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8/13/2019 Active versus passive restoration on restored tidal hummocks: does planting make a difference?
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Applied Ecological Restoration Page 1
Humboldt State University 2013
Active Versus Passive Revegetation on Recently
Restored Tidal Marsh Hummocks: Does Planting
Make a Difference?
By: Corinne Kennah, Katie White, Zachary Silber-Coats
Abstract
The Wood Creek property in Humboldt County, California is the site of a recent tidal marsh
restoration project. The property is owned by the Northcoast Regional Land Trust (NRLT), and
in 2010 the land trust conducted a restoration project that reestablished historic tidal influence to
the Wood Creek property, which had been used as pastureland for decades. This study compared
and analyzed the effectiveness of two revegetation techniques on eight constructed tidal marsh
hummocks, six of which were actively seeded and planted with native species while the
remaining two were left as-to to allow vegetation to passively establish over time. We found that
three years after the restoration there was no significant difference in total colonization by exotic
vegetation on actively seeded and planted hummocks compared to hummocks that were allowed
to passively revegetate over time. The dominant species on both hummock types was the non-
native pasture grass, Agrostis stolonifera. Planted and seeded native varieties did not flourish.
Some success of active revegetation efforts were seen by the decreased presence of A. stolonifera
on actively revegetated hummocks in relation to passively revegetated hummocks, and also by a
greater abundance of Deschampsia cespitosa subs. cespitosa (an actively planted and seeded
native species) on actively revegetated versus passively revegetated hummocks. However,
although these differences were visible in the field as well as in our data, analysis did not find
that these differences were significant.
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Introduction
According to the U.S. Fish and Wildlife Service, wetlands can be characterized by a water
regime intermediate between those of terrestrial and aquatic systems (Zedler and Kercher, 2005).
Estuarine systems are characterized as aquatic systems located at the interface between
freshwater and oceanic environments (Schlosser and Eicher, 2012). Brackish and salt marsh
habitats, as estuarine wetlands, thus form valuable edge habitats with respect to both salinity and
hydric influence.
Coastal wetland habitats are crucial to global biodiversity (Zedler and Kercher, 2005). Important
ecological services of coastal wetlands include water purification via nutrient uptake,
amelioration of the effects of severe storms and floods, and groundwater recharge (Zedler and
Kercher, 2005). Prominent biodiversity services provided by coastal wetlands include habitat for
juvenile fish and invertebrates of economic importance, alongside the crucial role that these
habitats play in both breeding and wintering habitat for migratory waterfowl (Zedler and
Kercher, 2005).
It is estimated that approximately half of all wetlands present worldwide have been lost to
competing land uses since the 1800’s, and coastal wetland loss may be greater (Zedler and
Kercher, 2005). The current extent of salt marsh ecosystems in North America is only a small
fragment of what occurred historically. Loss and degradation has occurred since European
settlement from land use changes and disturbances such as farming, grazing, dredging, and
altering of natural hydrologic regimes and sediment supply (Gedan et al ., 2009). It has been
conservatively estimated that 50% of all historic salt marshes in the United States have already
been lost (Kennish, 2001). Laws enacted in recent decades by federal and state governments
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have helped curtail large scale impacts to the nation's wetlands through direct causes. However,
non-direct impacts from exotic species invasion, eutrophication and climate change related sea
level rise are now beginning to be seen as major sources of salt marsh loss and degradation
(Gedan et al ., 2009).
In Humboldt Bay, salt marsh loss has been particularly dramatic. Since the beginning of
European settlement in the area in the 1850’s, over 90% of Humboldt Bay’s salt marsh has been
diked, drained, and converted to pasture land or developed (Pickart, 2001). Prior to European
settlement, Humboldt Bay had 9,000 acres of salt marsh habitat (Pickart, 2001). Following
settlement, the railroad, and extensive agricultural land conversions, Humboldt Bay now has less
than 900 acres of salt marsh remaining (Pickart, 2001). Salt marsh in Humboldt Bay has not only
faced direct destruction due to lack of tidal influence, but is also threatened by a highly invasive
species. Dense-flowered cordgrass (Spartina densiflora) arrived in Humboldt Bay in the late
1800’s, most likely from ship ballast carried from Chile (USFWS, 2013). As of 2001, S.
densiflora occupied 94% of the 900 acres of remaining Humboldt Bay salt marsh (Pickart, 2001).
