Wu et al. Ecological Processes 2012, 1:8
http://www.ecologicalprocesses.com/content/1/1/8
RESEARCH
Open Access
Impacts of fire and phosphorus on sawgrass
and cattails in an altered landscape of the
Florida Everglades
Yegang Wu3*, Ken Rutchey1, Susan Newman1, Shili Miao1, Naiming Wang1, Fred H Sklar1 and William H Orem2
Abstract
Introduction: Although fire as a critical ecological process shapes the Florida Everglades landscape, researchers lack
landscape-based approach for fire management. The interactive effect of fire, nutrients, water depth, and invasive
cattails (Typha spp.) on vegetation communities is of special concern for ecosystem restoration. In particular,
questions concerning the effect of fire on nutrient release and, by extension, the potential thereof to stimulate
sawgrass (Cladium jamaicense Crantz) re-growth and cattail expansion under varying hydrological conditions are of
immediate relevance to ecologists and land managers who work to restore the Everglades.
Methods: In late April of 1999, a 42,875 ha surface fire, including a 100 ha peat fire, burned the northern section of
Water Conservation Area 3A (WCA-3A) in the Everglades. In this study, total phosphorus (TP) in soil, surface water,
pore-water, and vegetation was sampled at non-burned, surface-burned and peat-burned areas within one and five
months after the burn. Four years after the initial fire, field data were collected in a large scale survey to analyze
how the 1999 fire affected cattail distribution in the altered landscape of high soil TP and cattail habitats. Existing
GIS maps were utilized to select field sampling locations and to provide additional information for the analysis.
Results: The analyses showed that five months after the fire, sawgrass biomass re-growth was about 5 times higher
in burned areas (611 ± 47 g/m2) than in non-burned areas (102 ± 18 g/m2). Sawgrass re-growth in water depths less
than 30 cm was 4.9 ± 0.4 g/m2/day while sawgrass re-growth in water depths deeper than 60 cm decreased to
0.5 ± 0.3 g/m2/day. Cattail biomass re-growth in peat-burned areas was as high as 1,079 ± 38 g/m2. The data also
showed that post-fire cattail expansion could be related to cattail stands existing before the fire. Furthermore,
post-fire cattail appeared more significant expansion in the areas with soil TP above 900 mg/kg than in that with
soil TP below 900 mg/kg.
Conclusions: The data showed that fire within altered landscapes (e.g. high soil TP and/or cattail) of the Everglades
could stimulate the re-growth and expansion of cattails, and post-fire re-growth of sawgrass could be severely
impeded by deep water after a surface-burn. This research indicates that fire continues to be an effective ecological
process for maintaining the Everglades; therefore, ecologists and land managers may have to reevaluate the future
management of natural fire with regard to its dynamic relationship with high soil TP and cattail expansion in the
altered Everglades landscape.
Keywords: Fire, Phosphorus, Sawgrass, Cattails, Everglades, Water depth, Altered landscape, Wetlands
* Correspondence: ywu_mail@yahoo.com
3
Cardno ENTRIX, 339 Whitecrest Drive, Maryville, TN 37801, USA
Full list of author information is available at the end of the article
© 2012 Wu et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
Wu et al. Ecological Processes 2012, 1:8
http://www.ecologicalprocesses.com/content/1/1/8
Introduction
Fire is recognized as a key historical factor in shaping
the Florida Everglades landscape and is being considered
now as one of the management tools appropriate for the
Everglades restoration (Slocum et al. 2003, Wu et al. 1996,
Wade et al. 1980). Before the construction of the approximately 2,000 km of canals and levees in the Everglades
for drainage and flood protection, fire occurred systematically and helped facilitate the maintenance of the ridge
and slough landscape with dotted tree islands (Beckage
et al. 2009; Ogden 2005; Wu et al. 2002). Fires ignited by
summer lightning storms burned sawgrass ridges until
they hit natural barriers such as sloughs or tree islands
or until they were extinguished by summer rains. Fires
ignited by rare winter lightning strikes could burn larger
areas due to dry conditions that typically persist during
that season. The pattern of ridge, slough, and tree islands
in the Everglades ensured that fires would burn and leave
a mosaic of burned and non-burned vegetation in the
landscape (Gunderson and Snyder 1994). This burned
and non-burned mosaic provided the diversity of habitats
and refuges for native plants and wildlife (Childers et al.
2006, Wu et al. 2002, Vogl 1973).
The Everglades ecosystems flourished and evolved uninterruptedly until the middle of the nineteenth century.
The construction of canal and levees in and adjacent to
the Everglades, which redirected water and compartmentalized the landscape, then began disrupting the natural
processes. This compartmentalization and water management activities altered the hydroperiod and produced
a gradient of drier to wetter conditions from north to
south, respectively, in each of the constructed impoundments. As a result, fire regimes changed greatly as natural or human-induced fires increased in frequency and
intensity in the northern areas of the impoundments;
however, the spatial extent of fire in the Everglades was
often restricted by the canal/levee system and also by
other landscape features such as airboat trails (Smith and
Newman 2001, Wu et al. 1996, Gunderson 1994). Water
management (diversion) further aggravated the situation
by creating drier dry seasons and wetter wet seasons,
which ultimately altered fire frequency and intensity.
