Using Soils to Understand Ecosystem Change in Wetlands in Palo Verde National Park, Costa Rica

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Using Soils to Understand Ecosystem Change in
Wetlands in Palo Verde National Park, Costa Rica

• Courtney M. Gallaher
• Cynthia A. Stiles
• University of Wisconsin-Madison

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Palo Verde National Park
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Dry tropical forests are presently one of the
most endangered ecosystems on earth, threatened
by both expanding development and agriculture.
Palo Verde National Park in northwestern Costa
Rica is an outstanding example of a large wetland
within a dry tropical forest. It is located in
the Guanacaste region along the lower reaches of
the Tempisque River, the second largest river in
Costa Rica. The climate for the area has a
pronounced dry and wet season, with most rain
(180-220 cm) falling between the months of April
and October. The mean annual temperature is 28C
and varies little throughout the year.
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Changes in the Palo Verde Wetlands-Aggressive
cattail invasion-
2002
1996
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The park provides diverse habitat for thousands
of birds that roost and nest here as they migrate
between the continents. Birdwatching attracts
many ecotourists and represents a vital source of
revenue for this area. Recently the native
vegetation in the open water marshes that were
once suitable to migratory waterfowl has been
replaced by dense stands of invasive cattails,
possibly a hybrid between the non-aggressive
native species and European strains. These
changes in vegetation have had severe
repercussions on the wildlife in the area which
may eventually impact the ecotourism industry,
which relies on the abundant bird populations
attracted to open water margins.
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Recent Changes in Vegetation

• Possible causes
• Changes in hydrology and sedimentation due to
  flooding during Hurricane Mitch in 1998
• Irrigation run-off from nearby rice agriculture

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What may be causing this recent drastic change in
the vegetation? A major problem in identifying
the cause of the cattail invasion is a general
lack of knowledge of the soils of Palo Verde
National Park. Because soils form the matrix in
which plants grow, a good understanding of soil
properties is necessary to understand the
ecological processes that have led to the cattail
invasion. A possible recent cause may be
flooding of the park by Hurricane Mitch, which
struck Central America in 1998. This area is
seasonally flooded by monsoons, but storms the
size of Hurricane Mitch hit western Costa Rica
approximately once every 100 years. Recent
agrarian activities (forest clearance and field
expansion) may have contributed to enhanced
sediment loads during this event.
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Hurricane Mitch Flooding
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When Hurricane Mitch flooded the wetland area, it
likely changed local hydrology and soil
properties. Specifically, the influx of a large
volume of rainwater may have changed the water
chemistry of the brackish wetlands and tipped the
balance to a strongly freshened condition, more
conducive to cattail growth. In addition,
flooding may have deposited fresh sediment that
dramatically altering the soil matrix, releasing
many water-soluble nutrients that would aid in
the establishment of cattail colonies and explain
the disappearance of the native vegetation.
Finally, irrigated rice agriculture, a fairly
recent component in agrarian component, may also
be enhancing marsh eutrophication through the
addition of soluble nutrients and possibly
pesticides.
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Research Questions

• Is the marsh experiencing passive salinization
  due to a wet-edge effect?
• Is the surface water of the marsh freshened due
  to changes in sedimentation and hydrology?