It was first identified as an invasive species in 1984 (USFWS, 2013). Invasive species such as S.
densiflora drastically outcompete native vegetation and therefore limit salt marsh plant diversity
(USFWS, 2013). This is of particular concern in the high salt marsh habitats of Humboldt
County, which support over 20 native plant species (USFWS, 2013).The high level of salt marsh
degradation around Humboldt Bay highlights the need for restoration. Restoration projects in the
area tend to focus either on converting agricultural land back to salt marsh, or on removing
invasive species from existing salt marsh.
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Post-project monitoring is an important aspect of every ecological restoration project.
Monitoring allows for evaluation of the project in order to assess impacts and the degree to
which restoration goals have been met. Post-project monitoring can also help identify areas that
require further treatment or a different management approach (Bash and Ryan, 2002; Thom,
2000). Post-project monitoring also has overarching importance in that the reporting of results
can help drive the success of future restoration projects and restoration knowledge (McTigue et
al ., 2005) Aquatic restoration projects are especially susceptible to failure (Thom, 2000). For this
reason, monitoring of projects in these ecosystems is of utmost importance. Unfortunately,
monitoring efforts on restoration projects are often constrained by a lack of funding. However,
large sums of money are now being spent on restoration activities. Between 2000 and 2007, over
$100 million dollars was allocated to restoration efforts in Humboldt County (Baker and Quinn-
Davidson, 2011). With so much being spent on restoration annually, it is important to increase
the success of projects so that restoration efforts are not done in vain.
The objectives of this study were to examine vegetation on tidal hummocks in the Wood Creek
Tidal Marsh Enhancement Project. The main objective was to compare current vegetation on
hummocks that were actively revegetated versus vegetation that passively colonized on
hummocks that were not actively revegetated.
Methods
Site Description and CharacteristicsThe Wood Creek Tidal Enhancement project site is located north of Myrtle Avenue, northeast of
Eureka, California (Figure 1). The project is located on over 35 acres on the western portion of
the Wood Creek property (McBain and Trush, 2007).
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Figure 1. Map of the Wood Creek property, showing the project site and constructed sloughchannels.
In 2005, the Northcoast Regional Land Trust (NRLT) acquired 54 acres of former tidal salt
marsh (NRLT, 2012). In the late 1800s or early 1900s, the land was diked by European settlers in
order to turn the salt marsh into viable agricultural land for growing crops and grazing livestock
(Schlosser and Eicher, 2012). The land has been farmed in the past, and in more recent decades
has been a cattle pasture (O’Dowd A; personal communication, 2013). Prior to site purchase in
2005 by the NRLT, the site was owned by Freshwater Farms. It was used to grow many different
species of native trees to provide propagules for sale at Freshwater Farms Nursery, a local
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restoration nursery that also provided thousands of propagules for the brackish marsh
enhancement.
The climate of the site is transitional between cool-summer Mediterranean and moist maritime
zones (McBain and Trush, 2007). Temperature variation is remarkably small; summer maximum
temperatures rarely exceed 70°F, and winter minima rarely drop below 30˚F (NWSFO-NOAA,
2013). Rainfall is approximately 40 inches per year, concentrated between October and April
(NWSFO-NOAA, 2013).
Soil at the project site is a silty clay loam of the Occidental series (USDA-NRCS, 2013). Depth
to the aquifer is commonly one meter or less, and surface water is frequently present (USDA-
NCCS, 2013). Current site elevation ranges between approximately 6.0 to 7.5 feet above mean
lower low water (MLLW), placing the site towards the upper limit of tidal influence (McBain
and Trush, 2007).
