Litter decomposition promotes differential feedbacksin an oligotrophic southern Everglades wetland

Tiffany G. Troxler Æ Daniel L. Childers

Received: 13 October 2006 / Accepted: 30 January 2008 / Published online: 17 March 2008

� Springer Science+Business Media B.V. 2008

Abstract The differential accumulation or loss of

carbon and nutrients during decomposition can promote

differentiation of wetland ecosystems, and contribute to

landscape-scale heterogeneity. Tree islands are impor-

tant ecosystems because they increase ecological

heterogeneity in the Everglades landscape and in

many tropical landscapes. Only slight differences in

elevation due to peat accumulation allow the differ-

entiation of these systems from the adjacent marsh.

Hydrologic restoration of the Everglades landscape is

currently underway, and increased nutrient supply that

could occur with reintroduction of freshwater flow

may alter these differentiation processes. In this study,

we established a landscape-scale, ecosystem-level

experiment to examine litter decomposition responses

to increased freshwater flow in nine tree islands and

adjacent marsh sites in the southern Everglades. We

utilized a standard litterbag technique to quantify

changes in mass loss, decay rates, and phosphorus (P),

nitrogen (N) and carbon (C) dynamics of a common

litter type, cocoplum (Chrysobalanus icaco L.) leaf

litter over 64 weeks. Average C. icaco leaf degrada-

tion rates in tree islands were among the lowest

reported for wetland ecosystems (0.23 ± 0.03 yr-1).

We found lower mass loss and decay rates but higher

absolute mass C, N, and P in tree islands as compared

to marsh ecosystems after 64 weeks. With increased

freshwater flow, we found generally greater mass loss

and significantly higher P concentrations in decom-

posing leaf litter of tree island and marsh sites. Overall,

litter accumulated N and P when decomposing in tree

islands, and released P when decomposing in the

marsh. However, under conditions of increased fresh-

water flow, tree islands accumulated more P while the

marsh accumulated P rather than mineralizing P. In

tree islands, water level explained significant variation

in P concentration and N:P molar ratio in leaf tissue.

Absolute P mass increased strongly with total P load in

tree islands (r2 = 0.81). In the marsh, we found strong,

positive relationships with flow rate. Simultaneous C

and P accumulation in tree island and mineralization

in adjacent marsh ecosystems via leaf litter decompo-

sition promotes landscape differentiation in this

oligotrophic Everglades wetland. However, results of

this study suggest that variation in flow rates, water

levels and TP loads can shift differential P accumula-

tion and loss leading to unidirectional processes among

heterogeneous wetland ecosystems. Under sustained

high P loading that could occur with increased

freshwater flow, tree islands may shift to litter miner-

alization, further degrading landscape heterogeneity in

this system, and signaling an altered ecosystem state.

T. G. Troxler (&) � D. L. Childers

Southeast Environmental Research Center, Florida

International University, 11200 SW 8th St., Miami,

FL 33199, USA

e-mail: troxlert@fiu.edu

T. G. Troxler � D. L. Childers

Department of Biological Sciences, Florida International

University, 11200 SW 8th St., Miami, FL 33199, USA

123

Plant Ecol (2009) 200:69–82

DOI 10.1007/s11258-008-9405-2

Keywords Decay rate � Heterogeneity �Accumulation � Mineralization � Phosphorus �Tree islands � Linear regression

Introduction

On a large scale, peatland development and mainte-

nance are influenced by regional climate, geomor-

phology, site history, and disturbance regimes including

fire and drought (Hogg et al. 1992; Almquist-Jacobson

and Foster 1995). Just as feedbacks driving differen-

tial processes of accumulation and loss are important

in maintaining hummock-hollow microtopography

within and resilience of peatlands (Nungesser 2003),

these differential processes may also be important at

the landscape-scale for maintenance of the peatland

ecosystem in the landscape (Almquist-Jacobson and

Foster 1995). Thus, differential processes of accu-

mulation and loss of carbon and nutrients during

decomposition may promote differentiation among

wetland ecosystems, and contribute to landscape-

scale heterogeneity. Decomposition processes in

wetlands are often nutrient limited, and hydrology

interacts with nutrient availability, and the extant

plant community, to influence peat accumulation or

loss (Pastor et al. 2002).

Exogenous sources of nutrients to wetland envi-

ronments can have deleterious effects on ecological

structure and function, and can alter mechanisms for

differentiation of wetland ecosystems in the landscape,

degrading landscape heterogeneity. In many peatland

studies, litter decomposition is cited as an important

driver of surficial peat dynamics (Middleton and

McKee 2001; Nungesser 2003). In oligotrophic envi-

ronments, especially the phosphorus (P)-limited

Everglades (Noe et al. 2001), litter decomposition

studies have demonstrated that P availability promotes

net immobilization of P by decomposing litter (Qualls

and Richardson 2000; Newman et al. 2001; Davis

et al. 2003; Debusk and Reddy 2005; Corstanje et al.

