Potential Occurrence of Natural Swamp Regeneration on the
Maurepas Land Bridge, Southeast Louisiana
Theryn Henkel
Eva Hillmann
Kristen Butcher
David Baker
John Lopez
March, 2017
Acknowledgements
This study was funded through the Restore the Mississippi River Delta with support from the Walton
Family Foundation. We would like to thank the Louisiana Department of Wildlife and Fisheries for giving
us permission to conduct this study in the Maurepas Swamp Wildlife Management Area and
Southeastern Louisiana University Turtle Cove Research Station for logistical support.
Suggested Citation:
Henkel, T. K., E. R. Hillmann, K. Butcher, D. B. Baker, and J. A. Lopez. 2017. Potential occurrence of
natural swamp regeneration on the Maurepas Land Bridge, Southeast Louisiana. Lake Pontchartrain
Basin Foundation 24pp.
2
Introduction
The Maurepas Land Bridge is a critical line of defense in reducing risk from storm surge for
communities around Lake Maurepas as well as East Baton Rouge (LPBF and CRCL 2008). Historically, the
land bridge was covered with swamp forest dominated by bald cypress (Taxodium distichum) trees. The
entire land bridge was clear cut in the early 1900’s when logging of cypress peaked in the region (Keddy
et al. 2007). While some natural regeneration occurred, it was limited by the introduction of nutria
(Conner and Toliver 1987), lack of sediment, freshwater, and nutrient input due to leveeing of the
Mississippi River, and subsidence (Keddy et al. 2007). The construction of the Mississippi River Gulf
Outlet (MRGO), which increased salinity in Lake Pontchartrain and Lake Maurepas, also had a negative
effect on the remaining cypress stands. Currently, natural cypress regeneration is not possible on much
of the land bridge because the swamps are permanently flooded and seedlings need a period of draw
down to germinate (Shaffer et al. 2009). In other areas, the swamp forest has already converted to
marsh (Shaffer et al. 2009). Additionally, a severe drought in 1999 and 2000 caused salinity in the area
to spike which killed many trees, both naturally occurring and planted (Shaffer et al. 2009).
Because the Maurepas Land Bridge is a critical risk reduction feature on the landscape, the Lake
Pontchartrain Basin Foundation (LPBF) is interested in restoring the swamp to maintain the land bridge,
as well as to expand this important habitat type in the region (LPBF 2006). Swamp forest is an effective
storm buffer by reducing and slowing storm surge. It is a more effective buffer than marsh, and
experiences less damage in high winds than bottomland hardwood forest (Touliatos and Roth 1971,
Doyle et al. 1995, Williams et al. 1999). Most of the Maurepas Land Bridge is managed by the Louisiana
Department of Wildlife and Fisheries (LDWF) as Wildlife Management Areas (WMA) (Figure 1). Three
large WMA’s exist in the area, the Joyce WMA (24,190 acres), Manchac WMA (7,355 acres) and
Maurepas Swamp WMA Eastern Tract (32,243 acres). The remaining area is privately owned.
LPBF has been restoring swamp in the Maurepas region since 2013 when 100 trees were test
planted. The initial test planting was successful and subsequently,11,000 trees were planted in the
winter of 2014/2015, 7,000 were planted in the winter of 2015/2016 and 6,500 more trees will be
planted by the end of the 2016/2017 planting season (total 24,600). After one year of monitoring the
survival rate is 87%, indicating that swamp restoration could be successful in this region.
3
Figure 1: Location of the area of possible natural regeneration (red), location of trees that have been planted as
part of LPBF's swamp restoration program (blue dots) and location of the Wildlife Management Area in the
region (green).
While scouting for planting areas in July of 2015, a large area of the land bridge was observed to
be undergoing natural regeneration (Figure 1). Adult swamp tree species were observed seeding, and
younger trees of different ages were growing and appeared healthy. Also, a swamp understory
community was developing. The region was scouted on August 25, 2015 to look at the regeneration in
more detail. Previous observations were confirmed and a possible gradient of regeneration with more
regeneration occurring on the southern end of the land bridge and less regeneration moving northward
was also noticed. Most of the region has been classified as a degraded forest and the rest as relic forest
by Shaffer et al. (2009) (Figure 2). Degraded forest, in this case, refers to areas prone to saltwater
intrusion which are transitioning to marsh. Relic forest refers to swamp areas where natural
4
regeneration cannot occur because of permanently flooded conditions. Conditions in the basin may
have changed since the closure of the MRGO in 2009, with the rock dam near the Bayou La Loutre Ridge
and the Inner Harbor Navigation Canal Lake Borgne Surge Barrier near the Golden Triangle, and the area
may now be fresh enough to support growth of swamp seedlings and saplings (Figure 3).
Figure 2: Areas of sustainable, relic and degraded swamp on the Maurepas land bridge delineated by Shaffer et
al. 2009. Relic swamp refers to areas that are currently swamp but no natural regeneration occurs and degraded
swamp are areas that are on a trajectory towards conversion to marsh.
