A Quantitative Investigation and Inventory of the Soils and Vegetation of
Coastal Lowland Hawaiian Wetlands
Meris Bantilan-Smith
M.S. Thesis Proposal
Advisor: Greg Bruland
Department of Natural Resources and Environmental Management, UHM
Spring 2008
Abstract
Coastal lowland wetlands are important features in the landscape that provide numerous
functions for people and wildlife, including water quality improvement, flood
attenuation, wildlife habitat, and biological productivity. Due to the loss and degradation
of wetlands throughout the Hawaiian Islands, created (CWs) and restored (RWs) wetland
projects are becoming more common. A comprehensive, quantitative assessment of the
conditions of these wetlands has yet to occur, and it has not been resolved whether CWs
and RWs provide the same environmental and ecological benefits as natural wetlands
(NWs). In light of this deficiency this project assessed the current water quality and
habitat functions of CWs, RWs, and NWs in Hawai'i. Forty coastal wetlands sites on
each of the five major Hawaiian Islands (Kauai, Oahu, Molokai, Maui, and Hawaii) were
intensively sampled. The sampling consisted of collecting soil samples and assessing
percent cover of vegetation at each site. Both position along the hydrological gradient
and status (NW vs. RW vs. CW) significantly affected the soil characteristics and
vegetation richness of Hawaiian wetlands. Natural wetlands had greater soil organic
matter, water content, and lower soil bulk density than RWs and CWs.
Literature Review
Introduction
Wetlands are characterized by saturated and/or inundated soil conditions, which
are physically, chemically, and biologically distinct from adjacent upland soils, and the
ability to support hydrophytic (water tolerant) vegetation (NRC 1995; Mitsch and
Gosselink 2000). Wetlands are unique ecosystems characterized by hydrology, soils, and
vegetation. Wetlands are important features in the landscape that provide numerous
functions and values for people and wildlife, including water quality improvement, flood
attenuation, wildlife habitat, and provide a means of economic livelihood for millions of
people. For these reasons, the United States has a “no-net-loss” policy for wetlands.
Compensatory wetland mitigation plays a central role in this policy and has increased the
interest in and practice of wetland creation and restoration by state and federal wildlife
agencies (Brown and Batzer 2001).
Wetland mitigation is defined as the " restoration, creation, or enhancement of
wetlands to compensate for permitted wetland losses" (EPA 2007). When damage or the
destruction of a wetland is unavoidable, the wetland must be mitigated by the
replacement or enhancement of the wetland elsewhere. Currently numerous wetland
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restoration and creation projects are being undertaken across the state of Hawaii by an
array of governmental and non-governmental organizations such as the Department of
Transportation (DOT), Hawaii Department of Land and Natural Resources (DLNR),
Ducks Unlimited (DU), and private land owners. The ability of created (CWs) and
restored (RWs) wetlands to functionally replace natural wetlands (NWs) has become a
topic of considerable debate (Kentula 2000; Zedler and Calloway 1999).
Studies comparing CWs and RWs to NWS have examined differences in
vegetation communities (Heaven et al. 2003; Spieles 2005), soil physical and chemical
properties (Campbell 2002; Bruland and Richardson 2006; Craft 2001), hydrology
(Zampella 2003), and biological communities (Hannaford 1998). Generally mitigation
sties differed significantly in their basic physical and chemical soil properties and species
composition and richness when compared to natural wetlands (Stolt et al. 2000;
Galatowitsch and Vander Valk 1996).
No comprehensive investigation of wetlands has previously been conducted in the
state of Hawaii. In order to provide base-line data about coastal lowland wetlands, soils
and vegetation will be discussed in relation to the semi-natural “reference” wetlands and
constructed and restored wetlands in Hawaii.