2005; Rubio and Childers 2006). Increased decompo-

sition and P accumulation have been observed in

areas where P loading is above oligotrophic levels

(Craft and Richardson 1993; Qualls and Richardson

2000; Noe et al. 2001). Thus, if feedbacks control-

ling differential processes of accumulation and loss

between ecosystems are maintained only under

conditions of oligotrophy, then external nutrient

inputs that change the oligotrophic status of inflow-

ing water to the system may subsequently degrade

landscape heterogeneity.

Tree islands are a unique component of the

Everglades landscape and an important component

of landscape heterogeneity, increasing the diversity of

Everglades flora and fauna (Sklar and van der Valk

2002; Troxler Gann et al. 2005; Wetzel et al. 2005).

Like other Everglades communities, tree islands have

experienced major changes in the last 100 years of

human alterations and are expected to undergo further

changes as a result of hydrologic restoration. In the

Everglades, nutrient loads delivered by actions taken

to restore freshwater flow to oligotrophic wetlands

may increase nutrient supply, degrade feedbacks that

promote differential processes of accumulation and

loss, and degrade landscape heterogeneity in the

system. In this study, we explored the importance of

decomposition processes as drivers of this differential

accumulation and loss between tree islands and the

adjacent marsh matrix, and the effects of reintroduced

freshwater flow to an oligotrophic, heterogeneous,

short hydroperiod wetland in the southern Everglades

with peatland tree islands. In 1999, we estab-

lished a landscape-scale, ecosystem-level manipula-

tive experiment to characterize tree island ecosystem

response to increased freshwater flow in the southern

Everglades. We sought to: (1) determine whether

decomposition of a common tree island litter type,

Chrysobalanus icaco (cocoplum), varied among treat-

ment levels of water flow rates and bulk nutrient

loads, (2) examine differences between two contrast-

ing ecosystems, tree islands and the adjacent sawgrass

marsh, and (3) investigate linear relationships between

hydrologic factors and decomposition processes in

these two contrasting ecosystems. We employed a

standard litter bag study and, using a common litter

type, controlled for the effects of litter quality, to

address these three objectives. We tested the follow-

ing hypotheses. (1) Overall, tree islands would accu-

mulate C and nutrients while, in marsh ecosystems,

mineralization would occur. (2) Increased freshwater

flow would result in increased litter decomposition

and nutrient accumulation in tree island and marsh

ecosystems if surface water P was above oligotrophic

levels.

70 Plant Ecol (2009) 200:69–82

123

Methods

Study area

In 1997, hydrologic restoration of the southern

Everglades began with the removal of the southern

levee of the C-111 canal along the segment of the

canal that traverses the easternmost southern Ever-

glades (the C-111 Basin or Everglades National Park

Panhandle; Fig. 1a). The goal was to increase fresh-

water flow to the southern Everglades wetland

landscape and to northeastern Florida Bay. Canal

inputs are currently the predominant source of water

to this region, and are controlled by water manage-

ment activities in the lower C-111 Basin (Light and

Dineen 1994). Tree islands in this study area are

seasonally flooded peatlands (6–11 months duration)

in a short hydroperiod marl marsh (Troxler Gann

et al. 2005), their development presumably is pri-

marily a function of paludification (‘‘the presence of

peat deposits directly over mineral soil’’; Anderson

et al. 2003). The tree islands are relatively low in

elevation with moderate microtopography and a thick

litter layer on the surface of peat soils up to 1 m

thick. The marsh matrix is a Cladium jamaicense

wetland that experiences annual dry downs of

4–6 months. The marsh soils are calcium-carbonate

based marls, derived from periphyton accumulation

during annual draw downs.

In August 1999 (2 years after levee removal), we

selected nine Chrysobalanus icaco-dominated, sea-

sonally flooded tree islands (based on similar island

Fig. 1 Study Site: (a)

South Florida and the

Everglades National Park

Panhandle (inset) with

experimental tree islands—

flow, no flow, and wall

locations shown, and (b)

wall experimental

manipulation

Plant Ecol (2009) 200:69–82 71

123

size and vegetation type) downstream of or adjacent

to the levee removal segment of the C-111 canal

(Fig. 1a). The post-levee increase in freshwater flow

to the C-111 Basin provides a unique opportunity to

investigate the effects of hydrologic restoration.

