ATLSS Vegetation Succession Model Project.
Scott M. Duke-Sylvester and Louis J. Gross
University of Tennessee, Knoxville, TN
Paul R. Wetzel
Smith College, Northampton, MA
The USGS's Across Trophic Level System Simulation (ATLSS) models in their current
form all assume that vegetation will remain unchanged over the period of the model
simulations. This is not likely to be true, as changes in hydrology are almost certain to
result in changes in vegetation patterns in many parts of the Everglades over periods of
decades. There is already some evidence of changes in vegetation types in some areas of
the Everglades over the last ten years.
The development of a set of succession models for the major vegetative types in the
Everglades region is generally regarded as being essential, if scientists and managers are
to be able to project the possible effects of changes in the hydrology of the region.
Vegetation response is also sensitive to changes in nutrient concentrations and fire.
Furthermore, important animal species, such as wading birds, the snail kite, and the Cape
Sable seaside sparrow, have specific habitat needs that are tied to particular types of
vegetation. The basic goal of this ATLSS project is to develop succession models that
estimate future patterns of vegetation for targeted habitats and to describe how these
habitats are affected by changes in hydrology, available nutrients, fires and the interaction
of these processes. Three basic community types, "pine/scrub/flatwood", "cypress
forest", and "herbaceous plant communities" will be included in the modeling. Within
these types, a more detailed structure using Florida GAP vegetation alliances has been
developed, allowing for explicit alliance responses to patterns of hydrology, nutrients,
and fire.
The vegetation succession model will cover ~48% of the total area in South Florida
(urban, agricultural, and mining areas excluded), uses all 22 of the Florida GAP alliances
present in the ATLSS study area and will provide yearly estimates of vegetation alliance
distribution at a 100x100 meter resolution. Model parameters have been determined from
an intensive literature review followed by an extensive compilation and synthesis of the
available data (available in two documents: Plant Community Parameter Estimates and
Documentation for the Across Trophic Level System Simulation (ATLSS), and Nutrient
and Fire Disturbance and Model Evaluation Documentation for the Across Trophic Level
System Simulation (ATLSS)). The information from the literature review was structured
around the Florida GAP vegetation alliances, which provided a basis for the model
parameters and succession models of the three basic community types.
Review of the data suggests that, at the spatial scale of the ATLSS succession model, the
effects of hydrology, phosphorous, and fire are very important, while the effects of
nitrogen enrichment are of lesser importance and will not substantially affect model
dynamics. Two different fire types will be included in the model: those that damage the
soil, commonly referred to as muck fires, and surface fires that destroy vegetation but do
not burn the soil. Making a distinction between muck and non-muck fires is important
for succession modeling because burning soil changes local topography, which in turn
changes the hydrology of local habitats. The interactions between fire and hydrology, and
fire and nutrients have also been estimated in the succession documentation, based upon
the limited available information on interaction effects for the vegetation alliances in
South Florida.
The model uses a two-part approach to simulating the process of succession. First, a static
look up table was constructed for each basic community type. Each look up table
describes the potential future states of a plant community as a function of the history of
hydrology, fire, nutrients and the interaction between these processes. The table also
provides parameters for the expected time for a change to take place for each of the
different processes affecting succession. The look up table is based on the literature
review and synthesis.
Secondly, once local environmental changes indicate that succession to a new vegetation
alliance is possible, the succession pathway is treated as a Markovian stochastic process.
The change to the new vegetative state is random with the probability distribution for the
various new states and the expected time for transformation given by the look up table.
Presenting the output of the vegetation succession model will follow the relative
assessment approach using three panel maps applied in the analysis of other ATLSS
models. This synthesis will present two maps representing the distribution of FGAP
vegetation alliances produced by two different hydrology scenarios with the third map
indicating locations and type of differences between the two distributions. The
classification of differences is still an open question and depends largely on what
information is most useful to the various scientific, managerial and policy making entities
operating in South Florida.
ATLSS model evaluation will begin in the earliest stages of model development. The
evaluation effort is necessary to determine the usefulness and accuracy of plant
community succession in ATLSS. Peer review, Turing tests, gradient response and
extreme condition tests, and tracing the behavior of specific variables or vegetation types
through a model run are all possible model evaluation procedures suitable for the plant
community succession module of ATLSS.
There are several goals for the succession model: it will provide an estimate of relative
changes in vegetation distribution in response to changes in the abiotic environment and
it will provide additional information about the potential effects of restoration. Model
results will also be useful as input to other ATLSS and non-ATLSS models, including
SIMSPAR and the ATLSS SESI models, that simulate processes sensitive to changes in
the distribution of vegetation in South Florida.
Duke-Sylvester, Scott M., University of Tennessee, 569 Dabney Hall, Knoxville, TN,
37996-1610
Phone: 865-974-0223, Fax: 865-974-3067, sylv@tiem.utk.edu