Large expanses of cattail invasion have also changed the
fire regime (in both fire frequency and fire intensity) dramatically (Beckage 2005, Wu et al. 1996). Soil oxidation
occurring in the northern sections of the impoundments
have caused the ridge and slough pattern to lose its
micro-topology and transition to a sawgrass/brush community with less slough. As a result the landscape has
become more homogeneous (Wu et al. 2006, Beckage
and Platt 2003) with fires now consuming even larger
uniform areas of the landscape. Many ecologists believe
that fire is a critical metric for restoring and maintaining
the altered Everglades and that it must be incorporated
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as part of a long-term maintenance regime (Hagerthey et al.
2008, Lockwood et al. 2003, Wu et al 1997, Gunderson
1994).
Fire intensity and frequency under different fuel loadings and environmental conditions is of interest to ecologists and landscape managers (Gilliam and Platt 2006,
Herndon et al. 1991). It has been hypothesized that
water depth and fire metrics are some of the key factors
influencing vegetation communities in the Everglades.
Newman et al. (1998) and Wu et al. (1997) conducted
research from these metrics and concluded that the
combination of water depth and fire shape or alter the
sawgrass (Cladium jamaicense) community and/or potentially stimulate the expansion of cattails (Typha spp.)
in the Everglades landscape.
Intensive fires under the current altered system have
increased the likelihood of severe peat fires, which damage sensitive resources like tree islands by burning away
precious soil that in many cases took decades to centuries
to form. This phenomenon was rare in the preimpoundment Everglades. In late April of 1999, a fire
consumed almost the entire north section of Water Conservation Area 3A (WCA-3A) (Figure 1a), a total of
42,875 hectares. Most of the fire was categorized as surface fire, which only burned the aboveground vegetation.
However, in the northwest corner of WCA-3A, approximately 100 ha of peat burns were observed. Wu et al.
(1996) reported that average and largest fire sizes for
WCA-2A (Figure 1a) during 1980 to 1991 were 3,505 ha
and 15,390 ha, respectively. Although these quantities
of ha are for a different impoundment (WCA-2A) of the
Everglades, they show that the April, 1999 fire in the
northern WCA-3A impoundment was an unusually large
fire that consumed much more of the landscape. This
large fire itself may have been the result of the altered
hydrological regime and spatial mosaic of vegetation, topography and landscape patterns, which therefore changed
the fire patterns and consequently the landscape patterns
(Young et al. 2011, Bruland et al. 2007, Scheidt and Kalla
2007, Wu et al. 2002, Arabas 2000).
Research efforts have suggested that fire alters not just
the landscape patterns but also the physical and chemical
properties of organic and mineral reserves and the microbial population of soil (Gu et al. 2008, Ilstedt et al. 2006,
Rashid 1987). The effects of fire on the nutrients of an
ecosystem depend on the type and frequency of fire, fuel
load, time and season of burn, nature of the plant tissues
burnt, topography, succession status of the community,
and post-fire climatic and biotic conditions (Anderson
2006, Certini 2005, Wu et al. 1996). A fire of mild intensity may stimulate high seedling establishment and
growth by raising the soil temperature and nutrient status
by removing plant litter and vegetation cover (Beckage
and Platt 2003, Wade et al. 1980). As a result, the
Wu et al. Ecological Processes 2012, 1:8
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Page 3 of 11
Figure 1 (a) Locations of Water Conservation Area 3A and Water Conservation Area 2A; (b) Water Conservation Area 3A north
(WCA-3A N, most of the area was burned by 1999 fire) and south (WCA-3A S, not burned by 1999 fire), the peat-burned, surfaceburned and non-burned sampling areas, and the distribution of three vegetation types of sawgrass, sawgrass-cattail mixed
(cattails <50%) and cattail-sawgrass mixed (cattails >50%).
nutrients for plant uptake can increase in availability or
be volatilized and lost from the site, especially with the
alternations in soil total phosphorus (TP) in the Everglades (Miao et al. 2010, Smith et al. 2001, DeBusk et al.
2001).
Marsh fire burns have been reported to occur at least
once every 9 years within the Everglades (Hagerthey
et al. 2008, Imbert and Delbé 2006, Wu et al. 1996). Fire
not only gives plant communities a mechanism by which
to remove decaying vegetation but also allows for new
robust plant growth that did not exist prior to the burn.
This robust plant growth following a fire has been a stable
metric within the Everglades, which cycles the variety and
availability of food sources necessary for much of the wildlife (Bruland et al. 2007, Scheidt and Kalla 2007).
Newman et al. (1997) reported that over-drainage of
peat soils may lead to soil compaction and phosphorus
nutrient accumulation. Agricultural runoff and urban
development have also caused elevated point source
loadings of phosphorus into the Everglades (Adorisio
et al. 2006; Davis 1994). These phosphorus loadings have
over time resulted in elevated soil P concentrations,
which are considered a primary factor accelerating cattail growth over sawgrass and influencing cattail invasion
and distribution in the Everglades (Beckage et al. 2005,
Miao and Debusk 1999, Rutchey and Vilchek 1999,
Doren et al. 1997, Newman et al. 1996, Davis 1991). The
issue regarding how fire and water depth interact and
affect nutrient release and, in particular, how this then
stimulates sawgrass re-growth and cattail expansion
within nutrient impacted areas has become an important
restoration consideration for ecologists and land managers (Hagerthey et al. 2008, Scheidt and Kalla 2007,
Bruland et al. 2007, Ross et al. 2006, Lockwood et al.
2003, Arabas 2000, Herndon et al. 1991).
