Occasionally users may want to alter certain values that get calculated by the model. Users can easily alter either volume calculations or crown width calculations.
5.3.1 Modifying Volume Calculations
Both the merchantable cubic foot volume and the board foot volume indirectly influence the frequency of thinning through the minimum harvest constraints. These volume predictions also directly influence any comparison of alternative management strategies. Therefore, volume calculations are included as a part of the discussion of management options.
All FVS variants contain a choice of volume equations. The default is to use volume equations contained in the National Volume Estimator Library. These equations are regarded as the “standard” for the Forest Service, and they are also used in other agency software such as the National Cruise Program or Regional databases. In addition, each variant usually contains another set of equations that are commonly used in that geographic area, or users can enter their own volume equation coefficients.
Individual tree volumes are computed to three merchantability standards. These are slightly different depending on whether the geographic area is in the eastern or western United States. In western variants, volumes are reported in total cubic feet from ground to
tip, total merchantable cubic feet from the stump to a specified cubic foot merchantable top, and total Scribner board feet from the stump to a specified board foot merchantable top. In eastern variants, volumes are reported in total merchantable cubic feet, total cubic feet of merchantable sawtimber from the stump to a specified sawtimber merchantable top, and total board feet of merchantable sawtimber from the stump to a specified sawtimber merchantable top. The total merchantable cubic feet in eastern variants includes: (1) pulpwood from the stump to a specified pulpwood cubic foot merchantable top for pulpwood trees; (2) the pulpwood portions of sawtimber trees; and (3) total cubic feet of merchantable sawtimber. Total pulpwood can be computed by subtracting
merchantable sawtimber cubic feet from total merchantable cubic feet.
Volume calculations may be modified in three ways. First, different volume equations can be chosen. Second, merchantability limits can be changed. Third, form and defect correction factors can be implemented.
5.3.1.1 Changing Volume Equations and Merchantability Limits
To select different volumes equation from the National Volume Estimator Library, select volume computation method that is different from the National Volume Estimator
Library equations, or to change the merchantability specifications for volume
computations, use the VOLEQNUM, VOLUME and BFVOLUME keyword records. The VOLEQNUM keyword record is used to select different equation numbers from the National Volume Estimator Library. The VOLUME keyword record pertains to
merchantable cubic foot volume in western variants and pulpwood cubic foot volume in eastern variants. The BFVOLUME keyword record pertains to board foot volume in western variants and sawtimber volume in eastern variants. Sawtimber volumes in eastern variants include the sawtimber portion of trees that contain both sawtimber and
pulpwood. Users can specify the volume computation method, stump height, minimum top diameter, and minimum DBH to be used in estimating volumes. Users should consult their variant overview document to determine which volume computation methods are available for the variant they are using. Form class can also be changed with these keywords for volume computation methods that use form class. All these factors can be altered by cycle and by species.
5.3.1.2 User-Specified Volume Equations
Users can enter coefficients for their own volume equations if the equation is of the form of equation {5.3.1.2.1}, where D is tree diameter and H is tree height.
{5.3.1.2.1} VOLUME = a0 + (a1 D) + (a2 D H) + (a3 D2 H) + (a4 Da5 Ha6)
Coefficients for total cubic foot volume are entered with the CFVOLEQU keyword record. Merchantable cubic foot volume is derived from total cubic foot volume by using a Behre hyperbola (Behre 1927) to approximate bole form, and the merchantability limits entered on the VOLUME keyword record. Coefficients for board foot volume are entered with the BFVOLEQU keyword record.
5.3.1.3 Entering Form and Defect Correction Factors
The other means of modifying volume predictions is through form and defect correction factors. Correction factors can be entered in three ways: (1) by entering the percent defect on the tree records, (2) by entering values for a linear interpolation function that
computes the defect percentage based on the diameter of the tree, or (3) by entering parameters for form and defect log-linear correction equations that are used to compute a percent defect. Tree volume is then corrected for form and defect by multiplying the percent defect times the predicted gross volume. The percent defect applied to the tree record is the largest percent defect resulting from the three options. Both the cubic and board foot defect percentages applied to the tree record are printed in columns on the Tree List Output File (see section 4.3.3.1).
Defect percentages are entered on the tree input records by using the damage codes 25, 26, or 27 on the input tree records, and entering the percent defect in the corresponding severity field (see section 4.2.1). Parameters for the linear interpolation function are entered using the MCDEFECT, BFDEFECT, or DEFECT keyword records. Parameters for the log-linear correction equations are entered with the MCFDLN and BFFDLN keyword records.
There may be cases when users may not want to use the volume defect percentages recorded in the data and they don’t want to change the data files. For example, they may want FVS to output gross volumes even when defects are included in the input data, or the defect percentages in the data are of questionable value. Coding a value of 1 or greater in field 3 of the DEFECT keyword record will instruct FVS to ignore the volume defect percentages read as part of the data input record.
5.3.2 Modifying Crown Width Calculations
Crown width estimates get printed in the Tree List Output File (see section 4.3.3.1) and are used in canopy cover calculations and some post-processing programs. Users can adjust crown width estimates by species or species group, and diameter range, using a multiplier entered on the FIXCW keyword (see section 6.5.2.4). Users can also enter coefficients for their own crown width equations if the equation is of the form of equation {5.3.2.1}, where D is tree diameter.
{5.3.2.1} CROWN WIDTH = a0 + (a1 D) + (a2 Da3)
Separate coefficients can be entered for trees less than a threshold diameter and for those that are greater than or equal to the threshold diameter. These coefficients and the
threshold diameter are entered using the CWEQN keyword.