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Modeling Hurricane Effects on Mangrove Ecosystems

Mangrove ecosystems are at their most northern limit along the coastline of Florida and in isolated areas of the gulf coast in

Louisiana and Texas. Mangroves are marine-based forests that have adapted to colonize and persist in salty intertidal waters.

Three species of mangrove trees are common to the United States, black mangrove (Avicennia germinans), white mangrove

(Laguncularia racemosa), and red mangrove (Rhizophora mangle). Mangroves are highly productive ecosystems and

provide valuable habitat for fisheries and shorebirds. They are susceptible to lightning and hurricane disturbance, both of

which occur frequently in south Florida. Climate change studies predict that, while these storms may not become more

frequent, they may become more intense with warming sea temperatures. Sea-level rise alone has the potential for increasing

the severity of storm surge, particularly in areas where coastal habitats and barrier shorelines are rapidly deteriorating. Given

this possibility, U.S. Geological Survey researchers modeled the impact of hurricanes on south Florida mangrove communities.

Field and experimental studies were conducted to improve our understanding of species tolerance and community

zonation and to aid modeling trials of

impacts on mangrove ecosystems.

Permanent plots and greenhouse

experiments were established to

determine growth habits and ecology

of mangrove species following

disturbance from Hurricane Andrew

(Fig. 1). Remote videography was

taken at low altitude by helicopter over

mangrove forests along the southwestern coast of Florida to derive a coastwide assessment of damage extent and

pattern. Coastal and inland transects

were flown within the mangrove forest

boundary over Ten Thousand Island

National Wildlife Refuge and Everglades National Park. Video frame

analysis assessed the degree of

damage, windfall orientation, and

canopy height of the forest below (Fig.

2). Aerial videography proved to be an

efficient and timely means to document

large-scale hurricane damage and may

also help to monitor ecosystem

recovery in the coming years.

Measurements of light penetration

through mangrove canopies differentially impacted by hurricane winds of

estimated force offer a means to

calibrate wind damage probabilities on

mangrove forests. Mangrove species

and forests are susceptible to catastrophic disturbance by hurricanes

such as Andrew that cause significant

changes in forest structure and

function. These functional relationships of hurricane impacts on mangrove species and systems have been

incorporated into a landscape simulation model of south Florida mangroves.

Hurricane Simulation

A hurricane model, HURASIM, and

a mangrove forest model, MANGRO,

were combined in a spatially distrib- Fig. 1. Mangrove forest struck by Hurricane Andrew, 1992.

Fig. 2. Mangrove forest damage

assessment of coastal transects based

on visual interpretation of canopy

damage across the hurricane-affected

zone.
Conclusions

Historical simulations of actual

hurricane tracks and conditions seem

to account for the structural composition of modern day mangrove forests

across south Florida. The occurrence

of major storms every 30 years in this

century may be the most important

factor controlling mangrove ecosystem

dynamics in south Florida. Should

storms become more intense over the

next century, they may further alter the

structure and composition of this

mangrove landscape. Model results of

global change scenarios (high damage

probability) indicate that future

mangrove forests may be diminished in

both stature and extent.

For more information, contact

Thomas W. Doyle

U.S. Geological Survey

National Wetlands Research Center

700 Cajundome Blvd.

Lafayette, LA 70506

318-266-8647

tom_doyle@usgs.gov

http://www.nwrc.gov

Fig. 3. Frequency of hurricane strikes

of category 2 or greater by quadrangle

across south Florida for 1886-1989,

estimated by the HURASIM model for

mangrove habitat.

uted landscape application to review

the impact of hurricanes over the last

century on forest structure of mangrove communities across south

Florida. The model uses historical

tracking and meteorological data of

dated North Atlantic tropical storms

from 1886 to 1990 (Fig. 3). Data input

for the model includes tracking

information of storm position, latitude

and longitude, and maximum sustained

wind speed every 6 hours or less.

HURASIM model output from Hurricane Andrew was correlated with field

data to construct data tables of

damage probabilities by site and

species and to determine critical

windspeeds and vectors of tree

mortality and injury.

Mangrove Simulation

MANGRO is a spatially explicit

stand simulation model constructed for

neotropical mangrove forests of black

mangrove, white mangrove, and red

mangrove. This individual-based

model comprises a species-specific set

of biological functions predicting the

growth, establishment, and death of

individual trees. MANGRO predicts

the tree and gap replacement process

of natural forest succession as

influenced by stand structure and

environmental conditions. For our

study, tree mortality was modeled as a

random process of age, suppression,

and hurricane impact derived from

damage probability curves developed

from observed data of Hurricane

Andrew (1992) effects. If in any given

year of the simulation a predicted

hurricane windforce exceeded 30

square meters per second, a probability

was derived based on windspeed from

which a percentage of the standing

crop or trees were randomly removed

from the forest simulation.

Model Application

Four treatment effects were

implemented including a no-hurricane

simulation contrasted with a low,

moderate, and high mortality effect that

increased with corresponding increases in windspeed. A historical

simulation for 1886 through 1989 was

achieved by passing hurricane and

site-specific information from the

HURASIM model to the associated

MANGRO simulation for common

cells. A cumulative assessment of

hurricane impact was achieved by

averaging stand attributes and size for

the entire simulated landscape and time

interval from 1890 to 1989. Simulations

of hurricane tracks and history for

south Florida showed that storm

frequency and intensity varied across

the landscape. Hurricane frequencies

by quadrangle for the period of record

showed that the number of storms with

winds exceeding 30 square meters per

second were more numerous on the

Atlantic side than the gulf side of

southern Florida. The southwest coast

of Florida has endured stronger storms

on record than the gulf and Atlantic

coasts to the north. The combined

layering of hurricane impact showed

that there are portions of the south

Florida landscape that have received

more frequent and more intense storm

activity than other portions.

U.S. Department of the Interior

U.S. Geological Survey

USGS FS-095-97

June 1997