We investigated the effects of this increase in

freshwater input by selecting tree islands in two

different locations relative to the C-111 canal (Trox-

ler Gann and Childers 2006). Six islands were located

directly downstream of this canal (i.e., islands with

increased freshwater flow) and three islands were in

an area west of the canal that had minimal influence

of the levee removal (we refer to the latter three

islands as ‘‘No Flow’’ islands; Fig. 1a). In six islands

near the canal, we identified the three nearest the

canal as receiving fully enhanced water flow (‘‘Flow’’

islands). With the other three islands, we tested the

effects of sheetflow by experimentally deflecting

surface water flow around the entire islands. We

experimentally reduced sheetflow with plastic walls

(constructed of 12 mil grade plastic and affixed with

PVC posts and cable ties, placed in the marsh

upstream from islands; = ‘‘Wall’’ islands; Fig. 1b).

See Troxler Gann and Childers (2006) for details of

our experimental design and other sampling.

Litter decomposition

In October 1999, we deployed 126 bags at nine sites

with paired tree island and adjacent marsh locations

in each site. Paired marsh locations were approxi-

mately 5 m downstream from each respective tree

island location. Each bag contained approximately

5 g of air-dried C. icaco leaf litter, and was placed on

the standing litter surface upon deployment. We

collected one bag per location (two bags per site) at 1,

2, 4, 8, 16, 32, 64 weeks. We applied an air-dried/

oven-dried weight conversion to determine final oven

dry weights for each bag at each collection period.

We quantified total mass remaining and loss, N and P

concentrations, C:N, C:P, and N:P molar ratios, mass

C, N and P for leaf litter at t0 and each collec-

tion period thereafter. Decay rates were calculated

using the first order, exponential loss decay model,

Mt = Moe-kt, where Mt is the dry mass at time t, Mo

is original dry mass, and k is degradation coefficient

(yr-1; Olson 1963). We calculated total percent loss

as (100 - (Mt/M0) 9 100) day-1 following Middle-

ton and McKee (2001). Turnover rates (T0.95) were

estimated for 95% turnover time as 3 k-1 (Olson

1963). In order to follow nutrient accumulation or

loss, absolute mass was calculated as the product of

dry mass remaining and C, N or P content of litter

following Davis et al. (2003).

After retrieving bags from the field, leaf litter was

gently washed of adhering soil (carefully, and to the

extent possible, leaving accumulated biofilms intact

and attached to the litter), dried to constant weight at

70�C, and weighed. Subsamples from each bag were

ground to a homogeneous powder (\500 lm), and

analyzed for total (T) N, TP and TC content. Leaf

tissue samples were analyzed for TC and TN with a

Carlo Erba elemental analyzer. The modified Solorz-

ano and Sharp (1980) method was used to analyze

for TP.

Environmental factors

From a larger dataset of environmental parameters

collected in association with this study, and published

elsewhere, we removed auto-correlated hydrologic

parameters using Pearson correlation coefficients

(Troxler Gann and Childers 2006). Hydroperiod,

average water level, wet season water level, water

flow rate, annual TP, TN and TOC loads were thus used

to determine which of these factors had the greatest

influence on leaf litter decomposition. See Troxler

Gann and Childers (2006) for details of these data.

Statistical analyses

We compared treatment means for decay rates and

total mass loss, and final values of C and nutrient

content, molar ratios and mass C and nutrients with a

randomized block analysis of variance (ANOVA)

using ecosystem as our blocking factor. Multiple

comparisons for treatment means were evaluated with

Tukey–Kramer tests. We then compared differences

among ecosystems in these same parameters using

t-tests. We used the paired t-test to evaluate differ-

ences between initial and final C, N and P content,

molar ratios and mass nutrients for each ecosystem.

We used simple linear regression analyses to quantify

relationships between hydrologic factors and mass

72 Plant Ecol (2009) 200:69–82

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loss, decay rates, and final nutrient values separately

for island and marsh ecosystems. In order to maintain

statistical robustness, we only considered island and

marsh regression relationships that were significant at

a Bonferroni-corrected value of P = 0.05/7 or

P B 0.0071 (Zar 1999).

Results

Effects of flow level

Mass loss averaged 41.69 ± 2.95, 34.98 ± 3.30, and

30.37 ± 3.30% over the 64 week decomposition

experiment for Flow, Wall, and No Flow treatment

levels, respectively (Fig. 2). Decay rates were 0.441

± 0.101, 0.318 ± 0.030 and 0.274 ± 0.046 yr-1 for

Flow, Wall, and No Flow treatment levels. Decay rates

were similar across treatment level but mass loss

values were significantly higher in the Flow level as

compared with the No Flow level (ANOVA:

F = 3.792, P = 0.0483). Based on decay rates, we

also calculated average 95% turnover rates (years) and

total percent loss (d-1; Table 1). Turnover rates were

10.3 ± 2.8, 11.1 ± 1.0, and 12.0 ± 2.1 yr and total

percent loss rates were 0.086 ± 0.01, 0.073 ± 0.01,

and 0.074 ± 0.01% d-1 for Flow, Wall, and No Flow

levels, respectively.