Overview of Study Methods
For this study we assessed post-restoration vegetation on four tidal hummocks. Two of the
hummocks were actively revegetated after restoration, while the other two hummocks were not
revegetated, and were instead left as controls to observe the passive colonization of vegetation
naturally over time. Data for actively revegetated hummocks was then compared with data from
passively revegetated hummocks to determine whether initial planting methods had a significant
influence on hummock vegetation. This study was conducted three years after final restoration
actions were implemented, leaving a substantial amount of time for hummock vegetation to
develop.
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Restoration Project Background
The Wood Creek Estuary Revegetation Project was a subset of a larger project, the Wood Creek
Tidal Marsh Enhancement Project. According to the Wood Creek Tidal Marsh Design Report for
the Wood Creek Tidal Marsh Enhancement Project (Anderson and Associates, 2008), the goals
of this project were to reestablish historic tidal hydrology to the site by opening the tide gate
located at the confluence of Wood Creek and Freshwater Slough and reestablishing a network of
slough channels (Figure 2). Through these actions, the project area was subject to tidal influence
for the first time in almost a century. Improvement of winter rearing habitat for salmonids, such
as Coho salmon (Oncorhynchus kisutch) and steelhead trout (Oncorhynchus mykiss), was also a
central goal of the restoration project (NRLT, 2012). Winter juvenile rearing habitat was
improved by increasing the cover of native wetland plant species, and increasing access to areas
of refuge from high velocity flow events by establishing connections to floodplains, side
channels, and off-channel ponds. Off-channel ponds were also created with the objective of
increasing and improving habitat for the federally endangered tidewater goby ( Eucyclogobius
newberryi) that has been seen on the site (NRLT, 2012). Heavy equipment was used to excavate
slough channels, create off channel ponds, remove a 300 foot berm, and build eight tidal
hummocks in the wetland area using sediment excavated throughout the project site, reducing the
exepenses associated with trucking sediments off-site (NRLT, 2012). Six of the eight hummocks
were actively revegetated with seed and plugs grown at Freshwater Farms Nursery, while the
remaining two hummocks were left as-is to allow for the monitoring of passively colonized
vegetation over time.
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Figure 2. Map showing restoration design elements of the Wood Creek Tidal MarshEnhancement Project, the larger project of which the Wood Creek Estuary Revegetation Projectwas a subset to (Image source: Anderson and Associates, 2008).
Actively revegetated hummocks were seeded and planted with a few different native brackish
habitat plant species (NRLT, 2012). The NRLT planted 37,000 plants on the actively revegetated
hummocks, including Deschampsia caespitosa (tufted hairgrass), Carex lyngbyei (Lyngbye’s
sedge), silverweed Potentilla anserina (silverweed), Distichlis spicata (saltgrass), Juncus effusus
(softstem rush), and Scirpus microcarpus (small-fruited bulrush) (NRLT, 2012). It was difficult
to find post-project information on the specific modes of planting ultimately used, but a
biological assessment completed in 2007 reported the NRLT’s original plans (McBain and
Trush, 2007). According to the report, the NRLT intended to plant seeds and nursery stock of
Deschampsia cespitosa ssp. cespitosa (tufted hairgrass), Carex lyngbyei (Lyngbye sedge), and
nursery stock of Potentilla anserina (silverweed), Distichlis spicata (saltgrass), and Juncus
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lescurii (salt rush; formerly J. lesueurii) (McBain and Trush, 2007). It is unclear whether or not
these species were ultimately planted in the modes specified by the 2007 assessment, but this
information may indicate the approximate forms of outplanting propagules.
Field Methods
We conducted line transects to compare vegetation on actively vegetated versus passively
vegetated hummocks within the restoration site. To do this, we recorded vegetation data on two
passively vegetated hummocks and two actively vegetated hummocks, as indicated in Figure 3.