Figure 3: Soil salinity (left) and surface salinity (right) at five CRMS stations on the Maurepas Land bridge (CPRA
2017a). Both soil and surface salinity show a decline following the closure of the MRGO in 2009.
LPBF recently completed a swamp restoration suitability assessment for the Pontchartrain Basin
(Henkel et al. 2016). In the Maurepas region, we identified over 50,000 acres (20,000 hectares, 75
square miles) where conditions are currently suitable for swamp restoration (Figure 4). Both surface and
5
soil salinity have decreased and remained below 2 ppt for the last three years; a threshold necessary to
support swamp species. This area can accommodate over 9.8 million trees (assuming 4.5 meter spacing).
Some natural regeneration is already occurring in areas identified as restoration ready, which indicates
that other restoration areas may be ready for natural regeneration, provided seed sources can reach
these areas and nutria do not destroy all of the new seedlings (Conner and Toliver 1987).
Figure 4: Area of the Maurepas land bridge that are swamp restoration ready and location of the trees already
planted.
In order to determine if natural regeneration is indeed occurring, LPBF set out to set up a study
that would investigate the extent and density on natural swamp regeneration in the Maurepas land
bridge. Our specific questions are:
1. Is natural swamp regeneration occurring in the region, indicated by the presence of swamp
tree species in the sapling layer?
2. Is there a gradient of increased natural regeneration moving from north to south on the
land bridge?
3. If any natural regeneration is observed, is regeneration sustainable towards a trajectory of
healthy natural swamp?
The results presented here report on the first year of data collection. This study is designed as a
long-term study that investigates natural regeneration over time. Natural regeneration has not been
detected in this region for decades and even as recently as 2010, high mortality of less salt tolerant
species, such as red maple and green ash has been observed (Shaffer et al. 2016). The documentation of
natural regeneration occurring in this area since the closure of the MRGO would show that large-scale
hydrologic restoration projects can have rapid and far reaching benefits, including the return of historic
ecosystems.
6
Methods
Study Site
The study was conducted on the Maurepas Land Bridge, west of I-55 and south of Pass Manchac
(Figure 5). Plots were set up along a gradient from minimal observed regeneration (north) to healthier
forest (south). Eight (50 x 50 meter) plots were randomly chosen in eight sub-areas on the land bridge.
Plots had to be located more than 50 meters from the water’s edge to avoid edge effects and within 400
meters of the water’s edge to be logistically feasible.
Figure 5: Plot locations along the Maurepas Land Bridge and location of the Maurepas Land Bridge in the greater
New Orleans Region (inset).
7
Data Collection
Plot set up and data collection occurred in August and September of 2016. In each plot, PVC
poles were used to mark the four corners and the center of each plot. Elevation measurements were
taken at all four corners and the center of each plot using a Trimble Geo Explorer 6000 GeoXR GPS
attached to a Zephyr Model 2 GNSS receiver. Capable of real time kinematic (RTK) data collection, this
survey grade GPS system provided latitude, longitude, and elevation with a horizontal precision of less
than two inches and vertical precision of less than three inches. A soil salinity measurement was taken
at the center of each plot. Soil salinity was measured using the soil sipper method (Howes et al. 1985,
Watson and Frickers 1990, Folse et al. 2012). Herbaceous vegetation was measured at two, 1m2 quadrat
vegetation plots for percent cover by species at the northwest and southeast corners of each plot
(Iwanaga et al. 2011). Woody vegetation was surveyed by tagging and measuring all saplings and
seedlings that could be found and all adult trees in each plot, identifying to species and recording
location coordinates using a handheld gps (Garmin GPSMAP 60csx) (Figure 6). Trees were measured for
diameter at breast height (DBH). Young trees (saplings) encountered in the plot that were too small to
measure DBH were measured for height
Figure 6: Pole marking the center of one of the plots and trees that have been tagged and flagged after they
were measured (left) and a different plot showing the trees with tags, flags and tree paint.
Analysis
Data was analyzed in Rstudio (RStudio 2012) and for all tests a significance value of p = 0.05 was
used. For each plot, number of individuals, average elevation (average of the five elevation
measurements taken), number of tree and vegetative species, percent vegetative cover and average
DBH were analyzed using analysis of variance (ANOVA) to determine if any of these factors were
significantly different by plot, elevation, or soil salinity, followed by post hoc comparison testing using
pairwise t-test with a Bonferroni adjustement. In addition, the Shannon’s Diversity Index and Floristic
Quality Index (FQI) (Johnson 1990, Walters and Yawney 1990) were calculated for both trees and
vegetative species in each plot. These were also used in ANOVA to determine significant difference by
plot, elevation or soil salinity. The main dispersal method of each tree was determined and ANOVA was
used to determine if there was a significant difference in DBH by dispersal method. Additionally, an
ANOVA of DBH versus plot and species was also conducted. Biomass for each tree was calculated using
allometric equations found in the literature (Chojnacky et al. 2014) (Table 1) and an ANOVA was
conducted to investigate if there were significant differences in total and average biomass by plot,
8
species, elevation, soil salinity, FQI and diversity. Lastly, density distributions of DBH by plot and by
species were created.
