Prepared in cooperation with the U.S. Fish and Wildlife Service

Effects of Prescribed Burning on MarshElevation Change and the Risk of Wetland 

Loss

Open-File Report 2012-1031

U.S. Department of the Interior

U.S. Geological Survey
i

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia 2012

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Suggested citation:

McKee, K.L. and Grace, J.B., 2012, Effects of prescribed burning on marsh-elevation change 

and the risk of wetland loss: U.S. Geological Survey Open-File Report 2012-1031, 51 p.

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.

Cover: The McFaddin National Wildlife Refuge marsh during a prescribed burn; photograph by Tommy 

McGinnis
ii

Acknowledgments

This project was funded in part by the U.S. Geological Survey-Fish and Wildlife Service 

(USGS-FWS) Science Support Partnership (Project Number 05-R2-02). We acknowledge 

associate USGS investigators, Don Cahoon and Glenn Guntenspergen (Patuxent Wildlife 

Research Center) and FWS partners, Andy Loranger and Patrick Walther (Texas Chenier 

Plain National Wildlife Refuge Complex). We greatly appreciate the cooperation and 

support provided by the personnel at the McFaddin National Wildlife Refuge, including 

Craig Crenshaw, Doug Head, and Mike Nance. Several people assisted in installation of 

plots and SETs or in data collection: Heather Baldwin, Julia Cherry, Wes Cochran, Kari 

Cretini, Ada Diz, Andy From, Amber Hanna, Philippe Hensel, Jim Lynch, Tommy 

McGinnis, Amy Trahan, Ellen Travis, and William Vervaeke. We also appreciate the 

comments and suggestions of Connie Herndon, Beth Middleton, and Chris Swarzenski on 

earlier versions of this report. 
iii

Contents

Abstract............................................................................................................................................ 1

Introduction ...................................................................................................................................... 4

Scope of Research ...................................................................................................................... 6

Study Area................................................................................................................................... 7

Experimental Design...................................................................................................................10

Methods ..........................................................................................................................................15

Measuring Elevation Dynamics...................................................................................................15

Measuring Aboveground and Belowground Plant Responses....................................................18

Measuring Decomposition ..........................................................................................................19

Assessing Physicochemical Conditions......................................................................................20

Performing Statistical Analyses ..................................................................................................21

Results and Discussion...................................................................................................................22

Conclusions ....................................................................................................................................42

General Implications for Resource Management........................................................................42

Research Implications ................................................................................................................44

References Cited ............................................................................................................................47
iv

Figures

Figure 1. Map of the study site at McFaddin National Wildlife Refuge (top panel) and views 

(lower panels) of an experimental plot through four seasons prior to the prescribed burn. .............. 8

Figure 2. Setup of experimental plots. Clockwise from top left: (1) construction of platforms, (2) 

study plot demarcated by access platform, (3) driving benchmark rods for installation of surface 

elevation tables (SET), (4) SET receiver cemented to deep benchmark rod...................................11

Figure 3. Schematic diagram of experimental plot showing positions of various measurements 

made during the study.....................................................................................................................12

Figure 4. Views of study area during and after burn treatment (December 2006). Clockwise from 

top left: (1) prescribed burn, (2) initial plant regrowth after burn, (3) elevation measurement after 

burn treatment, (4) plant regrowth in spring 2007. ..........................................................................13

Figure 5. Surface temperatures recorded by data loggers during the prescribed burn on 

December 3, 2006. The upper panel shows data for experimental plots M13-16 (black lines) and 

M1-4 (gray lines). The lower panel shows data for the surrounding marsh to the northwest (NW), 

southwest (SW) and northeast (NE) of the plots. ............................................................................14

Figure 6. Diagram showing the rod surface elevation table (SET) in relation to deep and shallow 

benchmarks and marker horizon, which demarcate three strata in the soil profile to allow vertical 

delineation of processes controlling elevation change. ...................................................................16

Figure 7. Methods used to measure elevation change, accretion, subsidence, and root zone 

movement. Clockwise from top left: (1) Adding feldspar clay to soil surface to create a marker 

horizon, (2) Measuring thickness of accreted sediment above the marker horizon, (3) measuring 

arm of the surface elevation table being attached to the deep benchmark rod, (4) Fiberglass pins 

being lowered to the soil surface and to the shallow benchmark (white cap)..................................17
v

Figure 8. Methods used to measure response to treatments. Clockwise from top left: (1) Clipping 

of aboveground biomass and litter collection, (2) Collection of porewater for measurement of 

salinity, pH, sulfide and nutrients, (3) Installation of in-growth bags to measure rates of 

belowground accumulation of root matter, (4) Installation of a standardized cellulose material to 

measure decomposition rates. ........................................................................................................19

Figure 9. Time-course of elevation change (σ) and accretion above marker horizon (θ) in 

nonburned and burned plots (mean ± standard error, n = 8, averaged over nutrient treatment). 

Note different scales for Y-axis before (left) and after (right) Hurricanes Gustav and Ike................25

Figure 10. Variation in elevation-change rate (Y-axis) in relation to vertical movement (X-axis) in 

three strata, including accretion (top panels), root zone (middle panels), and subroot zone (bottom 

panels). Rates of change measured before and after Hurricane Ike (September 9, 2008) are plotted 

in left or right panels, respectively. Treatment plots are indicated by closed (burned) or open 

(nonburned) symbols. Inset values are correlation coefficients......................................................27

Figure 11. Standing live and dead biomass and fallen litter in burned and unburned plots 

(averaged over nutrient treatments; mean value ± standard error; n = 8). The prescribed burn 

occurred on December 3, 2006.......................................................................................................31

Figure 12. Variation in live and dead stem density in burned and nonburned plots (averaged 

over nutrient treatment; mean value ± standard error; n = 8,). The prescribed burn occurred on 

December 3, 2006...........................................................................................................................32

Figure 13. Aboveground and belowground production of biomass (averaged over burn and 

nutrient treatments; mean value ± standard error; n =8). The prescribed burn occurred on 

December 3, 2006...........................................................................................................................34
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Figure 14. Depth variation in decomposition of standard cellulose material in burned and 

nonburned plots (summer of 2007) (averaged over nutrient treatment; mean value ± standard 

error; n = 8). 36

Figure 15. Soil redox potential (Eh) at soil depth (in centimeters) and porewater salinity, pH, 

sulfide, ammonium (NH4), and phosphate (PO4) measured seasonally in experimental plots 

(averaged over treatments; mean value ± standard error; n = 16)..................................................38

Figure 16. Conceptual model illustrating internal processes and external drivers influencing 

vertical movement in three soil strata in a brackish marsh dominated by Spartina patens, including 

accretion above a marker horizon, root zone, and subroot zone. Observed effects of burning, 

nutrient addition, and hurricanes are indicated as follows: + (positive), - (negative), and 0 (none). 46

Tables

Table 1. Summary of area (hectares) burned in McFaddin National Wildlife Refuge during the 

study period. .................................................................................................................................... 9

Table 2. Summary of relative sea-level rise (SLR) and elevation dynamics in burned and 

nonburned plots at McFaddin National Wildlife Refuge prior to Hurricanes Gustav and Ike 

