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U.S. Forest Service
Caring for the land and serving people

United States Department of Agriculture

FVS Technical Support

Green button, the FVS logo wearing a telephone headset.

The FVS Staff of the Forest Management Service Center provides technical support for the Forest Vegetation Simulator. You may contact the FVS Helpdesk by telephone or by e-mail. When contacting the FVS Helpdesk be prepared to provide the following information:

  1. Name the program and the version you are using. You can find the FVS version number in the top-center of the Main FVS output file.
  2. Provide a brief description of the problem.
  3. If the problem is reproducible, please list the steps required to cause it. If the problem is not reproducible (only happened once, or occasionally for no apparent reason), please describe the circumstances in which it occurred and the symptoms observed.
  4. Note whether the problem causes any error messages to appear, and note in your message the exact text displayed.
  5. Please provide us with the following contact information:
    1. Name
    2. Organization
    3. Phone/E-mail Address

E-mail Address: sm.fs.fvs-support@usda.gov

Frequently Asked Questions (FAQs)

The FVS Staff has summarized some of the questions most commonly asked regarding its products. Follow one of the links below to the related FAQs page to see if your question has already been answered.

Questions (with link to Answer)

  1. Why do I get an error when I import my FVS formatted Access Database into FVS?
  2. What does the warning message “WARNING: INITIAL STAND STOCKING OF x TREES/ACRE IS MORE THAN 5% ABOVE THE UPPER LIMIT OF y TREES/ACRE. UPPER LIMIT IS BASE ON A SDI MAXIMUM OF z , AND AN UPPER BOUND OF xx % OF MAXIMUM. MAXIMUM SDI BEING RESET TO xxx FOR FURTHER PROCESSING” mean?
  3. What causes the Summary Statistics table in the main FVS output file to be filled with all zeros?
  4. What does the merchantable cubic feet volume column include in the Summary Statistics Table in the Eastern variants (LS, SN, NE, SE, CS)?
  5. I have included a CFVOLEQU (or BFVOLEQU) keyword; however, my volumes have not changed. Why?
  6. Why is there regeneration in the simulation even though I did not schedule any?
  7. Why are the residual trees per acre or basal area higher than the value I set on the harvest keyword?
  8. How many growth sample trees does FVS need for local stand/plot calibration?
  9. How do I determine which geographic variant to use with my data?

Questions and Answers

1. Why do I get an error when I import my FVS formatted Access Database into new FVS?

New FVS converts the imported FVS formatted Access Database to an SQLite database for use in FVS. We believe the error in converting the database from Access to SQLite is related to the Microsoft settings on individual PCs. Use our web link to convert the file to an SQLite database and then import the SQLite file to new FVS.

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2. What does the warning message “WARNING: INITIAL STAND STOCKING OF x TREES/ACRE IS MORE THAN 5% ABOVE THE UPPER LIMIT OF y TREES/ACRE. UPPER LIMIT IS BASE ON A SDI MAXIMUM OF z , AND AN UPPER BOUND OF xx % OF MAXIMUM. MAXIMUM SDI BEING RESET TO xxx FOR FURTHER PROCESSING” mean?

In most variants, mortality is based on Stand Density Index (SDI). Stand density asymptotes at a certain percentage of the maximum SDI. This is generally 85% of maximum SDI, but a user can adjust that level. If the initial stand density is more than 5% above this asymptotic level, the model automatically adjusts the maximum SDI. This warning message indicates one of two things:

  1. The maximum SDI values (either input by the user, or set by model defaults) are too low; or,
  2. There is an error in data input, and stand densities are too high.

In the first case, the solution is to raise the SDI values using the SDIMAX keyword. In the second case, you probably have an error on your DESIGN keyword specifications.

