Ecological Response of Forested Wetlands with and Without Large-Scale

Ecological Engineering 46 (2012) 57–67

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Ecological Engineering

j ournal homepage: www.elsevier.com/locate/ecoleng

Ecological response of forested wetlands with and without Large-Scale

Mississippi River input: Implications for management

a,b b,∗ c a d a

John Day , Rachael Hunter , Richard F. Keim , Ronald DeLaune , Gary Shaffer , Elaine Evers ,

e f g b b

Denise Reed , Christopher Brantley , Paul Kemp , Jason Day , Montgomery Hunter

School of the Coast and Environment, Louisiana State University, Baton Rouge, LA 70803, United States

Comite Resources, Inc., 11643 Port Hudson Pride Rd., Zachary, LA 70791, United States

School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, United States

Department of Biological Sciences, Southeastern Louisiana University, Hammond, LA 70402, United States

Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, LA 70148, United States

Corps of Engineers US Army Corps of Engineers, New Orleans District, Bonnet Carré Spillway, PO Box 216, Norco, LA 70079, United States

National Audubon Society, 6160 Perkins Road, Suite 215, Baton Rouge, LA 70808, United States

a r t i c l e i n f o a b s t r a c t

Article history: We investigated two adjacent wetlands in the Lake Pontchartrain basin, one of which receives peri-

Received 3 November 2011

odic input of Mississippi River water and one which does not, to gain insight into how isolation from

Received in revised form 11 April 2012

river input impacts wetland loss in the Mississippi delta. The LaBranche (LB) wetlands bordering Lake

Accepted 28 April 2012

Pontchartrain are severely degraded due to saltwater intrusion, subsidence, leveeing of the river, and

Available online 26 May 2012

hydrologic alterations including partial impoundment. Directly adjacent is the Bonnet Carré (BC) spill-

way, a geomorphically similar area that contains healthy baldcypress swamp. The spillway carries river

Keywords:

water to the lake during high discharge years and has been opened eleven times in 80 years, with ﬂows as

River diversion 3 −1

high as 9000 m s . The primary hydrologic difference between the two areas is the regular input of River

Coastal wetlands

Baldcypress water to the BC wetlands while the LB wetlands are isolated from the river. The interior of the LB wetlands

Forested wetland is also isolated from sediment originating from Lake Pontchartrain. Long-term accretion, tree growth, and

137

Swamp elevation were measured in these two wetland areas to determine impacts of riverine input. Cs accre-

−1 −1

Louisiana tion rates in the BC wetlands were 2.6–2.7 cm yr , compared to 0.43 and 1.4 cm yr , respectively, in the

Sediment accretion

LB wetlands in areas without and with sediment input from Lake Pontchartrain. Baldypress growth in

Bonnet Carré spillway −1 −1

the BC averaged about 2.3 mm ring width yr , compared to 1.4 mm yr in LB. Trees are of relatively the

same age due to lack of recruitment and widespread logging. Tree height, an indicator of site quality, is

about 20% less at the LB sites compared to BC, even though the trees are approximately the same ages.

The average wetland elevation in the BC wetlands was about one meter with some areas higher than two

meters, and was signiﬁcantly higher than elevations in the LB (average sea level and 0.3 m, respectively,

in areas with and without input from Lake Pontchartrain).

1. Introduction of the deltaic plain from riverine input is one of the primary factors

contributing to this loss (Kesel, 1988, 1989; Mossa, 1996; Day

Freshwater, sediments and nutrients from the Mississippi et al., 2000, 2007). In an effort to restore marshes and freshwater

River formed the expansive network of deltaic wetlands in coastal swamps, water from the Mississippi River is being diverted into

Louisiana, including forested wetlands such as those that are the coastal wetlands, primarily in small-scale diversions less than

3 −1

focus of this study (Roberts, 1997; Day et al., 2007; Törnqvist et al., 200 m s such as the Caernarvon diversion (Day et al., 2009).

2006; Shaffer et al., 2009a). In the 20th century, there was a dra- Previous research has shown that these small river diversions

matic loss of about 25% of these coastal wetlands. This loss was due can increase marsh primary production, wetland surface elevation,

to a variety of factors such as pervasive hydrological alteration and and vertical accretion (DeLaune et al., 2003; Lane et al., 2006; Day

enhanced subsidence, but there is broad consensus that isolation et al., 2007, 2009). However, there is concern that these diver-

sions are so small compared to pre-levee ﬂooding of the Mississippi

River that they are not making a signiﬁcant contribution to coastal

∗ restoration and that they may make wetlands susceptible to hurri-

Corresponding author. Tel.: +1 225 439 3931.

E-mail address: [email protected] (R. Hunter). cane damage (Turner, 2010; Howes et al., 2010). In the past, large

58 J. Day et al. / Ecological Engineering 46 (2012) 57–67

Fig. 1. Location of the Bonnet Carré Spillway and LaBranche wetlands within the Pontchartrain Basin, Louisiana.

Figure reprinted with permission from author Lane et al. (2001).

crevasses on the Mississippi River were fairly common and they 4) Synthesize available information on the area to determine fac-

delivered large quantities of freshwater (generally with peak dis- tors responsible for change over time.