(September 2008). Values (in millimeters per year) are the mean value ± standard error.............40
vii

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 1.094 yard (yd) 

Area

square meter (m2) 0.0002471 acre 

hectare (ha) 2.471 acre

square millimeter (mm2) 0.001550 square inch (in2)

square meter (m2) 10.76 square foot (ft2) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

Energy

millivolt (mV) 0.001 volt

-Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

-Vertical coordinate information is referenced to North American Vertical Datum of 1988 (NAVD 88)

-Salinity is expressed in permille (‰) units, which is approximately equal to the grams of salt per kilogram of 

solution. In this study, salinity was determined based on the refractive index of the solution and converted to 

permille units by the instrument (refractometer). 
1

Effects of Prescribed Burning on Marsh-Elevation 

Change and the Risk of Wetland Loss

By Karen L. McKee and James B. Grace

Abstract

Marsh-elevation change is the net effect of biophysical processes controlling 

inputs versus losses of soil volume. In many marshes, accumulation of organic matter is 

an important contributor to soil volume and vertical land building. In this study, we

examined how prescribed burning, a common marsh-management practice, may affect 

elevation dynamics in the McFaddin National Wildlife Refuge, Texas by altering 

organic-matter accumulation. Experimental plots were established in a brackish marsh

dominated by Spartina patens, a grass found throughout the Gulf of Mexico and Atlantic 

marshes. Experimental plots were subjected to burning and nutrient-addition treatments 

and monitored for 3.5 years (April 2005 – November 2008). Half of the plots were 

burned once in 2006; half of the plots were fertilized seasonally with nitrogen, 

phosphorus, and potassium. Before and after the burns, seasonal measurements were 

made of soil physicochemistry, vegetation structure, standing and fallen plant biomass, 

aboveground and belowground production, decomposition, and accretion and elevation 

change (measured with Surface Elevation Tables (SET)). Movements in different soil 

strata (surface, root zone, subroot zone) were evaluated to identify which processes were 

contributing to elevation change. Because several hurricanes occurred during the study

period, we also assessed how these storms affected elevation change rates. The main 

findings of this study were as follows:
2

1. The main drivers of elevation change were accretion on the marsh surface and 

subsurface movement below the root zone, but the relative influence of these 

processes varied temporally. Prior to Hurricanes Gustav and Ike (September 

2008), the main driver was subsurface movement; after the hurricane, both 

accretion and subsurface movement were important.

2. Prior to Hurricanes Gustav and Ike, rates of elevation gain and accretion above a 

marker horizon were higher in burned plots compared to nonburned plots, 

whereas nutrient addition had no detectable influence on elevation dynamics.

3. Burning decreased standing and fallen plant litter, reducing fuel load. Hurricanes 

Gustav and Ike also removed fallen litter from all plots.

4. Aboveground and belowground production rates varied annually but were 

unaffected by burning and nutrient treatments.

5. Decomposition (of a standard cellulose material) in upper soil layers was 

increased in burned plots but was unaffected by nutrient treatments. 

6. Soil physicochemistry was unaffected by burning or nutrient treatments.

7. The elevation deficit (difference between rate of submergence and vertical land 

development) prior to hurricanes was less in burned plots (6.2 millimeters per 

year [mm yr-1

]) compared to nonburned plots (7.2 mm yr-1).

8. Storm sediments delivered by Hurricane Ike raised elevations an average of 7.4 

centimeters (cm), which countered an elevation deficit that had accrued over 11 

years.

Our findings provide preliminary insights into elevation dynamics occurring in 

brackish marshes of the Texas Chenier Plain under prescribed fire management. The 
3

results of this study indicate that prescribed burning conducted at 3- to 5-year intervals is

not likely to negatively impact the long-term sustainability of S. patens-dominated 

brackish marshes at McFaddin National Wildlife Refuge and may offset existing 

elevation deficits by ≈ 1 mm yr-1. The primary drivers of elevation change varied in time 

and space, leading to a more complex situation in terms of predicting how disturbances 

may alter elevation trajectories. The potential effect of burning on elevation change in 

other marshes will depend on several site-specific factors, including geomorphic/

sedimentary setting, tide range, local rate of relative sea level rise, plant species 

composition, additional management practices (for example, for flood control), and 

disturbance types and frequency (for example, hurricanes or herbivore grazing). 

Increasing the scope of inference would require installation of SETs in replicate marshes 

undergoing different prescribed fire intervals and in different geomorphic settings (with 

different hurricane frequencies and/or different sedimentary settings). Multiple locations

along the Gulf and Atlantic coasts where prescribed fire is used as a management tool

could provide the appropriate setting for these installations. 
4

Introduction

Global mean sea level has been increasing at a rate of 3.2 millimeters per year 

(mm yr-1

) since 1992 (based on satellite altimetry; Colorado Center for Astrodynamics 

Research, 2011), with predicted future rates due to global warming being at 6 mm yr-1 or 

more (Parry and others, 2007). Coastal marshes must keep pace with rising water levels 

or become submerged. Local subsidence rates may combine with eustatic rates of sealevel rise to cause even faster rates of inundation locally. Emergent vegetation not only

accelerates mineral sediment deposition and vertical accretion (through baffling of 

sediment-laden water by plant shoots) (Morris and others, 2002), but also directly 

contributes organic matter to soil formation (Nyman and others, 1993; Nyman and others, 

2006; McKee and others, 2007; Neubauer, 2008; McKee, 2011). In fact, habitat stability 

of many coastal wetlands in relation to sea level rise may be the result of a feedback 

relationship between environmental conditions and plant matter accumulation (Nyman 

and others, 2006; Mudd and others, 2009; McKee, 2011). In some cases, the decaying 

plant shoots and other organic detritus accumulate on the marsh surface, contributing to 

vertical accretion, but in many marshes and swamps, plant roots and shoot bases 

contribute to subsurface soil expansion (McKee and others, 2007; Cherry and others, 

2009; Langley and others, 2009; McKee and Cherry, 2009; McKee, 2011). 

Any factor that influences plant production or organic matter decomposition will 

alter the net amount of biomass that accumulates in the soil. From a few studies, 

scientists have inferred organic contributions to vertical accretion and soil expansion 

based on analysis of sediment cores (Turner and others, 2006; Sanders and others, 2008). 

Direct measurements of changes in accretion and elevation in relation to accumulation of 
5

organic matter (and processes that control it) are necessary, however, to understand and 

predict habitat stability (Neubauer, 2008; Mudd and others, 2009). For example, 

manipulation of nutrient availability in an oligotrophic mangrove ecosystem altered root 

production, which in turn caused a change in direction and magnitude of elevation change 

(McKee and others, 2007). Two additional studies have shown experimentally that 

increases in atmospheric CO2 can accelerate elevation gain in some coastal marshes by 

stimulating belowground production and upward expansion of the soil surface (Cherry 

and others, 2009; Langley and others, 2009). 