With the altered system now containing areas of
increased soil TP and with the subsequent invasion of
cattails, the impacts of fire within these areas require
evaluation. The objective of this current research is to
investigate how the 1999 northern WCA-3A fire affected
phosphorus releases and, consequently, how this may
have had an influence on the landscape dynamics for
this area. In particular, the study's focus is post-fire sawgrass and cattail change due to fire related phosphorus
releases from vegetation and/or soil. This study utilizes a
natural landscape fire to measure the effects of soil nutrient release and post-fire water quality on vegetation
growth and pattern. Drought with soil oxidation followed by increased water depth and duration of flooding
has been shown to significantly impact cattail expansion
(Newman et al. 1998, Wu et al. 1997). This study will explore this idea further by analyzing the synergistic effects
of fire on cattail invasion, which will help develop an appropriate management plan for the restoration of the
Wu et al. Ecological Processes 2012, 1:8
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altered Everglades ecosystem. The hypotheses in this
study could be set as follows:
(1) Soil, pore-water and surface-water TP may vary in
peat-burned, surface-burned and non-burned areas.
(2) TP and total nitrogen (TN) in sawgrass and cattails
may vary in peat-burned, surface-burned and
non-burned areas.
(3) Post-fire growth and biomass of sawgrass and
cattails may vary in peat-burned, surface-burned
and non-burned areas.
(4) An altered landscape in terms of higher soil TP and
cattail density could impact post-fire cattail
expansion.
Methods
Study area
The study was conducted in the northern sections of
Water Conservation Area 3A (WCA-3A), which is separated by Interstate Highway I-75 with WCA-3A North
in the north and WCA-3A South in the south. Both
WCA-3A North and WCA-3A South are covered primarily with sawgrass and cattails (Rutchey et al. 2005).
Unlike WCA-3A South, which was not burned by the
1999 fire, WCA-3A North was burned mostly by a surface fire with about 100 ha peat burned in the northwest
corner (Figure 1b). High soil TP patches (> 700 mg/kg)
are distributed along the northern portions of the Miami
Canal, in the very northwest, and in the east central
Page 4 of 11
portions of the impoundment (Figure 2) based on a soil
TP map by Reddy et al. (1994). From late April to early
May of 1999, a 42,875 ha fire consumed almost the entire north section of WCA-3A. Field surveys conducted
during the period of the 1999 fire found that there was
very little surface water in the area and that most of the
fires that occurred were surface-burns, which only
burned the aboveground vegetation. The fire burned a
mosaic pattern in which non-burned areas were patches
of tree islands, alligator holes and wet prairies. It was
evident that the micro topography played an important
role in the patchiness and patterns of vegetation, fuel,
and fire. In the northwest corner of WCA-3A, approximately 100 ha of peat-burned patches varied from several square meters to 200 square meters. Many of the
peat fires were associated with woody vegetation or tree
islands, since peat surrounding these communities tends
to be at higher elevation and thus thicker and drier. Peat
fires burned out all the organic materials, and casual
observations indicated that the burn lowered the elevation by several inches to a foot.
Sampling methods
Spatially explicit data sampling and analyses have
increased substantially and become important means for
fire study in the past 20 years (Nelson 2012, Wu et al.
1996). In our study, existing GIS maps were utilized to
select field sampling locations and to provide additional
information for the analysis. A complete GIS vegetation
Figure 2 (a) Water Conservation Area 3A north (WCA-3A N) and north part of Water Conservation Area 3A south (WCA-3A S); (b) the
soil total phosphorus map by Reddy et al. (1994) and the distribution of 134 survey sites (in circle) in six categories of soil total
phosphorus (mg/kg) and three vegetation types of sawgrass, sawgrass-cattail mixed (cattails <50%) and cattail-sawgrass mixed
(cattails >50%) in both the burned (WCA-3A N) and the non-burned (WCA-3A S) areas.
Wu et al. Ecological Processes 2012, 1:8
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Page 5 of 11
database of WCA-3 was obtained from the WCA-3
Vegetation Map, which utilized 1:24000 scale color infrared aerial photographs and coded vegetation communities using a single hierarchal classification system by
Rutchey et al. (2005). Sawgrass and wet prairie communities accounted for the greatest area (205,564 ha) and
percentage (87.5%) of polygons categorized for the entire
project. Cattail habitat encompassed 5% of the project
area or 11,751 ha.
A GIS map of soil TP distribution (Figure 2) was
obtained from field survey data of 1992 by Reddy et al.
(1994). The GIS map of soil TP was classified into six
categories of (1) 0-299, (2) 300-499, (3) 500-699, (4)
700-899, (5) 900-1099 and (6) >= 1100 mg/kg.