3 −1

charge between 5000 and 10,000 m s ) and sediment to adjacent

wetlands (Davis, 2000; Day et al., 2009). With larger river diver-

sions planned for future restoration projects, it is important to 3. Methods

understand how discharges of large volumes of water will impact

wetlands. 3.1. Site description

One example of an area with large-scale riverine input is the

Bonnet Carré spillway, which carries ﬂoodwaters from the river 3.1.1. Bonnet Carré Spillway

to Lake Pontchartrain Basin when high river levels threaten New The BC Spillway was completed in 1931 in response to the

Orleans (Fig. 1). The LB wetlands, located adjacent to the BC spill- great ﬂood of 1927 (Barry, 1997). The purpose of the spillway is to

way and separated from it by an earthen levee, are an example of decrease river stage to reduce ﬂooding threat to New Orleans. The

wetlands that have been isolated from the river by levees for more 3.4 km wide spillway is conﬁned by two 8.6 km levees, and connects

than a century. In this paper we compare baldcypress growth, sedi- the Mississippi River to Lake Pontchartrain. A water ﬂow regu-

ment accretion, elevation in wetlands and other information in the lation structure, consisting of 350 ﬂoodgates (each with twenty

BC wetlands with adjacent LB wetlands to determine impacts of an 20 cm 30 cm creosoted wooden timbers 3.1–3.6 m in length), is

infrequent, large-scale diversion on forested coastal wetlands. located at the Mississippi River inlet. The structure is opened and

closed by removing or replacing the timbers one at a time. Thus,

it can take a week or more to completely open or close the struc-

2. Objectives of research

ture. The spillway is located just downstream of the Bonnet Carré

crevasse, one of the many natural crevasses that occurred in the

We hypothesized that previous openings of the BC spillway have 3 −1

1800s, introducing up to 10,000 m s of river water into Lake

led to higher elevation, increased accretion, and increased cypress

Pontchartrain during high ﬂow events (Kesel, 1989; Davis, 1993).

growth rates than in the LB wetlands that have been isolated from

The spillway has 1300 ha of forested wetlands; approximately 50%

ﬂooding from the Mississippi River for over a century. We also

of the total spillway area (Lane et al., 2001). Although the spillway

hypothesized that accretion rates in the BC wetlands are sufﬁcient

was designed for ﬂood control, its use as a freshwater diversion to

to offset current and projected increases in the rate of sea-level rise.

manage salinity levels for oyster production and to provide sedi-

To test our hypotheses, the objectives of this research were to:

ments and nutrients for wetland restoration has been considered

(Lane et al., 2001).

1) Utilize LIDAR data to determine and compare elevations Since 1931, the spillway has been opened eleven times during

between the two study sites; high water events of the Mississippi River, with ﬂows ranging from

3 −1

2) Determine long-term patterns of sediment accretion in the BC 3100 to 9000 m s (Table 1; Sikora and Kjerfve, 1985; Day et al.,

137

and LB wetlands using Cs; 1999; Lane et al., 2001). Spillway openings generally correspond to

3) Determine long-term patterns of baldcypress growth in the the peak hydrograph of the Mississippi River when snowmelt and

study sites and relate patterns to environmental conditions such rainfall in the upper basin increase ﬂow and stage in the lower

as precipitation, drought, and storms; and river. When open, water ﬂows through the spillway into Lake

J. Day et al. / Ecological Engineering 46 (2012) 57–67 59

Table 1

Opening dates of the Bonnet Carré Spillway, maximum water discharge, and estimated sediment deposition within the spillway (USACE, 2009). The 1994 opening was for

experimental purposes and not for ﬂood control.

3 −1 3

Year Dates spillway open Maximum number of bays open Maximum discharge (m s ) Sediment deposition (m )

1937 Jan 28–March 16 285 5976 7,800,000

1945 March 23–May 18 350 9006 9,500,000

1950 February 10–March 19 350 6315 3,800,000

1973 April 8–June 21 350 5523 11,500,000

1975 April 14–26 225 3115 1,500,000

1979 April 17–May 31 350 6457 3,800,000

1983 May 20–June 23 350 7590 3,800,000

1994 May 16–26 30 400 –

1997 March 17–April 18 298 6797 6,900,000

2008 April 11–May 8 160 4531 1,500,000

2011 May 9–June 20 330 8949 7,200,000

Pontchartrain and then to the Gulf of Mexico. River water dis- Basin (Shaffer et al., 2009a). In 1965, Hurricane Betsy caused a surge

charged into Lake Pontchartrain often tends to ﬂow along the that overtopped the railroad and inundated the wetlands with salt

southern portion of the Lake in an easterly direction adjacent to the water. This occurred again during hurricanes Katrina and Rita in

LB wetlands, exiting through Chef Menteur Pass and the Rigolets 2005. The embankment associated with the railroad is a barrier

(White et al., 2009). to sheet ﬂow and drainage, thus reducing sediment input from

With each opening of the BC spillway, the river deposits an Lake Pontchartrain to wetlands south of the railroad (Pierce et al.,

average of about 5 million m of sediment, consisting mostly of 1985). The construction of Interstate I-10 altered hydrology of the

silts and sands, in the spillway (Table 1; Lane et al., 2001). wetlands through the excavation of permanent access canals and

During an experimental opening of the spillway in 1994 (peak extensive borrow pits (Nesbit et al., 2004).