We evaluated differences in final nutrient content,

ratios and mass due to litter decomposition among

treatment levels. Final C, N, and P concentrations

ranged from 45.72 to 46.48%, 1.25 to 1.42%, and

0.012 to 0.033%, respectively (Fig. 3). Phosphorus

concentrations were significantly higher in the Flow

treatment level after 64 weeks of decomposition

(ANOVA: F = 5.280, P = 0.0196; Fig. 3e, f), despite

similar initial P concentrations across all treatments.

Final molar ratios of the remaining tissue averaged

from 4318 to 11916 (C:P), 38 to 43 (C:N), and 150 to

291 (N:P; Fig. 4). We found significantly lower N:P

and C:P ratios in the Flow treatment as compared

with the No Flow level (ANOVA: F = 6.286,

P = 0.0113; F = 5.138, P = 0.0212, respectively).

The wide range in C:P values was a function of very

low Flow level values. Final absolute mass of C, N,

and P averaged 1.15–1.35 g C, 0.034–0.037 g N, and

0.381–0.797 mg P, with no differences in C or N

mass, but the highest P mass in the Flow level

(ANOVA: F = 5.443, P = 0.0178; Fig. 5).

Variation across ecosystem types

We compared decomposition parameters between

tree island and marsh ecosystems and between initial

and final values for each ecosystem type. Comparing

ecosystem types, we found a significant difference in

total mass loss, and island and marsh values averaged

27.10 ± 2.78 and 44.27 ± 2.41%, respectively (t =

-3.684, P = 0.0025). Ecosystem differences in

decay rates were also significant, and these values

were 0.241 ± 0.033 and 0.456 ± 0.040 yr-1 in islands

and adjacent marsh, respectively (t = -3.472, P =

0.0049). Turnover rates and total percent loss rates

followed values for decay rates. Turnover rates (T0.95)

were 14.5 ± 1.5 and 7.8 ± 0.8 yr and total percent

loss rates were 0.064 ± 0.007 and 0.092 ± 0.007% d-1

in tree islands and marsh, respectively.

Fig. 2 Percent mass remaining of decomposing cocoplum leaf

litter in tree island (a) and adjacent marsh sites (b) of wall,

flow, and no flow treatment levels

Plant Ecol (2009) 200:69–82 73

123

Differences between initial and final values (after

64 weeks decomposition) in tree islands were pro-

nounced and significant for all parameters except N:P

molar ratio and absolute mass P (Table 2). Overall, in

islands, there were declines in C concentration, C

absolute mass, C:P and C:N ratios, and increases in N

and P concentrations and N absolute mass. Similar

increases in both N and P concentrations yielded

Table 1 Summary of leaf degradation rates (k), turnover times (T) and total percent loss (% loss) of C. icaco litter in tree island and

marsh ecosystems with treatment effect of different flow levels

Treatment Ecosystem -k (d-1) -k (yr-1) T0.95 (yr) % loss (d-1) r2 P

Wall Tree island 0.0006 (0.00001) 0.226 (0.003) 13.3 (0.2) 0.065 (0.002) 0.47–0.82 */ns

Marsh 0.0009 (0.00006) 0.341 (0.026) 8.9 (0.7) 0.081 (0.005) 0.83–0.95 **

Flow Tree island 0.0006 (0.00015) 0.222 (0.055) 15.3 (3.8) 0.057 (0.012) 0.76–0.94 **

Marsh 0.0016 (0.00025) 0.596 (0.096) 5.3 (0.8) 0.116 (0.012) 0.96–0.98 ***

No flow Tree island 0.0006 (0.00018) 0.237 (0.073) 14.9 (3.6) 0.068 (0.021) 0.81–0.83 *

Marsh 0.0009 (0.00012) 0.339 (0.042) 9.2 (1.2) 0.079 (0.008) 0.89–0.96 ***

The coefficient of determination (r2) is variation explained by the model to predict k. P values indicate significance level of regression

coefficient at * P \ 0.05, ** P \ 0.005, and *** P \ 0.0005 unless otherwise noted

ns, Tree island 1.3 of the Wall island group was not significant at P \ 0.05

Fig. 3 C, N, and P

concentrations of

decomposing cocoplum leaf

litter in tree island (a, c, e)

and adjacent marsh sites

(b, d, f) of wall, flow, and

no flow treatment levels

74 Plant Ecol (2009) 200:69–82

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insignificant changes in N:P molar ratio, and the

standard error for final absolute mass P value were

high (Table 2). In the marsh, we found declines in C

concentration, C:N ratio, and P absolute mass but

increases in N concentration and N:P ratio (Table 2).

In marsh ecosystems, we found no change in C:P

reflecting higher variability in P concentrations, but

an increase in N:P, despite this variability in P

concentrations (Table 2).

