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, 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 (Fig. 1).
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 four surveys are shown. The upper left 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 upper right figure is the “actual”
state at the initial sampling. The situation in Jan 1995 is shown in the lower
left. Recruitment is well underway and consists of an even mix of Rhizophora
and Laguncularia. The bottom right 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.