A Decade of Mangrove Forest Change Following Hurricane Andrew
Thomas J. Smith III
U.S. Geological Survey, Center for Water and Restoration Studies, St.
Petersburg, FL
Kevin R.T. Whelan & Gordon H. Anderson
U.S. Geological Survey, Center for Water and Restoration Studies, Miami, FL
Christa L. Walker
Dyncorp Systems & Solutions LLC, U.S. Geological Survey, Homestead, FL
Jeffrey S. Dismukes
ETI Professionals, Inc., USGS Center for Coastal and Watershed Studies, St.
Petersburg, FL
Thomas W. Doyle
U.S. Geological Survey, National Wetlands Research Center, Lafayette, LA
Hurricane Andrew crossed the southern Florida peninsula on the morning of
August 24, 1992. Following the storm, the National Park Service conducted
an “Environmental Damage Assessment” to gauge its’ impacts on the natural
resources of south Florida Park Service holdings. Although hurricanes had
impacted Park’s lands, such as the Everglades, in the past, no systematic,
permanent sampling scheme had ever been established to monitor long-term
recovery (or lack of recovery) following catastrophic disturbance.
In October 1992 six large plots were established in the heavily damaged areas
of mangrove forest on the southwest coast of the Everglades, along the
Lostmans and Broad Rivers. The plot network was expanded during the next
24 months and now encompasses more than 20 permanent plots. Each plot is
circular. For each stem > 1.4 m in height, the distance and bearing from the
stem to a permanent center stake was measured. The stem was identified to
species and its diameter at breast height measured. The condition of each stem
was recorded (alive, killed by Andrew). Each living stem was marked with an
aluminum tree tag for future identification. Since establishment, each plot has
been sampled from 6-9 times. During a resampling, all tagged stems were
located and their dbh and condition recorded. Recruits (previously untagged
stems now having grown to 1.4m in height) were identified, measured and
mapped. Changes in condition of all stems were noted (e.g. mortality from
various causes). Increases or decreases in biomass were calculated for
individual stems based on allometric equations relating biomass to dbh. Total
biomass was determined by summing individual changes and changes due to
addition of recruits and losses from mortality. Co-incident with the
establishment of the permanent vegetation plot network, researchers from the
USGS were constructing a network of hydrological monitoring stations in the
southwest coastal Everglades. Each hydrology monitoring station has one or
more vegetation plots nearby (but not all vegetation plots have an adjacent
hydrology station).
The trajectory of vegetation change, growth, mortality and recruitment has
been highly variable among plots. Indeed, most plots have followed unique
patterns. The only overall pattern was the increase in stem density observed
in all plots. The species that dominated recruitment varied, sometimes being
Rhizophora, often Laguncularia, but never Avicennia. The rate of stem
density increase varied among plots. After 10 years, Second Onion Bay had
>1,900 stems, whereas Johnson Mound Creek had 325, about an order of
magnitude difference. Individuals are continuing to recruit into the population
at all plots except at Lostmans Ranger Station and Broad River Mid. The 10th
year survey at both of these showed slight declines in stem density (figs. 1, 2).
Mortality is occurring at all sites. Sources of stem death have included:
continuing mortality from damage initially induced by Hurricane Andrew,
stems being killed by falling debris, lightning, wind-throw during winter cold
fronts, freeze, fire and several smaller hurricanes since Andrew such as
George, Harvey, Irene and Mitch. Stems in the smaller size classes are
beginning to perish due to suppression (that is, being overtopped and heavily
shaded by larger neighbors).
Growth by stems that survived Andrew or which have since recruited into the
plots is difficult to explain. Productivity cannot be explained simply by
sediment porewater nutrient concentrations, which are highly variable.
Salinity and hydrologic parameters seem most promising to explain patterns
of biomass increase following the catastrophic disturbance from Hurricane
Andrew. Sampling of these plots will continue in order to monitor the effect
of increasing freshwater inflow that will occur as a major component of the
Everglades restoration.
Contact: Smith, Thomas J., III. USGS, Center for Water & Restoration
Studies, 600 Fourth St., South, St. Petersburg, FL, 33199. Phone: 727-8038747 x 3130, FAX 727-803-2030, Email Tom_J_Smith@usgs.gov. Ecology
& Ecological Modeling
Figure 1. Illustrative data for the Lostmans Ranger Station plot. Results from
the first surveys are shown. The top figure depicts the plot as it appeared prior to
Hurricane Andrew and is based on a “reconstruction” from the initial sampling in
October 1992. The lower figure is the “actual” state at the initial sampling.
Figure 2. Subsequent surveys for the Lostmans Ranger Station plot. The
situation in Jan 1995 is shown at top. Recruitment is well underway and consists
of an even mix of Rhizophora and Laguncularia. The bottom figure shows 10
years after the hurricane, October 2002. Rhizophora has now come to dominate
the pool of recruits and total biomass is beginning to exhibit an increasing trend.