Comparing final nutrient and C values between

tree island and marsh ecosystems, we found signif-

icantly lower C concentrations in cocoplum litter

decomposing in the marsh, but no difference in N and

P concentrations (Table 2). We found significantly

higher C:P and N:P ratios in marsh locations, with no

difference in C:N ratios. We also found significantly

lower C, N, and P mass in marsh locations than in

tree island locations after 64 weeks of decomposition

(Table 2).

Relationships to explore potential hydrologic

controls on litter decomposition

As for the tree islands, we found significant positive

relationships between average water levels and

average wet season water levels and final litter P

concentration, and negative relationships with final

litter N:P ratio (Table 3). Hydroperiod also explained

considerable variation in final litter N:P (71%), but at

P = 0.0085 (see Methods for Bonferonni-corrected

P-value). We also found a significant positive rela-

tionship between annual TP load and absolute mass

P, where TP load explained 81% of the variation in

final mass of P in decomposing litter in tree islands

(Table 3).

In the marsh, we found that flow rate was the only

significant predictor of decomposition values

(Table 3). Flow rate explained the most variation in

decay rate and mass loss (85 and 82%, respectively,

Fig. 4 C:P, C:N, and N:P

molar ratios of decomposing

cocoplum leaf litter

in tree island (a, c, e)

and adjacent marsh sites

(b, d, f) of wall, flow, and

no flow treatment levels

Plant Ecol (2009) 200:69–82 75

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Table 3). Flow rate also described positive relation-

ships with P and N concentrations and final mass P, and

negative relationships with C:N molar ratio and

absolute mass C (Table 3). Notably, flow rates mea-

sured in the marsh varied from 0.87 to 1.23 cm s-1,

0.63 to 0.82 cm s-1, and 0.62 to 0.72 cm s-1 in Flow,

Wall and No Flow treatments, respectively (Troxler

Gann and Childers 2006).

Discussion

In wetland landscapes, regardless of latitude, nutrient

status or precipitation patterns, heterogeneity com-

monly emerges as a landscape feature. Landscape

heterogeneity occurs with the differentiation of an

oligotrophic wetland feature from a more nutrient rich

matrix in the case of pocosin wetlands of the SE US

(Bridgham and Richardson 1993), raised bogs of New

Zealand (Bragazza and Gerdol 2002; Clarkson et al.

2005) and coastal Panama (Troxler 2007) and pat-

terned peatlands of circumpolar latitudes (Glaser and

Janssens 1986). Heterogeneity also occurs as the

development of a more nutrient rich feature over a

nutrient depauperate landscape as in the case of tree

islands in alpine tundra (Seastedt and Adams 2001), the

Okavango delta (Ellery et al. 1998), seasonally-inun-

dated savannas of Brazil and Bolivia (Ponce and Cunha

1993; Langstroth 1996) and the subtropical Everglades

(Troxler Gann et al. 2005; Wetzel et al. 2005).

Regardless of the origin of emergent heterogeneous

landscape components, feedbacks driving differential

processes of accumulation and loss, including litter

decomposition, have been suggested as important

mechanisms in the maintenance of wetland heteroge-

neity at the landscape-scale (Bauer 2004). In many

wetland landscapes, heterogeneous features appear to

differentiate as C is accumulated at a higher rate

Fig. 5 Absolute mass C, N,

and P of decomposing

cocoplum leaf litter in tree

island (a, c, e) and

adjacent marsh sites

(b, d, f) of wall, flow, and

no flow treatment levels

76 Plant Ecol (2009) 200:69–82

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relative to the landscape matrix. This process has been

linked to differences in hydrology, nutrient availabil-

ity, standing biomass, and litter quality between the

heterogeneous feature and its matrix system (Glaser

and Janssens 1986; Bridgham and Richardson 1993;

Rietkerk et al. 2004; Couwenberg and Joosten 2005;

Belyea and Baird 2006). This study suggested that this

process also occurs where peatland tree islands emerge

as heterogeneous features of the Everglades wetland

landscape.

Table 2 Paired differences between initial and final values

(viewed horizontally across table) and ecosystem differences

between final values (viewed vertically down ‘‘final mean’’

column) of %C, %N, and %P, C:P, C:N, and C:P molar ratios,

and mass C, N, and P of leaf litter decomposing in island and

marsh ecosystems with t-ratio and P-value shown (P [ |t|)

