Using Soils to Understand Using Soils to Understand Ecosystem Change in Ecosystem Change in

Wetlands in Palo Verde Wetlands in Palo Verde National Park, Costa RicaNational Park, Costa Rica

CCourtney M. Gallaherourtney M. Gallaher

CynthiaCynthia A. Stiles A. StilesUniversity of Wisconsin-MadisonUniversity of Wisconsin-Madison

Palo Verde National ParkPalo Verde National Park

Dry tropical forests are presently one of the most Dry tropical forests are presently one of the most endangered ecosystems on earth, threatened by endangered ecosystems on earth, threatened by both expanding development and agriculture. both expanding development and agriculture. Palo Verde National Park in northwestern Costa Palo Verde National Park in northwestern Costa Rica is an outstanding example of a large Rica is an outstanding example of a large wetland within a dry tropical forest. It is located wetland within a dry tropical forest. It is located in the Guanacaste region along the lower reaches in the Guanacaste region along the lower reaches of the Tempisque River, the second largest river of the Tempisque River, the second largest river in Costa Rica. The climate for the area has a in Costa Rica. The climate for the area has a pronounced dry and wet season, with most rain pronounced dry and wet season, with most rain (180-220 cm) falling between the months of April (180-220 cm) falling between the months of April and October. The mean annual temperature is and October. The mean annual temperature is 28°C and varies little throughout the year. 28°C and varies little throughout the year.

Changes in the Palo Verde WetlandsChanges in the Palo Verde Wetlands-Aggressive cattail invasion--Aggressive cattail invasion-

19961996

20022002

The park provides diverse habitat for thousands The park provides diverse habitat for thousands of birds that roost and nest here as they migrate of birds that roost and nest here as they migrate between the continents. Birdwatching attracts between the continents. Birdwatching attracts many ecotourists and represents a vital source of many ecotourists and represents a vital source of revenue for this area. revenue for this area.

Recently the native vegetation in the open water Recently the native vegetation in the open water marshes that were once suitable to migratory marshes that were once suitable to migratory waterfowl has been replaced by dense stands of waterfowl has been replaced by dense stands of invasive cattails, possibly a hybrid between the invasive cattails, possibly a hybrid between the non-aggressive native species and European non-aggressive native species and European strains. These changes in vegetation have had strains. These changes in vegetation have had severe repercussions on the wildlife in the area severe repercussions on the wildlife in the area which may eventually impact the ecotourism which may eventually impact the ecotourism industry, which relies on the abundant bird industry, which relies on the abundant bird populations attracted to open water margins.populations attracted to open water margins.

Recent Changes in VegetationRecent Changes in Vegetation

Possible causes;Possible causes;– Changes in hydrology and Changes in hydrology and

sedimentation due to flooding during sedimentation due to flooding during Hurricane Mitch in 1998 Hurricane Mitch in 1998

– Irrigation run-off from nearby rice Irrigation run-off from nearby rice agricultureagriculture

What may be causing this recent drastic change in What may be causing this recent drastic change in the vegetation? A major problem in identifying the the vegetation? A major problem in identifying the cause of the cattail invasion is a general lack of cause of the cattail invasion is a general lack of knowledge of the soils of Palo Verde National Park. knowledge of the soils of Palo Verde National Park. Because soils form the matrix in which plants grow, Because soils form the matrix in which plants grow, a good understanding of soil properties is a good understanding of soil properties is necessary to understand the ecological processes necessary to understand the ecological processes that have led to the cattail invasion. A possible that have led to the cattail invasion. A possible recent cause may be flooding of the park by recent cause may be flooding of the park by Hurricane Mitch, which struck Central America in Hurricane Mitch, which struck Central America in 1998. This area is seasonally flooded by monsoons, 1998. This area is seasonally flooded by monsoons, but storms the size of Hurricane Mitch hit western but storms the size of Hurricane Mitch hit western Costa Rica approximately once every 100 years. Costa Rica approximately once every 100 years. Recent agrarian activities (forest clearance and field Recent agrarian activities (forest clearance and field expansion) may have contributed to enhanced expansion) may have contributed to enhanced sediment loads during this event.sediment loads during this event.