In this study, samples of soil, surface water, pore-water,
and vegetation were collected from three sampling sites
such as (1) non-burned (26°07004.5”N latitude, 80°
34004.4”W longitude), (2) surface burned (26°11013.3”N
latitude, 80°44025.3”W longitude) and (3) peat burned
(26°19017.7”N latitude, 80°47043.1”W longitude) located
within WCA-3A north (Figure 1b). Three replicates
were made in each site on June 1, 1999 (about one
month after the fire). Soil samples were collected using a
thin-walled 10 cm diameter Al coring tube to a depth of
10 cm and were sent to lab for nutrient analysis (e.g. TP
concentration). Airstones connected to Tygon tubing
were inserted 10-15 cm below the soil surface for porewater sampling. Monoject 140 cc syringes were used to
collect at least 80 ml of porewater sample, which was
immediately placed in an ice filled cooler and sent to lab
for analysis. Surface water samples were collected at
each porewater sampling location. Three replicate samples of post fire sawgrass and cattail were randomly
collected from 1 x 1 m quadrats in non-burned, surfaceburned and peat burned locations on June 1 and
October 5, 1999. Number of leaf shoots, leaf heights and
surface water depth (<30 cm and >60 cm) were recorded
for each quadrat. In peat-burned areas, various cattail
leaves were harvested from the new growth in the peatburned patches, and sawgrass collected at the edge of
peat-burned patches since there was no sawgrass in the
peat-burned patches. Leaf materials from the 1 x 1 m
quadrats were then harvested and weighed and then
oven-dried at 70-75°C (Dash and Frith 2011) and
weighed for dry-weight data. Leaf nutrient content (TN
and TP) and total dry biomass were measured and
analyzed.
In May 2003, four years after the initial fire, a largescale field survey was conducted over the burned and
non-burned areas to analyze how the 1999 fire affected
cattail distribution in the altered landscape in terms
of both high soil TP and cattail invasion. A total of
134 sampling sites were selected from overlaying three
GIS maps of (1) fire map, (2) vegetation map and (3) soil
TP map. The fire map includes two categories of surfaceburned (WCA-3A North) and non-burned (WCA-3A
South) areas (Figure 1b). A vegetation map from Rutchey
et al. (2005) contains three vegetation types (Figure 1b)
such as (1) sawgrass, (2) sawgrass-cattail mix (cattails
<50%) and (3) cattail-sawgrass mix (cattails >50%). Soil
TP map (Figure 2) from Reddy et al. (1994) classified
soil TP into six categories of (1) 0-299, (2) 300-499,
(3) 500-699, (4) 700-899, (5) 900-1099 and (6) >=
1100 mg/kg). The 134 sampling sites were distributed
in 2 categories of burned and non-burned, 3 categories
of vegetation types and 6 categories of soil TP. For the
survey, the percentage of existing sawgrass and cattail
were recorded at each sampled site. A comparison of cattail distribution before and after the fire was analyzed
using t-test (Alder and Roessler 1972).
Results
Results showed that post fire TP values measured on
June 1, 1999 within soil, surface water and porewater
samples were different in non-burned, surface-burned
and peat-burned areas (Table 1). TP values were
Table 1 t-Test for the soil total phosphorus (STP, mg/kg), surface water total phosphorus (SWP, μg/L), pore-water total
phosphorus (PWP, μg/L) after 1999 fire in peat-burned area (PBA), surface-burned area (SBA), non-burned area (NBA),
sampling on the day of June 1, 1999 (JUN)
Mean (s.d.)
n
Σ(X-M)2
Y
Mean (s.d.)
n
Σ(Y-M)2
t
JUN-NBA-STP
398 (123)
3
30421
JUN-SBA-STP
580 (49)
3
4835
−2.37
JUN-SBA-STP
580 (49)
3
4835
JUN-PBA-STP
853 (36)
3
2581
−7.78*
JUN-PBA-STP
853 (36)
3
36303
JUN-NBA-STP
398 (123)
3
30421
6.14*
JUN-NBA-SWP
18 (4)
3
25
JUN-SBA-SWP
22 (5)
3
43
−1.28
JUN-SBA-SWP
22 (5)
3
43
JUN-PBA-SWP
52 (16)
3
522
−3.08*
X
JUN-PBA-SWP
52 (16)
3
522
JUN-NBA-SWP
18 (4)
3
25
3.57*
JUN-NBA-PWP
20 (2)
3
9
JUN-SBA-PWP
23 (2)
3
7
−2.20
JUN-SBA-PWP
23 (2)
3
7
JUN-PBA-PWP
181 (9)
3
170
−28.96*
JUN-PBA-PWP
181 (9)
3
170
JUN-NBA-PWP
20 (2)
3
9
29.51*
* Statistically significant.
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Table 2 t-Test of vegetation TP and TN in peat-burned area (PBA), surface-burned area (SBA), non-burned area (NBA)
after 1999 fire for sawgrass total phosphorus (SVP, mg/kg), cattail total phosphorus (CVP, mg/kg), sawgrass total
nitrogen (SVN, mg/kg), cattail total nitrogen (CVN, mg/kg), cattail biomass (CBM, g/m2), and sawgrass biomass (SBM,
g/m2), sampling on the day of June 1, 1999 (JUN) and October 5, 1999 (OCT)
Mean (s.d.)
n
Σ(X-M)2
JUN-NBA-SVP
344 (47)
3
4376
JUN-SBA-SVP
689 (56)
3
JUN-PBA-SVP
935 (70)
3
X
Mean (s.d.)