Ecosystem Initial mean Final mean* t-Ratio P-value

%C Island 49.23 (0.18) 47.05 (0.21)a -9.459 \0.0001

Marsh 49.23 (0.18) 44.85 (0.56)b -6.931 0.0001

%N Island 0.619 (0.042) 1.299 (0.039)a 12.80 \0.0001

Marsh 0.619 (0.042) 1.333 (0.072)a 15.80 \0.0001

%P Island 0.013 (0.001) 0.025 (0.003)a 4.277 0.0037

Marsh 0.013 (0.001) 0.017 (0.006)a 0.780 0.4578

C:P Island 10519 (937) 5582 (794)b -3.875 0.0061

Marsh 10519 (937) 12401 (2374)a 0.872 0.4088

C:N Island 96 (6) 43 (1)a -8.510 \0.0001

Marsh 96 (6) 40 (2)a -12.38 \0.0001

N:P Island 110 (7) 130 (18)b 0.954 0.3716

Marsh 110 (7) 300 (55)a 3.524 0.0078

Mass C (g g-1) Island 2.121 (0.008) 1.460 (0.045)a -12.77 \0.0001

Marsh 2.119 (0.008) 1.079 (0.073)b -12.85 \0.0001

Mass N (g g-1) Island 0.027 (0.002) 0.041 (0.001)a 4.931 0.0026

Marsh 0.027 (0.002) 0.031 (0.001)b 1.792 0.1109

Mass P (mg g-1) Island 0.560 (0.058) 0.815 (0.082)a 2.156 0.0744

Marsh 0.560 (0.058) 0.343 (0.086)b -3.868 0.0024

Different letters indicate significant differences between final means at P \ 0.05 with t-test*

Table 3 Linear regression relationships between hydrologic variables and decomposition values

Ecosystem x y Equation r2 F ratio P value

Island Average water level %P y = -0.0045 + 0.0033x +0.73 16.59 0.0065

N:P y = 325.6 - 21.68x -0.78 21.27 0.0036

Wet season water level %P y = -0.0162 + 0.0033x +0.73 16.08 0.0070

N:P y = 410.0 - 22.65x -0.82 27.49 0.0019

TP Load Mass P y = -0.0313 + 0.0028x +0.81 21.25 0.0058

Marsh Flow rate %P y = -0.0514 + 0.0844x +0.81 30.06 0.0009

%N y = 0.5385 + 0.9820x +0.78 24.24 0.0017

C:N y = 60.81 - 25.72x -0.67 14.55 0.0066

Mass P y = -0.5363 + 1.087x +0.67 14.22 0.0070

Mass C y = 1.890 - 1.002x -0.78 24.88 0.0016

Decay rate y = -0.1844 + 0.7538x +0.85 38.55 0.0004

Mass loss y = 3.980 + 49.78x +0.82 31.60 0.0008

Only relationships with Bonferroni-corrected P values (P B 0.0071) were considered significant at P \ 0.05

Plant Ecol (2009) 200:69–82 77

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Previous studies have investigated litter decompo-

sition in heterogeneous elements of wetland landscapes

(i.e., Thormann and Bayley 1997). This approach

provided an opportunity to explore the importance of

ecosystem characteristics and hydrologic interventions

on this important process in the oligotrophic Ever-

glades. In wetland environments, leaf litter decom-

position rates have been shown to vary from 0.06 yr-1

in a monocot bog in Alaska to 5.11 yr-1 in a riverine

mangrove forest of Ecuador (Brinson et al. 1981;

Twilley et al. 1997). In this study, degradation rates in

peatland tree islands were comparable to, but on

average lower than, rates for temperate flooded forests

(0.42–1.3 yr-1; Baker et al. 2001), and among the

lowest rates reported for wetland ecosystems (this

study: 0.23 ± 0.03 yr-1). This is a surprising result

given the subtropical climate of south Florida but is

likely in part due to the extremely oligotrophic status of

the Everglades (Noe et al. 2001).

Despite the relatively small difference in litter

decomposition rates between ecosystems and among

flow levels, decomposition processes resulted in large

differences in C and nutrient concentrations, molar

ratios, and absolute mass after 64 weeks (Table 2).

Although large scale disturbances like fire and

drought are thought to be important drivers in

wetlands (Hogg et al. 1992), the differences we

found likely serve to maintain differential processes

of accumulation and loss between tree islands and

adjacent marsh in the periods between disturbance

events.

Tree islands in the marsh landscape matrix

Litter quality is often invoked as an important control

on litter decomposition rates and the nutrient dynam-

ics of litter decomposition (Brinson et al. 1981;

Webster and Benfield 1986; Villar et al. 2001). In

this study, we used leaf litter of similar initial quality

across all treatment levels and ecosystems so as to

evaluate the effects of flow and ecosystem indepen-

dently of litter quality. Despite the fact that C. icaco

litter has slightly lower N:P and C:P ratios than

C. jamaicense (Rubio and Childers 2006), the dom-

inant species that would otherwise be decomposing in

the marsh, C. icaco litter served as an acceptable

substrate with which to evaluate decomposition

processes.