3 −1

discharge was 396 m s ), Lane et al. (2001) estimated that Natural water features such as Bayou Trepagnier and Bayou

6 3

approximately 6.4 × 10 m of Mississippi River water was diverted LaBranche and canals such as Parish Line Canal, Walker Canal, and

into Lake Pontchartrain over 42 days. During this time, there were several I-10 access canals serve as hydrological conduits for brack-

3.9 ± 1.1 cm of accretion in the spillway (Lane et al., 2001). Research ish and/or saline waters from Lake Ponchartrain into the LaBranche

by Snedden et al. (2007) indicated that sediment inputs are maxi- wetlands (Nesbit et al., 2004). Prior to disruption of the natu-

mized when diversion events coincide with the hydrograph peak. ral hydrology, there was regular riverine input and ﬂow of fresh

water through the LB wetlands before reaching the lake. Currently,

3.2. LaBranche wetlands because of isolation from the Mississippi River and the access

canals, salinities in the LB wetlands can reach more than 10 ppt

The 8100-hectare LB wetlands consist primarily of non- during drought, as occurred during the 2000–01 regional drought

regenerating baldcypress-water tupelo (Taxodium distichum – (Shaffer et al., 2009a). As a result of these changes, from 1952 to

Nyssa aquatica) swamp and freshwater herbaceous wetlands in 1983, much of the interior marsh converted to open water in the

the southern areas, grading to intermediate, brackish, and saline LaBranche wetlands and portions of the swamp were reclaimed

marsh and shallow open water ponds closer to Lake Pontchar- (Pierce et al., 1985, Table 2, Fig. 2).

train. The wetlands border Lake Pontchartrain to the north while The climate in the area is subtropical with hot summers and

◦

the rest of the area is bordered by ﬂood control levees (Cramer mild winters. Summers have an average daily temperature of 28 C,

◦

et al., 1981; Pierce et al., 1985; Boumans et al., 1997). The average daily maximum temperature of 33 C, and high average

major factors contributing to the deterioration of the LB wet- humidity (National Climate Data Center, New Orleans International

lands are isolation from riverine input by Mississippi River levees, Airport). Winters are inﬂuenced by cold, dry, polar air masses mov-

hydrologic alterations, erosion, saltwater intrusion, hurricanes, ing southward from Canada, with an average daily temperature

◦ ◦

semi-impoundment, nutria herbivory, and soil subsidence (Pierce of 12 C and an average daily minimum of 7 C. Estimated annual

et al., 1985). These factors have interacted in a cumulative way to evapotranspiration is 108 cm and is relatively constant from year to

create non-sustainable ecosystems in much of the Pontchartrain year and annual precipitation averages 137 cm (USDANRCS, 2002).

Basin (Shaffer et al., 2009a,b). Generally, water deﬁcits occur in the summer growing season

The Mississippi ﬂood control levees have eliminated most river when evapotranspiration often exceeds precipitation.

input to Mississippi delta wetlands. Relatively healthy cypress Lake Pontchartrain is a 163,000-hectare oligohaline body of

swamps occur only in freshwater areas experiencing minimal daily water with a mean depth of 3.7 m and approximate volume of

9 3

tidal action and where the salinity range does not normally exceed 1.66 10 m . Tides are diurnal with a mean range of 11 cm (Sikora

two parts per thousand (ppt; Conner et al., 2007; Krauss et al., and Kjerfve, 1985). Although the average salinity of Lake Pontchar-

2009; USACE, 2009). Fragmentation of the LB wetlands occurred train is less than 6 ppt, salinity can vary widely (e.g., from fresh

due to digging of canals, oil and gas extraction, and construc-

tion of the Illinois Central Gulf Railroad in 1854 and highway I-10

Table 2

in the 1960s. The railroad sits atop an embankment with seven

Area (ha) of different habitats in the LaBranche wetlands quantiﬁed using aerial

small openings along a total length of approximately 10.4 km that photographs (Pierce, 1985). See also Fig. 2.

severely restricts water exchange between Lake Pontchartrain and

Habitat Year

the wetlands south of the railroad. The dredging of several canals

to Lake Pontchartrain and the opening of the Mississippi River Gulf 1952 1956 1965 1972 1978 1983

Outlet (MRGO) east of New Orleans in 1963 caused saltwater intru- Open water 108 151 1911 1893 1633 1722

sion into the wetlands (Shaffer et al., 2009b). A severe drought in Marsh 4353 4327 3647 3190 3385 3061

Swamp forest 4084 4036 3681 3260 3316 3417

2000–2001 led to high salinities that killed large areas of cypress

Developed land 60 97 107 264 281 410

in the Labranche wetlands and other parts of the Pontchartrain

60 J. Day et al. / Ecological Engineering 46 (2012) 57–67

Fig. 2. Changes in the areas of open water and wetlands in the LaBranche area between 1952 (left) and 1983 (right).

to 18 ppt) depending on location, season, frequency of storms, The LIDAR image for the study area was derived from LIDAR data

and freshwater input. The normal elevation of Lake Pontchartrain captured in 2002 (Source: http://atlas.lsu.edu). The LIDAR data

is about 0.1 m above mean sea level (MSL). At least 40% of the are accurate to 15–30 cm root mean square error (RMSE). The

Pontchartrain basin consists of swamps and marshes. The dom- bare earth returns were used to create the LIDAR digital eleva-

inant hydrological inﬂuence on the LB wetlands is the surface tion maps (DEMs) with a 5 m pixel size. Most LIDAR signal is

elevation of Lake Pontchartrain. Although the average tidal range absorbed by water and so gives fewer returns. The water areas

in the lake is about 11 cm, during major storm events the surface within this collection of DEMs were assigned a value that was lower

elevation can range from 0.6 m above MSL during frontal passages than the surrounding land (−0.6 m) and were therefore ignored.

to about 4 m or more above MSL during hurricanes. Elevated water The LIDAR elevations are relative to 1988 NAVD (North Ameri-

levels can completely submerge the LB and BC wetlands (Nesbit can Vertical Datum). In the Lake Pontchartrain side of the project

et al., 2004). area the local MSL is −0.11 m below NAVD88 and on the Mis-

sissippi River side of the project area the difference is −0.16 m (http://vdatum.noaa.gov/welcome.html).