Table 1: Allometric equations used to calculate biomass (kg) for all trees.
a
Scientific Name
Acer rubrum
Fraxinus pennsylvanica
Common Name
Red Maple
Green Ash
Allometric Biomass Equation
Y= -2.0470 + 2.3852ln(dbh)
Y= -2.0314 + 2.3524ln(dbh)
Nyssa aquatica
Nyssa sylvatica
Quercus virginiana
Water Tupelo
Blackgum
Live Oak
Y= -2.2118 + 2.4133
Y= -2.2118 + 2.4133ln(dbh)
Y= -3.0304 + 2.4982ln(dbh)
3-64
3-64
3-74
Salix nigra
Taxodium distichum
Triadica sebifera
Ulmus americana
Black Willow
Bald Cypress
Chinese Tallow
American Elm
Y= -2.4441 + 2.4561ln(dbh)
Y= -2.6327 + 2.4757ln(dbh)
Y= -2.5095 + 2.5437ln(dbh)
Y= -2.2118 + 2.4133ln(dbh)
3-70
3-109
4-42
3-64
ln(dbh)
Diameter range (cm)
3-109
3-43
Y= Biomass (kg); dbh = diamter at breast height (cm)
a
Equations from Chojnacky et al.
Results
Elevation
Elevation ranged from -0.26 to 0.26 meters (-0.88 to 0.88 ft.) across all plots. Elevation was
significantly different by plot (p=0.0014) but there was no pattern or gradient for elevation moving from
north to south. Plot 1 had the highest mean elevation (0.19 m, 0.656 ft.) and Plot 7 had the lowest (0.01 m, -0.046 ft).
Soil Salinity
Soil salinity ranged from 0.8 ppt to 2.7 ppt. Soil salinity was not significantly different by plot or
elevation and there was no trend or pattern in soil salinity from north to south. Plot 2 had the lowest
soil salinity and Plot 3 had the highest.
Herbaceous Vegetation
There were 13 herbaceous vegetation species found across all plots. Total percent cover ranged
from 15% to 50% across the 16 vegetation plots (two in each large plot). Species that were found most
commonly were alligator weed (Alternanthera philoxeroides) in 87.5% of the plots, bull tongue
(Sagittaria lancifolia) in 62.5% of the plots, pickerelweed (Pontederia cordata) in 56% of the plots and
cattail (Typha latifolia) and dotted smartweed (Polygonum punctatum) both in 50% of the plots. The
species with the highest average percent cover was pale spikerush (Eleocharis macrostachya). The FQI
ranged from 4 to 23 across all plots, which is a low FQI. Percent cover, species richness and FQI of
herbaceous vegetation were not significantly different by elevation, soil salinity or the interaction of
elevation and soil salinity.
Woody Vegetation
Plot Description
There were 647 trees measured across all plots (Figure 7). Number of individuals was
significantly different by plot (p=0.004) with the highest number of individuals found in plot 8, the
9
southernmost plot (Figure 8). Tree density ranged from 0.0016 trees per m2 (6.5 trees per acre) to 0.09
trees per m2 (374 trees per acre). In general, there was a trend of more individuals present moving
south, from plot 1 to plot 8. Soil salinity was not significantly different by plot but there was a trend of
decreasing soil salinity moving south (Figure 9). Therefore, more individuals, in general, were present at
lower soil salinities.
10
Figure 7: Tree locations in all plots. The GPS used to mark the trees locations has some error, and that is why
some trees appear to be outside the plot.
11
Number of individual Trees per Plot
250
p = 0.004
Number of Individual Trees
200
150
100
50
0
1
2
3
4
5
Plot
6
7
8
Figure 8: Number of individuals per plot. In general, there is a trend of increasing numbers of individuals moving
from plot 1 to plot 8 or moving from north to south on the Maurepas Land Bridge.
Soil Salinity by Plot
3
p = 0.59
Soil Salinity (ppt)
2.5
2
1.5
1
0.5
0
1
2
3
4
5
6
7
8
Plot
Figure 9: Soil salinity (ppt) by plot. Soil salinity was not significantly different by plot but in general, there is a
trend of decreasing soil salinity moving from plot 1 to plot 8 or from north to south on the Maurepas Land
Bridge.
12
Species composition
The 647 trees measured across all plots represented nine species in eight plant families. The
species observed were red maple (Acer rubrum), green ash (Fraxinus pennsylvanica), water tupelo
(Nyssa aquatica), blackgum (Nyssa sylvatica), black willow (Salix nigra), live oak (Quercus virginiana),
baldcypress (Taxodium distichum), Chinese tallow (Triadica sebifera), and American elm (Ulmus
Americana). Water tupelo had the most individuals across all plots (237) and red maple had the second
most (121). The remaining species had fewer individuals with Green ash at 85, baldcypress at 73,
Chinese tallow at 67, blackgum at 28, black willow at 13, and American elm and live oak at one.