Here, the environment created by tree island

ecosystems is markedly different from the marsh

landscape of the study area. Tree islands maintain

higher standing biomass, greater total litter deposition

and standing litter cover, deposition of more refrac-

tory materials (i.e., wood) with lower light

penetration to the forest floor. This results in large

variation in biogeochemical characteristics between

the two ecosystem types (Troxler Gann et al. 2005).

For example, when compared with marsh sites, tree

island surface water contains two times greater TN

and TOC concentrations (tree island: 39.28 ± 5.71

and 1816.67 ± 250.00 lM l-1, respectively; marsh:

15.00 ± 10.71 and 658.33 ± 42.5 lM l-1, respec-

tively), and an order of magnitude greater TP

concentrations (tree island: 2.32 ± 0.21 lM l-1;

marsh: 0.09 ± 0.01 lM l-1) in the early wet season

(Troxler and Childers, unpublished data). These

ecosystem differences likely exert the strongest

controls on differences in mass loss, decay rates,

and nutrient dynamics promoting the differential

processes that lead to lower C loss and net P

accumulation in tree islands and loss of C and P in

the marsh (Brinson et al. 1981; Webster and Benfield

1986; Baker et al. 2001; Middleton and McKee 2001,

Table 2).

Other interesting and noteworthy considerations to

further explain differential processes of P accumula-

tion in tree islands and P loss in the marsh include

acidity effects on microbially-mediated enzyme

activity and soil/water temperature. Enzyme activity

can influence strong controls over mineralization of C

and nutrients (Wetzel 1991; Sinsabaugh and Moore-

head 1994; Corstanje et al. 2005). Newman et al.

(2001) observed that higher Ca+2 concentrations in

surface waters in the Water Conservation Area

(WCA) 1, a northern component of the WCA system

of the Everglades, may have promoted greater mass

loss because enzyme activities may be depressed

where humic acids interfere with enzyme-substrate

complexes that would otherwise promote litter deg-

radation (Wetzel 1991). Temperature also likely

contributed to greater mass loss and P mineralization

in the marsh ecosystem as surface soils (10 cm depth)

were, on average, 2�C warmer than tree island soils

during the period of study (Webster and Benfield

1986; Troxler Gann and Childers 2006; FCE LTER

2006). Besides nutrient availability, acidity and

temperature, hydrology is often invoked as having

78 Plant Ecol (2009) 200:69–82

123

strong influences on nutrient and C accumulation in

peatlands (Webster and Benfield 1986). For instance,

the hydraulic properties of a peatland landform

promote a positive feedback on water table height

of the peatland (Belyea and Baird 2006). Thus, even

though annual hydroperiod and surface water level

are on average lower in these tree islands than

adjacent marsh, soil moisture is likely higher in tree

islands when water levels have receded in the

landscape. Some studies have shown that peatland

vegetation can influence hydrology and solute trans-

port that in turn promotes a high water table

throughout the year (Rietkerk et al. 2004). These

interacting factors likely promote the feedbacks that

control differential processes of accumulation and

loss between tree islands and adjacent marsh and

likely contribute to landscape heterogeneity in this

oligotrophic, southern Everglades wetland (Fig. 6,

scenario A).

Hydrologic modifications alter mechanisms

promoting landscape differentiation

In oligotrophic wetland environments, the decompo-

sition of low nutrient quality leaf litter typically takes

place in a soil and water environment of similarly low

nutrient status. In general, studies of litter decompo-

sition in oligotrophic Everglades wetlands (soil TP:

100–400 lg P g-1; water TP: 0.1–0.2 lM l-1; Noe

et al. 2001) have shown net N immobilization and

either mineralization or no change in P content

(Newman et al. 2001; Debusk and Reddy 2005;

Corstanje et al. 2005; Rubio and Childers 2006).

However, when P concentrations of surface water or

soil exceed oligotrophic levels, either as a function of

natural or human-induced P sources, net immobiliza-

tion is the most consistent result (Newman et al. 2001;

Davis et al. 2003; Debusk and Reddy 2005; Corstanje

et al. 2005; Rubio and Childers 2006). Thus, as tree

islands have moderate (above oligotrophic levels) soil

and surface water P concentrations, P immobilization

would be expected. Whereas in the marsh, we found

net P mineralization under conditions where inflowing

water was of low nutrient status (Figs. 4 and 5). Yet,

with greater than oligotrophic levels of inflowing P,

net P immobilization becomes independent of eco-

system type leading to similar net accumulation of P

(Fig. 5, Table 2). This can be conceptualized with two

contrasting models depicting the net response of leaf

litter decomposition in tree islands and adjacent marsh

in conditions of low and high P in inflowing surface

water (Fig. 6, scenarios A and B). Thus, under

unimpacted conditions, positive and negative feed-

backs occurring in contrasting environments via

decomposition processes promote the maintenance

of heterogeneous landscape features in this system.