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3. What causes the Summary Statistics table in the main FVS output file to be filled with all zeros?
When the Summary Statistics table is filled with zeros (or when the first cycle values are all zeros) it means that there were no projectable tree records in the input tree data file. Projectable tree records are those for live trees with the species and DBH specified. The Options Selected By Input section of the main output file will contain the warning:

******** FVS08 WARNING: TOO FEW PROJECTABLE TREE RECORDS

There are three main causes of this problem. The first is that all of the trees in the input tree data are coded as dead. The second is that the input tree data information is not in the correct format, or it does not contain species codes or DBH values. The third is that the reference to the tree data is incorrect so FVS can’t find the tree data.

If the tree data are in a database, this error usually means the “From” clause in the query that selects the tree data represents a table that is empty, formatted incorrectly, does not exist, or does not contain species or DBH values. This query is either in the FVS_GroupAddFilesAndKeywords table in the input database or in the TREESQL keyword record. Refer to the Users Guide to the Database Extension of the Forest Vegetation Simulator Version 2.0 for more information.

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4. What does the merchantable cubic feet volume column include in the Summary Statistics Table in the Eastern variants (LS, SN, NE, SE, CS)?
In the eastern variants only, the merchantable cubic feet volume column (McuFt) in the Summary Statistics Table includes volume on all trees meeting the pulpwood volume minimum merchantability specifications (pulpwood + sawlog + topwood). The SCuFt field includes sawlog volume to the minimum merchantability specifications.

When using the Event Monitor to compute volume in the eastern variants, the BTCUFT variable returns values from the merchantable cubic feet volume column in the Summary Statistics table (pulpwood + sawlog + topwood). The BMCUFT variable returns values from the sawlog cubic feet volume column in the Summary Statistics Table.

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5. I have included a CFVOLEQU (or BFVOLEQU) keyword; however, my volumes have not changed. Why?
When using the CFVOLEQU (or BFVOLEQU) keyword, you also need to include the VOLUME (or BFVOLUME) keyword and indicate in field 7 that you were using your own equation. Also, if you want FVS to calculate volumes with your equation for the first cycle you must put a zero in field 1 (year). If you use the inventory year FVS will use the default equations for the first cycle and your equation for the rest of the cycles. Refer to the document Keyword Reference Guide for the Forest Vegetation Simulator for more information.

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6. Why is there regeneration in the simulation even though I did not schedule any?
Most of the geographic variants include regeneration from roots or stumps whenever trees of species that exhibit this behavior are cut or killed by fire. Several stems will regenerate from each tree cut or killed by fire. This type of regeneration can be suppressed using the NOSPROUT keyword.

A few of the geographic variants (AK, CI, EM, KT and NI) also have the capability to include regeneration automatically after some types of harvests or after periods during which no regeneration was scheduled by the user. The easiest way to suppress this type of regeneration is with the NOAUTOES keyword.

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7. Why are the residual trees per acre or basal area higher than the value I set on the harvest keyword?
The residual targets specified with the harvest keywords are not stand residuals, but are rather for specific ranges of diameter and height, and in some cases for particular species. Anything not within the specified range is not even considered for removal by that keyword. The stand residual will be the sum of the residual from the harvest keyword and all of the trees not within the range specified on the keyword. Refer to the description of a particular harvest keyword in the document Keyword Reference Guide for the Forest Vegetation Simulator for more detail.

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8. How many growth sample trees does FVS need for local stand/plot calibration?
If growth/increment information is provided in the FVS Input Tree data, then the large tree diameter model and/or the small tree height model can be scaled to reflect local conditions relative to regional growth trends. In order to compute scale factors, there must be five or more observations supplied per tree species. Diameter growth/increment is accepted for trees greater than a designated threshold diameter, whereas height growth/increment is accepted for trees less than a designated threshold diameter. The threshold diameter varies by variant, but it is generally 3-inches for large tree diameter growth and 5-inches for small tree height growth. Consult the Variant Overview for the threshold limits.

There is also a height-diameter relationship calculated in FVS. At least 3 height observations are required for a species in order to calibrate the height-diameter relationship.

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9. How do I determine which geographic variant to use with my data?
If the data files have already been correctly created by another program such as FSVEG or FIA, the correct variant will be specified in the input file.

The What is FVS? page contains a shape file a user can use to determine the geographic areas covered by the different variants.

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