Initial performance measures and information related to the
ATLSS Vegetation Succession Model.

by
The Institute for Environmental Modeling
University of Tennessee, Knoxville
Contact:

Introduction
      This page contains information describing the Across Trophic Level System Simulation (ATLSS) Vegetation Succession Model (VSMod), the preliminary runs of this model, as well as the data produced by these runs. These data are provided as sample output of the type available from this model. Our objective in posting these preliminary results is to provide an opportunity for comments and feedback from those interested in the model, and to illustrate the types of results obtainable from the model.
      The VSMod projects the spatial distribution of vegetation on a yearly basis. The model can be used to cover any period of time for which appropriate input and initialization data can be provided. Typically the model is run for 31 to 36 year time periods for which hydrology data from the South Florida Water Management Model (SFWMM) are available. The yearly time step was chosen because we felt that this was the shortest period of time over which successional changes would occur at the spatial resolution projected by the model.
      The spatial domain of the model includes most of the natural areas of south Florida including A.R.M. Loxahatchee National Wildlife Refuge, Water Conservation Areas (WCA) 2 and 3, the Everglades National Park, most of Big Cypress National Preserve as well as numerous surrounding natural areas. The spatial domain is divided into a regular grid where each plot or cell is 500x500 meters in size. This spatial resolution was chosen to match the resolution of many of the other ATLSS models, including the ATLSS Spatially Explicit Species Index (SESI) models, that will eventually use the output of the VSMod as inputs.
      The vegetation types used in the VSMod are currently based on the 24 of the 58 natural types present in version 6.6 of the Florida GAP (Fl-GAP) map (Pearlstine et al. 2002). The 24 types used in the VSMod are those that have some representation within the spatial domain of the model. The remaining 34 types have very little ( less than 5 cells ) or no representation within the spatial domain of the model. Adding any or all of the unused 34 types is a matter of obtaining appropriate model parameters for each type. Future model development may include estimating parameters for these types to allow those types not currently present within the model domain to invade over time. In addition, the VSMod can be used with any other collection of vegetation types and vegetation maps for which appropriate model parameters can be estimated.
      To make projections, the model incorporates the effects of hydrology, fires and nutrients as well as information about how each of the 24 vegetation types included in the model respond to these factors. The parameter estimates for the response of vegetation are documented in two ATLSS reports : Plant Community Parameter Estimates and Documentation for the Across Trophic Level System Simulation (ATLSS) (Wetzel 2001), Nutrient and Fire Disturbance and Model Evaluation Documentation for the Across Trophic Level System Simulation (ATLSS) (Wetzel 2003).
      The primary source of hydrology data is the output of the SFWMM. Hydrology data from this model are post-processed using the ATLSS High Resolution Hydrology (HRH) model to produce variation at the 500x500 meter resolution within the 2x2 mile model cells of the SFWMM.
      The VSMod output presented here are based on hydrology data from the Calibration/Verification run of the SFWMM, version 3.7 (CalVer-v3.7). The CalVer-v3.7 data includes the period from January 1, 1979 to December 31, 1995. A description of this data set and the SFWMM are provided at the
South Florida Water Management Model (SFWMM) website.
      The primary source for fire data for the vegetation succession model is the ATLSS Fire model (FMod). This model estimates the spatial pattern of annual area burned by fires within the region covered by the ATLSS VSMod. The spatial distribution of fires projected by this model is based on fire history, hydrology and the distribution of vegetation. A description of the FMod is provided at the ATLSS FMod website. Fire data can also be provided to the VSMod in the form of input files. This feature can be used to run the VSMod using historical fire patterns, or to evaluate the results of fire management scenarios.
      The VSMod output presented here are based on three different fire scenarios, labeled LowFire, MedFire and HighFire. Each fire scenario was created using the ATLSS FMod. The LowFire fire scenario is based on fire model parameters that represent low probability of fire spread, the MedFire fire scenario is based on fire model parameters that represent medium probability of fire spread and the HighFire fire scenario is based on fire model parameters that represent a high probability of fire spread. The parameterization of the fire model is incomplete as of this writing, and the parameter values used to represent low, medium and high probability of spread are best estimates based on the experience of the ATLSS staff and collaborators.
      Nutrient data for the VSMod is produced by the ATLSS Nutrient Model. This is a spatially explicit model that projects changes in Total Phosphorous (TP) levels resulting from TP enriched water being discharged into the natural regions from various water control structures. A basic description of this model is provided in Wetzel (2003). Nutrient data can also be provided to the VSMod in the form of input files. As done in the case of input fire data files, nutrient data files can be used to run the VSMod on historical nutrient patterns or to evaluate alternative nutrient delivery scenarios.
      The vegetation succession model itself is a stochastic cellular automata (SCA) model. For each of the 500x500 meter cells the model uses a SCA to determine the transition from one vegetation type to another. The model is a cellular automata because at any given time a cell is on one of a finite number of states (a state being a particular vegetation type). The model is stochastic because transitions between states do not occur deterministically, but occur stochastically with certain transition probabilities. Typically, models of this sort assume that the transition probabilities do not change in time and do not vary across space. The ATLSS VSMod is different in that transition probabilities vary in both space and time. The transition probabilities depend on the local hydrologic and fire history, the current level of TP as well as the current vegetation type. At each time step the effects of these factors are used to update the transition probabilities and are evaluated at each location across the landscape. The updated transition probabilities are then used to determine the next state of each cell across the landscape.
      Because the vegetation succession model is stochastic, it must be run several times to obtain an estimate of the average trend in succession projected by the model as well as to estimate the variability produced by the model. For the results presented here 20 replicate runs were used, but the number of replicates can be chosen by the user.