Species richness was significantly different by soil salinity (p=0.029, r2=0.54) with higher species
richness at lower soil salinities (Figure10). Species richness was not significantly different by elevation or
the interaction of elevation and soil salinity. Shannon’s Diversity was not significantly different by plot or
elevation but was by soil salinity (p=0.019, r2=0.63) with greater diversity at lower soil salinities (Figure
11). The FQI for the trees was not significantly different by plot, elevation, or soil salinity. Lastly, number
of individual trees in a plot was not significantly different by elevation or soil salinity.
Soil Salinity vs Species Richness
9
P=0.029
8
y = -2.3081x + 8.7026
R² = 0.54
Species Richness
7
6
5
4
3
2
1
0
0
0.5
1
1.5
2
2.5
3
Soil Salinity (ppt)
Figure 10: Species richness was significantly different by soil salinity with more species found at lower salinity.
13
Soil Salinity vs Shannon's Diversity Index
90
p=0.019
Shannon's Diversity Index
80
y = -23.383x + 97.532
R² = 0.63
70
60
50
40
30
20
10
0
0
0.5
1
1.5
Soil Salinity (ppt)
2
2.5
3
Figure 11: Shannon's Diversity Index was significantly different by soil salinity with higher diversity found at
lower salinity.
Tree Size
The diameter at breast height was measured for all trees. DBH ranged from 0.8 cm to 90.8 cm
across all plots. The average DBH across all plots was 17.6 cm ± 0.55. When a density distribution was
conducted for DHB across all plots and species, the majority of the trees had a DBH around 10 cm
(Figure12). There was a second peak at approximately 24 cm and then there were few larger trees
(Figure12). Mean DBH was significantly different by plot (p < 0.001) with plot 3 having the largest mean
and plot two having the smallest mean (Figure 13). However, the standard deviation in all plots was
large, indicating a range of sizes. Figure 14 shows the DBH density by plot, with all plots having many
small trees and a few plots having no large trees.
14
Figure 12: Density plot of DBH (cm) across all plots and species, showing that the majority of the trees were
approximately 10 cm DBH.
Mean DBH by Plot
70
p < 0.001
60
DBH (cm)
50
40
30
20
10
0
1
2
3
4
Plot
Figure 13: Mean DBH by plot.
15
5
6
7
8
Figure 14: DBH (cm) density by plot, showing that all plots had many smaller trees but some plots had no large
trees.
In addition to being significantly different by plot, DBH was also significantly different by species
across all plots (Figure 15). Baldcypress and water tupelo had the largest mean DBH (29.5cm ± 21.9 and
27.12cm ± 8.2, respectively), blackgum and black willow had intermediate mean DBH (16.2cm ± 6.7 and
15.7 cm ± 7.9, respectively) and Chinese tallow, red maple and green ash had the smallest mean DBH
(6.2cm ± 2.7, 6.7cm ± 2.6, 6.5cm ± 2.9, respectively). Both the one live oak and American elm individuals
had a DBH of 10.6cm. Baldcypress had a large standard deviation, indicating a large range of sizes for
this species. Density distribution of DBH by species showed that red maple, green ash and Chinese
tallow trees had the high density of trees at small DBH, mostly less than 10 cm (Figure 16). Blackgum
had the highest density of trees around the 15 cm DBH size. Black willow had a range of sizes with the
highest density, ranging from 10 cm to 20 cm in DBH. Water tupelo had the highest density at a range of
sizes as well, from 20 cm to 35 cm DBH. Baldcypress had a small peak of higher density at 12 cm and 50
cm but overall was evenly distributed across all sizes, except for fewer trees in the very large size classes
(75 cm and above).
16
Mean DBH by Species
60
P<0.001
b
50
DBH (cm)
40
b
30
c
c
20
10
a
a
Red Maple
Green Ash
a
0
Water Tupelo
Blackgum
Tupelo
Black Willow
Baldcypress Chinese Tallow
Figure 15: Mean DBH (cm) by species, across all plots.
Figure 16: DBH (cm) distribution by species across all plots. Red maple, green ash and Chinese tallow had no
large individuals while baldcypress had individuals across all size classes.
Mean DBH per plot was also significantly different by soil salinity (p = 0.01, r2 = 0.49) with larger
trees, on average, found at higher soils salinities (Figure 17). Given the data presented above, this shows
17
that a few, larger trees were found at higher soil salinities while more individuals over a variety of sizes
were found at lower soil salinities. The largest tree (90 cm baldcypress) was found in plot 3, the plot with
the highest soil salinity (2.7 ppt).
Soil Salinity v Average DBH
40
P=0.01
Average DBH per plot (cm)
35
y = 10.718x + 1.456
R² = 0.49
30
25
20
15
10
5
0
0
0.5
1
1.5
2
2.5
3
Soil Salinity (ppt)
Figure 17: Average DBH (cm) per plot was significantly different by soil salinity with larger trees, on average,
found at higher soil salinities.