On the other hand, disturbance processes may cause negative effects on 

accumulation of organic matter. Biological agents that damage or remove vegetation 

include grazers such as geese, nutria, muskrat, and snails (Gauthier and others, 1995; 

Ford and Grace, 1998; Silliman and others, 2005). Physical processes include sediment or 

wrack burial, erosion or scouring, and fire (Guntenspergen and others, 1995; Nyman and 

Chabreck, 1995). Of particular concern are marsh management practices that may alter 

accumulation of soil organic matter and vulnerability to submergence. Prescribed burning 

is a common management practice used in coastal marshes to reduce hazardous buildup 

of fuel, enhance wildlife habitat or food sources, and promote rare or endangered species

(Nyman and Chabreck, 1995). Burning may stimulate marsh productivity through release 

of nutrients or may alternatively cause a net loss of organic matter through direct 

combustion of peat and/or a change in the species composition or plant tissue chemistry, 

which affects community production or decomposition rates (Hackney and De La Cruz, 

1977; Schmalzer and others, 1991; Nyman and Chabreck, 1995; Ford and Grace, 1998; 

Gabrey and Afton, 2001; Smith and others, 2001; Ponzio and others, 2004). Burning not 
6

only removes aboveground biomass but may also remove upper layers of peat that may 

have accumulated over long periods of time (Nyman and Chabreck 1995). Depending on 

burn frequency and intensity, however, plant biomass production could be stimulated 

without substantial loss of accumulated organic material (Nyman and Chabreck 1995).

Scope of Research

The goal of this cooperative research was to address an information need 

identified by managers at McFaddin National Wildlife Refuge (NWR): Does the 

prescribed fire regime currently in use adversely affect marsh elevations and increase the 

risk of wetland loss? This study experimentally investigated how fire management and 

nutrient addition (with and without fire) affected elevation dynamics in a brackish marsh 

dominated by Spartina patens (Ait.) Muhl. (marshhay cordgrass), a grass found 

throughout Gulf of Mexico and Atlantic marshes. A prescribed burn was applied once 

during the study, after one year of preburn measurements. We hypothesized that burning 

and/or nutrient addition would alter the net accumulation of organic matter, which would 

affect the rate of soil elevation change. Because several hurricanes occurred during the 

study, we also assessed the relative impact of these storms in relation to experimental 

treatment effects. Thus, our research was aimed at addressing the following questions:

1. What are the main drivers of elevation change and do they vary over time?

2. Does burning or nutrient enrichment have a positive or negative effect on 

elevation change?

3. To what extent do biotic processes (shoot or root production, decomposition) 

contribute to elevation change in the experimental marsh and how are they affected by 

burning and nutrient enrichment? 
7

The information gained in this study will be useful in designing and planning 

future fire regimes and will provide insight into controls on accretion and elevation 

change in brackish marshes. Together with other studies of fire effects on marsh elevation 

(Cahoon and others, 2010), this work will lead to better management strategies for 

coastal wetlands nationwide. 

Study Area

The study was conducted at the McFaddin NWR, a 23,843 ha area, which is part 

of the Texas Chenier Plain National Wildlife Refuge Complex. Chenier plain marshes are 

typically more stable than those in more rapidly subsiding areas, such as in deltaic areas, 

but are still subject to rising sea level. The current rate of relative sea level rise at 

Galveston Pier 21, TX (tide gauge ID 8771450, 1908-2010) is 6.3 mm yr-1 (NOAA, 

2011). The McFaddin NWR contains extensive expanses of freshwater, brackish, and salt 

marsh. To maintain plant diversity in this system, several management techniques are

used, including prescribed burns, grazing, water level management, and exotic species 

control. The site selected for this study was a brackish marsh located along a bayou 

within the McFaddin NWR (29o 41’41.47”N, 94o 04’04.85”W) (fig. 1). The dominant 

plant species was S. patens with minor occurrence of Bolboschoenus robustus (Pursh) 

Soják. At this site, a thin organic mat (< 1 m thick) overlies deeper sediments composed 

of compacted silt and clay. Brackish marshes, including the area used in this study, are 

burned at about 3-year intervals, usually at the end of the growing season. 
8

Figure 1. Map of the study site at McFaddin National Wildlife Refuge (top panel) and views (lower panels) of an experimental plot through four 

seasons prior to the prescribed burn. 
9

Experimental plots were established in April 2005, and measurements were made 

seasonally through November 2008. During the study period, a total of 18,653 ha in the 

Refuge were treated by prescribed fire, and another 12,701 ha were burned by wildfire 

(table 1). The marsh encompassing the experimental plots was burned once in 2006 (see 

below for details). Also, four major hurricanes occurred during the study, including

Hurricanes Katrina (August 29, 2005), Rita (September 24, 2005), Gustav (September 1, 

2008), and Ike (September 13, 2008).

Table 1. Summary of area (in hectares) burned in McFaddin National Wildlife Refuge during the 

study period.

 

Year Wild Prescribed

2005 7,998 6,359

2006 509 1,917

2007 2,613 10,369

2008 1,581 8

Total 12,701 18,653

Combined 31,354
10

Experimental Design

The experimental design was completely randomized with a 2 x 2 factorial 

treatment arrangement. Two rows of eight plots each were established in the interior 

marsh ≈ 120 m from the bayou. The plots were positioned several meters apart to isolate 

treatment effects. The perimeter of each plot (5.2 m x 3.2 m) was defined by a wooden 

platform that facilitated access and minimized disturbance (fig. 2). An extendible plank 

was laid across the platform to provide access to the plot interior. The plots were each

divided into a section for nondestructive measurements and another section for 

destructive sampling (fig. 3). 

Within the nondestructive segment of the plots, a rod surface elevation table 

(hereafter, SET) along with deep and shallow benchmarks and marker horizons were

installed (see below for details) (fig. 3). Destructive measurements, including 

aboveground and belowground production, decomposition, and physicochemistry, were 

made in the other half of the plot (see below for details) (fig. 3). 

Plots were randomly assigned to a nutrient treatment (none, nutrient addition) and 

a fire treatment protocol (burned, nonburned). Each nutrient-fire treatment combination 

was replicated four times for a total of 16 experimental plots (M1-16). Fertilized plots 

received a slow-release fertilizer (Vigoro Tree, Shrub, and Evergreen Food; Spectrum 

Brands; Madison, WI, USA; containing a 16:4:8 ratio of nitrogen, phosphorus, and 

potassium), which was broadcast onto the marsh surface during low water levels 3 times 

per year. The fertilizer treatment was initiated in April 2005 and continued until the end 

of the study in 2008.
11

The prescribed burn occurred on December 3, 2006 (fig. 4). Fire crews burned the 

surrounding marsh and allowed the fire to move across the study area. In preparation for 

the burn, platforms in burn plots were partially disassembled, and lumber was stored in 

adjacent (nonburned) plots. Deep and shallow benchmarks were protected by fire 

blankets. Half of the plots were prevented from burning by operation of a sprinkler 

system installed prior to the fire. The sprinkler system protected all but one nonburned 

plot where the sprinklers failed; this plot was partially burned. Temperature loggers were 

deployed in all burn plots and at three points in the surrounding marsh and retrieved after 

the burn. Surface temperatures reached a peak of 274o C in the marsh just northwest of

the site; the maximum temperature in burn plots ranged from 32 to 185 o C (fig. 5). 