Whereas, with P loading associated with hydrologic

interventions, at least in the short term, there are

similarly positive feedbacks (unidirectional) that

emerge in contrasting environments that degrade this

mechanism for landscape differentiation (Fig. 6).

TREE ISLAND

MARSH

Low P High P

TREE ISLAND

MARSH

Immobilization

Immobilization

ACCUMULATION +LOSS =

DIFFERENTIATION

ACCUMULATION +ACCUMULATION ≠DIFFERENTIATION

LOSS +ACCUMULATION ≠DIFFERENTIATION

SHORT TERM LONG TERM

TREE ISLAND

MARSH

High P

A B C

Mineralization

Immobilization

Immobilization

Mineralization

ytinum

moC

tfihS

Fig. 6 Three conceptual scenarios depicting how P supply

affects feedbacks driving differential processes of accumula-

tion and loss via leaf litter decomposition in tree island and

marsh ecosystems. Scenario A: Ecosystem divergence via

accumulation in tree islands and marsh ecosystems. Scenario

B: Ecosystems begin to converge as marsh ecosystems

accumulate rather than mineralize C and P. Scenario C:

Ecosystems converge after prolonged P loading shifts litter

decomposition in tree islands to mineralization with a shift in

plant community structure or nutrient status

Plant Ecol (2009) 200:69–82 79

123

Strong linear relationships between hydrologic factors

and decomposition values further elucidated these

patterns.

This study suggested that both water level and the

supply of P by inflowing water were important factors

regulating tree island P immobilization via litter

decomposition (Table 3). Peatland tree islands of this

area have soils with greater P concentrations than

surrounding marshes, but are dominated by tree

species delivering senesced leaves of very low P

content and high C content (N:P and C:P ratios

typically 100 and 8,500, respectively; Troxler Gann

et al. 2005). This constant influx of low quality litter

coupled with slightly acidic soil and surface water

conditions promote a substrate for accumulation of

microbial biomass P and CaPO4 precipitation (Keuhn

et al. 2000; Noe et al. 2001). If plant species maintain

delivery of low quality litter (high N:P, high C:P),

despite increased surface water loads of P, tree

islands will likely accumulate P. However, under

circumstances in which hydrologic modifications

alter litter nutrient quality or species composition

favoring plants with higher quality litter, C and P

mineralization of leaf litter in peatland tree islands is

a potential outcome, i.e., convergence of ecosystem

types (Fig. 6, scenario C).

The marsh ecosystem, on the other hand, appeared

to be more sensitive to flow rate directly (Table 3).

While nutrient loads into the C-111 would co-vary

with flow rates, nutrient loads were not similarly

correlated with decomposition values after 64 weeks

(Table 3). Thus, advective sheet flow with even a

minor change in P concentrations of surface water

appeared to influence N and P dynamics and P

accumulation by decomposing leaf litter within the

marsh ecosystem. This coincided with positive rela-

tionships with mass loss and decay rates, and

negative relationships with absolute C mass, suggest-

ing a mechanism that promotes maintenance of

mineral soils in the marsh landscape. Furthermore,

considering that mass loss was highest where flow

rates were highest (in the marsh of the Flow treatment

level; 55.7 ± 4.1%) as compared with mass loss in

the marsh of Wall and No Flow levels (38.9 ± 4.17

and 38.1 ± 4.1%) suggests an additive factor influ-

encing C and nutrient remineralization in the marsh

system. It is interesting to note that given a situation

where light, nutrient and temperature-limited condi-

tions are removed (marsh, flow treatment), flow rates

promote the highest mass loss and decay rates, as has

been found in some stream studies (Lepori et al.

2005; Ardon et al. 2006).

The similar P concentration and accumulation

responses that occurred in contrasting wetland eco-

systems of the Flow treatment level strongly suggests

that canal influences modify mechanisms for land-

scape differentiation in this study. Furthermore, small

differences in P loading can determine whether tree

island and marsh ecosystems have differential P

dynamics or similarly accumulate P. Our study

suggests another mechanism by which reduced

landscape heterogeneity may occur in hydrologi-

cally-altered wetland landscapes, at least in the short

term. With long term P loading, and changes in tree

island canopy structure or leaf quality, convergence

of ecosystem types, and loss of landscape heteroge-

neity, is a possible result, and may signal an altered

tree island ecosystem state.

Acknowledgments We would like to thank the Wetland

Ecosystems Ecology group for field and lab support, as well as

critical reviews that greatly improved this manuscript. Critical

to the accomplishment of this work were Damon Rondeau and

undergraduate research assistants Josh Mahoney and Simone

Normile. This research was partially supported by the South

Florida Water Management District under several sequential

contracts, and by the National Science Foundation through the

Florida Coastal Everglades Long-Term Ecological Research

Program (DEB-9901514).

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