Wetland Functions and Values
Wetlands are a diverse natural resource, which provide many useful functions and
values for society. Costanza et al. (1997) estimate that wetlands provide a total of
$14,785 (1994 US$) per hectare in annual ecosystem services. The Natural Resource
Council (1995) defines function as “processes and manifestations of processes that occur
within wetlands.” Most functions fall into three broad categories: biogeochemical,
hydrologic, and habitat and food web maintenance (NRC 1995; Brinson and Rheinhardt
1996). Some of these functions include maintenance of water quality such as promoting
denitrification, trapping sediment, and phosphorus sorption; serving as critical habitat for
many endemic waterfowl, macroinvertebrates, and algae; and minimizing flood surges by
storing storm water (EPA 2001).
The U.S. Fish and Wildlife Service, National Wetlands Inventory, estimates that
over 50% of U.S. wetlands have been lost since the 1780’s. The state of Hawaii has lost
approximately 7,000 acres which amounts to a 12-percent loss of wetlands statewide
(Dahl 1990). Kosaka (1990) analyzed wetlands below 1000 ft (305m) elevation and
estimated a 31-percent loss of coastal wetlands between 1780s and 1980s. According to
Dahl (1990) Hawaii currently has 22,474 acres of lowland wetlands and 36,328 acres of
upper/mid-elevation wetlands remaining.
Wetland Policy and Mitigation
Through most of the twentieth century wetlands have been viewed narrowly as
either wastelands or as areas providing little benefits beyond support of waterfowl
populations. This negative perception of wetlands was encouraged by policies and
incentives of the United States federal government which encouraged or subsidized the
conversion of wetlands to filled or drained lands used for agriculture. Due to these
policies total wetland acreage in the United States has been reduced to half of the original
total by the mid-1980s (NRC 1995; Kusler and Kentula 1990; Dahl 2006). In recent
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years the negative trend of wetland loss has shifted, and as of 2004 the contiguous United
States has experienced an annual net gain of wetland acres (Dahl 2006).
The shift in trends of wetland acreage in the U.S. is the result of changes in
nation-wide wetland public policy as well as increased scientific knowledge of wetland
values over the past 35 years. The origin of the shift can be traced to the Clean Water
Act of 1972, followed by Coastal Barrier Resource Act of 1972, the 1985 Food Security
Act, and the 1986 Tax Reform Act (Mitsch and Gosselink 2000). A more significant
initiative in developing U.S. wetland policy was undertaken in 1987, when a National
Wetland Policy Forum convened and set a significant goal for the nations remaining
wetlands, “no net loss.” The “no net loss” policy introduced wetland mitigation as the
leading tool in combating wetland loss, and is now the cornerstone of wetland
conservation in the U.S. (Zedler 1996; Mitsch and Gosselink 2000).
Reference Wetlands and Mitigation Success
Although the increased pace of wetland restoration and construction is
encouraging, a key question still remains, “do the created and restored wetland sites
provide the same ecological and environmental functions as the natural wetlands they
were created to replace?” The ability of CWs and RWs to functionally replace natural
wetlands has become a topic of considerable debate (Kentula 2000; Zedler and Calloway
1999). A key tool used to determine the functional equivalence or the level of success of
a wetland mitigation project is reference wetlands.
Reference wetlands are naturally occurring, relatively undisturbed wetlands, with
hydric soils, support some native vegetation, and has a water table near the surface during
the growing season (Kentula 2000; Brinson and Rheinhardt 1996). The reference
wetlands, which for the purpose of this study will be referred to as a natural wetland
(NW), are the long term ecosystem target for mitigation projects. Natural (reference)
wetlands serve as a standard for comparison when evaluating the success of mitigation
projects. Generally the ecological attributes of a similar or nearby wetland is most
commonly used. Single site studies and comparisons of pairs of sites have been used to
effectively evaluate the status of individual projects, however the results cannot be
extrapolated beyond the specific study sites. Kentula (2000) recommends comparing
samples of populations of NWs, RWs, and CWs within an area in order to capture an
entire range of conditions and factors important to the function of wetlands.