Figure 2. Setup of experimental plots. Clockwise from top left: (1) construction of platforms, (2) 

study plot demarcated by access platform, (3) driving benchmark rods for installation of surface 

elevation tables (SET), (4) SET receiver cemented to deep benchmark rod.
Figure 3. Schematic diagram of experimental plot showing positions of various measurements made during the study.
13

Figure 4. Views of study area during and after burn treatment (December 2006). Clockwise from top left: (1) prescribed burn, (2) initial plant 

regrowth after burn, (3) elevation measurement after burn treatment, (4) plant regrowth in spring 2007. 
Figure 5. Surface temperatures recorded by data loggers during the prescribed burn on 

December 3, 2006.The upper panel shows data for experimental plots M13-16 (black lines) 

and M1-4 (gray lines). The lower panel shows data for the surrounding marsh to the northwest 

(NW), southwest (SW) and northeast (NE) of the plots.
15

Methods

Measuring Elevation Dynamics

Surface elevation change in each plot was determined to the nearest millimeter

with SETs (Cahoon and others, 2002). Change in the elevation of the soil surface relative 

to the base of a benchmark reflects expansion or contraction over different portions of the 

sediment profile (fig. 6). The rod SET setup consisted of a benchmark rod driven to the 

point of refusal, four shallow benchmarks (30 cm deep), and a portable measuring arm. 

Deep benchmark rods were driven with a slide-hammer to depths of 15 to 16 m. Shallow 

benchmarks, which were constructed of 5-cm diameter x 40-cm long aluminum pipe 

capped at the top with an acrylic disc, were inserted into the soil to a 30-cm depth. 

During measurements, the portable arm was attached to the deep benchmark in one of 

four fixed directions and leveled in both horizontal and vertical planes. On each sampling 

date, seven fiberglass pins were lowered to the soil surface, and the extension distances 

above the arm were recorded. Changes in pin extension distance corresponded to changes 

in soil elevation relative to the baseline measurement. Two pins were lowered to the top 

of the shallow benchmark, which allowed movement in the root zone to be isolated from 

movement in the sub-root zone by partitioning change in the upper 30 cm from the total 

elevation change over the entire soil profile (figs. 6-7). Accretion of sediment on the soil 

surface was determined by using marker horizons (3 per plot) of white feldspar clay sown 

onto the soil surface (fig. 7). Marker horizons were ≈ 1 cm thick and covered an area of 

0.25 m2

. Accumulation of sediment was determined at 4-month intervals by collecting

cryogenic cores through each marker horizon (fig. 7) (Cahoon and others, 1996).
16

Figure 6. Diagram showing the rod surface elevation table (SET) in relation to deep and shallow 

benchmarks and marker horizon, which demarcate three strata in the soil profile to allow 

vertical delineation of processes controlling elevation change.

The marker horizon depth was measured at four positions around the core to the nearest 

millimeter and averaged. For each SET, a total of 28 deep-benchmark, 8 shallowbenchmark, and 3 marker-horizon measurements were recorded.

The elevations of the marsh surface in each SET plot were determined by using 

real-time kinematic (RTK) Global Positioning System measurements, with a minimum of 

15-second (s) observations at four points within each plot. RTK elevations were 

expressed with respect to the North American Vertical Datum of 1988 (NAVD88).
17

Figure 7. Methods used to measure elevation change, accretion, subsidence, and root zone 

movement. Clockwise from top left: (1) Adding feldspar clay to soil surface to create a marker 

horizon, (2) Measuring thickness of accreted sediment above the marker horizon, (3) 

measuring arm of the surface elevation table being attached to the deep benchmark rod, (4) 

Fiberglass pins being lowered to the soil surface and to the shallow benchmark (white cap). 
Measuring Aboveground and Belowground Plant Responses

Standing plant biomass was harvested at 4-month intervals by clipping all shoots 

at the soil surface within a 0.1-square meter (m2) circular quadrat (fig. 8). All fallen litter 

was collected within the quadrat and bagged separately. Clipped plots were randomly 

selected from a gridded area and marked to avoid resampling. Standing and fallen 

material was later sorted by species and condition (live, dead), oven-dried (60o C), and 

weighed. Annual aboveground production was estimated by summing end-of-growing 

season standing live and dead biomass (Morris, 2007).

Belowground accumulation of roots and rhizomes was determined by using the 

in-growth method (McKee and others, 2007). Duplicate in-growth bags (5-cm diameter x 

30-cm length) containing a root-free, standardized organic substrate (milled sphagnum 

peat) were installed in all plots at the beginning of the study (fig. 8). The bags were 

constructed of a flexible, large-mesh material (3 square millimeters (mm2

), J&M 

Industries, Ponchatoula, LA, USA), which allowed in-growth of both fine and coarse 

roots. A core of soil was removed initially, and a prefilled bag was inserted into the hole;

the top of the bag was secured with twine to a stake. Bags were retrieved after one year 

by recoring; new bags were installed in the same location. Bags with ingrown material

were divided into 10-cm segments, placed into ziplock bags, and kept cool until 

processing. All ingrown material was washed over a 1-mm sieve, separated into size 

classes (fine roots [<2 mm], coarse roots [>2 mm, rhizomes]) and condition [live or dead 

based on appearance and flotation], dried at 60 oC, and weighed. Rates of belowground 

production (to a 30-cm depth) were calculated by dividing the mass per unit area by the 

time interval to give an annual value (grams per square meter per year [g m-2

yr-1]).
19

Figure 8. Methods used to measure response to treatments.Clockwise from top left: (1) clipping 

of aboveground biomass and litter collection, (2) collection of porewater for measurement of 

salinity, pH, sulfide and nutrients, (3) installation of in-growth bags to measure rates of 

belowground accumulation of root matter, (4) installation of a standardized cellulose material to 

measure decomposition rates.

Measuring Decomposition

Decomposition rates in each experimental plot were measured by using the 

“cotton-strip technique” (Slocum and others, 2009). Canvas material (100 percent undyed 
20

cotton, style number 568, Tara Materials, Inc., Lawrenceville, Ga., 

www.taramaterials.com) was cut into strips (10 cm x 30 cm) and inserted vertically into 

the soil with a spade so that about 4 cm extended above the soil surface (fig. 8). The exact 

position of the soil surface was marked by clipping a notch in the strip. Samples remained 

in the ground for 10 days and then were retrieved; at the time of sample retrieval, 

reference strips were inserted and immediately removed. At the laboratory, all strips were 

washed with deionized water, air-dried, and stored away from sunlight. Sampling depths 

were marked with a pencil on each strip and then sub-strips were cut by using a rotary 

blade. Tensile strength of each sub-strip was measured with a tensometer (Slocum and 

others, 2009). Rates of loss of cotton tensile strength were calculated as CTSL (%) = [1-

(N/C)/D] x 100, where N is the strength of the experimental sub-strip in Newtons; C is 

the mean tensile strength of the reference sub-strips; and D is the number of days in the 

ground. 

Assessing Physicochemical Conditions

Duplicate samples of pore water were collected at a 25-cm depth in the 

undisturbed section of the plot and analyzed for salinity, pH, sulfide, and nutrients as 

described previously (McKee and others, 1988; McKee and McGinnis, 2002) (fig. 8). 