n
Σ(Y-M)2
JUN-SBA-SVP
674 (56)
3
9231
−8.19*
6301
JUN-PBA-SVP
935 (70)
3
9715
−4.75*
9715
JUN-NBA-SVP
344 (47)
3
4376
12.19*
Y
t
JUN-NBA-SVN
7668 (267)
3
142617
JUN-SBA-SVN
15500 (200)
3
80000
−37.78*
JUN-SBA-SVN
15500 (200)
3
80000
JUN-PBA-SVN
20333 (1102)
3
2426667
−7.48*
JUN-PBA-SVN
20333 (1102)
3
2426667
JUN-NBA-SVN
7668 (267)
3
142617
19.22*
JUN-PBA-SVP
935 (70)
3
9715
OCT-PBA-SVP
727 (89)
3
15923
3.17*
JUN-SBA-SVP
689 (56)
3
6301
OCT-SBA-SVP
285 (9)
3
171
12.32*
JUN-NBA-SVP
344 (47)
3
4376
OCT-NBA-SVP
299 (65)
3
8467
0.97
OCT-PBA-CVP
1503 (144)
3
41667
OCT-PBA-SVP
727 (89)
3
15923
7.92*
OCT-PBA-CVN
8827 (498)
3
496267
OCT-PBA-SVN
11100 (985)
3
1940000
−3.57*
OCT-NBA-SBM
102 (18)
3
67
OCT-SBA-SBM
611 (47)
3
444
−17.43*
OCT-SBA-SBM
611 (47)
3
444
OCT-PBA-CBM
1079 (38)
3
282
−13.46*
OCT-PBA-CBM
1079 (38)
3
282
OCT-NBA-SBM
102 (18)
3
67
40.53*
* Statistically significant.
significantly higher within the peat-burned areas (853 ±
36 mg/kg) than that in surface burned areas (580 ±
49 mg/kg) and non-burned areas (398 ±123 mg/kg). Soil
TP values in surface-burned area was higher than those
in non-burned areas, but not statistically significant.
These same trends were also noted for TP in surface
water in that TP in peat-burned areas (52 ± 16 μg/L) was
significantly higher than that in surface-burned (22 ±
5 μg/L) and non-burned (18 ± 4 μg/L) areas. The same
relative pattern appeared for porewater TP, with values of
181 ± 9, 23 ± 2 and 20 ± 2 μg/L, for the peat-burned,
surface-burned and non-burned areas, respectively. The
post-fire porewater TP value in peat-burned areas was
significantly higher than that in surface-burned (t = 28.96*)
and non-burned areas (t = 29.51*) (Table 1).
Significant differences (t = -8.19*, -4.75*, and 12.19*) of
sawgrass TP values were noted in the non-burned,
surface-burned and peat-burned areas with 344 ± 47,
689 ± 56, and 935 ± 70 mg/kg, respectively (Table 2).
Sawgrass TN values in surface-burned areas (15,500 ±
200 mg/kg) were approximately doubled (t = -37.78*) to
that of non-burned areas (7,668 ± 267 mg/kg) with
that in peat-burned areas (20,333 ± 1,102 mg/kg) about
twenty-five percent higher than that in surface-burned
areas (t = -7.48*). Post fire sawgrass TP values in peatburned areas significantly dropped (t = 3.17*) twenty-five
percent from 935 ± 70 mg/kg to 727 ± 89 mg/kg between
June 1 and October 5, 1999. There was also a significant
(t = 12.32*) fifty percent drop of sawgrass TP values in
surface-burned areas from 689 ± 56 mg/kg on June 1 to
285 ± 9 mg/kg on October 5. There were no significant
differences between sawgrass TP in non-burned areas
in the period of June 1 (344 ± 47 mg/kg) to October 5
(299 ± 65 mg/kg) of 1999 (Figure 3). However, cattail
TP (1503 ± 144 mg/kg) collected on October 5, 1999
was twice (t = 7.92*) that of sawgrass (727 ± 89 mg/kg)
collected at that same time (Table 2). Furthermore, there
were significant differences (t = -3.57*) between cattail
TN (8,827 ± 498 mg/kg) and sawgrass TN (11,000 ±
985 mg/kg) in the peat-burned areas (Table 2).
This study also showed that post fire sawgrass regrowth (611 ± 47 g/m2) from June 1 to October 5 was approximately 5 times higher (t = -17.43*) than sawgrass
growing in non-burned stands (102 ± 18 g/m2) (Table 2).
In another comparison, cattails (1,079 ± 38 g/m2) in
peat-burned areas were growing approximately twice
Figure 3 Comparison of sawgrass total phosphorus (mg/kg)
between 30 days post-fire and 150 days post-fire in
peat-burned, surface-burned and non-burned areas.
Wu et al. Ecological Processes 2012, 1:8
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Table 3 t-Test of cattail coverage (CTC,%) within sawgrass stand (SAW), sawgrass-cattail stand (SCT, cattails <50%),
cattail-sawgrass stand (CTS, cattails >50%) in surface-burned areas (SBA) and non-burned areas (NBA) based on 134
sampling sites, collected four years after 1999 fire, in May, 2003
X
Mean (s.d.)
n
Σ(X-M)2
Y
Mean (s.d.)
n
Σ(Y-M)2
t
SBA-SAW-CTC
8 (7)
30
1524
NBA-SAW-CTC
6 (9)
15
1023
0.77
SBA-SCT-CTC
49 (21)
25
10864
NBA-SCT-CTC
31 (20)
16
5711
2.71*
SBA-CTS-CTC
77 (18)
31
9869
NBA-CTS-CTC
61 (20)
17
6612
2.75*
SBA-SAW-CTC
8 (7)
30
1524
SBA-SCT-CTC
49 (21)
25
10864
−9.81*
SBA-SCT-CTC
49 (21)
25
10864
SBA-CTS-CTC
77 (18)
31
9869
−5.28*
NBA-SAW-CTC
6 (9)
15
1023
NBA-SCT-CTC
31 (20)
16
5711
−4.49*
NBA-SCT-CTC
31 (20)
16
5711
NBA-CTS-CTC
61 (20)
17
6612
−4.32*
* Statistically significant.
