U.S. Department of the Interior
U.S. Geological Survey
FS-146-96
Introduction |
Reconstruction of Ancient Communities |
Vegetation along a Nutrient Gradient: Water
Conservation Area 2A |
Anticipated Schedule |
Summary
Plant and animal communities of the historic Everglades have undergone striking changes over the last few decades, including declines in wading bird populations, invasion by exotic (non-native) plant and animal species, and areal expansion of cattails into sawgrass marshes. Many of these changes have been attributed to human activities in the region, and efforts are underway to restore the Everglades to a more pristine state.
Restoration plans include alteration of the present levee and canal system that controls water flow throughout southern Florida and changes in agricultural and land development activities. However, can we be sure that the observed changes are related to human practices rather than representing an extreme in the natural variability of the system? This question should be answered before changing current land-use practices to alleviate environmental concerns. To address this question, it is necessary to look at patterns of floral and faunal change over the last century as well as over the last few millennia.
This project aims to reconstruct floral and faunal composition at selected time intervals throughout southern Florida to determine: 1) detailed biotic changes over the last 150 years; 2) the natural range of biotic variability over the last few thousand years; and 3) determine whether any cause-and-effect relationships exist between biotic and environmental changes. Such questions are being addressed by scientists in the South Florida Initiative of the Ecosystem Program at the U.S. Geological Survey.
A series of surface samples and shallow cores are being collected
from sites throughout southern Florida for analysis of pollen and
spores, mollusks, foraminifers, ostracodes, diatoms, and any
other biotic elements present, as well as for radiometric dating,
charcoal, and other geochemical analyses. Age control for the
last 150 years is provided by analysis of the radioisotope
lead-210(210Pb) from each sample; radiocarbon (14C) dating is
used on samples in the middle and basal parts of the cores to
date sediments too old to be dated using lead-210.
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.
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.
For more information contact:
Related information:
U.S. Department of the Interior, U.S. Geological Survey
Pollen preserved in the cores provide a proxy record of
vegetational patterns throughout the history of the site.
Because pollen typically is transported from the source plant
before being incorporated into sediments, it provides a record of
the regional vegetation rather than just the vegetation growing
at a site, and this is useful in interpreting broad patterns of
vegetational distribution. However, the percent abundance of
pollen in a sample is not directly related to the abundance of
its source plant in the standing vegetation, because plants
differ in pollination method and produce different quantities of
pollen. To cite one example, a wind-pollinated species such as
pine releases much more pollen into the atmosphere (and,
ultimately, into the sediment) than an insect-pollinated
plant. To correct for such biases, it is useful to analyze pollen
from surface (modern) samples to determine what pollen
assemblages are characteristic of different vegetation types.
For this project, surface samples have been collected from 23
sites in WCA 2 and 3 and from the mangrove fringe along Florida
Bay. Statistical comparison of downcore assemblages with these
modern assemblages allows identification of modern analogs for
the downcore assemblages and, therefore, interpretation of
standing vegetation throughout the history of the site.
Debra A. Willard
U.S. Geological Survey
926A National Center
Reston, VA 20192
(703) 648-5320
Email: dwillard@usgs.gov
SOFIA Project: Ecosystem History: Terrestrial and Fresh-water Ecosystems of Southern Florida
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Last updated: 03 November, 2004 @ 03:04 PM(TJE)