Soil samples were collected with a piston corer for measurement of bulk density (dry 

mass per volume) and percent organic matter (loss on ignition) according to standard 

methods (Parent and Caron, 1993). Soil redox potentials at 1-, 15-, and 30-cm depths 

were measured with bright platinum electrodes equilibrated in situ for 30 min (McKee 

and others, 1988). Each electrode was checked before use with quinhydrone in pH 4 and 

7 buffers (millivolt [mV] reading for quinhydrone is 218 and 40.8, respectively, at 25 C). 
21

The potential of the calomel reference electrode (+244 mV) was added to each reading to 

calculate redox potential (Eh). 

Performing Statistical Analyses 

The elevation data (total change and vertical movement of each depth interval) 

were divided into the period prior to and after Hurricanes Gustav and Ike (September

2008) and analyzed separately. The total vertical departure (cumulative) from the 

baseline for each fiberglass pin (36 per SET) was calculated for each sampling date. The 

cumulative changes for the 28 pins lowered to the soil surface were averaged to provide 

an overall rate of mean elevation change (relative to the base of the deep benchmark) per 

SET per date. Similarly, the cumulative changes for the 8 pins lowered to the shallow 

benchmarks (4 directions per SET) were averaged and represented the movement 

between the base of the shallow benchmarks to the depth of the deep benchmark. The 

depths of sediment accreted above the three marker horizons per SET were also averaged 

to provide a single mean value for each SET per sampling date. Movement within the 

root zone was calculated by subtracting the shallow benchmark movement and accretion 

from total elevation change. Total subsurface change (subsidence or expansion) was 

calculated by subtracting accretion (above the marker horizon) from total elevation 

change (soil surface movement relative to the base of the deep benchmark). Thus, each 

SET plot provided five linear trajectories representing total elevation change, shallow 

subsidence, and movement in three strata, including surface accretion, root zone, and 

subroot zone. The slopes of these linear relationships were determined to provide a rate 

of change for each SET, which was considered to be the experimental unit. 
22

Rates of change for elevation variables were analyzed by analysis of covariance 

(ANCOVA) using the NAVD88 elevation of the SET plots as a covariate and burn and 

fertilizer treatments as categorical grouping factors. Other variables (biomass and 

production) were analyzed by using a repeated-measures analysis of variance (ANOVA)

with burn and nutrient treatments as grouping factors and time as the repeated measure.

Decomposition data were assessed with repeated measures ANOVA by using burn and 

nutrient treatments as the grouping factors and depth as the repeated measure. If 

necessary, data were log-transformed to meet assumptions of ANOVA. Statistical 

analyses were performed with JMP® Version 9.0.0 (SAS Institute, Inc., Cary, N.C.).

Results and Discussion

Several considerations must be kept in mind when interpreting the results of this 

study. First, the study was conducted in one location within the McFaddin NWR and in 

one marsh habitat. Thus, the results may not be applicable to other locations or marsh 

types within the refuge or in other geographic locations, especially where conditions (for 

example, subsidence) are considerably different. Nevertheless, the natural mechanisms 

involved in controlling elevation dynamics in the experimental plots and the effects of 

burning and nutrient treatments on these mechanisms are generally relevant to other 

coastal marshes. Second, the responses to treatments do not necessarily indicate longterm responses to burning or nutrient treatments and, therefore, cannot be extrapolated 

beyond the time interval of the study. Third, these results reflect the effect of a single 

prescribed burn (simulating a 3- to 5-year burn interval); more frequent (annual) burning 

may produce different results. Fourth, several hurricanes occurred during the study and 
23

apparently influenced some of the response variables, including biomass accumulation, 

soil chemistry, and elevation dynamics. 

With these caveats in mind, we describe the key results and their implications in 

the following sections.

1. Result: The main drivers of elevation change in the brackish marsh studied at 

McFaddin NWR were accretion on the marsh surface and subsurface movement 

below the root zone, but the relative influence of these processes varied 

temporally (with hurricane occurrence).

Movements in different soil strata contribute to elevation change of the marsh 

surface (Cherry and others, 2009; McKee and Cherry, 2009; McKee, 2011). Several 

biological and physical processes control these movements by promoting vertical 

expansion or contraction of these soil strata. Mineral and/or organic detritus is deposited 

on the soil surface and can undergo subsequent compaction or erosion or microbial 

decomposition (organic). Root-zone movement is driven by belowground production of 

roots, rhizomes, and shoot bases; decomposition of organic matter; and physical 

compaction. Below the root zone, subsidence and shrink-swell of the substrate (due to 

changing water content) predominate. The relative contribution of these processes in each 

soil strata varies among marshes and over time. This study quantified the rate and 

direction of movement in each of these soil strata, which allowed identification of the 

main processes controlling elevation change in the study site at McFaddin NWR. 
24

Elevation and accretion in burned and nonburned plots varied over time relative to 

the initial baseline (fig. 9). Prior to Hurricanes Gustav and Ike, the rate of elevation 

change varied across all experimental plots from -9.1 to 7.9 mm yr-1, with an overall 

average of -0.4 mm yr-1. Elevation change was positively correlated with plot elevation, 

which varied from 0.235 to 0.301 m (NAVD88) (r = 0.71, P = 0.0021). This correlation 

likely reflects a feedback relationship between elevation and flooding-related factors 

influencing sedimentation and production-decomposition processes (Neubauer, 2008; 

Mudd and others, 2009; McKee, 2011). Accretion above marker horizons varied from 0.4 

to 7 mm yr-1

, with an overall average of 3.2 mm yr-1. Accretion exceeded elevation 

change throughout the 3.5 year observation period, indicating that subsidence was 

contributing to vertical movement of all plots regardless of treatment. The rate of average 

shallow subsidence across the study site was -3.6 mm yr-1. The rate of shallow subsidence 

plus relative sea level rise (SLR) (6.3 mm yr-1) yields a total submergence rate of 9.9 mm 

yr-1. Vertical change in the root zone varied from -0.6 to 0.9 mm yr-1 (average = 0.2 mm 

yr-1), indicating a slight, but overall expansion. Total elevation gain (accretion plus root 

zone) was 3.4 mm yr-1

. Thus, there was an overall elevation deficit (submergence minus 

elevation gain) of 6.5 mm yr-1 at this site prior to the 2008 hurricanes. 
25

Figure 9. Time-course of elevation change (σ) and accretion above marker horizon (θ) in 

nonburned and burned plots (mean ± standard error, n = 8, averaged over nutrient treatment). 

Note different scales for Y-axis before (left) and after (right) Hurricanes Gustav and Ike. 
26

To determine which processes were driving vertical movement of the marsh 

surface, elevation change rates across plots were assessed in relation to movement in the 

three main strata (surface, root zone, subroot zone) (fig. 10). Prior to Hurricanes Gustav 

and Ike, the primary driver of change was sub-root zone movement (r = 0.92, P <0.0001). 

Movement below the root zone thus explained 85 percent of the variation in elevation 

change before this hurricane, whereas accretion and root zone expansion together 

explained only 15 percent. The storm surge associated with Hurricane Ike (East and 

others, 2008) delivered substantial sediment to experimental plots, which strongly 

influenced elevations (fig. 9). After this hurricane, accretion explained 53 percent and 

subroot zone movement explained 46 percent of the variation in elevation change (fig. 