On each hummock, we conducted at least two transects, the combined length of which totaled
200 feet per hummock (Figure 4). The number of transects and individual transect lengths on
each hummock was determined by the size and shape of each hummock. The transect locations
were determined by a stratified sampling method (Neyman, 1934). Once the first transect on a
hummock was placed, we then placed all following transects parallel to the first at a distance of
20 feet apart. The location of the first transect on each hummock was determined by first
defining the border of the hummock and then measuring 30 feet from the northwestern edge of
the hummock, to eliminate edge effects. Dominant and subdominant vegetation were identified
directly under the transect tape until we reached a distinct change in dominant vegetation. Each
change in dominant vegetation was considered a separate sampling unit within each transect. We
then calculated percent cover of dominant and subdominant vegetation on each hummock by
comparing the total length occupied by each species from each transect to the total length of all
the transects (200 feet). We then used these data to analyze the success of native vegetation
versus nonnative or invasive vegetation on the two different types of hummocks. To analyze our
data, we ran t-tests of log-normalized percent cover by species and made bar graphs to compare
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the percent cover of each dominant or subdominant species between actively revegetated and
passively revegetated hummocks.
Figure 3. Map showing the tidal hummocks that were constructed within the project area,distinguishing between actively revegetated and passively revegetated hummocks. The map alsoshows the four hummocks that were surveyed for this study (Image source: NRLT, 2009).
Figure 4. Map showing approximate locations of transects conducted on each hummocksurveyed (Image source: NRLT, 2009).
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Results
Surveyed transects on hummocks included both native and non-native plant species. Plant
species found, and their status as native or non-native, are listed below (Table 1).
Table 1. Plant species present on transects surveyed.Latin name Common name Native status
Agrostis stolonifera Creeping bentgrass Non-native
Holcus lanatus Velvet grass Non-native
Lotus corniculatus Birdsfoot trefoil Non-native
Elymus repens Quackgrass Non-native
Rumex sp. Dock Non-native
Agrostis sp. Bentgrass Non-native
Hordeum brachyantherum Native barley Native
Atriplex sp. Salt bush Native
Juncus lesueurii Salt rush Native
Carex lyngbyei Lynbye Sedge Native
Aster chilensis Chilean Aster Native
Triglochin maritima Arrow-grass Native
Deschampsia cespitosa ssp. cespitosa Tufted hairgrass Native
Non-native Agrostis stolonifera (creeping bentgrass), an exotic rangeland grass, was the most
common plant species observed on both actively revegetated and passively-revegetated transects,
forming approximately 40% and 80% of the total transect distances, respectively. Native
Deschampsia cespitosa subsp. cespitosa (tufted hairgrass) was the second most common species,
forming over 15% of surveyed vegetation on passively revegetated hummocks, and
approximately 30% on actively revegetated hummocks. Non-native Holcus lanatus (velvet grass)
was a substantial component of actively revegetated hummocks, but nearly absent from passively
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revegetated hummocks. All other species occurred sporadically and as small percentages of the
total (Figure 5). Agrostis stolonifera was prominent as a dominant component of both actively
and passively revegetated hummocks. However, A. stolonifera occupied nearly twice as much of
the passively revegetated hummocks, as compared to its presence in actively-revegetated
hummocks. Conversely, Holcus lanatus, also an invasive grass, was dominant on over a quarter
of the linear feet surveyed in actively revegetated hummocks, but all but absent on passively
revegetated hummocks.
Figure 5. Dominant nonnative (exotic) and native plant species by percentage of total transectlength, in actively and passively restored hummocks.
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Agrostis stolonifera was also prominent as a subdominant component of vegetation on both
hummock types (Figure 6). In general, subdominant vegetation along transects was more broadly
divided among species than was the dominant vegetation, in the case of both actively and
passively revegetated hummocks.