3.3. LIDAR

Light Detection and Ranging (LIDAR) is an optical sensing tech- 3.4. Accretion

nology that measures properties of scattered light to ﬁnd range

137

and/or other information of a distant target. LIDAR can be used Six sediment cores were collected for Cs dating to determine

to determine tree canopy heights down to bare earth elevations. long-term accretion rates. Two cores were collected in each of

Fig. 3. Location of soil (SC1–6) and tree core (TC1–5) collection sites in the LaBranche wetlands and Bonnet Carré Spillway.

J. Day et al. / Ecological Engineering 46 (2012) 57–67 61

Fig. 4. Digital LIDAR elevation maps showing elevation (NGVD 1984) in the Bonnet Carré Spillway (A–A’) and the LaBranche wetlands (B–B’) displayed using Global Mapper

9.0. Elevations should be reduced by 14 cm for reference to local mean sea level. Open water areas in B–B’ are set to −0.6 m. Low elevations represented by the bottom of

the chart at −0.6 m represent water and are not true elevations. This image is a compilation of sections of four DEMs (North American Vertical Datum (NAVD) elevations of

1988) displayed using Global Mapper 9.0. Transect lines are shown traversing the two study areas. Transect A–A’ crosses the Bonnet Carré Spillway and transect B–B’ crosses the LaBranche wetlands.

137

three areas between April and July 2009 (Fig. 3): BC wetlands Sedimentation rates were calculated from initial Cs concentra-

(Cores 1 and 2), LB wetlands south of the railroad (Cores 3 and 4), tion in the proﬁle that correlates to 1953, the year of the beginning

137

and LB wetlands north of the railroad (Cores 5 and 6). Wetlands of Cs fallout.

north and south of the railroad differ in hydrology because the rail- On January 3, 2006 recent sediment accretion due to hurricanes

road restricts surface water ﬂow to wetlands south of the railroad. Katrina and Rita was measured at sites both north and south of

Thus, wetlands north of the railroad exchange water with Lake the railroad from Bayou Labranche to the Pipeline Canal (Fig. 3).

Pontchartrain while those south of the railroad have limited water Cores were collected using 2-cm diameter piston corer to mea-

exchange. sure the depth of the unconsolidated root-free sediment layer due

Cores were collected using 15 cm-diameter thin wall aluminum to hurricane sediment deposition. The upper recently deposited

cylinders. Cores were extruded with a wooden plunger and sec- material readily separated from the lower part of the core that

tioned at 3-cm intervals. Compaction as a consequence of the coring contained roots. We then measured the depth of this recently

procedure was minimal as determined by DeLaune et al. (1990). deposited material by gently pushing a thin metal ruler into the

Core sections were placed into individual aluminum foil cups, dried upper unconsolidated sediment until penetration was stopped by

◦

at 80 C, weighed, and crushed with a mortar and pestle. Sediment the underlying sediments that contained roots. This depth was

−3

bulk density (g cm ) was calculated from the sediment dry weight measured at 55 sites north of the railroad and 50 sites south of

137

and the known volume of each sediment section. Cs activity of the railroad. Several additional cores were collected using the pis-

the bulk sediment was counted with a Lithium-drifted Germanium ton corer to check against the measurements using the ruler and

detector and multichannel analyzer (DeLaune et al., 1978, 2003). all gave the same results using the ruler.

62 J. Day et al. / Ecological Engineering 46 (2012) 57–67

3.5. Tree cores

Two cores were collected from each of 10–12 baldcypress trees

at each of three sites in the LB wetlands and two sites in the adja-

cent BC spillway between January and March 2009 (TC1–5; Fig. 3).

Sampled trees were in dominant or co-dominant crown positions,

so that climatic and soil variations most likely affected tree ring

variations more than did competition from neighboring trees. Trees

from each area were growing in the same stand, so that a stand-

scale estimate of annual growth variations was obtained. Cores

were taken with a 5-mm diameter increment borer above the but-

tress base to avoid distorted rings. The cores were dried, glued to

wooden mounts, and sanded using progressively ﬁner sandpaper

to 600-grit, so that individual cells were visible under the micro-

scope at 10–100× magniﬁcation. Ring widths were then measured

using a Velmex sliding stage (model A60, Bloomﬁeld, New York)

and recorded to the nearest 0.001 mm. The cores were crossdated 137 −1

Fig. 5. Cs activity (pCi segment ) in soil cores of sites located south of the rail-

to assign correct calendar year to each ring and to identify miss-

road in the LaBranche wetlands.

ing rings and then the crossdating was veriﬁed using the standard

dendrochronological software COFECHA (Holmes, 1983; Grissino-

Table 3

Mayer, 2001). Bulk density of LaBranche wetland sites.

ARSTAN (Cook, 1985) was used to create tree ring chronologies 3

Depth (cm) Bulk density (g/cm )

for each site (mean growth index 1.0), using only negative expo-

Site 1 Site 2

nential detrending to account for age effects on ring width but to

allow other long-term trends in growth (such as periods of poor 0–3 0.11 0.23

or good growth) to remain. Correlations of tree growth to weather 3–6 0.19 0.31

6–9 0.17 0.15

(data from the NOAA National Climate Data Center) and salinity in

9–12 0.18 0.22

Lake Pontchartrain at Pass Manchac also were calculated using SAS.

12–15 0.33 0.23

Surface water salinity was collected from the EPA STORET database 15–18 0.25 0.26

(http://www.epa.gov/storpubl/legacy/gateway.htm) for Pass Man- 18–21 0.15 0.25

21–24 0.15 0.23

chac in Lake Pontchartrain from January 1951 through April 1998.