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In order to understand whether the observed
changes in vegetation are symptoms of a larger
ecosystem change, I asked two questions. First,
is the marsh experiencing passive salinization
due to a wet-edge effect? In essence, are salts
in the ground water being drawn up into the
wetland soils, causing soil salinization? And
second, is the surface water of the marsh
freshened due to changes in sedimentation and
hydrology? As previously discussed, flooding may
have added sediment layers to the wetland soils
changing local hydrological patterns.
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Palo Verde National Park
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This map shows the location of Palo Verde
National Park. The Tempisque River is an estuary
along the lower margins of the park and, along
the extent of the park, reverses its flow with
the tide. Initial fieldwork for the study was
conducted by a team of researchers from the
University of Tennessee, including Cynthia Stiles
who is presently at the University of Wisconsin.
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Site LocationsChamorro and Laguna Bocana
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Fieldwork included soil profile descriptions at
two sites within the park, the Chamorro
floodplains and the Laguna Bocana wetlands. Soil
samples were collected from these soil profiles.
The Chamorro site is close to the river itself
(within the reversing flow regime) and represents
a normal floodplain sequence. Three profiles
were excavated near the essentially closed basin
wetland in Bocana. Note the network of
irrigation canals outside the limits of the park
and how the surface hydrologic pattern suggests
that excess run-off from irrigation is directed
down into the restricted wetland system. Bocana
is in essence a sump for the drainage water
from the upstream irrigation, which introduces
excessive water to the system during the driest
part of the year. Remember that these are soils
originally formed under monsoonal climate cycles.
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Chamorro alluvial plain Vertisol
A
Bss1
Bss2
Bss3
Bsskg
Bssg
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The baseline soil for the study, from near the
Tempisque River, is a Vertisol. This is a very
clay-rich soil formed from overbank deposition
from the Tempisque. The floodplain on which this
soil is found extends for a considerable distance
along the Tempisque and ranges from 2-6 km wide.
The soil has slickensided aggregate structure,
most strongly expressed at 0.5 1.0 m depth (Bss
horizons). There is also some accumulation of
pedogenic carbonate below 1.5 m (Bssk horizons).
Embedded within the profile are occasional layers
of erosional pedorelicts that may have been
carried down from past large volume flood events.
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Laguna Bocána wetland margin
A
Bw
2Byk
2Byg
3Bwb
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Other profiles excavated closer to the edge of
the Bocana marsh had lithologic discontinuities
which represent deposition from different storm
events. The textures vary irregularly, as will
be shown in the particle size distribution
figures. The horizon designation By indicates
that there is at least 5 gypsum (by volume)
present in this profile and the designation 3Bwb
indicates a buried soil subsurface horizon from
an earlier sequum that contains relict mangrove
roots.
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Methods of Analysis

• Particle size
• Soil texture
• Sediment deposition
• Hydrology
• Water soluble elements
• Nutrients released during flooding
• Salt accumulations