(t = -13.46*) as fast as sawgrass (611 ± 47 g/m2) in
surface-burned areas. In terms of average daily growth
rates in the period of June 1 to October 5 of 1999,
sawgrass grew 0.8 g/m2/day in non-burned areas and
4.9 g/m2/day in surface-burned areas while cattail grew
8.6 g/m2/day in peat-burned areas. Post-fire water depths
also showed an impact on sawgrass re-growth. Sawgrass
re-growth in surface-burned areas reached 4.9 ± 0.4 g/
m2/day in water depths less than 30 cm but decreased
significantly (t = 6.44*) to 0.5 ± 0.3 g/m2/day in water
depths greater than 60 cm.
Survey results showed that post-fire community
structures and cattail increase seemed to be influenced
by initial presence of cattail, which meant that postfire community structures for sawgrass, sawgrass-cattail
mix and cattail-sawgrass mix appeared to form differently depending on the amount of initial cattail found
within each of these categories (Table 3). Figure 4 shows
the percent distribution of cattail collected from those
134 sampling sites in May, 2003 (four years after the initial fire) for these categories in surface-burned and nonburned areas. Sawgrass areas that were void of cattail
Figure 4 Potential cattail increases (%) in different stands of
sawgrass, sawgrass-cattail mix (cattails <50%) and cattailsawgrass mix (cattails >50%) stands between surface-burned
and non-burned areas in WCA-3A, four years after the 1999 fire
(1999-2003).
before the 1999 fire were found to have very similar percentages of cattail for the surface-burned (8 ± 7%) and
non-burned (6 ± 9%) areas four years after the fire
(Table 3). In contrast, sawgrass-cattail mix and cattailsawgrass mix categories showed significant differences
(t = 2.71* and 2.75*) in percentage of cattail increases between surface-burned and non-burned areas. However,
the amount of cattails was greater for the surface-burned
(sawgrass-cattail mix: 49 ± 21%; cattail-sawgrass mix: 77
± 18%) areas as opposed to the non-burned areas (sawgrass-cattail mix: 31 ± 20%; cattail-sawgrass mix: 61 ±
20%) for these two categories (Table 3). These data show
cattail increases of 2%, 18% and 16% coverage in surfaceburned sawgrass, sawgrass-cattail mix and cattailsawgrass mix categories, respectively compared to nonburned areas. These data also show that higher cattail
increases in both surface-burned and non-burned areas
depend on the amount of cattail initially present. Cattail
growth in surface-burned areas of cattail-sawgrass mix
stands (77 ± 18%) was significantly (t = -5.28*) higher
than in sawgrass-cattail mix stands (49 ± 21%), which
again was significantly (t = 9.81*) higher than that of sawgrass stands (8 ± 7%). The same trend could be observed
in non-burned areas with t-values of -4.32* and -4.49*
(Table 3). Figure 4 shows that cattail increases in coverage were 5.5%, 24% and 27% in the surface-burned sawgrass, sawgrass-cattail mix and cattail-sawgrass mix
categories, respectively, compared to 3.5%, 6% and 11%
increases, respectively, in the non-burned areas four
years after the 1999 fire. These data suggest that fire in
cattail environments might facilitate cattail expansion.
This study suggested a strong positive relationship between cattail distribution and soil TP (Figure 5). Cattail
distribution in six categories of soil TP concentration
increased linearly from 8 ± 4% cattail with 0-299 mg/kg
of soil TP to 76 ± 17% cattail with >1100 mg/kg of soil
TP in surface-burned areas (Table 4). The same trend
could be seen in non-burned areas (Figure 5) with distribution of 4 ± 5% cattail with <300 mg/kg of soil TP to
70 ± 15% cattail with >1100 mg/kg of soil TP (Table 4).
Wu et al. Ecological Processes 2012, 1:8
http://www.ecologicalprocesses.com/content/1/1/8
Page 8 of 11
Figure 6 The correspondence of soil total phosphorus and the
distribution of cattails and sawgrass in non-burned area.
Figure 5 The relations between cattail density (%) and soil total
phosphorus (mg/kg) in surface-burned and non-burned areas of
WCA-3A based on 134 survey sites, sampled four years after 1999
Fire.
There were significant (3.51* and -2.75*) impacts of high
soil TP on cattail expansion in surface-burned areas in
the 900-1099 mg/kg and >=1100 mg/kg of soil TP as
opposed to cattail expansion in the 500-699 mg/kg and
700-899 mg/kg of soil TP, respectively. Similar significant (-2.55* and -2.64*) results were also found in the
non-burned areas within those same ranges, but there
was no significant difference between the surface-burned
and non-burned areas (Table 4).
Figure 6 shows the mean TP concentrations of different soil layers (0-2 cm and 2-10 cm) and floc layers
within cattail and sawgrass stands in non-burned areas.
The mean TP concentration of the floc layer within cattail stands was 1080 mg/kg, but only 400 mg/kg within
sawgrass stands (Figure 6). Similar concentrations to floc
were found in the 0-2 cm soil layer with mean TP concentrations in cattail and sawgrass stands of 930 and
390 mg/kg, respectively. Mean TP concentrations in the
2-10 cm soil layer were not significantly different between cattail (480 mg/kg) and sawgrass (390 mg/kg)
stands. Mean TP concentrations in all three layers (floc,
0-2 cm and 2-10 cm) of sawgrass stands were roughly
within the range of 390-400 mg/kg while higher levels of
TP concentration were found in the floc and 0-2 cm soil
layer within cattail stands.