Lastly, DBH was significantly different by life history (p<0.001), specifically seed dispersal
method (Figure 18). Trees with seeds that are dispersed by water (baldcypress, water tupelo and
Chinese tallow) were significantly larger than trees dispersed by bird (blackgum) or wind (black willow,
red maple and green ash). However, Chinese tallow is also commonly dispersed by birds and is an
invasive species. Therefore, the analysis was repeated without Chinese tallow and the significant
differences between the dispersal methods was larger. The analysis was repeated with Chinese tallow in
the bird dispersed category rather than water dispersed and while the overall relationship between DBH
and dispersal method was still significant (p<0.001), DBH of trees in the wind and bird dispersed
categories were no longer significantly different.
18
Average DBH by Seed Dispersal Method
40
b
P<0.001
35
DBH (cm)
30
25
a
20
15
c
10
5
0
Bird
Water
Wind
Seed Dispersal Method
Figure 18: Mean DBH (cm) by seed dispersal method.
Tree Biomass
As expected, trends in tree biomass closely followed trends in tree DBH. Biomass was calculated
using allometric equations based on DBH. Average biomass was significantly different by plot (p=0.0003)
and by species (p<0.001). The plot with the highest meant biomass was plot 3 (also the plot with the
largest tree) and the lowest was plot 2. The species with the highest mean biomass was baldcypress
(650 kg ± 925) followed by water tupelo (367 kg ± 247). The species with the lowest mean biomass was
Chinese tallow (12 kg ± 11.9). The plot with the highest total biomass was plot 6 (44,673 kg) which had
101 fewer individuals than plot 8 which had the second highest biomass (31,485 kg). The plot with the
total lowest biomass was plot 2 (590kg). The species with the highest total biomass was water tupelo
(86,646 kg) followed by baldcypress (46,843 kg). The species with the lowest total biomass was Chinese
tallow (759 kg). Total biomass was not significantly different by plot, elevation, soil salinity diversity or
FQI. Average biomass per plot was significantly different by soil salinity (p = 0.043, r2=0.51) with higher
mean biomass with greater soils salinity (Figure 19). This was similar to the average DBH results.
Average biomass per plot was not significantly different by elevation, diversity or FQI.
19
Soil Salinity v Average Biomass per Plot
1,200
p = 0.043
1,000
Average Biomass (kg)
y = 400.06x - 378.23
R² = 0.51
800
600
400
200
0
0
0.5
1
1.5
2
Soil Salinity (ppt)
2.5
3
Figure 19: Average biomass per plot (kg) was significantly different by soils salinity with higher biomass found at
higher soil salinities.
Discussion
The species composition found in all plots was expected and included species that are normally
part of the swamp community. Swamp communities are usually low in diversity, with species that are
adapted to prolonged flooding, such as baldcypress and water tupelo, dominating (Blair and Langalinais
1960, Conner and Day Jr. 1976, Conner et al. 2002). Baldcypress, water tupleo and blackgum are
common dominant species in these ecosystems and red maple, green ash and black willow are common
associates and understory species in a climax swamp forest (Blair and Langalinais 1960). The presence
of the invasive species Chinese tallow in abundance, in some plots, at small sizes, speaks to a recent
invasion of the species. The presence of Chinese tallow in high numbers in some plots also may indicate
that these plots experience dry-down phases and are not permanently flooded. While Chinese tallow
thrives in wet soils, it is not adapted to long-term flooding and displays decreased growth rates under
these conditions (Conner 1994).
In most of the plots, the density of trees was not what is expected of a normal swamp although
the plot with the most trees (plot 8) did fall in the normal range of density for swamps. Plot tree density
ranged from 16 to 924 trees per hectare. Common density for Louisiana ranges from 460 to 1030 trees
per hectare with flooded areas having higher density (Conner et al. 2002). In general, swamps tend to
be dense with trees growing close together, forming a closed canopy. This indicates that, in this region,
much of the swamp is not as healthy or dense as typical swamps. The history of the region, including
intense logging (Mancil 1980, Keddy et al. 2007), nutria invasion (Blair and Langalinais 1960, Conner and
Toliver 1987, Conner and Toliver 1990, Keddy et al. 2007), saltwater intrusion (Keddy et al. 2007), severe
drought (Shaffer et al. 2009, Shaffer et al. 2016), and hydrologic disturbance offers an explanation for
why this area is not currently a thriving swamp. However, there are some indications, described below,
that this trajectory could be reversing.
20
There was a general trend of decreasing soil salinity, increased species richness, and increased
number of individuals when moving south on the Maurepas Land Bridge. This is to be expected, as most
swamp tree species are affected by high salinities, experiencing decreased growth and mortality under
those conditions (Conner et al. 1997, Conner and Inabinette 2003, Effler and Goyer 2006, Hoeppner et
al. 2008, Shaffer et al. 2016). The tree species that dominated the small size classes tended to be
considered understory to mid-story species in swamps, including red maple, green ash, and black willow
and show pioneer species characteristics (wind dispersed seeds, not part of the climax canopy). These
species were more abundant in fresher plots and were absent from plots with elevated soil salinity.