24–27 0.20 0.18

Palmer Drought Severity Index data (PDSI) were obtained from

27–30 0.19 0.22

the National Climate Data Center (NOAA, 2010). PDSI is a mete-

orological drought index that is calculated based on precipitation,

temperature, and available soil water content. 137

In cores collected from the BC wetlands, the Cs activity in

137

core proﬁles did not have a distinct 1963/64 peak because Cs

137

4. Results was introduced primarily during spillway openings and Cs activ-

ity was measured throughout the core. Therefore, sediment was

137

4.1. LIDAR not sampled deep enough to reach a depth where no Cs was

137

present (i.e., prior to 1953 when Cs ﬁrst entered the environ-

The LIDAR image (Fig. 4) showed striking differences in wetland ment; DeLaune et al., 1978). We therefore took deeper cores to

137

elevation between BC and LB. BC wetland elevations (transect line determine where Cs activity ceased. Based on the proﬁle distri-

137

A–A’; ranging from less than 0 to 2.9 m MSL) are higher than ele- bution of Cs (assuming that the 140–160 cm depth was near the

vations (transect line B–B’) in the LB wetlands (ranging from about 1953 marker) we estimate accretion for the two east levee sites

−1

0 to almost 1 m MSL), with the exception of the railroad embank- in the BC wetlands was approximately 2.6 and 2.7 cm yr (Fig. 7).

ment. The low areas in the spillway are due to periodic removal of

sediment for ﬁll material while low spots in the LB wetlands are

due to subsidence and conversion to open water. It is interesting

to note that in the LB transect, areas south of the railroad (average

marsh elevation near sea level) are lower in elevation than areas

north of the railroad (average marsh elevation about 0.2–0.3 m).

4.2. Accretion

137

The peak Cs distribution, representing 1963/64, was

observed in two cores taken from the sites south of the railroad

in the LB wetlands (Fig. 5). Based on this distribution, the accretion

rate of the LB wetlands south of the railroad between 1953 and

−1

2009 was estimated as approximately 0.43 cm yr . Bulk density at

−3

these two sites was between 0.20 and 0.30 g cm (Table 3), which

is typical of highly organic marsh soils receiving little mineral sed-

137

iment input (DeLaune et al., 1978, 1987). Accretion based on Cs

proﬁle distribution was much higher at sites north of the railroad, 137 −1

Fig. 6. Cs activity (pCi segment ) in soil cores of sites located north of the rail-

−1

averaging 1.4 cm yr (Fig. 6). road in the LaBranche wetlands.

J. Day et al. / Ecological Engineering 46 (2012) 57–67 63

Table 5

Tree characteristics at the sample sites.

Site Area Trees Cores Age (yr) Height (m) Mean ring

width (mm)

Mean Max Mean Max

TC1 LaBranche 10 24 130 190 17 23 1.47

TC2 LaBranche 11 20 120 130 19 23 1.44

TC3 LaBranche 12 24 120 130 16 18 1.26

TC4 Bonnet Carré 11 20 110 140 23 27 2.20

TC5 Bonnet Carré 11 20 110 140 21 24 2.31

Estimated from age at coring height and assuming 0.3 m height growth per year.

width per year for trees in LB wetlands (Table 5). Tree height was

about 20% less at the LB sites than at the BC sites, even though the

trees are approximately the same ages (Table 5).

Despite multiple missing and false rings typical of baldcypress

(Ewel and Parendes, 1984), especially when growing under stress-

137 −1

Fig. 7. Cs activity (pCi segment ) in soil cores of sites located in the Bonnet Carré

ful conditions, crossdating for three of the ﬁve sites was sufﬁcient

wetlands.

to meet the stringent standard of less than 10% problem segments

required for inclusion in the NOAA International Tree-Ring Data

Results suggest appreciable sediment was obviously deposited dur-

Bank (Table 6). Trees at site TC2 had up to 10 missing rings after

ing the openings of the spillway. The BC soil proﬁles had bulk

1960 (about 20% missing), so were essentially impossible to cross-

−3

density of over 1.0 g cm (Table 4), which is typical of mineral

date correctly in the period 1960–2008. The resulting ring widths

soils with low organic matter content or a depositional environ-

used to estimate growth and corresponding ARSTAN chronologies

137

ment with high sediment input. Total Cs inventory in the BC soil

are shown in Fig. 8.

proﬁles were also greater than the LB marsh sites suggesting the

The environmental variable most highly correlated with tree-

137 137

deposited riverine sediment was a source of Cs. Cs is readily

ring chronologies was salinity in Lake Pontchartrain (Fig. 9). These

adsorbed to silt & clay and transported to other sites such as the BC

correlations are high despite the fact that the measurement site for

spillway.

salinity was about 10–20 km from the sampling sites. The climate

Short-term sediment deposition due to hurricanes Katrina and

variables most correlated with tree-ring chronologies were pre-

Rita was signiﬁcantly higher north compared to south of the rail-

cipitation and temperature during April and May (Fig. 10). Warm,

road. Deposition north of the railroad was 4.7 ± 1.8 cm while that

dry spring weather reduces growth at these sites, but cool, wet

south of the railroad was 1.8 ± 1.8 cm.

spring weather increases growth. May PDSI was the single climatic

variable most related to growth (Fig. 11).