Discussion and conclusion
This study suggests that there is initially (June 1) significantly higher TP in peat-burned and surface-burned
areas than in non-burned areas after the 1999 fire in
Table 4 t-Test for cattail expansion (CTC% increases) with different soil total phosphorus in both surface-burned area
(SBA) and non-burned area (NBA) four years after the 1999 fire, sampling in May, 2003. Soil total phosphorus (mg/kg)
categories: P01: 0-299; P02: 300-499; P03: 500-699; P04: 700-899; P05: 900-1099; P06: >= 1100
Y
Mean (s.d.)
n
Σ(Y-M)2
t
80
NBA-P01-CTC
4 (5)
5
120
1.27
5
580
NBA-P02-CTC
13 (10)
5
480
0.00
23 (25)
6
3088
NBA-P03CTC
22 (15)
6
1083
0.07
36 (28)
5
3120
NBA-P04-CTC
40 (0)
2
0
−0.19
SBA-P05-CTC
59 (18)
11
3241
NBA-P05-CTC
53 (23)
3
1067
0.47
SBA-P06-CTC
76 (17)
5
1120
NBA-P06-CTC
70 (15)
11
2323
0.77
X
Mean (s.d.)
n
SBA-P01-CTC
8 (4)
5
SBA-P02-CTC
13 (12)
SBA-P03CTC
SBA-P04-CTC
Σ(X-M)2
NBA-P01-CTC
4 (5)
5
120
NBA-P03CTC
22 (15)
6
1083
−2.53*
NBA-P02-CTC
13 (10)
5
480
NBA-P04-CTC
40 (0)
2
0
−3.29*
NBA-P03CTC
22 (15)
6
1083
NBA-P05-CTC
53 (23)
3
1067
−2.55*
NBA-P04-CTC
40 (0)
2
0
NBA-P06-CTC
70 (15)
11
2323
−2.64*
SBA-P01-CTC
8 (4)
5
80
SBA-P03CTC
23 (25)
6
3088
−1.28
SBA-P02-CTC
13 (12)
5
580
SBA-P04-CTC
36 (28)
5
3120
−1.69
SBA-P03CTC
23 (25)
6
3088
SBA-P05-CTC
59 (18)
11
3241
−3.51*
SBA-P04-CTC
36 (28)
5
3120
SBA-P06-CTC
76 (17)
5
1120
−2.75*
* Statistically significant.
Wu et al. Ecological Processes 2012, 1:8
http://www.ecologicalprocesses.com/content/1/1/8
northern WCA-3A. TP in surface-burned areas was noticeably higher than that in non-burned areas and, in
comparison, 46% higher in soil, 22% in surface water,
15% in pore-water and 100% in sawgrass tissue. Soil,
pore-water, surface water and sawgrass tissue. TP in
peat-burned areas was 47%, 136%, 687% and 36% significantly higher than that in surface-burned areas, respectively. Bostic and White (2007) indicated that phosphorus
enrichment of marsh soils can act as an internal source
of nutrients to the water column and drive existing
wetland eutrophic conditions. Steward and Ornes (1975)
found a strong nutrient relationship in sawgrass re-growth
after fire which differed considerably from sawgrass in
non-burned areas. This current research also indicates
that the amount of sawgrass TN was significantly higher
in surface-burned (15,500 mg/kg) and peat-burned areas
(20,333 mg/kg) than in non-burned areas (7,668 mg/kg).
Gunderson and Snyder (1994) also spectulated that
nutrients released as a result of fire appear to promote
luxuriant plant re-growth and improve the overall marsh
habitat.
This research is consistent with the hypothesis that
fire, as an ecological process in the Everglades, facilitates
nutrient release and subsequent plant re-growth (Miao
et al. 2010). This conclusion is supported by a comparison of surface-burned sawgrass biomass (611 g/m2) and
average daily growth rate (4.9 g/m2) to non-burned sawgrass biomass (102 g/m2) and average daily growth rate
(0.8 g/m2). These post-fire cattail and sawgrass biomass
accumulated values appear to correspond to the available TP concentrations found in the soil, pore-water and
surface water within each of the non-burned, surfaceburned and peat-burned areas. Fire in the Everglades
may continue to shape the marsh landscape and substrate composition and provide a mechanism whereby
essential nutrients are released and whereby vegetation
communities are maintained by the recycling of accumulated litter. Fire influenced in the evolution of the Everglades, and fire management must be understood in
order to maintain the current natural ecosystem of the
Everglades (Miao et al. 2010, Wu et al. 2002, 1996).
The data from this study suggest that TP released after
a fire may not last more than 125 days within the environment. TP concentrations in sawgrass were reduced
from 689 mg/kg (30 days post-fire) to 285 mg/kg
(125 days post-fire) after the burn, which is not significantly different from the 299 mg/kg found in nonburned areas. Deep water (> 60 cm) may also delay
sawgrass re-growth (from 4.9 to 0.5 g/m2/day) following
a fire. This supports Main and Barry’s results (2002), for
their results indicated that wet prairie grasses are promoted by early growing season fires, which cause an initial fast growth rate before the rainy season brings deeper
water to the wetland. These data suggest that post-fire
Page 9 of 11
nutrient availability will initially enhance sawgrass regrowth, but high water levels could delay that growth.