Hurricane Mitch FloodingHurricane Mitch Flooding

When Hurricane Mitch flooded the wetland area, it When Hurricane Mitch flooded the wetland area, it likely changed local hydrology and soil properties. likely changed local hydrology and soil properties. Specifically, the influx of a large volume of rainwater Specifically, the influx of a large volume of rainwater may have changed the water chemistry of the may have changed the water chemistry of the brackish wetlands and tipped the balance to a brackish wetlands and tipped the balance to a strongly freshened condition, more conducive to strongly freshened condition, more conducive to cattail growth. In addition, flooding may have cattail growth. In addition, flooding may have deposited fresh sediment that dramatically altering deposited fresh sediment that dramatically altering the soil matrix, releasing many water-soluble the soil matrix, releasing many water-soluble nutrients that would aid in the establishment of nutrients that would aid in the establishment of cattail colonies and explain the disappearance of the cattail colonies and explain the disappearance of the native vegetation. Finally, irrigated rice agriculture, a native vegetation. Finally, irrigated rice agriculture, a fairly recent component in agrarian component, may fairly recent component in agrarian component, may also be enhancing marsh eutrophication through the also be enhancing marsh eutrophication through the addition of soluble nutrients and possibly pesticides. addition of soluble nutrients and possibly pesticides.

Research QuestionsResearch Questions

• Is the marsh experiencing passive Is the marsh experiencing passive salinization due to a wet-edge effect? salinization due to a wet-edge effect?

• Is the surface water of the marsh Is the surface water of the marsh freshened due to changes in freshened due to changes in sedimentation and hydrology? sedimentation and hydrology?

In order to understand whether the observed In order to understand whether the observed changes in vegetation are symptoms of a larger changes in vegetation are symptoms of a larger ecosystem change, I asked two questions.ecosystem change, I asked two questions.

First, is the marsh experiencing passive salinization First, is the marsh experiencing passive salinization due to a wet-edge effect? In essence, are salts in the due to a wet-edge effect? In essence, are salts in the ground water being drawn up into the wetland soils, ground water being drawn up into the wetland soils, causing soil salinization? And second, is the surface causing soil salinization? And second, is the surface water of the marsh freshened due to changes in water of the marsh freshened due to changes in sedimentation and hydrology? As previously sedimentation and hydrology? As previously discussed, flooding may have added sediment layers discussed, flooding may have added sediment layers to the wetland soils changing local hydrological to the wetland soils changing local hydrological patterns.patterns.

Palo Verde National ParkPalo Verde National Park

This map shows the location of Palo Verde National This map shows the location of Palo Verde National Park. The Tempisque River is an estuary along the Park. The Tempisque River is an estuary along the lower margins of the park and, along the extent of lower margins of the park and, along the extent of the park, reverses its flow with the tide. the park, reverses its flow with the tide.

Initial fieldwork for the study was conducted by a Initial fieldwork for the study was conducted by a team of researchers from the University of team of researchers from the University of Tennessee, including Cynthia Stiles who is presently Tennessee, including Cynthia Stiles who is presently at the University of Wisconsin. at the University of Wisconsin.

