BA is total stand basal area

QMD is stand quadratic mean diameter

This measure of relative density is derived from Reineke’s SDI formulation and provides a simple and convenient scale of relative stand density. It is used extensively in the Pacific Northwest region of the United States.

The THINRDSL keyword thins to a relative density target as defined in the SILVAH model (Marquis and Ernst 1992) using equation {5.2.4.2.4.2}. This keyword is only for use in the Northeast variant and only when the stand is classified as one of the following five SILVAH forest types:

Northern Hardwood – stands that have at least 65% of total stand basal area in sugar maple, red maple, American beech, yellow birch, sweet birch, eastern hemlock, American basswood, cucumbertree, black cherry, white ash, or yellow-poplar. Northern hardwood-hemlock – stands meeting the requirements for the northern hardwood type which also have at least 50% of their basal area in eastern hemlock. Allegheny hardwood – stands meeting the requirements for the northern hardwood type that also have at least 25% of their basal area in black cherry, white ash, and yellow- poplar, and less than 50% of their basal area in eastern hemlock.

Oak-hickory – stands that have at least part of their basal area in any oak or hickory species.

Transition – stands that have at least 65% of their basal area in species of either the northern hardwood or oak-hickory types, but do not qualify for any of the other forest types alone.

{5.2.4.2.4.2} RD =• _i • _j{(ai + bi · DBHj + ci · DBHj2) · TPAj}

where:

RD is relative density

TPAj is trees per acre represented by the jth tree of ith species

DBH is diameter at breast height of the jth tree of ith species ai, bi, ci are coefficients for species group i

• _i is summation over all species

• _j is summation over all trees in the species group Species Group 1: a = 0.0033033, b = 0.020426, c = 0.0006776 Species Group 2: a = -0.027142 , b = 0.024257, c = 0.0015225 Species Group 3: a = -0.0027935, b = 0.0058959, c = 0.0047289

Species groups for the Northeast variant were provided by Susan Stout and Todd Ristau (USFS, Northern Research Station). Northeast variant species not specifically assigned to a group use the coefficients for species group 2.

Species Group 1: ash species, white ash, yellow-poplar, black cherry, pin cherry Species Group 2: eastern white pine, eastern hemlock, red maple, yellow birch, sweet birch, sweetgum, cucumbertree, quaking aspen, balsam poplar, eastern cottonwood, bigtooth aspen, northern red oak, other hardwood species, hackberry, common persimmon, American holly, magnolia species, blackgum, sourwood, black locust, sassafras, American basswood, white basswood, other elm species, American elm, slippery elm, boxelder, serviceberry, American hornbeam, flowering dogwood, hawthorn species, eastern hophornbeam

Species Group 3: sugar maple, hickory species, pignut hickory, shellbark hickory, shagbark hickory, mockernut hickory, American beech, white oak, bur oak, chinkapin oak, post oak, other oak species, scarlet oak, shingle oak, water oak, pin oak, chestnut oak, swamp white oak, swamp chestnut oak, black oak, cherrybark oak, willow oak, non- commercial hardwood species, striped maple

The residual relative density on either a THINRDEN or THINRDSL thinning request pertains only to the specified species or species group, and diameter range, and does not represent the total stand residual unless the request includes all species and all diameters in the stand.

5.2.4.2.5 Thinning Using a Q-Factor Approach

The THINQFA keyword record schedules a thinning from a specified diameter range for any species, species group, or all species to a residual density target specified as basal area per acre, trees per acre, or stand density index. The diameter distribution of the residual trees in the specified diameter range (trees per acre vs. diameter class) approximates, as closely as possible, an inverse, J-shaped curve determined by the specified Q-factor (or diminution quotient, often denoted Dq) and associated diameter class sizes (Meyer 1952).

In simple terms, the Q-factor is a value that, when multiplied by the desired number of trees in a diameter class, gives the desired number of trees in the adjacent class of smaller diameter trees.

Using the specified Q-factor, the residual trees-per-acre target is calculated for each specified diameter class so that the total specified residual density would be met. If the existing trees-per-acre value in any diameter class is less than the target, the targets of all classes with excess trees are increased in equal proportion so the total specified residual density will be met. Thinning occurs uniformly throughout each of the specified diameter classes. The cutting priority algorithm is not used. The keyword acts like a series of THINDBH keywords; one for each of the diameter classes. If the specified residual density for the entire specified diameter range is greater than the existing density for that range the keyword is canceled.

If the specified diameter range is not equally divisible by the specified diameter class width, the class representing the smallest diameter trees will be incomplete (i.e., too narrow). When this occurs the incomplete class is excluded from the harvest, so no trees are cut in the incomplete class of the smallest diameter trees.