Many of the functions discussed in previous sections, such as traps for sediments,
sinks for non-point source pollution, and denitrification of ground water are difficult and
costly to directly measure. Thus the use of reference wetlands is based on the underlying
assumptions that natural wetlands represent high levels of functioning and that wetlands
sharing similar soil properties, hydrology characteristics, vegetation community, or
environmental conditions, function in a similar or equivalent manner (Brinson and
Rheinhardt 1996; Stolt et al. 2000; Zampella 2003).
Many different variables have been used when evaluating compensatory
mitigation wetlands, with measurements of vegetation being most common (Kentula
2000; Mitsch and Wilson 1996; Galatowitsch and van der Valk 1996), followed by soils
(Stolt et al. 2000; Craft 2001; Bruland and Richardson 2004), hydrology (Ashworth
1996), and macrofauna (Brown and Batzer 2001). In this particular study, a quantitative
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investigation and inventory of coastal lowland Hawaiian wetlands, I will be considering
soil properties and vegetation community composition of CWs, RWs, and NWs.
Wetland Soils
Current monitoring of mitigation wetlands nearly always includes some measure
of vegetation as a performance standard (Spieles 2005; Mitsch and Wilson 1996; Cole
2002), and does not require soil properties and processes to be monitored. This is of
great concern because wetland soils are the physical foundation for every wetland
ecosystem (Stolt et al. 2000).
Wetland soils, also referred to as hydric soils, are defined by the U.S. Department
of Agriculture’s Natural Resource and Conservation Service (NRCS 2007) as “soil that
formed under conditions of saturation, flooding or ponding long enough during the
growing season to develop anaerobic conditions in the upper part.” Both their physical
and chemical properties reflect the condition of the wetland environment, such that
proper hydrological function will result in characteristic pH (Bishel-Machung et al. 1996)
and soil organic matter content (Campbell et al. 2002). Wetland soils serve as a storage
reservoir of available chemicals and nutrients for most wetland plants and are also the
medium which many nutrient and chemical transformations take place (Mitsch and
Gosselink 2000). Furthermore, soils can greatly effect the distribution, abundance, and
composition of vegetation, as when extreme nutrient levels, salinities, or pH levels
prevent colonization or persistence of some species (Neckles et al. 2002).
The relationships between wetland soil properties and vegetation and hydrology
are dynamic, and thus by measuring soil characteristics on a wetland site and comparing
them to levels found in natural reference wetlands it can provide insights into the
hydrology and vegetation of a site.
Wetland Vegetation
Vegetation plays a major role in the identification and delineation of wetland
systems (Tiner 1999). Certain plant species and communities are characteristic of
wetlands. The Natural Resource Conservation Service (2007) has estimated that 7000
species of plants are reported growing in U.S. wetlands. The affinity of the plants to
grow in wetlands varies among plant species and regions. Based on differences in
expected frequency of occurrence in wetlands the species were designated to one of four
“wetland indicator categories.” These include obligate wetland species (OBL) that occur
in wetlands greater than 99% of the time. Facultative wetlands plants (FACW) occur in
wetlands 67-99% of the time. Facultative plants (FAC) occur in wetlands 34-66% of the
time and facultative upland plants (FACU) occur in wetlands 1-33% of the time. Upland
plants (UPL) occur in wetlands less than 1% of the time (Erickson and Puttock 2006).
Besides their use in wetland identification and delineation, plants can also be used
as indicators of other environmental conditions. Plants with low tolerances are simple
and reliable indicators of changes in environmental conditions, such as hydrology or
water chemistry (Tiner 1999). Herbaceous plants respond rapidly to both degradation
and improvement of wetland health and integrate disturbance from point-source pollutant
discharge to non-point source factors (Ervin et al. 2006). Research has shown that
vegetation composition and structure influence quantity and quality of plant foods
available, quantity and type of substrate available for invertebrates, and water quality
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(Balcombe et al. 2005). For these reasons, vegetation has been widely used to assess
compensatory mitigation wetlands.
Objectives and Hypotheses
Overall objective: To document the edaphic and vegetative attributes of created, restored,
and semi-natural coastal lowland Hawaiian wetlands.