Site LocationsSite LocationsChamorro and Laguna BocanaChamorro and Laguna Bocana

Fieldwork included soil profile descriptions at two Fieldwork included soil profile descriptions at two sites within the park, the Chamorro floodplains and sites within the park, the Chamorro floodplains and the Laguna Bocana wetlands. Soil samples were the Laguna Bocana wetlands. Soil samples were collected from these soil profiles. The Chamorro site collected from these soil profiles. The Chamorro site is close to the river itself (within the reversing flow is close to the river itself (within the reversing flow regime) and represents a normal floodplain regime) and represents a normal floodplain sequence. Three profiles were excavated near the sequence. Three profiles were excavated near the essentially closed basin wetland in Bocana. Note the essentially closed basin wetland in Bocana. Note the network of irrigation canals outside the limits of the network of irrigation canals outside the limits of the park and how the surface hydrologic pattern park and how the surface hydrologic pattern suggests that excess run-off from irrigation is suggests that excess run-off from irrigation is directed down into the restricted wetland system. directed down into the restricted wetland system. Bocana is in essence a “sump” for the drainage Bocana is in essence a “sump” for the drainage water from the upstream irrigation, which introduces water from the upstream irrigation, which introduces excessive water to the system during the driest part excessive water to the system during the driest part of the year. Remember that these are soils originally of the year. Remember that these are soils originally formed under monsoonal climate cycles.formed under monsoonal climate cycles.

Chamorro alluvial plain VertisolChamorro alluvial plain Vertisol

A

Bss1

Bss2

Bss3

Bsskg

Bssg

The baseline soil for the study, from near the The baseline soil for the study, from near the Tempisque River, is a Vertisol. This is a very clay-Tempisque River, is a Vertisol. This is a very clay-rich soil formed from overbank deposition from the rich soil formed from overbank deposition from the Tempisque. The floodplain on which this soil is Tempisque. The floodplain on which this soil is found extends for a considerable distance along the found extends for a considerable distance along the Tempisque and ranges from 2-6 km wide. The soil Tempisque and ranges from 2-6 km wide. The soil has slickensided aggregate structure, most strongly has slickensided aggregate structure, most strongly expressed at 0.5 – 1.0 m depth (Bss horizons). There expressed at 0.5 – 1.0 m depth (Bss horizons). There is also some accumulation of pedogenic carbonate is also some accumulation of pedogenic carbonate below 1.5 m (Bssk horizons). Embedded within the below 1.5 m (Bssk horizons). Embedded within the profile are occasional layers of erosional pedorelicts profile are occasional layers of erosional pedorelicts that may have been carried down from past large that may have been carried down from past large volume flood events.volume flood events.

Laguna BocLaguna Bocáána wetland marginna wetland margin

A

Bw

2Byk

2Byg

3Bwb

Other profiles excavated closer to the edge of the Other profiles excavated closer to the edge of the Bocana marsh had “lithologic discontinuities” which Bocana marsh had “lithologic discontinuities” which represent deposition from different storm events. represent deposition from different storm events. The textures vary irregularly, as will be shown in the The textures vary irregularly, as will be shown in the particle size distribution figures. The horizon particle size distribution figures. The horizon designation “By” indicates that there is at least 5% designation “By” indicates that there is at least 5% gypsum (by volume) present in this profile and the gypsum (by volume) present in this profile and the designation 3Bwb indicates a buried soil subsurface designation 3Bwb indicates a buried soil subsurface horizon from an earlier sequum that contains relict horizon from an earlier sequum that contains relict mangrove roots. mangrove roots.

Methods of AnalysisMethods of Analysis

• Particle sizeParticle size – Soil textureSoil texture– Sediment depositionSediment deposition– HydrologyHydrology

• Water soluble elementsWater soluble elements– Nutrients released during floodingNutrients released during flooding– Salt accumulationsSalt accumulations

To help address the hypotheses, I analyzed the soil To help address the hypotheses, I analyzed the soil samples taken from the Palo Verde profiles for samples taken from the Palo Verde profiles for particle size distribution and nutrient contents.particle size distribution and nutrient contents.

Particle size distributions allow you to draw Particle size distributions allow you to draw conclusions about soil texture, sediment deposition conclusions about soil texture, sediment deposition in the basin as well as local hydrology, and I in the basin as well as local hydrology, and I measured this using the pipette method. measured this using the pipette method.

