Yegang Wu

ANALYSIS AND SIMULATIONS OF FRAGMENTATION PATTERNS IN THE EVERGLADES

Sawgrass (Cladium jamaicense) communities of Water Conservation Area 2A (WCA-2A), a 43281 ha Northern Everglades impoundment, are being invaded by cattail (Typha spp.). Results from analyses suggest that the yearly invasion rate of cattails has increased from 1% in 1973 to 4% by 1987. The total area of the landscape impacted by cattail increased from <5% (2054 ha) in 1973 to more than one-third of the landscape (16017 ha) by 1991. The landscape also became more fragmented. The number of sawgrass patches increased from 173 in 1973 to 5709 by 1991. During the same period the lacunarity index of sawgrass changed from 2.8 to 3.9. The effects of agricultural phosphorus (P) runoff and water depth (D) on invasion of cattail were expressed as, Probpe = 1/(1 + αe−βP) + ζD/P. The threshold for accelerated cattail invasion was estimated at ≈650 mg/kg soil total phosphorus. Cattail dispersal was mostly spatially dependent. For a given year, the probabilities of sawgrass changing to cattail based on the number of 1–8 adjacent cattail cells (20 × 20 m) were calculated to be Probad = [0.049 0.052 0.061 0.065 0.069 0.072 0.076 0.094]. The probabilities Probpe and Probad were used as Markov chain probabilities in a spatial model to simulate vegetation dynamics. The simulated landscape matched the actual landscape with an overall accuracy of 72.8% and predicted that cattail would invade 50% of WCA-2A in another 6–10 yr if the driving forces remain unchanged.

Fire simulations in the Everglades Landscape using parallel programming

Abstract Fire can significantly influence vegetation patterns in the Everglades. Unfortunately, fire is a difficult process to experimentally manipulate, especially at a landscape level. An Everglades Landscape fire model (ELFM) was developed using parallel-processing algorithms and transputer-processors to understand fire behavior in Water Conservation Area 2A (WCA 2A) in the Everglades. Fuel characteristics, water depth, wind velocity and direction, rainfall, lightning, and humidity determined the physical state and rate at which fire spreading and spotting occurred in the ELFM. The ELFM simulated fire spread across a heterogeneous landscape using a grid-based system. Parallel processing enabled the model to simulate fire on a large spatial scale with fine resolution (i.e., 1755 × 1634 pixels with 20 × 20 m resolution). The model was designed as a multiprocessor program with the ability to compile and run on UNIX workstations, the CM-5 supercomputer, and Mac Transputers with no change in the code. The ELFM was used to conduct a series of fire experiments that indicated how current fire regimes differ from historical ones due to cattail ( Typha spp.) invasion and longer and deeper water depths. In an Everglades dominated by cattail, the predicted average annual area burned and fire frequency were significantly reduced by 23% and 21%, respectively. The ELFM experiments also suggested that altered hydroperiod have changed fire patterns by reducing fire frequency 63% while increasing fire size during drought years. Airboat trails did not significantly influence total area burned in the ELFM. However, they did seem to function as breaks in upwind fires and tended to reduce the size of potentially large fires.

Spatial Simulations of Tree Islands for Everglades Restoration

The Florida Everglades, a vast wetland dotted with diverse tree islands, is a uniquely difficult wetland to manage because of competing urban, agricultural and environmental water demands. Tree islands in certain sections of the Everglades have experienced altered hydroperiod due to water management practices that has, at times, caused tree island vegetation to die. This study uses the Everglades Landscape Vegetation Model (ELVM) to investigate whether an observed trend in tree island loss is reversible, and if tree islands can be used as performance measures or ecological indicators for the success of Everglades restoration actions. The ELVM was developed and designed to be a tool to understand the spatial and temporal interactions among vegetation, water, fire and nutrients. Simulation results from this model suggest that hydroperiod is a major factor contributing to tree island development and stability in the Everglades. Simulations of the ELVM indicated that tree island water depths greater than 30 cm and hydroperiods longer than 150 days were decreasing tree island survival rates. According to the model, about 60% of tree islands lost in the Water Conservation Area 2A (WCA-2A) in the last few decades can be recovered by restoring the natural hydrological regimes. As a result, tree island health could be used as a performance measure to evaluate the effects of various hydrological restoration alternatives in the Everglades.

Parallelization of an ecological landscape model by functional decomposition

A functional scheme is described to parallelize computer simulations of grid-based ecological landscape models. The method is implemented using the Message Passing Interface protocol and is applied to the Everglades Landscape Vegetation Model. On a two-processor system, the speed-up is satisfactory and the overall performance of the program is competitive with traditional parallelization techniques such as geometrical decomposition. The method is discussed, timing information is provided for three different parallel machines, and some further developments are indicated.