4.3. Tree cores

5. Discussion

Tree cores revealed that all sites were logged between approxi-

mately 1880–1920 and the majority of the cores are from re-growth

5.1. Wetland accretion and elevation

since then. The average growth of baldcypress trees at the BC sites

was about 2.3 mm ring width per year compared to 1.3 mm ring

Wetland elevation is directly inﬂuenced by a complex relation-

ship between subsidence and accretion (Cahoon et al., 1995; Lane

Table 4 et al., 2006), and the BC spillway serves as an example of how a

Bulk density of Bonnet Carré sites. very large diversion can lead to high rates of accretion and eleva-

3 tion gain. The spillway has been opened eleven times since 1937

Depth (cm) Bulk density (g/cm )

(Table 1). There were ten openings of the spillway for ﬂood control

−

Site 1 Site 2 3 1

(maximum discharge 4500–9000 m s ) and a small experimen-

3 −1

0–3 0.38 0.35 tal opening (400 m s ) in 1994 (Lane et al., 2001). In addition,

3–6 0.38 0.50

leakage through the spillway when river levels are high can deliver

6–9 0.71 0.91 3 −1

up to 300 m s . These openings introduce fresh water, sediment

9–12 0.71 0.93

and nutrients into wetlands in the spillway. As sediments are intro-

12–15 0.90 1.17

15–18 1.28 1.36 duced in the spillway, elevation increases as sediments drop out of

18–21 1.08 1.29

21–24 1.51 1.26

Table 6

24–27 1.53 1.24

Tree ring chronology characteristics.

27–30 1.27 1.26

30–33 1.49 1.65

Site Area Mean inter-series Problem

33–36 1.35 1.74 correlationa segmentsb (%)

36–39 1.56 1.9

39–42 1.49 1.42 TC1 LaBranche 0.548 3

42–45 1.50 0.68 TC2 LaBranche 0.461 21

45–48 1.37 0.75 TC3 LaBranche 0.589 12

48–51 1.33 0.95 TC4 Bonnet Carré 0.544 5

51–54 1.54 1.32 TC5 Bonnet Carré 0.585 8

54–57 1.53 1.53 a

Mean of correlations between detrended ring widths in single cores and the

57–60 1.42 1.63

mean of all other detrended rings widths at that site.

60–63 1.63 1.62 b

50-year periods in a tree that match the rest of the chronology poorly (Grissino-

63–66 1.38 N/A

Mayer, 2001).

64 J. Day et al. / Ecological Engineering 46 (2012) 57–67

5.0 Bonnet Carré 4.0 LaBranche 3.0

2.0

Ring Width (mm) 1.0

0.0

1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

2.5 LaBranche 2.0 Bonnet Carré 1.5

1.0

Growth Index 0.5

0.0

1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Fig. 8. Ring widths (top) and ARSTAN chronologies (bottom). Each value is the mean across sampling locations within each site. Vertical gray bars indicate years when the

Bonnet Carré Spillway was opened.

the water column. For example, sediment elevation near the cen- the Mississippi River. The accretion rates measured in the spillway

ter of the spillway increased by ﬁve to ten feet between 1936 and show that very large but infrequent diversions of river water will

1969 (Fig. 12). In the 1994 opening, Lane et al. (2001) reported lead to accretion sufﬁcient to keep up with predicted relative sea

that about 83% of total suspended sediments introduced from the level rise.

river were deposited in the spillway. The percentage of sediments Accretion was lower south of the railroad, indicating that the

retained in the spillway during large ﬂood control diversions is railroad acts as a barrier to water ﬂow to wetlands to the south

likely considerably less because of the high discharge. and this contributes to sediment deﬁcits in this area. The seven

−1

Long-term accretion rates in the BC wetlands (2.6–2.8 cm yr ) openings under the railroad through which water can exchange

are more than twice the rate necessary to keep pace with current have a total width of about 127 m and a cross section area of only

relative sea-level rise (RSLR), the combination of subsidence plus about 150 m . The length of the railroad running through the LB

−1

eustatic sea level rise. RSLR is ≥1 cm yr in many areas of the Mis- wetlands is about 10,400 m. Thus, much of the water exchange is

sissippi River delta (Day et al., 2007). The IPCC (2007) predicted an restricted to openings that total about 1% of the previous exchange

increase in eustatic sea level rise of about 40 cm and some projec- area. The differences in sediment deposition south and north of the

tions are as high as one meter or more (i.e., Vermeer and Rahmstorf, railroad during hurricanes Katrina and Rita also demonstrate the

2009). The accretion rates measured in the BC Spillway can keep impact of the barrier formed by the railroad.

up with even these high rates (see also Coleman et al., 1998; Day

et al., 2005). In contrast, accretion rates in the LB wetlands south of

−1

the railroad (0.36–0.40 cm yr ) are less than half of what is nec- 5.2. Forested wetlands

essary to keep pace with current RSLR. Accretion rates in LB north

of the railroad are sufﬁcient to survive current RSLR but not pre- Baldcypress trees in the BC wetlands grew almost twice as fast

dicted RSLR by 2100, which is 1.5–2.0 m based on the sea level as in the LB wetlands. By looking at cross-dated tree ring data it is

projections cited above. Lane et al. (2006) reported that elevation possible to gain an understanding of ﬂuctuations in growth of trees

gains at marsh sites at two smaller diversions were sufﬁcient to off- in the two sites (Fig. 11). Several milestone years in the history of

set local RSLR, as is also the case for the Atchafalaya Delta (Shaffer growth in these sites have been accompanied by unusual spring

et al., 1992). Blum and Roberts (2009) predicted most coastal wet- weather. The 1963 spring drought, for example, occurred at the

lands in the Mississippi delta will disappear by 2100 as a result of onset of the poor growth period of the 1960s. This period of poor

subsidence, eustatic sea-level rise, and reduced sediment load in growth also was correlated with the opening of the MRGO shipping