10). In Louisiana brackish marshes impacted by Hurricane Katrina, storm sediments also

increased elevations by several centimeters, but post-hurricane elevation movement in 

upper soil layers was determined by the texture (for example, particle size and organic 

matter content) of the deposited sediment (McKee and Cherry, 2009).

Authors of a companion fire study conducted at Blackwater NWR (on the eastern 

shore of Maryland) found that marsh-elevation change, averaging 4 mm yr-1

across 

treatment plots, was driven by three main factors, including accretion, root-zone 

subsidence, and shrink-swell of sediments below the root zone (Cahoon et al. 2010). At 

Blackwater NWR, surface accretion was higher (5.9 – 9.7 mm yr-1

) and caused by litter 

accumulation rather than mineral sedimentation, but there was root-zone collapse (<-1 to 

-6 mm yr-1). Overall, shallow subsidence varied from -1.7 to 5.9 mm yr1.
27

Figure 10. Variation in elevation-change rate (Y-axis) in relation to vertical movement (X-axis) in 

three strata, including accretion (top panels), root zone (middle panels), and subroot zone 

(bottom panels). Rates of change measured before and after Hurricane Ike (September 9, 

2008) are plotted in left or right panels, respectively. Treatment plots are indicated by closed 

(burned) or open (nonburned) symbols. Inset values are correlation coefficients.
28

These results illustrate how relative movement in different soil strata (and associated 

processes) can vary substantially within and among marshes over time. Consequently, it 

is important to avoid viewing such controlling factors as static, either in time or space.

2. Result: Burn treatment increased rates of elevation gain and accretion compared 

to those in nonburned plots, whereas nutrient treatment had no detectable 

influence on elevation dynamics.

Because of the overriding effect of storm sediment on elevation dynamics, effects 

of fire and nutrient treatments were assessed by using data collected prior to the 2008 

hurricanes (earlier hurricanes had no detectable effect). Before the 2008 hurricanes, 

nutrient addition did not significantly affect elevation-change rates, but burning had a 

significant effect on both accretion and elevation change. The burned plots gained 

elevation (+0.14 mm yr-1

), whereas nonburned plots lost elevation (-0.87 mm yr-1) (main 

effect of fire, F = 5.34, P = 0.0496) (fig. 9). Thus, burning not only affected the 

magnitude of elevation change, but also the direction, and the difference caused by 

burning was approximately 1 mm yr-1. Accretion above feldspar marker horizons also 

was higher in burned (4.5 mm yr-1

) compared to nonburned (1.9 mm yr-1) plots. Subsurface change indicated that some plots were subsiding, whereas others were undergoing 

expansion, but there was no effect of burning or nutrient treatments on root zone or 

subroot zone movement. These findings indicate that the effect of burning on soil 

expansion occurred in the surface layers (above the marker horizon). This effect may 

result from deposition of residual ash from the burned plant material that was deposited 
29

on the soil surface. The lack of a nutrient effect on elevation further suggests that burning 

mainly acted by causing a physical contribution to vertical accretion, rather than through 

stimulation of plant growth, such as roots on the marsh surface.

Similarly, a previous study conducted at McFaddin NWR found that marsh 

burning slowed the rate of elevation loss compared to a nonburned marsh after hurricaneinduced flooding killed S. patens (the dominant vegetation), causing root zone collapse

(Cahoon and others, 2004). The positive effect of burning appeared to be related to 

increased root growth resulting in an increase in organic matter volume, but effects on 

decomposition could not be ruled out. Results of the study at Blackwater NWR indicated

no effect of burning on elevation change (Cahoon et al. 2010). Although not significantly 

different, there was a trend for faster accretion with decreasing burn frequency. Rates of 

accretion in marshes with different fire (frequency) regimes were 5.9 mm yr-1 (annual), 

6.9 mm yr-1 (3–5 year), 8.4 mm yr-1 (7–10-year), and 9.7 mm yr-1 (control). Shallow 

subsidence tended to be slower in annually burned marshes (1.7 mm yr-1) compared to 

control marshes (5.9 mm yr-1) or plots undergoing burn frequencies of 3–5 year (3.4 mm 

yr-1

) and 7–10 year (3.3 mm yr-1). Lower litter and accretion on the marsh surface may 

have offset root-zone subsidence, leading to similar rates of elevation change in annually 

burned marshes (4.0 mm yr-1), in marshes burned at other frequencies (3.9 mm yr-1 [3–5 

year], 5.0 mm yr-1 [7–10 year]), and in control marshes (3.8 mm yr-1).

3. Result: Burning decreased standing and fallen plant litter, reducing fuel load; 

Hurricanes Gustav and Ike also removed fallen litter from all plots.
30

Aboveground biomass (live and dead standing and fallen litter) varied over time 

and in response to burn and nutrient treatments and to hurricane impact (fig. 11). 

Aboveground live biomass was increased by nutrient addition before the prescribed burn 

(479 vs. 715 grams per square meter [g m-2] in control and fertilized plots) but it did not 

differ among treatment and control plots after the burn and averaged 516 g m-2 (time by

nutrient interaction, F = 6.8, P = 0.012). Prior to the prescribed burn, standing dead 

biomass averaged 703 g m-2 in subsequently burned and 841 g m-2 in nonburned plots; 

litter averaged 181 g m-2 in subsequently burned and 164 g m-2 in nonburned plots. After 

the burn, dead biomass was significantly lower in burned (326 g m-2, standing dead; 112 

g m-2, litter) compared to nonburned (736 g m-2, standing dead; 365 g m-2, litter) plots. 

After Hurricanes Gustav and Ike, litter in all plots was decreased to approximately 51 g 

m-2 and did not differ among burn and nutrient treatments. At Blackwater NWR, annual 

burning led to lower standing dead biomass (38 g m-2yr-1) compared to less frequently 

burned marshes (175–216 g m-2yr-1 (Cahoon et al. 2010).

Live and dead stem densities also varied over time and in response to burning 

(fig. 12). Prior to the burn, live stem density was similar in subsequently burned (1021 

stems m-2) and nonburned (964 stems m-2) plots; after the burn, density was two times

higher in burned (2309 stems m-2) than in nonburned (1054 stems m-2) plots (fire by time 

interaction, F = 5.38, P = 0.0235). Live density decreased to zero after Hurricanes Gustav 

and Ike. Dead stem density varied over time, but differences between burned and 

nonburned plots were not significant. 
31

Figure 11. Standing live and dead biomass and fallen litter in burned and unburned plots 

(averaged over nutrient treatments; mean value ± standard error; n = 8). The prescribed burn 

occurred on December 3, 2006.
32

Figure 12. Variation in live and dead stem density in burned and nonburned plots (averaged over 

nutrient treatment; mean value ± standard error; n = 8,). The prescribed burn occurred on 

December 3, 2006. 
33

These findings support one goal of prescribed burning, which is to reduce fuel 

loads, but the findings also show that standing stocks of dead material rapidly return to 

pre-burn levels (within 1–2 years). Consequently, more frequent burning may be 

necessary to maintain low amounts of litter. Storm surge from hurricanes may also reduce 

fuel loads by physically flushing out litter. 