This study also supports Herndon et al.’s (1991) notion
of water depth and fire shaping sawgrass community.
Fire may cause significant TP changes in peat-burned
areas and impact cattails and sawgrass differently. Post
peat-fire soil TP concentrations (853 mg/kg) were much
higher than surface-burned areas (580 mg/kg). These
post-peat-fire soil TP concentrations are also much higher
than the 650 mg/kg threshold TP level that would promote and accelerate cattail expansion (Wu et al. 1997).
The fast growth rate of cattails (8.6 g/m2/day) after peatfires enabled them to outgrow the slower growing sawgrass (4.9 g/m2/day). Cattail biomass (1,079 g/m2) in
150-day post-peat-burn areas almost doubled that of
sawgrass (611 g/m2) in surface-burned areas and was ten
times greater than that of sawgrass in non-burned areas
(102 g/m2). Comparisons of TP concentration in cattails
(1,503 mg/kg) to that in sawgrass (727 mg/kg) provide
further evidence that, under the right conditions, postfire, available TP releases may cause cattail expansion
(Miao et al. 2001, Smith and Newman 2001).
This study also suggests that, under some situations,
fire may provide undesirable effects within certain
regions of the altered Everglades. Cattail increases in
sawgrass stands were 5.5% in surface-burned areas vs.
3.5% in non-burned areas whereas sawgrass-cattail mix
stands (24% vs. 6%) and cattail-sawgrass mixed stands
(27% vs. 11%) saw greater cattail invasion within the
surface-burned than non-burned areas, respectively. These
data indicate that post-fire cattail expansion is related to
cattail stands existing before the fire. The data also suggest that cattails in the altered landscape grow in proportion to cattails after a surface-burn.
Smith et al. (2001) speculated that soil TP transformations may encourage the growth of invasive cattails. Similarly, this current study shows a positive relationship
between cattail distribution and high soil TP. The survey
data analyses show that cattail distribution in six categories
of soil TP concentration of 0-299, 300-499, 500-699, 700899, 900-1099, and >1100 mg/kg was 8%, 13%, 23%, 36%,
59%, and 76%, respectively in surface-burned areas. Cattails
in the soil TP of 900-1099 and >=1100 mg/kg have significantly higher cattail distributions than that in the soil TP of
500-699 and 700-899 mg/kg in post-fire surface-burned
environments. These data again are consistent with research suggested by Wu et al. (1997) that higher TP soil
concentrations facilitate faster cattail expansion.
Soil sampling data from WCA-3A indicate that there
are significantly higher concentrations of soil TP in cattail stands than in sawgrass stands; furthermore, cattails
are more related to TP concentrations in the top layers
of floc and 0-2 cm of soil. The TP of floc and 0-2 cm soil
layer in cattail stands was 1,080 mg/kg, and 930 mg/kg,
Wu et al. Ecological Processes 2012, 1:8
http://www.ecologicalprocesses.com/content/1/1/8
respectively, whereas it was 400 mg/kg vs. 390 mg/kg,
respectively, in sawgrass areas. TP in the 2-10 cm soil
layer was not significantly different between cattail stands
(480 mg/kg) and sawgrass stands (390 mg/kg), which
indicates that the top soil layers (floc and 0-2 cm) contribute most of the phosphorus that allows cattail to expand and outgrow sawgrass. Wu et al. (1997) and Smith
and Kadlec (1985) also showed that the effects of fire and
TP on cattail expansion can be predicted. Behind both
changes in cattail expansion (Rutchey et al. 2005) and
increases in soil TP (Bruland et al. 2007), fire might be
another force altering the Everglades.
In summary, although fire continues to be a positive
metric for nutrient release and sawgrass re-growth, the
data also suggest that fire within altered areas (e.g. high
soil TP and/or high cattail density) of the Everglades could
be more beneficial to cattails’ growth and expansion than
sawgrass. This conclusion holds especially true for peatfires, which may dramatically increase the expansion of
cattails. This research further indicates that post-fire cattail expansion is related to original cattail distribution.
The continued use of fire as a driving metric within the
altered Everglades needs re-evaluation (LaPuma et al.
2007). This research indicates that ecologists and managers must redefine the role of fire as a natural process
shaping the historical Everglades wetland, since fire may
not act the same in all areas of an altered landscape.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
YW, SN, and FS participated in the post-fire survey and sampling. KR
provided the GIS vegetation map and soil map. NW designed the GPS points
of 134 sampling sites. SM provided the lab for sampling data measurement.
WO carried out the soil and pore-water total phosphorus measurement. KR
and FS made great efforts in editing the manuscript. All authors read and
approved the final manuscript.
Authors’ information
Ken Rutchey: **Retired from South Florida Water Management District.
Acknowledgements
This research resulted from the many discussions with our colleagues at the
South Florida Water Management District and support from colleagues at
Cardno ENTRIX.
Author details
1
Everglades Division, South Florida Water Management District, 3301 Gun
Club Road, West Palm Beach, FL 33406, USA. 2U.S. Geological Survey, MS 956,
12201 Sunrise Valley Drive, Reston, VA 20192, USA. 3Cardno ENTRIX, 339
Whitecrest Drive, Maryville, TN 37801, USA.
Received: 7 June 2012 Accepted: 14 August 2012
Published: 11 October 2012
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Cite this article as: Wu et al.: Impacts of fire and phosphorus on
sawgrass and cattails in an altered landscape of the Florida Everglades.
Ecological Processes 2012 1:8.
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