Additionally, these species’ seeds are wind dispersed. Therefore, these trees had a larger dispersal range
from seed sources and most likely have been reaching the area continuously but did not grow or thrive
until soil salinities were reduced, after the closure of the MRGO in 2009. Lastly, there were no larger
representatives of red maple and green ash, indicating that they have all germinated in recent years
(most were around 10 cm in DBH). There were larger individuals of black willow (15 to 25 cm DBH).
Black willow seeds are extremely light weight and small (2.3 million/lb) and have a larger dispersal range
than the other two species and are produced in copious amounts (Pitcher and McCarron 1990). Red
maple seeds are heavier (23,000/lb) and have a large dispersal range but not as large as black willow
(Walters and Yawney 1990). Green ash seeds are not dispersed very far from parent trees, indicating
heavier seeds but a weight could not be found (Kennedy 1990). Black willow is present in abundance on
local spoil banks. Due to its abundance of seed output and long distance dispersal, this species mostly
likely found small pockets of acceptable soil salinity, where they could thrive, earlier than the other
species.
In contrast to the pioneer or mid-story species discussed above, baldcypress was represented by
a wide range of sizes, from small saplings to large trees over 60 cm in DBH. Baldcypress was also found
across soil salinities. In the plots with elevated soil salinity, there were a few larger baldcypress and no
other species, indicating that these individuals survived the period of elevated salinity while the other
swamp species experienced mortality. The trend of larger trees in higher soil salinity was in contrast to
the trend found by Shaffer et al. (2016) where larger trees were found at lower soil salinities over an 11year study. The Shaffer et al. study spanned 2000-2010. The opposite trends in DBH vs. soil salinity
between this study and Shaffer et al. (2016) may be another indicator that this region in changing and in
transition. Also, the Shaffer et al. (2016) study surveyed trees 5 cm DBH and larger while this study
included smaller trees, which could be another reason for the different results. The lack of smaller trees
or saplings in plots with elevated salinity indicate that little to no natural regeneration is occurring in
these areas. Of the most common swamp species, baldcypress is the most salt tolerant (Krauss et al.
2000, Shaffer et al. 2009, Iwanaga et al. 2011). While there were some small cypress found, there was
substantially fewer than the pioneer species discussed above. Additionally, water tupelo, the other
common swamp climax species, had a range of larger trees from 25-60 cm DBH. Water tupleo was not
found in plots with elevated soil salinity and is not known to be very salt tolerant (Pezeshki et al. 1989,
Conner et al. 1997). However, larger trees were found in areas with lower soil salinity and there were a
few in the sapling age class. Both of the species rely on flooding and dry down to disperse seeds away
from parent trees (Johnson 1990, Wilhite and Toliver 1990). This method of dispersal, when compared
to wind dispersal, relies on chance events to distribute seeds to areas that were previously inhospitable
to swamp species but have become adequate with the closure of the MRGO. Therefore, it is expected
that it would take these species longer to naturally regenerate due to dispersal limitations.
The study described here is meant to be a multi-year study, where these plots are resurveyed
every year, new trees in the sapling layer are accounted for, and existing trees are measured for growth
rates. While this one year survey revealed many interesting patterns currently existing, including the
element of time in future analysis will elucidate the trajectory of natural regeneration on the land bridge
and help determine if what was observed this year is part of an ongoing course towards a naturally
21
restored swamp or a temporary phenomenon. Additionally, resurveying these plots over many years will
capture any stochastic events that may occur, such as hurricane storm surge, rainfall induced river
flooding like occurred in 2016, drought, etc. From this, the effects of stochastic events on natural
regeneration can be determined.
In general, a healthy swamp in this region would be dominated by baldcypress and water
tupelo, with a closed canopy that produces a shaded understory. The current abundance of understory
species, such as red maple and green ash, with sporadic occurrence of the climax species in the sapling
layer, indicates the beginning of natural regeneration which will need to be confirmed over time. If, in
the following years of data collection, the survival and growth of saplings of all species and additional
recruitment of climax species is observed, there will be more confidence that the swamp forest in this
region is on a trajectory towards naturally regenerating. The occurrence of natural regeneration in the
area would be impactful to the region by providing habitat and storm surge risk reduction benefits to
surrounding communities. Swamp restoration through plantings is costly and there are areas that are
inaccessible by boat or land that need to be restored. Natural regeneration would provide a substantial
costs savings and is synergistic with the plantings already taken place. The planted trees can become
additional seed sources in the region, given time. Lastly, natural regeneration could be augmented by
seeding the region by aerial deployment or other means, if proven successful.