4. Result: Burning had no effect on aboveground or belowground production.

Burning may stimulate plant growth by rapidly mineralizing organic matter and 

adding nutrients or by removing standing and fallen dead litter and increasing light 

penetration to support new shoot growth. Although aboveground and belowground 

production rates varied annually, they were unaffected by the experimental treatments

and averaged 1256 and 155 g m-2

yr-1, respectively, across all plots and years (fig. 13). 

Belowground production varied significantly with depth; most root matter accumulated in 

the upper 10 cm (121 g m-2

yr-1) compared to the deeper layers (16 – 19 g m-2yr-1). 

At Blackwater NWR, annual burning increased both aboveground and 

belowground production (Cahoon et al. 2010). The lack of response at McFaddin NWR 

may be due to the lesser frequency of burn treatment, compared to that at Blackwater 

NWR. Another difference between the two studies is that the plant community at 

Blackwater NWR contained Schoenoplectus americanus in addition to S. patens, whereas 

the marsh at McFaddin NWR was dominated by a near-monoculture of S. patens. The 

two species differ in their responses to disturbance, and growth of S. patens may not be as 

responsive to burning as S. americanus. Another possibility is that annual burning 
34

removed more dead standing and fallen litter at Blackwater NWR, which promoted 

greater production of new shoots in spring. 

Figure 13. Aboveground and belowground production of biomass (averaged over burn and

nutrient treatments; mean value ± standard error; n =8). The prescribed burn occurred on 

December 3, 2006.
35

5. Result: Decomposition in upper soil layers was increased by burning.

Accumulation of organic matter requires slower rates of decomposition than 

production. Burning may accelerate decomposition of soil organic matter by stimulating 

the activity of the soil microbial community via changes in nutrients or other 

environmental conditions. Burial of a standard cellulose material in the experimental 

plots allowed assessment of the decomposition potential—essentially the activity of the 

microbial community--at different soil depths within the root zone where most of the 

belowground production occurred. 

Decomposition of the standard cellulose material (summer 2007) varied by soil 

depth (main effect, P < 0.001) and was faster in upper soil layers in burned plots 

(interaction of depth by fire, P = 0.05) (fig. 14). There was no effect of nutrient addition 

on tensile strength loss, suggesting that the effect of burning on decomposition was not 

related to changes in availability of nitrogen, phosphorus, and potassium. Although 

decomposition appeared to be stimulated in burned plots, the greater accretion above the 

marker horizon counterbalanced any losses in organic matter, so that the net effect was a 

greater gain in elevation. If there had been lower surface accretion at this site or higher 

subsidence rates, however, the increased rate of decomposition could have had a greater 

influence on elevation.
36

Figure 14. Depth variation in decomposition of standard cellulose material in burned and 

nonburned plots (summer of 2007) (averaged over nutrient treatment; mean value ± standard 

error; n = 8).
37

6. Result: Soil physicochemistry was unaffected by burning or nutrient addition.

Soil bulk density across all plots averaged 0.1 ± 0.01 grams per cubic centimeter 

(g cm-3), and percent organic matter averaged 56 ± 2. There were no differences in these 

variables attributable to burning, nutrient addition, or season. Also, there were no 

substantial effects of either burning or nutrient treatments on soil or porewater chemistry, 

although these parameters varied significantly over time (fig. 15). Soil redox potential 

and sulfide concentration indicate flooding-related conditions and typically vary 

seasonally with soil flushing and plant growth. The levels measured during this study

suggested low to moderate flooding stress at the study site, which increased over the 

period of observation. Salinity increased after Hurricane Ike, but values remained within 

the tolerance range of S. patens. Salinity was significantly higher in burned (11 parts per 

thousand [ppt]) versus nonburned (10 ppt) plots, but this difference was slight and not 

biologically significant. Porewater concentrations of nutrients (ammonium [NH4] and 

phosphate [PO4]) fluctuated seasonally and annually but did not show a significant 

response to treatments. Plant uptake and/or chemical transformations likely explain why 

porewater concentrations of these nutrients were not elevated in nutrient-amended plots. 
38

Figure 15. Soil redox potential (Eh) at soil depth (in centimeters) and porewater salinity, pH, 

sulfide, ammonium (NH4), and phosphate (PO4) measured seasonally in experimental plots 

(averaged over treatments; mean value ± standard error; n = 16). 
39

7. Result: The elevation deficit was lower in burned plots (6.2 mm yr-1) compared to 

nonburned plots (7.2 mm yr-1).

The vulnerability of marshes to sea-level rise is determined by the rate of vertical 

marsh building relative to the rate of submergence, that is, relative SLR (table 2). The 

relative SLR for the region is based on measurements from local tide gages. In this case, 

mean sea level rise was estimated at 6.3 mm yr-1 (NOAA, 2011). This value reflects 

changes in water level as well as land movement below the base of the tide gage. To

determine the true relative SLR, however, the rate of shallow subsidence (between the 

base of the tide gage and the land surface) must be added to the rate of relative SLR 

based on the tide gage. This value is provided by the SET measurements. 

The subroot-zone movements were -4.6 mm yr-1 (burned plots) and -3.1 mm yr-1

(nonburned plots), giving a total submergence rate of 10.9 mm yr-1 and 9.4 mm yr-1, 

respectively. To maintain soil elevations relative to submergence, land building must 

occur at equivalent rates. The rate of land building is the combination of root zone 

expansion plus accretion on the soil surface. The combined rates of gain prior to 

Hurricanes Gustav and Ike were 4.7 mm yr-1 (burned plots) and 2.2 mm yr-1 (nonburned 

plots). Thus, the elevation deficit was lower in the burned plots (6.2 mm yr-1

) compared 

to nonburned plots (7.2 mm yr-1

) prior to these hurricanes (main effect of fire, F = 5.41, P 

= 0.0484). Because storm sediments subsequently altered elevation dynamics at this site, 

it was not possible to determine how much longer this difference in elevation deficit 

would have been maintained.
40

Table 2. Summary of relative sea-level rise (SLR) and elevation dynamics in burned and 

nonburned plots at McFaddin National Wildlife Refuge prior to Hurricanes Gustav and Ike

(September 2008). Values (in millimeters per year) are the mean value ± standard error.

Burned Nonburned

Relative SLR1 6.3 6.3

Sub-root zone subsidence 4.6 ± 2.2 3.1 ± 1.3

Total Submergence 10.9 ± 2.2 9.4 ± 1.3

Root zone expansion 0.2 ± 0.2 0.3 ± 0.1

Surface accretion 4.5 ± 0.5 1.9 ± 0.6

Total Land Building 4.7 ± 0.5 2.2 ± 0.6

Elevation Deficit2 6.2 ± 2.0 7.2 ± 1.0

1

Relative SLR equals the change in mean sea level plus deep land movement (from tide 

gauge records) (NOAA, 2011).

2 Elevation deficit equals total submergence minus total land building.

8. Result: Storm sediments delivered by Hurricanes Gustav and Ike raised elevations 

an average of 7.4 cm, which countered an elevation deficit accrued over the 

course of 11 years.