Water soluble elements characterize nutrients Water soluble elements characterize nutrients released during a flood event such as Hurricane released during a flood event such as Hurricane Mitch, as well as soluble salts accumulated in the Mitch, as well as soluble salts accumulated in the soil horizons. To measure this, I used a 1:10 pore soil horizons. To measure this, I used a 1:10 pore volume to water solutionvolume to water solution

Bocana 3

Chamorro Bocana 1Bocana 2

Clay

Sandy loam

Silt

Silt loam

Silt

Clay

Clay

Loam

Clay

Clay loam

Silt

Clay

Clay

Clay

Silt loam

Silt

Clay

Clay

Clay

Clay

Clay

Clay

Clay

Clay

Clay

Clay

Particle Size DistributionParticle Size Distribution

2 m

This is a simplistic diagram outlining the toposequence along This is a simplistic diagram outlining the toposequence along which the soil samples were collected, and the textures of each which the soil samples were collected, and the textures of each horizon within the soil profiles. The Chamorro soils are horizon within the soil profiles. The Chamorro soils are classified as Vertisols because they had high clay content. classified as Vertisols because they had high clay content. The Bocana soil textures on the other hand, contain far more The Bocana soil textures on the other hand, contain far more sand and silt than Chamorro, indicating their more active role in sand and silt than Chamorro, indicating their more active role in intercepting sediment during erosional events. Yet, all three intercepting sediment during erosional events. Yet, all three Bocana soils have clayey A horizons. This strongly suggests Bocana soils have clayey A horizons. This strongly suggests deposition of clays due to erosion of upland soils during the deposition of clays due to erosion of upland soils during the monsoons. The large floods of Hurricane Mitch probably monsoons. The large floods of Hurricane Mitch probably carried large quantities of clays from the uplands down into the carried large quantities of clays from the uplands down into the Bocana wetland basin. Bocana wetland basin.

This simple change in particle size within the sediments would This simple change in particle size within the sediments would have a profound effect on the hydrology of the wetland system. have a profound effect on the hydrology of the wetland system. Clays have a restrictive influence on infiltration and tend to act Clays have a restrictive influence on infiltration and tend to act as localized aquitards. There is a shallow aquifer within the as localized aquitards. There is a shallow aquifer within the soils (1-1.5 m deep) that carries dissolved salts from the soils (1-1.5 m deep) that carries dissolved salts from the irrigated fields. Clay caps seal this from meteoric interaction.irrigated fields. Clay caps seal this from meteoric interaction.

ChamorroChamorroRelationship between NaRelationship between NaH2OH2O and soil texture and soil texture

SandSandSodiumSodium

One of the most striking results of the water soluble nutrient One of the most striking results of the water soluble nutrient analysis was the high concentrations of salts in many of the analysis was the high concentrations of salts in many of the Bocana soil horizons. To better understand why the salt has Bocana soil horizons. To better understand why the salt has concentrated in the Bocona soils, on the graph I have plotted concentrated in the Bocona soils, on the graph I have plotted both the percent sand and the ppm of sodium against the soil both the percent sand and the ppm of sodium against the soil depth. This allows us to see any significant textural variations depth. This allows us to see any significant textural variations that would influence water flow as well as salt accumulations in that would influence water flow as well as salt accumulations in the horizons. the horizons.

The Chamorro soils are considered the control for this The Chamorro soils are considered the control for this investigation and show little response of sodium to the textural investigation and show little response of sodium to the textural increase in sands at about 100 cm depth. These soils are increase in sands at about 100 cm depth. These soils are largely clay-dominated and sodium accumulation is more an largely clay-dominated and sodium accumulation is more an effect of the proximity of the lowest B horizon to the brackish effect of the proximity of the lowest B horizon to the brackish water from the Tempisque estuarine system. The next three water from the Tempisque estuarine system. The next three slides graph sand and salt vs. depth for the three Bocana soils. slides graph sand and salt vs. depth for the three Bocana soils.