Although this data set is currently limited, it does suggest that the swamp forest on the
Maurepas Land Bridge is in the beginning stages of natural regeneration. Pioneer species have colonized
and climax species are present in lower numbers. The future of natural regeneration in the region is
uncertain but there a few factors that increase the chance of successful regional swamp regeneration.
The closure of the MRGO has decreased both the surface and soil salinity experienced in the region and
also seems to have decreased the large fluctuations in salinity experienced when the MRGO was open
(CPRA 2017a). Whether or not salinity spikes will be less severe during drought than previously
experienced in the region (during 1999-2000) remains to be seen. In addition to decreasing salinities
from the MRGO closure, there is also freshwater and sediment diversions proposed for the region, that
would help maintain fresh salinities. The 2012 Louisiana State Master Plan proposes the West Maurepas
Diversion which is a freshwater diversion into the southern end of the land bridge (CPRA 2012). The final
design of this project has been funded. In the 2017 Draft Louisiana State Master Plan, there are three
diversions in the region proposed, including the East Maurepas, Manchac Land Bridge and Union
Diversions, all proposed for implementation within the first 10 years (CPRA 2017b). Therefore, there
seems to be an emphasis on salinity control in the region which would promote natural regeneration
and guard against future salinity spikes. Over time, this study will reveal the dynamics of natural swamp
regeneration, including shifts in species composition, mortality, and the next generation of saplings that
would become the future tree canopy.
22
References
Blair, R. M. and M. J. Langalinais. 1960. Nutria and swamp rabbits damage baldcypress plantings. Journal
of Forestry 58(5):388-389.
Chojnacky, D. C., L. S. Heath, and J. C. Jenkins. 2014. Updated generalized biomass equations for North
American tree species. Forestry 87:129-151.
Conner, W. H. and J. W. Day Jr. 1976. Productivity and composition of a baldcypress-water tupelo site
and a bottomland hardwood site in a Louisiana Swamp. American Journal of Botany
63(10):1354-1364.
Conner, W. H. and J. R. Toliver. 1987. The problem of planting Louisiana swamplands when nutria
(Myocastor coypu) are present in Third Eastern Wildlife Damage Control Conference. University
of Nebraska-Lincoln.
Conner, W. H. and J. R. Toliver. 1990. Long-term trends in the bald cypress (Taxodium distichum)
resource in Louisiana (USA). Forest Ecology and Management 33-4(1-4):543-557.
Conner, W. H. 1994. The effect of salinity and waterlogging on growth and survival of baldcypress and
Chinese tallow seedlings. Journal of Coastal Research 10(4):1045-1049.
Conner, W. H., K. W. McLeod, and J. K. McCarron. 1997. Flooding and salinity effects on growth and
survival of four common forested wetland species. Wetlands Ecology and Management 5:99109.
Conner, W. H., L. Mihalia, and J. Wolfe. 2002. Tree community structure and changes from 1987 to 1999
in three Louisiana and three South Carolina forested wetlands. Wetlands 22(1):58-70.
Conner, W. H. and L. W. Inabinette. 2003. Tree growth in three South Carolina (USA) swamps., after
Hurricane Hugo: 1991-2001. Forest Ecology and Management 182(1-3):371-380.
CPRA. 2012. Louisiana's Comprehensive Master Plan for a Sustainable Coast. Coastal Protection and
Restoration Authroity, Baton Rouge, Louisiana, 188 p.
CPRA. 2017a. Coastwide Reference Monitoring System-Wetlands Monitoring Data. Retrieved from
Coastal Information Management System (CIMS) database. http://cims.coastal.louisiana.gov.
Accessed DD MONTH YYYY. Coastal Protection and Restoration Authority of Louisiana, New
Orleans Louisiana(Retrieved from Coastal Information Management System (CIMS) database.
http://cims.coastal.louisiana.gov.).
CPRA. 2017b. Louisiana's Comprehensive Master Plan for a Sustainable Coast - DRAFT. Coastal
Protection and Restoration Authority, Baton Rouge, New Orleans Louisiana, 168 p.
Doyle, T. W., B. D. Keeland, L. E. Gorham, and D. J. Johnson. 1995. Structural impact of Hurricane
Andrew on the forested wetlands of the Atchafalaya Basin in south Louisiana. Journal of Coastal
Research SI(21):354-364.
Effler, R. S. and R. A. Goyer. 2006. Baldcypress and water tupelo sapling response to multiple stress
agents and reforestation implications for Louisiana swamps. Forest Ecology and Management
226(1-3):330-340.
Folse, T. M., J. L. West, M. K. Hymel, J. P. Troutmann, L. Sharp, D. Weifenbach, T. McGinnis, and L. B.
Rodrigue. 2012. A standard operating procedures manual for the Coast Wide Referencing
System-Wetlands: Methods for site establishments, data collection, and quality
assurance/quality control. The Louisiana Coastal Protection and Restoration Authority. Baton
Rouge, LA. 207 pp.