The study site exhibited an overall elevation deficit of 6.7 mm yr-1 from April 

2005 to April 2008. In September 2008, 3 to 9 cm of sediment were deposited in plots, 

which raised elevations an average of 7.4 ± 0.9 cm (fig. 9). The bulk of this sediment was 

likely delivered by Hurricane Ike, based on storm surge heights for the area (East and 

others, 2008). This single event countered an elevation deficit accrued over the course of
41

11 years (estimated by dividing storm-induced elevation gain by an annual average 

deficit of 6.7 mm yr-1

). This finding was in contrast to Hurricane Rita, which did not 

deliver measurable sediment to study plots (fig. 9). A similar effect on elevation due to 

sediment input by Hurricane Katrina was found in subsiding brackish marshes in 

Louisiana (McKee and Cherry, 2009). Between 3 and 8 cm of sediment were added to the 

soil surface at Big Branch Marsh NWR and Pearl River Wildlife Management Area 

(WMA) during the storm surge. At those sites, where the submergence rate (mean 

relative SLR + subsidence) was 21 mm yr

-1

, the hurricane sediment was important in 

countering elevation loss. Although the elevations measured after Hurricane Katrina 

continued to decline (because of continuing subsidence, root-zone collapse, and/or 

compaction of deposited sediment), net elevation gain was still positive two years later. 

In addition, belowground accumulation of root matter was stimulated 10-fold at Big 

Branch Marsh NWR, which further contributed to upward expansion of the soil surface.

In contrast to the above effects, marsh flooding caused by Tropical Storm Charley

(August 1998) and Tropical Storm Frances (September 1998) led to plant dieback and 

soil collapse at McFaddin NWR (Cahoon and others, 2004). In that instance, floodwaters 

from the storm surge covered the marsh surface to a maximum depth of 1.5 m, and 

marshes remained flooded for about two months. Since the study by Cahoon et al. (2004) 

was initiated after floodwaters had receded, it is unknown how much sediment may have 

been deposited. The rate of accretion measured during the year following the storms, 

however, was 4 to 5 mm yr-1

, which was insufficient to counterbalance the rate of 

collapse (37 to 68 mm yr-1).
42

The results of studies in Texas and Louisiana illustrate the potentially positive 

effects of storm sediment on elevation dynamics in coastal marshes. Although some 

coastal marshes may suffer collapse, erosion, or other negative impacts from hurricane 

disturbance, subsiding areas that receive sufficient sediment to alter elevation trajectories 

may benefit in the long-run. However, not all hurricanes will deliver sediment sufficient 

to alter accretion and elevation trajectories, as suggested by the differences between 

Hurricanes Ike and Rita. Additional monitoring of elevations at McFaddin NWR will be 

required to assess if sediments deposited by 2008 hurricanes might influence future

elevation gain via stimulation of belowground production.

Conclusions

General Implications for Resource Management

1. Prescribed burning conducted at 3- to 5-year intervals does not seem to pose 

an additional risk to long-term sustainability of S. patens-dominated brackish 

marshes at McFaddin NWR. 

The results of this cooperative study indicate that the current prescribed fire 

regime (3–5 year frequency) at McFaddin NWR seems to pose no additional threat to 

marsh sustainability and may offset existing elevation deficits by approximately 1 mm yr1

. Although this specific offset cannot be extrapolated beyond the study period or to other 

coastal refuges, the findings at McFaddin NWR and Blackwater NWR together suggest 

that prescribed fire may provide benefits to future sustainability of some brackish 

marshes. The potential effect of burning on elevation change will depend on several sitespecific factors, including geomorphic/sedimentary setting, tide range, local rate of 
43

relative SLR, plant species composition, additional management practices (e.g., for flood 

control), and disturbance types and frequency (for example, hurricanes and herbivore 

grazing). Sites undergoing high rates of subsidence and/or intense herbivory, for 

example, might be negatively affected by burning. Further studies conducted in different 

marsh types and hydroedaphic settings and at different burn intervals will be required for 

further understanding.

2. The primary drivers of elevation change vary in time and space, leading to a 

more complex situation in terms of predicting how disturbances may alter 

elevation trajectories.

A growing body of information suggests that various processes, both physical and 

biological, contribute to vertical movement in different soil layers. In this study, we 

partitioned movement in three main zones, including the soil surface (above a marker 

horizon), root zone, and subroot zone. Besides this study, two other studies have 

examined effects of movement in discrete strata on total elevation change in brackish 

marshes (McKee and Cherry, 2009; Cahoon and others, 2010). Movement in each of 

these strata is controlled by a different suite of processes and, consequently, may be

influenced by different physical and biological drivers (fig. 16). Mineral sedimentation 

on the soil surface is influenced by tides and currents; by proximity to rivers, creeks and 

open ocean; by the composition of the sediment (particle-size distribution); by barriers to 

water movement; and by plant type and stem density (which baffles water movement). 

Expansion or contraction of the root zone is influenced by the growth cycle of the rooted 

vegetation, by factors influencing plant growth (nutrients, climate), and by factors 

influencing decomposition of organic matter. Subroot-zone movement is due to physical 
44

processes such as compaction (reduction in pore space) and by shrink-swell movement 

caused by surface and groundwater influx and steric changes in adjacent bays and oceans. 

Because these biophysical drivers may vary seasonally and/or annually and in response to 

stochastic events such as hurricanes, their relative influence also varies temporally. Also, 

the influence of seasonal or annual processes may obscure finer movements responsible 

for directional trends, making quantification of effects of burning and other management 

practices difficult to determine without long-term records (>5 years). 

Research Implications

Long-term monitoring of elevation changes will provide additional insights into 

how processes controlling marsh elevations may change over time and how long 

elevation trajectories in burned plots will persist. The study sites at McFaddin NWR in 

Texas and Blackwater NWR in Maryland are the only two known locations where effects 

of prescribed fire on marsh-elevation dynamics have been evaluated. These two locations 

are very similar in that they are characterized by microtidal to nontidal astronomical 

ranges, are in subsiding geomorphic settings, and have low sediment supplies (during 

nonhurricane years). The McFaddin NWR site also provides a unique opportunity to 

follow long-term impacts of hurricane sedimentation on elevation dynamics. Together 

with sites such as those at the Big Branch Marsh NWR and the Pearl River WMA, the 

SET-instrumented plots represent a valuable resource that can continue to produce 

information in the future about hurricane effects on coastal marshes. To increase the 

scope of inference, SETs could be installed in replicate marshes undergoing different 

prescribed fire intervals and in different geomorphic settings (with different hurricane 

frequencies and/or different sedimentary settings). Multiple locations along the Gulf and 
45

Atlantic coasts in which prescribed fire is used as a management tool would provide the 

ideal setting for such installations. 
Figure 16. Conceptual model illustrating internal processes and external drivers influencing vertical movement in three soil strata in a brackish 

marsh dominated by Spartina patens, including accretion above a marker horizon, root zone, and subroot zone. Observed effects of burning, 

nutrient addition, and hurricanes are indicated as follows: + (positive), - (negative), and 0 (none).
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