Description of output
      For each fire scenario the results from the replicates are summarized into a set of spatial data layers. One layer is produced for each vegetation type and each year. For example, for sawgrass (Fl-GAP type 43), 17 layers are produced, one for each year from 1979 to 1995, inclusive. Each layer summarizes results across replicates and is composed of a set of 500x500 meter cells arranged in a raster grid. Each cell contains a single value that represents the fraction of replicates in which the particular vegetation type being mapped occupied that cell.
      For each fire scenario the summary layers produced by the VSMod are stored in a set of output files. These output files provide the same information in three different formats. Each format is designed to facilitate the analysis of model results using different tools.
      The first file format presents the data as a set of binary sequential (BSQ) files. In this format, all the yearly layers for a single vegetation type are stored together in a single file. The filenames for these files have the format, VSMod{fire scenario}_{index}.bin and where {fire scenario} is replaced by the fire scenario name, {index} is replaced by the Fl-GAP index for the vegetation type. Fire scenario names are LowFire, MedFire and HighFire. For the low fire scenario, the data file for sawgrass ( which is Fl-GAP type 43) is VSModLowFire_43.bin. These data files are for use with the ATLSS Data Viewer .
      The second file format provides the summary layers as a set of standard ASCII Grid files. In this format, each year and vegetation type is stored as a separate file. The filenames for these files have the format VSMod{fire scenario}_{index}_{year}.asc where {fire scenario}, {index} is replaced by the Fl-GAP index and {year} is replaced by the year. For the low fire scenario the data files for sawgrass are :
VSModLowFire_43_1979.asc
VSModLowFire_43_1980.asc
VSModLowFire_43_1981.asc
...
VSModLowFire_43_1994.asc
VSModLowFire_43_1995.asc