Bocana 1Bocana 1Relationship between NaRelationship between NaH2OH2O and soil texture and soil texture

SodiumSodiumSandSand

In contrast to the Chamorro soils you can see a In contrast to the Chamorro soils you can see a strong correlation between the soil texture and the strong correlation between the soil texture and the salt concentration in the Bocana soils. Also note that salt concentration in the Bocana soils. Also note that sand and sodium axes are an expanded range from sand and sodium axes are an expanded range from the previous Chamorro slide. Bocana 1 is located the previous Chamorro slide. Bocana 1 is located closest to the wetland. Sand percentages, indicated closest to the wetland. Sand percentages, indicated by the blue line, are highest at the top and bottom of by the blue line, are highest at the top and bottom of the soil profile as are the salt concentrations. the soil profile as are the salt concentrations. However, salt is concentrated in the finer textured However, salt is concentrated in the finer textured horizons just below the coarser sandy horizonhorizons just below the coarser sandy horizon

Bocana 2Bocana 2Relationship between NaRelationship between NaH2OH2O and soil texture and soil texture

SodiumSodiumSandSand

Bocana 2 is located between the wetland and the Bocana 2 is located between the wetland and the upland area. This profile shows two pronounced upland area. This profile shows two pronounced sand lenses at 40 cm and 80 cm with roughly sand lenses at 40 cm and 80 cm with roughly corespondent increases in sodium. This bimodal corespondent increases in sodium. This bimodal distribution is due to the geomorphically active distribution is due to the geomorphically active nature of this position – it is close enough to the nature of this position – it is close enough to the wetland to experience overbank deposition during wetland to experience overbank deposition during flooding, yet far enough away to avoid erosion. It is flooding, yet far enough away to avoid erosion. It is important to note that the lower sodium increase important to note that the lower sodium increase occurs slightly below the sand increase, suggesting occurs slightly below the sand increase, suggesting that sodium is diffusing downward from this that sodium is diffusing downward from this conductive layer into the finer texture sediments. conductive layer into the finer texture sediments. The offset here is not as pronounced as the offset The offset here is not as pronounced as the offset found closer to the wetland. This soil also has the found closer to the wetland. This soil also has the most elevated concentrations of sodium.most elevated concentrations of sodium.

Bocana 3Bocana 3Relationship between NaRelationship between NaH2OH2O and soil texture and soil texture

SodiumSodiumSandSand

Finally, Bocana 3 is on a slight upland rise away Finally, Bocana 3 is on a slight upland rise away from the wetland. An increase of sand occurs most from the wetland. An increase of sand occurs most prominantly at 35cm with sodium again increasing prominantly at 35cm with sodium again increasing below the sandy horizon, possibly due to diffusion of below the sandy horizon, possibly due to diffusion of salts from the hydraulically conductive horizon salts from the hydraulically conductive horizon (shallow aquifer). Although the data is not shown (shallow aquifer). Although the data is not shown here, the trends noted in sodum distribution are also here, the trends noted in sodum distribution are also found in sulfate and also in other cation found in sulfate and also in other cation concentrations (data not shown here) indicating concentrations (data not shown here) indicating increasing soil salinization. So what causes this increasing soil salinization. So what causes this pattern of salt accumulation beneath the coarser pattern of salt accumulation beneath the coarser textured soils? textured soils?

Wet-Edge EffectWet-Edge Effect

Steinwand and Richardson, 1989

Most likely, the salt accumulations seen in the Bocana soils are due to Most likely, the salt accumulations seen in the Bocana soils are due to the wet-edge effect. Flow reversal, when recharge changes to the wet-edge effect. Flow reversal, when recharge changes to discharge or vice versa, often occurs near wetland edges such as the discharge or vice versa, often occurs near wetland edges such as the Bocana soils. After a rainfall event, water shunted to the pond edge Bocana soils. After a rainfall event, water shunted to the pond edge creates a mounded water table. Because the water table is already creates a mounded water table. Because the water table is already near the surface of the soil, the shunted water table will actually rise 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 above the level of pond and creates a miniature drainage divide. When this happens, the soil is leached. The water table mound is removed 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 by water losses, and replaced soon after the rain by a depression in the water table. the water table.

