The FVS Model

The Forest Vegetation Simulator (FVS) is an individual-tree, distance-independent growth and yield model (Dixon 2002). It has been calibrated for specific geographic areas (variants) of the United States (Figure 1). FVS can simulate a wide range of silvicultural treatments for most major forest tree species, forest types, and stand conditions.

The original Prognosis model (Stage 1973) was developed for northern Idaho and western Montana. The Prognosis model framework was retained as data from other geographic areas was analyzed to develop new model variants. In the early 1980s, Prognosis became the Forest Vegetation Simulator (FVS) and was adopted by the USDA Forest Service National Forest System as the national standard for forest growth and yield modeling.

The FVS Staff of the Forest Management Service Center (FMSC) in Fort Collins, Colorado, maintains, supports, develops, and provides training for FVS. The FMSC performs a technology transfer role, working with researchers and National Forest staff from various geographical areas to incorporate their findings into the FVS framework. The FMSC is constantly upgrading existing variants and developing additional variants.

Model Structure

The FVS model has several components that work together to simulate forest growth and management actions. In addition to a mortality model, there are three main growth components of the FVS:

1. Large-tree model
2. Small-tree model
3. Establishment model

The stand is the population unit used to model individual tree interactions. Forest inventories or stand examination data can be used to describe the initial stand conditions. Input files include "keywords" the user can manipulate to simulate different management scenarios (Van Dyck 2001). There are extensions to FVS variants that simulate the influence of other agents upon tree growth, such as insects and disease. Post-processors are other programs that use FVS output for further reporting, display, or analysis.

Model Execution

The following summarizes basic FVS operation and Figure 2 displays the major model execution steps. Specific operation details will vary for different variants.

1. Process keywords and input data - FVS begins by reading keywords to establish location, environment, and stand parameters; then reads the tree record file.
2. Compute initial stand characteristics - FVS computes stand characteristics for the initial year (cycle 0). This is typically the inventory date or the stand regeneration date. Input tree records with missing heights or crown ratios have these dubbed in.
3. Backdate densities and compute calibration statistics - If sufficient large tree diameter increment data is contained in the input tree data (and/or small tree height increment data), FVS back-dates the stand one default cycle length. FVS then “grows” the stand back to the inventory date, compares the values from the actual inventory date with simulated values, and computes scale factor adjustments to account for differences in actual and model values.
4. Check event monitor (pre-thin) - FVS continues with the steps it repeats every cycle. FVS checks the Event Monitor keywords and functions to see if the user scheduled any activities based on existing stand conditions at the start of the cycle. Event Monitor capabilities are powerful and very useful for modeling situations and creating variables not covered in standard FVS output. Crookston (1990) describes Event Monitor functions and processes along with several good examples.
5. Perform thinning - Almost any silvicultural operation imaginable can be structured using one or more thinning keywords in a direct or conditional (If..Then) context.
6. Check event monitor (post-thin) - FVS Event Monitor checks for conditions and scheduled operations that are based on post-thinning activity conditions within the same cycle.
7. Grow large trees - If “large” trees exist in the tree-list, FVS estimates their new diameter (first) and height (second) one growth cycle into the future. The user can specify a different time interval length.
8. Grow small trees - FVS estimates small tree height (first) and diameter (second) one growth cycle into the future. FVS uses a weighting procedure to compute tree height increment to obtain a smooth height-growth transition from small to large tree models.
9. Mortality - Following growth estimation, FVS estimates mortality based on individual tree variables such as diameter and crown ratio, and on stand variables such as maximum stand density index or basal area.
10. Insect and disease impacts - If the variant has available extensions to cover specific disease or insect agents, and if the user calls for these, FVS estimates and incorporates these effects.
11. Regeneration - FVS adds new seedlings to FVS tree-lists in the regeneration step. Some FVS variants have natural regeneration routines, but most variants depend on the user to specify the species and number of trees to plant. Some variants have a stump sprouting algorithm, whereby certain species will sprout after a harvest.
12. Crown change - FVS estimates crown ratio change for all trees based on stand density and the trees position in the density distribution.
13. Update stand characteristics and compute volume - After projecting the stand for the growth cycle, FVS computes, summarizes, and records the stand attributes, including volume. Volume is computed using corporate equations from the National Volume Estimation Library.
14. Test for more cycles to be projected - FVS repeats the sequence from the initial "Check Event Monitor", per Figure 2, to this point for each cycle until the specified number of cycles is completed.
15. Final FVS Output file reports and post-processor files - After FVS completes the scheduled cycles, it prints final output file reports and generates files specified by the user for running post-processors, including SVS.

References

Dixon, Gary E. comp. 2002. Essential FVS: A User's Guide to the Forest Vegetation Simulator. Internal Rep. Fort Collins, CO: U. S. Department of Agriculture, Forest Service, Forest Management Service Center. 193p.

Crookston, Nicholas L. 1990. User's Guide to the Event Monitor: Part of Prognosis Model Version 6. Gen. Tech. Rep. INT-275. U.S. Department of Agriculture. Forest Service. Intermountain Research Station. Ogden, UT. October 1990. 21 pp.

Stage, A.R. 1973. Prognosis Model for stand development. Research Paper INT-137. Ogden, UT: USDA Forest Service, Intermountain Forest and Range Experiment Station.

Van Dyck, Michael. 2000. Keyword Reference Guide for the Forest Vegetation Simulator. WO-TM Service Center, USDA Forest Service. Fort Collins, CO. June 6, 2001. 97 pp.

Last modified: Friday, 02-Dec-2005 18:14:29 EST