These files can be read into any ESRI product for visualization.
      The third file format provides the summary layers as a set of standard TIFF image files. Like the ASCII Grid files, each year and vegetation type is stored as a separate file. The filenames for these files have format, VSMod{fire scenario}_{index}_{year}.tif, where {fire scenario} is replaced with the fire scenario name, {index} is replaced with the vegetation index and {year} is replaced with the year. These are graphic image files that can be viewed with a wide range of programs. Unlike the other two file formats, the frequency data are interpreted as a set of colors. Figure 1. provides the color legend for these images.
      The data files have been placed into a set of standard zip files to reduce their size and gather related files into a single resource. For the BSQ files each file is placed into its own zip file. The filename for the zip files is the same as the original BSQ filename, with the .zip file type extension added to the end of the filename.
      For the ASCII Grid and TIFF files, all the files for a vegetation type have been gathered into a single zip file. For these files the filename format is VSMod{fire scenario}_{index}.asc.zip for the ASCII Grid files and VSMod{fire scenario}_{index}.tif.zip for the TIF files, where the substitution of {fire scenario} and {index} are the same as those described above.
      Rather than displaying all the vegetation types simulated by VSMod, this page is limited to 4 types: 43 (Sawgrass), 44 (Eleocharis), 45 (Muhly Grass) and 46 (Cat tail). This was done to reduce the volume of output in this preliminary release of VSMod output. Even with this reduction in output there are 420 separate files (aggregated into 36 separate zip files).
      These four vegetation types were chosen because they are major constituents of the Everglades ecosystem. Each type is a defining member of important natural habitats within the Everglades. Cat tail is important because of its encroachment into Sawgrass marshes resulting from the nutrification of parts of the Everglades. In addition, theses type show large responses to changes in hydrology and fire over the 1979 to 1995 time frame.
      Based on the response to this initial release we will add additional types to the list provided or will make the selection user selectable. The capability to carry out simulations of VSMod using the ATLSS Model Interface will be provided to authorized users following a comment period for the preliminary simulations presented here.

Vegetation Succession binary output files for Calval-v3.7
High Fire Sawgrass Eleocharis Muhly Grass Cat tail
Medium Fire Sawgrass Eleocharis Muhly Grass Cat tail
Low Fire Sawgrass Eleocharis Muhly Grass Cat tail

Vegetation Succession Tiff graphical output files for Calval-v3.7
High Fire Sawgrass Eleocharis Muhly Grass Cat tail
Medium Fire Sawgrass Eleocharis Muhly Grass Cat tail
Low Fire Sawgrass Eleocharis Muhly Grass Cat tail

Vegetation Succession ASCII Grid output files for Calval-v3.7
High Fire Sawgrass Eleocharis Muhly Grass Cat tail
Medium Fire Sawgrass Eleocharis Muhly Grass Cat tail
Low Fire Sawgrass Eleocharis Muhly Grass Cat tail





Disclaimer
The version of the vegetation succession model which produced these results has not been evaluated with respect to detailed historical data. Thus the changes in vegetation observed in these files does not represent what the completed model projections will be when the ATLSS Fire model is parameterized. These sample data files are designed to provide an example of the output the model will produce. If you have any questions please feel free to contact:

Figures


Citations
Pearlstine, L. S., S. Smith, et al. (2002). "Assesing State-Wide Biodiversity in the Florida Gap Analysis Project." Journal of Environmental Management 66: 127-144.

Wetzel, P. R. (2001). Plant Community Parameter Estimates and Documentation for the Across Trophic Level System Simulation (ATLSS). Knoxville, TN 37996-1610, The Institute for Environmental Modeling: 58.

Wetzel, P. R. (2002). Nutrient and Fire Disturbance and Model Evalutation Documentation for the Across Trophc Level System Simulation (ATLSS). Knoxville, TN 37996-1610, The Institute for Environmental Modeling: 46.


The ATLSS Project at The University of Tennessee is supported by Cooperative Agreement #1445-CA09-95-0094 with the U.S. Geological Survey and the National Park Service, The University of Tennessee, and Awards DMS-0010920, DEB-0219269 and IIS-0427471 to the University of Tennessee from the National Science Foundation.

© 2005 ATLSS TIEM/UTK