Figure 6. Subdominant exotic and native plant species by percentage of total transect length, inactively and passively restored hummocks.We conducted paired t-tests of log-normalized vegetation percentages for the three most
common plant species to evaluate the null hypothesis that mean measured length of dominant
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plant species did not differ between actively and passively revegetated hummocks. Results of
this test were insignificant, suggesting that transects on active and passive hummocks did not
differ conclusively in the three most common dominant plant species (t = 0.9319, df = 2.019, p =
0.449). Results of a similar t-test on subdominant vegetation were similarly insignificant
(t=1.1626, df = 2.869, p = 0.3325).
We then calculated a Spearman’s rank-correlation coefficient (rho) to determine the extent to
which percentages of plant species on passively revegetated hummocks correlate with species
percentages on actively-revegetated hummocks. Spearman coefficients for exotic and native
species are reported below (Table 2).
Table 2. Spearman’s correlation coefficient (Rho) for subdivisions of plants surveyed.
Plant type Vegetation Component Spearman’s Rho
Non-native Dominant 0.169
Non-native Subdominant 0.514
Native Dominant 0.368
Native Subdominant 0.419
The low Spearman correlation coefficient for exotic dominant plant species indicates that
dominant exotic plant composition differs between actively and passively revegetated
hummocks. Higher Spearman correlation coefficients for dominant native vegetation, and for
subdominant vegetation both native and exotic, indicate that vegetation is more similar by
hummock type for these vegetation categories, as compared to the dominant exotic vegetation.
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Discussion
Before delving into the potential implications of our findings, there are a few factors which are
important to take into consideration. Our transect results may have been affected by the seasonal
timing of our survey. We observed large areas of dead thatch underneath much of the vegetation
we surveyed. While this could in many cases have belonged to the same species as the actively
growing vegetation, much of this thatch was in an advanced state of decomposition, rendering
moot any attempts at identification. In addition, we observed evidence that several species in the
larger marsh habitat had already fully senesced. In particular, dead leaves of dormant Potentilla
anserina were conspicuous in several areas outside the hummocks.
Of the native species sown and planted on the actively revegetated hummocks, only D. cespitosa
ssp. cespitosa was particularly successful. At 30% dominance, D. cespitosa ssp. cespitosa had
twice the percent dominance on actively revegetated hummocks, as compared to 15 % on
passively revegetated hummocks. Neither C. lyngbyei nor J. lescurii achieved a dominant
presence on actively revegetated hummocks, though they were 5% and 4% dominant on
passively revegetated hummocks, respectively. While the planting of D. cespitosa ssp. cespitosa
appears to have had an impact on the species’ success, planting of other native species appears to
have had no impact. The lack of success among native species other than D. cespitosa ssp.
cespitosa on actively revegetated hummocks suggests that the revegetation strategy employed
could have been more economically efficient. Specifically, restoration practitioners might have
had more success by planting native species better able to compete with invasive grasses. Based
on the results of coastal prairie plots in more southerly areas of California, inclusion of faster-
growing native prairie plants such as Calamagrostis nutkaensis (Pacific reedgrass), and Festuca
rubra (red fescue), would likely have improved exclusion of invasive grass species (Corbin et
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al ., 2004). Differences in seed germination, growth, and seed bank may each have had a role in
the lack of success of the planted native species at this site. The low success rate of native seeds
and plugs may also be due to the significant presence of the invasive species, A. stolonifera. It is
likely that much of the native seed and plants were outcompeted by A. stolonifera, which was
found to have the highest percent cover of all species on both actively and passively revegetated
hummocks (80% and 40% respectively).
Dominant invasive vegetation between actively and passively restored hummocks was found to
have a lower Spearman correlation coefficient (0.169) than other components of vegetation
sampled, suggesting high dissimilarity in invasive plant cover between hummock types. This
can be explained primarily by the predominance of H. lanatus on approximately 25% of the
transect length in actively restored hummocks. In contrast, this species was nearly absent on
passively revegetated hummocks. If we consider the sum of A. stolonifera and H. lanatus
percentages on actively revegetated hummocks, invasive grasses occupied 65% of transect length
on these hummocks, as compared to 80% of transect length on passively restored hummocks.
