MADISON, WI, September 7, 2010 – Rising sea levels and
coastal development are threatening coastal freshwater wetlands
with saltwater intrusion. While most ecosystem restoration projects
have focused on surface water and groundwater, new research finds
that conditions in the vadose zone, the unsaturated soil below the
surface but above the water table, are of particular importance to
seedling survival in coastal floodplain ecosystems.
Scientists at the University of Florida, the South Florida Water
Management District (SFWMD), and the Florida Park Service
investigated key measurements of the vadose zone, soil moisture,
and porewater salinity, in a historically freshwater floodplain
forest of the Loxahatchee River in southeastern Florida. Reduced
flows have resulted in the loss of bald cypress communities in
favor of drier, more salt-tolerant species.
Combining this new information with surface water, groundwater,
and meteorological data allowed the team to develop hydrological
relationships that drive ecosystem changes and inform proposed
restoration and management plans. Results from the study (funded by
the SFWMD) are published in the September-October 2010 issue of the
Journal of Environmental Quality.
The researchers measured soil moisture and porewater salinity
for four years at two sites – one in an upstream, freshwater
location and one in a downstream, tidal area. The team was
particularly interested in determining the relationship between
soil moisture and river levels to determine whether proposed
restoration flows would provide good conditions for of bald cypress
seeds, a valued ecosystem component in the area. A second major
research goal was to explain the distribution of freshwater and
salt-tolerant plants in the floodplain based on observed
salinities.
The unique data collection allowed researchers to measure
differences in vadose zone conditions between sites and over a wide
range of climatic conditions (two years with above-average
rainfall, followed by two years of extreme drought). They found
that soil moisture in upriver areas can be closely predicted based
on river level and topographic elevation in the floodplain.
The authors developed a calculator for land managers to estimate
average soil moisture under different river flow conditions during
restoration efforts. In downstream areas, river levels that
consistently saturate the soil will likely limit seed germination
and seedling survival to isolated high points.
The salinity tolerance threshold for bald cypress, 2 parts per
thousand (ppt), was rarely exceeded in upstream areas, but was
exceeded for considerable durations in downstream areas during dry
seasons. High porewater salinity provided the best explanation for
observed floodplain vegetation, which transitions from freshwater
species near the upland to salt-tolerant species near the river's
edge.
From a management perspective, porewater salinity peaks were at
most 63% of surface water salinity peaks, suggesting that
restoration flows that maintain downstream river salinity below the
2 ppt threshold will also sufficiently prevent floodplain porewater
salinities from exceeding this level.
"These results highlight the importance of understanding what's
happening in the root zone of plant species or communities you are
trying to conserve or restore. We believe this work offers a
framework for extending floodplain monitoring into the vadose zone
in other locations," says David Kaplan, one of the study's
authors.
Regarding future research, he adds, "Restoration efforts in
coastal floodplain forests would be further improved by
species-specific studies of moisture requirements for seed
germination as well as studies on the effects of variable tidal
inundation on the seeds and seedlings of important floodplain
species."
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