Henkel, T., A. Preau, and J. Lopez. 2016. Swamp restoration suitability assessment for the Pontchartrain
Basin. Lake Pontchartrain Basin Foundation. 15pp.
23
Hoeppner, S. S., G. P. Shaffer, and T. E. Perkins. 2008. Through droughts and hurricanes: Tree mortality,
forest structure, and biomass production in a coastal swamp targeted for restoration in the
Mississippi River Deltaic Plain. Forest Ecology and Management 256(5):937-948.
Howes, B. L., J. W. H. Dacey, and S. G. Wakeham. 1985. Effects of sampling technique on measurments
of porewater constituents in salt-marsh sediments. Limnology and Oceanography 30(1):221227.
Iwanaga, F., M. Hirazawa, T. Takeuchi, and F. Yamamoto. 2011. Effects of irregular saltwater
submergence on Taxodium distichum seedlings. Journal of Coastal Research 27(6A):193-198.
Johnson, R. L. 1990. Nyssa aquatica L. in R. M. Burns and B. H. Honkala, tech. coords., editors. Silvics of
North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654. U. S. Department of
Agriculture, Forest Service, Washington, D.C. vol 2., 877 p.
Keddy, P. A., D. Campbell, T. B. McFalls, G. P. Shaffer, R. Moreau, C. Dranguet, and R. Heleniak. 2007. The
wetlands of Lakes Pontchartrain and Maurepas: Past, present and future. Environmental
Reviews 15:43-77.
Kennedy, H. E. 1990. Fraxinus pennsylvanica Marsh. in R. M. Burns and B. H. Honkala, tech. coords.,
editors. Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654. U. S.
Department of Agriculture, Forest Service, Washington, D.C. vol 2., 877 p.
Krauss, K. W., J. L. Chambers, J. A. Allen, D. M. Soileau, and A. S. DeBosier. 2000. Growth and nutrition of
baldcypress families planted under varying salinity regimes in Louisiana, USA. Journal of Coastal
Research 16(1):153-163.
LPBF. 2006. Comprehensive habitat management plan for the Lake Ponchartrain Basin. Lake
Pontchartrain Basin Foundation 142 pp. http://saveourlake.org/PDF-documents/ourcoast/CHMP_final_%2022706.pdf.
LPBF and CRCL. 2008. Comprehensive recommendations supportung the use of the Mulitple Lines of
Defense Strategy to sustain coastal Louisiana. Lake Pontchartrain Basin Foundation and the
Coalition to Restore Coastal Louisiana http://saveourlake.org/PDFdocuments/MLODSreportFINAL-12-7-08with-comments.pdf 408pp.
Mancil, E. 1980. Pullboat logging. Journal of Forest History 24(3):135-141.
Pezeshki, S. R., W. H. Patrick, R. D. Delaune, and E. D. Moser. 1989. Effects of waterlogging and salinity
intteraction on Nyssa aquatica seedlings. Forest Ecology and Management 27(1):41-51.
Pitcher, J. A. and J. K. McCarron. 1990. Salix nigra Marsh. in R. M. Burns and B. H. Honkala, tech. coords.,
editors. Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654. U. S.
Department of Agriculture, Forest Service, Washington, D.C. vol 2., 877 p.
RStudio. 2012. RStudio. Version 2.14.1, Boston, MA http://www.rstudio.org/.
Shaffer, G. P., W. B. Wood, S. S. Hoeppner, T. E. Perkins, J. Zoller, and D. Kandalepas. 2009. Degradation
of baldcypress-water tupelo swamp to marsh and open water in southeastern Louisiana, USA:
An irreversible trajectory? Journal of Coastal Research SI 54:152-165.
Shaffer, G. P., J. W. Day, D. Kandalepas, W. B. Wood, R. G. Hunter, R. R. Lane, and E. R. Hillmann. 2016.
Decline of the Maurepas Swamp, Pontchartrain Basin, Louisiana, and approaches to restoration.
Water 8(3).
Touliatos, P. and E. Roth. 1971. Hurricanes and trees - 10 lessons from Camille. Journal of Forestry
69(5):285-289.
Walters, R. S. and H. W. Yawney. 1990. Acer rubrum L. in R. M. Burns and B. H. Honkala, tech. coords.,
editors. Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654. U. S.
Department of Agriculture, Forest Service, Washington, D.C. vol 2., 877 p.
Watson, P. G. and T. E. Frickers. 1990. A multilevel, insitu pore-water sampler for use in intertidal
sediments and laboratoy microcosms. Limnology and Oceanography 35(6):1381-1389.
24
Wilhite, L. P. and J. R. Toliver. 1990. Taxodium distichum (L.) Rich. in R. M. Burns and B. H. Honkala, tech.
coords., editors. Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654.
U. S. Department of Agriculture, Forest Service, Washington, D.C. vol 2., 877 p.
Williams, K., Z. S. Pinzon, R. P. Stumpf, and E. A. Raabe. 1999. Sea level rise and coastal forests on the
Gulf of Mexico. USGS-99-441, 63p.
25