channel that increased salinity levels in Lake Pontchartrain (Shaffer

et al., 2009b). Other spring droughts occurred in 1999, 2000, 2001,

2.5

and 2006, coinciding with periods of reduced tree growth in both

LaBranche r= -0.44

areas (Fig. 11 bottom). High production years occurred in conjunc-

2.0 Bonnet Carré r= -0.45

tion with cool, wet weather in the spring of 1919, 1939, 1961,

1.5 1991–1993, and 2005. Although it may not be intuitive that trees

growing in wetlands grow better in rainy weather, this relationship

1.0 has been found for baldcypress elsewhere in Louisiana (Keim and

Growth Index Amos, 2012), as well as in North Carolina (Stahle and Cleaveland,

0.5

1992), Arkansas (Cleaveland, 2000), and Mississippi because rain

0.0 and ﬂowing water increase oxygen to the root zone when compared

0 1 2 3 4 to stagnant water (Davidson et al., 2006) and may increase nutrient

loading from non-point source runoff (Shaffer et al., 2009a).

Mean Annual Salinity (ppt)

Long-term growth records from tree rings provide informa-

tion to help understand the long-term implications of subsi-

Fig. 9. Correlation between mean annual salinity and tree growth at the LaBranche

wetlands and the Bonnet Carré Spillway. dence, increased ﬂooding (Keeland and Young, 1997; Keim and

J. Day et al. / Ecological Engineering 46 (2012) 57–67 65

0.5

et al., 1996; Hoeppner et al., 2008; Shaffer et al., 2009a), few stud-

0.4 Bonnet Carré ies have combined productivity data with historical records of soil

0.3 LaBranche accretion and tree rings.

The openings of the BC spillway in 1937, 1945, 1950, 1973, 1975,

0.2

1979, 1983, 1997, and 2008 coincided with years of good growth in

0.1 both the BC and LB sites (Fig. 11). Most of these openings occurred

0.0 during times of wet weather. Fresh water from rainfall and from

the spillway combats saltwater intrusion and the deposition of

-0.1

sediments during spillway openings counteracts subsidence so the

-0.2

PDSI BC wetlands are not as subject to saline intrusion and drain bet-

-0.3 ter than the LB sites. Even if the BC sites are inundated with salt

J F M A M J J A S O N D J F M A M J J A water during storm surges, the generally higher elevations and fre-

year prior to formation | year of formation quent freshwater input appear to prevent retention of salinity for

extended periods.

0.5 The combination of dry spring weather, drought, and hurricanes

caused an extended period of poor growth in the 1960s and early

0.4 Bonnet Carré

1970s in both the BC and LB wetlands (Figs. 8 and 11). As mentioned

LaBranche

0.3 above, the opening of the MRGO in 1963 increased salinity in Lake

0.2 Pontchartrain, which contributed to the growth declines in the LB

wetlands at that time (Shaffer et al., 2009b). It increased salinities

r 0.1

in wetlands as remote as the Manchac/Maurepas Swamp (Shaffer

0.0

et al., 2009a). Similarly, the drought of 1998–2001 was responsible

-0.1 for the most recent period of poor growth. This drought also caused

the mortality of a large number of baldcypress in the LB wetlands.

-0.2 Precipitation

Hurricane Katrina also acted in concert with a drought in 2006 to

-0.3

cause another period of poor growth, most likely due to saltwater

J F M A M J J A S O N D J F M A M J J A intrusion.

year prior to formation | year of formation

Degradation of baldcypress—water tupelo swamps at the LB

wetlands is not a unique phenomenon in coastal Louisiana (Keim

0.5

et al., 2006). Declines have been measured in swamps from the

0.4 Bonnet Carré edges of Lakes Pontchartrain and Maurepas to the deep interior

(Hoeppner et al., 2008; Shaffer et al., 2009a). Reduced freshwater

0.3 LaBranche

inﬂows to the Lake since the construction of levees over the past

0.2

100–200 years, and lack of accompanying sediment and nutrient

r 0.1 additions to the wetlands have resulted in widespread decreases

0.0 in tree growth and outright loss of wetland forests. For example,

large areas of the Manchac land bridge between Lake Pontchartrain

-0.1

and Lake Maurepas were forests in the mid-20th century but today

-0.2

Temperature are largely treeless marshes and open water (Barras et al., 1994;

-0.3 Shaffer et al., 2009a).

J F M A M J J A S O N D J F M A M J J A

year prior to formation | year of formation 5.3. Management implications

Fig. 10. Correlation coefﬁcient (r) between monthly climate variables and tree-ring

The health of baldcypress wetlands in the BC Spillway indicates

chronologies from January of the year prior to ring formation through August of the

that periodic, large diversions of river water can be effective in

year of ring formation.

maintaining sustainable coastal forested wetlands. All other river

diversions constructed thus far are at least one order of magnitude

Amos, 2012), and saltwater intrusion on forested wetlands and to smaller than the BC Spillway and, thus, are inadequate to restore

quantify the potential effects of river diversions. Subsidence can the coast on a large scale (see Day et al., 1999). The spillway delivers

prolong ﬂooding in wetlands and, when ﬂooded by salt water, may river water on both temporal and spatial scales that mimic historic

intensify salt stress. Although there has been extensive research crevasses along the river (Davis, 2000). Our research shows that

conducted to understand how forested wetlands respond to these a large-scale diversion would most likely beneﬁt the LB wetlands

stressors (e.g., Conner and Day, 1988; Conner and Day, 1992; Allen and lead to sustainable management of the area.

2.5 8 LaBranche 6 2.0 Bonnet Carré 4 PDSI May 1.5 2

1.0 0 PDSI -2 Growth Index 0.5 -4 0.0 -6

1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Fig. 11. Time series of May PDSI superimposed on tree-ring chronologies.

