Vegetation along a Nutrient Gradient: Water Conservation Area
2A
Cattails are native to the Everglades, typically occurring in
small stands throughout the region. Disturbances, such as
increased water depths associated with soil subsidence and fires,
tend to favor colonization by cattails. They also are very
opportunistic in utilizing increased nutrient levels,
particularly phosphorus, and the relative contribution of each of
these factors to their increased abundance in key regions needs
to be ascertained. Cattails are particularly abundant along the
canals and levees in the Water Conservation Areas, forming single
species stands that are visible on satellite photographs. Along
the Hillsboro Canal, between WCA 1 and WCA 2A, cattails began to
dominate vegetation shortly after construction of WCA 2A in the
1960s, probably in response to the deeper water levels and
disturbance resulting from construction of the Water Conservation
Area. In the 1980s, the area covered by cattail stands
doubled, expanding southward into the sawgrass marshes.
A nutrient gradient exists from the Hillsboro Canal south into
WCA 2A, and a series of monitoring stations have been installed
along this gradient. Cores have been collected at several of
these stations to document the floral, faunal, and fire history
of the sites, and pollen data from two of the stations are
presented below. At site F1, <2 km from the canal, highconcentrations of phosphorus and other nutrients are present,
grading down to normal nutrient levels at site U3, approximately
13 km from the canal. Pollen assemblages and radioisotopes
(210Pb and 14C) have been analyzed from these cores to compare
both long-term and recent temporal and spatial vegetational
trends and to determine whether any changes or differences are
attributable to documented environmental change.
Site F1 - Cattail Site
Pollen assemblages from site F1 document several noteworthy
changes through time in vegetational composition. From about
1,100 years before present until the early 1900s, marsh and
slough vegetation, including sawgrass, water lily, and arrowhead,
were the dominant elements of the pollen assemblages. In the
early 1900s, pollen of the pigweed family doubled in
abundance, completely dominating the assemblage. Since 1960,
cattail pollen abundance has increased five-fold from 3% to 15%.
The shift from the original marsh and slough vegetation to a
weedy type of vegetation, represented by pollen of the pigweed
family, coincides approximately with construction of the
Hillsboro Canal in the early 1900s and associated disturbance
of the original vegetation. The initial increase in abundance of
cattail pollen at about 1960 occurred at about the same time as
construction of WCA 2A; subsequent disturbances and addition of
nutrients may have served to maintain and expand the area covered
by cattails.
Site U3 - Pristine Site
Site U3 is pristine in terms of nutrient content but has
undergone a great deal of physical disturbance associated with
scientific investigations. The peat accumulation rate is much
lower at this site (0.083 cm/yr as compared to 0.173 cm/yr at
site F1). Pollen assemblages from this site generally resemble
the pre-1900 patterns at site F1. Cattail pollen is present in
low abundances throughout the core, and pigweed pollen abundance
is similar to that in pre-1900 samples at site F1. Also,
patterns of sawgrass, water lily, and arrowhead abundance mirror
those of site F1. Apparently, site U3 is distant enough from the
canal that disturbances associated with its construction did not
alter the vegetation. The primary difference from site F1 is the
higher abundance of aster (daisy) pollen in the upper 10 cm at
site U3. Analysis of surface samples from the area should
provide new evidence on the distribution and abundance of aster
pollen throughout the region, enabling a better interpretation of
the unusually high abundance of the pollen at site U3.
Anticipated Schedule
- Collect cores in Taylor Slough and Buttonwood Embankment.
- Produce reports on vegetational history from sites in Water
Conservation Area 2A and Taylor Creek. Produce report on modern
pollen assemblages; correlate pollen record with standing
vegetation.
- Complete coring in Buttonwood Embankment and begin coring in
new regions of interest.
- Produce reports on cores from Taylor Slough and Big Cypress.
- Complete analyses of cores; synthesize data to produce
regional overview of vegetational changes. Develop vegetational
reconstructions for selected time intervals over the last 150 years.
Summary
Pollen assemblages document the normal variability of the
vegetation over the last millennium and suggest that some changes
over the last 150 years are beyond the bounds of normal
variability. The patterns and timing of vegetational change
established throughout southern Florida will provide critical
information for calibration of models designed to predict
vegetational response to future environmental change and will
help to determine whether past changes were caused by human
activities or whether they represent a normal element of
community variation.
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For more information contact:
Debra A. Willard
U.S. Geological Survey
926A National Center
Reston, VA 20192
(703) 648-5320
Email: dwillard@usgs.gov
Related information:
SOFIA Project: Ecosystem History: Terrestrial and Fresh-water Ecosystems of Southern Florida