According to the results of the t-test of dominant vegetation, there is not a significant difference
in total cover of these two invasive species between hummock types, so we can interpret the
approximately 15% difference in invasive grass cover as insignificant.
The dominant presence of A. stolonifera on the hummocks is not surprising considering the
nature of the species. Agrostis stolonifera is a perennial pasture grass that reproduces both
sexually by seed and vegetatively through stolons (Zapioloa et al ., 2008). Unlike many perennial
grasses, A. stolonifera is able to set seed in a single growing season (Esser, 1994). This allows
for the establishment of a large bank. In addition to having a large seed bank, A. stolonifera seeds
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are very small, making them more easily dispersed by wind than many other grasses (Zapiola et
al ., 2008). Due to the ease of dispersal through both seed and stolons, A. stolonifera readily
colonizes disturbed sites. The species is also known to exhibit high levels of phenotypic
plasticity (Zapiola et at., 2008), making it extremely tolerant to changing environmental
conditions and management actions employed to control the spread of the species. The seedbank
of A. stolonifera is viable in soil for one to four years (Esser, 1994; Zapiola et al ., 2008). The
material used to create the eight tidal hummocks on the project site came from the sediment
excavated during restoration activities. The site was formerly used as a cattle pasture for several
decades prior to the restoration project (O’Dowd, A.; personal communication, 2013), and
because of this, the sediment was probably dominated by the seed banks of pasture grasses such
as A. stolonifera and H. lanatus. Therefore, it is likely that a viable A. stolonifera seedbank
existed in the excavated soils that were used to build the hummocks.
Since D. cespitosa spp. cespitosa was dominant on substantial proportions of both hummock
types, its success may also be due to the existing seedbank for this species. Despite the higher
success rate of D. cespitosa spp. cespitosa on actively revegetated hummocks than on passively
revegetated hummocks, passively revegetated hummock transects suggest that the species can
compete with reasonable success against A. stolonifera up to a 15% cover dominance without the
need for active outplanting.
Although revegetation efforts may have decreased the success rate of A. stolonifera, the
apparently lower success rate of A. stolonifera on actively revegetated hummocks than on
passively revegetated hummocks might be attributed to the relative success of another invasive
grass, Holcus lanatus, or might instead reflect the relatively greater success of native
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Deschampsia cespitosa ssp cespitosa. While the active revegetation was not successful in
repopulating the hummocks with only native species, the slight difference in A. stolonifera
success suggests that further research and management actions might yield effective strategies
for limiting A. stolonifera on the site.
The successful predominance of Agrostis stolonifera on both hummock types, and the low
percent dominance of planted and unplanted native species, indicate that the active revegetation
conducted by the NRLT was somewhat ineffective. However, as suggested in a grant proposal
prepared by NRLT for the project, the revegetation effort can still benefit restoration knowledge
as a whole. By collecting data on the success of the different hummock types, the effort will
provide background research on the effectiveness of active revegetation on excavated soils,
particularly in historical brackish marshland impacted by former agricultural use. Future research
may lead to insight on effective strategies to decrease A. stolonifera success, to mitigate negative
impacts of revegetation (such as the increased success of H. lanatus on actively revegetated
hummocks), in addition to insight as to outplanting choices better able to establish in favor of
invasives.
Long-term monitoring at the site may also yield results in contrast to earlier data. For instance, it
is possible that planted native seed or plugs may result in the establishment of those native
species several years after this study. Given the findings of this study, similar restoration projects
conducted in the future may save resources by either foregoing active restoration or researching
and utilizing native plant species known to be competitive with invasive pasture grasses, such as
Calamagrostis nutkaensis (Pacific reedgrass), and Festuca rubra (red fescue) in their active
revegetation efforts (Corbin et al., 2004).
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