Alternation between mound and depression phases restricts water Alternation between mound and depression phases restricts water movement to the wetland, and during the evaporative phase soils movement to the wetland, and during the evaporative phase soils accumulate sodium and sulfur salts. Palo Verde experiences intense accumulate sodium and sulfur salts. Palo Verde experiences intense dry seasons during 6 months of the year, and salts accumulate due to dry seasons during 6 months of the year, and salts accumulate due to evaporative draw up of the brackish ground water. evaporative draw up of the brackish ground water.

More porous sandy layers impede the upward movement of the salt More porous sandy layers impede the upward movement of the salt water through the soils and the salts accumulate in the horizons just 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 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 surface of the horizons, salts are no longer rapidly leached from the soils during rainfall events as they once were, which contributes to the soils during rainfall events as they once were, which contributes to the salinization of these soils. salinization of these soils.

Bocana 3Bocana 3

Bocana 1Bocana 1Bocana 2Bocana 2

Clay

Sandy Sandy loamloam

SiltSilt

Silt loam

Silt

Clay

Clay

LoamLoam

ClayClay

Clay loam

Silt

Clay

Clay

Clay

Silt Silt loamloam

Silt Silt

Clay

Clay

Particle Size DistributionParticle Size Distribution

XX

X

WaterWaterSaltSalt

In the Bocana soils, before the arrival of the clay In the Bocana soils, before the arrival of the clay textured A horizons, the upper horizons of each textured A horizons, the upper horizons of each profile went from high porosity to low porosity, profile went from high porosity to low porosity, which meant that water easily flowed through the which meant that water easily flowed through the upper horizons via normal seasonal infiltration. The upper horizons via normal seasonal infiltration. The addition of a clayey A horizon changed the scenario, addition of a clayey A horizon changed the scenario, in a sense sealing off water movement through the in a sense sealing off water movement through the surface horizon. A large influx of fresh water, with a surface horizon. A large influx of fresh water, with a clayey A horizon that impedes rapid filtration of this clayey A horizon that impedes rapid filtration of this fresh water down into the brackish groundwater, fresh water down into the brackish groundwater, likely caused basin wide freshening of the water. likely caused basin wide freshening of the water. Because cattails germinate and thrive in less saline Because cattails germinate and thrive in less saline conditions than many of the native species adapted conditions than many of the native species adapted to brackish conditions, this sediment deposition is to brackish conditions, this sediment deposition is likely part of the reason for the cattail invasion of the likely part of the reason for the cattail invasion of the marsh.marsh.

SummarySummary

• Is the surface water of the marsh freshened Is the surface water of the marsh freshened due to changes in sedimentation and due to changes in sedimentation and hydrology? hydrology?

• YES- deposition of clay sediments now YES- deposition of clay sediments now impedes water flow through the soil impedes water flow through the soil causing basin wide fresheningcausing basin wide freshening

SummarySummary

• Is the marsh experiencing passive Is the marsh experiencing passive salinization due to a wet- edge effect? salinization due to a wet- edge effect?

• YES- salts accumulate during the dry YES- salts accumulate during the dry season in finer textured horizons. season in finer textured horizons.

ConclusionConclusion

• Changes Changes in vegetation in vegetation are are symptoms of symptoms of much larger ecological changesmuch larger ecological changes– Soil salinizationSoil salinization– Surface water fresheningSurface water freshening

• Restoration of the wetland involves Restoration of the wetland involves many natural and social factors and will many natural and social factors and will take a long timetake a long time

Thanks!Thanks!

CCynthia ynthia StilesStilesUniversity of University of

Wisconsin-MadisonWisconsin-Madison

Drs. Sally Horn, Steve Drs. Sally Horn, Steve Driese, and Claudia MoraDriese, and Claudia Mora

University of TennesseeUniversity of Tennessee