66 J. Day et al. / Ecological Engineering 46 (2012) 57–67

Fig. 12. Changes in the elevation of the Bonnet Carré wetlands between 1936 and 1969. Elevations are height (ft) above mean sea level.

Accretion rates in the LB wetlands compared to the BC wetlands Cook, E.R., 1985. A Time Series Approach to Tree-Ring Standardization. Ph.D. Dis-

sertation. University of Arizona.

show the effect of isolation of the area from the river while rates in

Conner, W.H., Krauss, K.W., Doyle, T.W., 2007. Ecology of tidal freshwater forests

the spillway demonstrate the effectiveness of periodic high river-

in coastal deltaic Louisiana and northeastern South Carolina. In: Conner, W.H.,

ine input. Although the spillway was opened only about once a Doyle, T.W., Krauss, K.W. (Eds.), Ecology of Tidal Freshwater Forested Wetlands

of the Southeastern United States. Springer, Dordrecht, The Netherlands, pp.

decade, accretion in the BC wetlands is almost an order of mag-

223–253.

nitude higher than in the LB wetlands south of the railroad. Some

Conner, W.H., Day Jr., J.W., 1988. Rising water levels in coastal Louisiana: implica-

areas of the BC wetlands are nearly two meters above sea level. This tions for two forested wetland areas in Louisiana. J. Coast Res. 4, 589–596.

is sufﬁcient to offset the highest predictions of sea-level rise for the Conner, W.H., Day Jr., J.W., 1992. Diameter growth of Taxodium distichum (l.) Rich.

and Nyssa aquatica l From 1979-1985 in four Louisiana swamp stands. Am. Mid.

21st century. By contrast, large areas of the LB wetlands are near

Nat. 127, 290–299.

sea level. In addition, baldcypress production is much greater in the Cramer, G., Day, J., Conner, W., 1981. Productivity of four marsh sites surrounding

BC wetlands. Baldcypress in the LB wetlands are slowly dying out Lake Pontchartrain, Louisiana. Am. Mid. Nat. 106, 65–72.

Davidson, G.R., Laine, B.C., Galicki, S.J., Threlkeld, S.T., 2006. Root-zone hydrology:

due to periodic intrusions of salt water during droughts, excessive

why baldcypress in ﬂooded wetlands grow more when it rains. Tree-Ring Res.

ﬂooding, and lack of recruitment.

62, 3–12.

Long-term restoration of the LB wetlands will require the rein- Davis, D.W., 1993. Crevasses on the lower course of the Mississippi River. Coastal

Zone. 1, 360–378.

troduction of Mississippi River water if the wetlands are to be

Davis, D.W., 2000. Historical perspective on crevasses, levees, and the Mississippi

sustainable into the next century. One possible location for a diver-

River. In: Colten, C.E. (Ed.), Transforming New Orleans and its Environs. Univer-

sion is in the southeast corner of the wetlands because water would sity of Pittsburgh Press, Pittsburgh, PA, pp. 84–106.

Day Jr., J.W., Lane, R.R., Mach, R.F., Brantley, C.G., Daigle, M.C., 1999. In: Rozas, L.P.,

ﬂow over most of the wetland before reaching Bayou LaBranche,

Nyman, J.A., Profﬁtt, C.E., Rabalais, N.N., Reed, D.J., Turner, R.E. (Eds.), Water

the major conduit for water exchange between the north and south

Chemistry Dynamics in Lake Pontchartrain, Louisiana, during the 1997 Opening

portions of the wetland. Water control structures in the openings of the Bonnet Carré Spillway in Recent Research in Coastal Louisiana: Natural

System Function and Response to Human Inﬂuence.

under the railroad could be used to manage ﬂow both south and

Day, J.G., Shaffer, L., Britsch, D., Reed, S., Hawes, D., Cahoon, 2000. Pattern and process

north of the railroad. Pierce et al. (1985) also proposed a river diver-

of land loss in the Mississippi delta: A spatial and temporal analysis of wetland

sion for the LB wetlands. habitat change. Estuaries 23, 425–438.

In summary, the BC Spillway serves as an example of the size Day Jr., J.W., Barras, J., Clairain, E., Johnston, J., Justic, D., Kemp, G.P., Ko, Y., Lane, R.,

Mitsch, W.J., Steyer, G., Templet, P., Yanez-Arancibia, A., 2005. Implications of

of a river diversion that can maintain and build wetlands while the

global climatic change and energy cost and availability for the restoration of the

LB wetlands demonstrate the effects of hydrologic isolation and Mississippi delta. Ecol. Eng. 24, 253–265.

saltwater intrusion. Without large diversions such as the Bonnet Day, J., Boesch, D., Clairain, E., Kemp, P., Laska, S., Mitsch, W., Orth, K., Mashriqui,

H., Reed, D., Shabman, L., Simenstad, C., Streever, B., Twilley, R., Watson, C.,

Carré, we believe that there is little hope for successful restoration

Wells, J., Whigham, D., 2007. Restoration of the Mississippi Delta: lessons from

of the delta, especially with massive wetland losses projected as

Hurricanes Katrina and Rita. Science 315, 1679–1684.

eustatic sea levels continue to rise (i.e., Blum and Roberts, 2009). Day, J.W., Cable, J.E., Cowan, J.H., DeLaune, R., Fry, B., Mashriqui, H., Justic, D., Kemp, P.,

Lane, R.R., Rick, J., Rick, S., Rozas, L.P., Snedden, G., Swenson, E., Twilley, R.R., Wis-

sel, B., 2009. The impacts of pulsed reintroduction of river water on a Mississippi

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