Soils:
1) Document soil properties such as bulk density, soil moisture content, organic
matter content, pH, extractable P, and total N and C in created, restored, and
semi-natural wetlands.
2) To compare soil properties of created and restored wetlands to those of seminatural wetlands as well as compare differences in soil properties among sites on
different islands.
Alternative Hypothesis 1: Soil properties are different in created/ restored
wetlands than in semi-natural wetlands as well as among islands.
Null Hypothesis 1: Soil properties are the same in created/ restored wetlands
than in semi-natural wetlands as well as among islands.
3) To examine differences in soil properties across hydrologic (wetness) gradient in
created/ restored and semi-natural wetlands.
Alternative Hypothesis 2: There are differences in soil properties across the
hydrologic gradient.
Null Hypothesis 2: There are no differences in soil properties across the
hydrologic gradient.
4) Determine the correlation structure among the measured soil properties.
Vegetation:
1) Compare vegetative community composition of created, restored, and seminatural wetlands as well as the community composition among sites on different
islands.
Alternative Hypothesis 3: Vegetative community composition is different in
created/ restored wetlands and semi-natural wetlands as well as among
islands.
Null Hypothesis 3: Vegetative community composition is the same in created/
restored and semi-natural wetlands as well as among islands.
2) Examine differences in vegetation across a hydrologic (wetness) gradient in
created, restored, and semi-natural wetlands.
Alternative Hypothesis 4: There are differences in vegetative community
composition across the hydrologic gradient.
Null Hypothesis 4: The vegetation communities do not differ across the
hydrologic gradient.
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3) To examine differences in community composition across a salinity gradient in
created/ restored and semi-natural wetlands.
Alternative Hypothesis 5: There are differences in community composition
across the salinity gradient.
Null Hypothesis 5: There are no differences in community composition across
the salinity gradient.
4) To determine if certain vegetative species are indicators of created/restored or
semi-natural wetlands.
Alternative Hypothesis 6: Specific vegetative species are significant indicators
of wetland status (i.e. restored/created versus semi-natural wetlands).
Null Hypothesis 6: Specific vegetative species are not useful indicators of
wetland status.
Methods
Sampling Design
Forty coastal lowland wetland sites were sampled for soils and vegetation
between March and April 2007. The sites sampled met three criteria: 1) they spanned
across the major Hawaiian Islands (Hawaii, Kauai, Maui, Molokai, and Oahu), 2) were
located between 0 – 300 meters in elevation, and 3) were representative of natural,
restored, and created wetlands found within the state.
For each coastal lowland wetland site, sampling locations were established by
randomly selecting two transects which spanned the major hydrological gradient present.
Transects began at the wetter edge of the site and continued to the drier end of the site.
Three sample locations, corresponding to the wet, dry, and intermediate zones, along
each transect were sampled for soils and vegetation (Figure 1). Zones were identified
using visual cues such as, wetness, vegetation communities, and elevation changes. This
sampling design corresponds to a stratified random sample and follows a protocol
recommended by the Hawaii Wetland Field Guide (Erickson and Puttock 2006).
Vegetation Survey
Vegetation monitoring consisted of recording percent cover and species composition
from one 1-m2 quadrat within each zone along both transects. Using a standard data
sheet, in each quadrat along each transect, each plant occurring within that quadrat was
identified by genus and species. For each unique species within the quadrat, the
abundance of that species was determined using visual estimates of the percentage of the
quadrat occupied by that species. At least two investigators generated coverage values
independently and then compared values with one another. Voucher specimens were
collected when additional identification was necessary. Upon returning to UHM, the
voucher specimens were dried and pressed at 60oC for a minimum of 5 days then
delivered to Bishop Museum for further identification.
Soil Survey
Soil samples were taken in the center of each 1-m2 quadrat identified during the
vegetation survey. The GPS coordinates of each sampling location was recorded. Soil
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cores were collected from the upper 0 – 20 cm of the soil profile. Cores were collected
using a stainless steel auger with a circular plastic sleeve insert. The inner diameter of
the plastic sleeve is 4.8cm. The soil cores were oven dried at 105oC to a constant weight
to determine soil moisture content and bulk density. After oven-drying, the soil was
passed through a 2-mm mesh sieve. Representative 20 gram and 10 gram sub-samples
were used for texture and soil organic matter content analysis. Representative 30 gram
sub-samples were placed in small plastic vials and delivered to the ADSC for pH, total
carbon, nitrogen, and extractable phosphorus analysis.
