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 or 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. Coincident 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 (see figs.).

Mortality is occurring at all sites. Sources of stem death have included: continuing mortality from damage initially induced by Hurricane Andrew; and 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.

Click on images below for larger versions.

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Contact: Smith, Thomas J., III; USGS, Center for Water & Restoration Studies, 600 Fourth St., South, St. Petersburg, FL, 33199. Phone: 727-803-8747 x 3130, FAX 727-803-2030, Email Tom_J_Smith@usgs.gov. Ecology & Ecological Modeling.

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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