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To help address the hypotheses, I analyzed the
soil samples taken from the Palo Verde profiles
for particle size distribution and nutrient
contents. Particle size distributions allow you
to draw conclusions about soil texture, sediment
deposition in the basin as well as local
hydrology, and I measured this using the pipette
method. Water soluble elements characterize
nutrients released during a flood event such as
Hurricane Mitch, as well as soluble salts
accumulated in the soil horizons. To measure
this, I used a 110 pore volume to water solution
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Particle Size Distribution
Bocana 3
2 m
Bocana 2
Chamorro
Bocana 1
Clay
Sandy loam
Silt
Silt loam
Silt
Clay
Clay
Loam
Clay
Clay loam
Silt
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Silt loam
Silt
Clay
Clay
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This is a simplistic diagram outlining the
toposequence along which the soil samples were
collected, and the textures of each horizon
within the soil profiles. The Chamorro soils are
classified as Vertisols because they had high
clay content. The Bocana soil textures on the
other hand, contain far more sand and silt than
Chamorro, indicating their more active role in
intercepting sediment during erosional events.
Yet, all three Bocana soils have clayey A
horizons. This strongly suggests deposition of
clays due to erosion of upland soils during the
monsoons. The large floods of Hurricane Mitch
probably carried large quantities of clays from
the uplands down into the Bocana wetland basin.
This simple change in particle size within the
sediments would have a profound effect on the
hydrology of the wetland system. Clays have a
restrictive influence on infiltration and tend to
act as localized aquitards. There is a shallow
aquifer within the soils (1-1.5 m deep) that
carries dissolved salts from the irrigated
fields. Clay caps seal this from meteoric
interaction.
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ChamorroRelationship between NaH2O and soil
texture
Sodium
Sand
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One of the most striking results of the water
soluble nutrient analysis was the high
concentrations of salts in many of the Bocana
soil horizons. To better understand why the salt
has concentrated in the Bocona soils, on the
graph I have plotted both the percent sand and
the ppm of sodium against the soil depth. This
allows us to see any significant textural
variations that would influence water flow as
well as salt accumulations in the horizons. The
Chamorro soils are considered the control for
this investigation and show little response of
sodium to the textural increase in sands at about
100 cm depth. These soils are largely
clay-dominated and sodium accumulation is more an
effect of the proximity of the lowest B horizon
to the brackish water from the Tempisque
estuarine system. The next three slides graph
sand and salt vs. depth for the three Bocana
soils.
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Bocana 1Relationship between NaH2O and soil
texture
Sodium
Sand
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In contrast to the Chamorro soils you can see a
strong correlation between the soil texture and
the salt concentration in the Bocana soils. Also
note that sand and sodium axes are an expanded
range from the previous Chamorro slide. Bocana
1 is located closest to the wetland. Sand
percentages, indicated by the blue line, are
highest at the top and bottom of the soil profile
as are the salt concentrations. However, salt is
concentrated in the finer textured horizons just
below the coarser sandy horizon
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Bocana 2Relationship between NaH2O and soil
texture
Sodium
Sand
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Bocana 2 is located between the wetland and the
upland area. This profile shows two pronounced
sand lenses at 40 cm and 80 cm with roughly
corespondent increases in sodium. This bimodal
distribution is due to the geomorphically active
nature of this position it is close enough to
the wetland to experience overbank deposition
during flooding, yet far enough away to avoid
erosion. It is important to note that the lower
sodium increase occurs slightly below the sand
increase, suggesting that sodium is diffusing
downward from this conductive layer into the
finer texture sediments. The offset here is not
as pronounced as the offset found closer to the
wetland. This soil also has the most elevated
concentrations of sodium.
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Bocana 3Relationship between NaH2O and soil
texture
Sodium
Sand
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Finally, Bocana 3 is on a slight upland rise away
from the wetland. An increase of sand occurs
most prominantly at 35cm with sodium again
increasing below the sandy horizon, possibly due
to diffusion of salts from the hydraulically
conductive horizon (shallow aquifer). Although
the data is not shown here, the trends noted in
sodum distribution are also found in sulfate and
also in other cation concentrations (data not
shown here) indicating increasing soil
salinization. So what causes this pattern of
salt accumulation beneath the coarser textured
soils?
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Wet-Edge Effect
Steinwand and Richardson, 1989
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Most likely, the salt accumulations seen in the
Bocana soils are due to the wet-edge effect.
Flow reversal, when recharge changes to discharge
or vice versa, often occurs near wetland edges
such as the Bocana soils. After a rainfall
event, water shunted to the pond edge creates a
mounded water table. Because the water table is
already near the surface of the soil, the shunted
water table will actually rise above the level of
pond and creates a miniature drainage divide.
When this happens, the soil is leached. The
water table mound is removed by water losses, and
replaced soon after the rain by a depression in
the water table. Alternation between mound and
depression phases restricts water movement to the
wetland, and during the evaporative phase soils
accumulate sodium and sulfur salts. Palo Verde
experiences intense dry seasons during 6 months
of the year, and salts accumulate due to
evaporative draw up of the brackish ground water.
More porous sandy layers impede the upward
movement of the salt water through the soils and
the salts accumulate in the horizons just below.
In addition to this, because of the recent clay
layer at the surface of the horizons, salts are
no longer rapidly leached from the soils during
rainfall events as they once were, which
contributes to the salinization of these soils.
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Particle Size Distribution
Bocana 3
Bocana 2
Clay
Sandy loam
Silt
Silt loam
Silt
Bocana 1
X
Clay
Clay
Loam
Clay
Clay loam
Silt
Clay
Clay
Clay
Silt loam
Silt
Clay
Clay
X
X
Water
Salt
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In the Bocana soils, before the arrival of the
clay textured A horizons, the upper horizons of
each profile went from high porosity to low
porosity, which meant that water easily flowed
through the upper horizons via normal seasonal
infiltration. The addition of a clayey A horizon
changed the scenario, in a sense sealing off
water movement through the surface horizon. A
large influx of fresh water, with a clayey A
horizon that impedes rapid filtration of this
fresh water down into the brackish groundwater,
likely caused basin wide freshening of the water.
Because cattails germinate and thrive in less
saline conditions than many of the native species
adapted to brackish conditions, this sediment
deposition is likely part of the reason for the
cattail invasion of the marsh.
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Summary

• Is the surface water of the marsh freshened due
  to changes in sedimentation and hydrology?
• YES- deposition of clay sediments now impedes
  water flow through the soil causing basin wide
  freshening

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Summary

• Is the marsh experiencing passive salinization
  due to a wet- edge effect?
• YES- salts accumulate during the dry season in
  finer textured horizons.

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Conclusion

• Changes in vegetation are symptoms of much larger
  ecological changes
• Soil salinization
• Surface water freshening
• Restoration of the wetland involves many natural
  and social factors and will take a long time

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Thanks!
La Pura Vida!
Cynthia Stiles
University of Wisconsin-Madison
Drs. Sally Horn, Steve Driese, and Claudia
Mora University of Tennessee