Figure 1. Sampling Design
a. Riparian or tidal
sites
b. Isolated wetland sites
S1 S2 S3
Hydrologic
Gradient
Transect 2
S6
Drier
Intermediate
S4 S5
S2
Wet
Transect 1
Drier
Intermediate
S3
Wet
Stream / Tidal Creek
Transect 1
S4
S5
S6
S1
Pond
Hydrologic
Gradient
Transect 2
Laboratory Analysis
Soils were analyzed at for water content (WC), soil organic matter (SOM), and texture by
standard methods at the Soil and Water Conservation Lab at the University of Hawaii,
Manoa. Additional analyses conducted by the UHM Agricultural Diagnostic Service
Center include (ADSC) pH, total carbon (TCs), total nitrogen (TNs), and extractable
phosphorus (ExPs).
Soil Moisture Content (Bruland and Richardson 2004)
Soil Moisture content will be measured by drying a subsample from each core in an
aluminum tin at 105oC for a minimum of 24 hours or until a constant weight is reached.
The difference in mass of the sample before and after drying will be used at the percent
moisture.
Textural analysis, pipette (Tan 1996)
This method determines the approximate proportions of sand (2000 – 50μm), silt (50 –
2.0μm), and clay (<2.0μm) particles in a soil. Approximately 20 – 30 g of oven- dried
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soil (sieved to <2.0μm) is placed into a 500 mL Erlenmeyer flask with 100 mL of 0.5%
sodium metaphosphate solution. The sample is allowed to soak overnight (for a
minimum of 8 hours). The treated sample is transferred to a dispersing cup and mixed
for 2 minutes with an electric mixer. The entire suspension is then transferred to a
sedimentation cylinder and brought to a total volume of 1L with deionized water. The
suspension is left to equilibrate with room temperature and the temperature is recorded.
After the soil has been dispersed, the different particle size fractions are sorted out by
sedimentation. A pipette is used to withdraw an aliquot of the soil suspension at a
predetermined time and depth, then dried and weighed.
Soil Organic Matter, loss on ignition (Wilke 2005)
Soil organic matter content will be measured by igniting a subsample from each core.
Approximately 5 – 10 g of oven-dried soil (sieved to <2.0μm) is placed into a crucible
and ignited at 450oC for 2 – 4 hours until a constant mass is reached. The difference in
mass of the sample before and after ignition will be used as the percent soil organic
matter.
Data Analysis
The soils and vegetation data will be analyzed with traditional parametric
statistical tests such as Analysis of Variance (ANOVA), regression, and Pearson
correlations. For the ANOVA, we will test differences in mean values of individual water
quality parameters or soil properties across the different wetland types (created versus
restored versus natural; Hawaii versus Maui versus Molokai versus Kauai versus Oahu;
freshwater versus estuarine). The alpha level for significance for the ANOVA will be
0.05.
The regression analysis will involve testing relationships between water, soil,
vegetation, fish communities, and independent variables such as watershed size, and
watershed landuse characteristics. The alpha level for significance for the regression
analysis will be 0.05.
Data transformations (i.e. log base 10, square-root, natural log, etc.) may need to
be performed on the raw data in order to make it conform to the assumptions of
normality, equal variances, and linearity. In addition to ANOVA and regression
analyses, we will also analyze the data with multivariate statistical techniques such as
Principle Components Analysis (PCA) to determine which variables have the greatest
explanatory power in accounting for variability in the dataset.
Proposed Thesis Outline
Chapter 1: Introduction and Literature Review
Chapter 2: Wetland Soils
Chapter 3: Wetland Vegetation
Chapter 4: Summary and Conclusions
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