Growth, decomposition, and nutrient retention of Cladium jamaicense Crantz and Typha domingensis Pers. in the Florida Everglades

Aquatic Botany, 40 ( 1991 ) 2 0 3 - 2 2 4

203

Elsevier Science Publishers B.V., A m s t e r d a m

Growth, decomposition, and nutrient retention of Cladium jamaicense Crantz and Typha domingensis Pers. in the Florida Everglades Steven M. Davis South Florida WaterManagement District, P.O. Box 24680, West Palm Beach, FL 33416-4680, USA (Accepted for publication 7 January 1991 )

ABSTRACT Davis, S.M., 1991. Growth, decomposition, and nutrient retention of Cladiumjamaicense Crantz and Typha domingensis Pers. in the Florida Everglades. Aquat. Bot.,40: 203-224. Estimates of phosphorus (P) and nitrogen (N) gains and losses during annual macrophyte growth, death and 2 years decomposition were made along a gradient of surface water nutrient concentrations in the Florida Everglades. Annual rates of P and N allocation to growing leaves, translocation or leaching from dying leaves, and retention in dead leaves of Cladiumjamaicense Crantz and Typha domingensis Pers. were correlated to soluble reactive P and nitrate concentrations in surface water. Rates of each of these processes were higher in T. domingensis than in C. jamaicense, Cladium jamaicense rates increased linearly along the nutrient gradients, but did not fluctuate with yearly variations in soluble reactive P or nitrate concentrations. For T. domingensis, annual rates were strongly correlated with mean annual soluble reactive P and nitrate concentrations during specific sampling years. Responses of C. jamaicense to the nutrient gradient were characteristic of species competitive in an infertile habitat, while responses of 72 domingensis were characteristic of species competitive in a fertile habitat. The main effect of P and N enrichment on leaf nutrient flux was to accelerate translocation or leaching from dying tissue, rather than to increase retention in standing dead leaves. Freshly dead leaves retained only slightly greater quantities of P and N under enriched conditions in comparison to background conditions. After 2 years of decomposition, approximately half of the leaf litter mass remained intact. Increasing P and N concentrations in decomposing leaf litter resulted in net uptake or retention of these elements after 2 years despite decreasing litter mass. The total amounts of P and N that were sequestered annually by T. domingensis after processes of leaf production, mortality and 2 years decomposition were lowest under non-enriched conditions and reached a maximum under a moderate level of enrichment. Wetland ecosystems such as the Everglades, which developed under conditions of low nutrient supply, may offer a finite potential for accelerated nutrient retention when the exogenous nutrient supply increases as a result of human activities. However, a plant species such as C. jamaicense, that is adapted to a low-nutrient environment, may have a low nutrient threshold before it loses its competitive capability and its habitat is invaded by a species such as T. domingensis that is better adapted to a highnutrient environment.

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204

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INTRODUCTION

The use of wetlands to remove anthropogenic nutrient inputs from surface water has gained popularity since Boyd (1970) and Steward (1970) noted the nutrient uptake potentials of various macrophyte species. The role of plants in wetland nutrient uptake, however, has remained poorly understood until recently. Organic matter accumulation, plus soil adsorption, appear to be two major processes controlling long-term nutrient immobilization in wetlands (Richardson and Marshall, 1986). Wetland nitrogen (N) budgets are also influenced by denitrification and nitrogen fixation. The role of plants in longterm nutrient retention appears to be largely related to detritus production (Davis and van der Valk, 1978) and the resulting accumulation of organic matter. At an ecosystem level, wetland plants appear to affect nutrient budgets to the extent that they ( 1) accumulate nutrients in biomass as they grow, (2) retain nutrients in biomass as they die and (3) retain or accumulate nutrients as they decompose in conjunction with the accretion of organic sediment. In this context, the effectiveness of wetland plants in retaining nutrients appears to be limited and to vary from one ecosystem to another. Senescing plant tissues release nutrients which they accumulated during growth (Davis and van der Valk, 1983; Hopkinson and Schubauer, 1984). Leaching of soluble organic and inorganic compounds during the first few weeks of decomposition results in further loss of nutrients from dead plant tissue (Howard-Williams and Howard-Williams, 1978; Webster and Benfield, 1986). Detritus may continue to lose phosphorus (P) and N after the initial leaching period (Latter and Cragg, 1967; Davis and van der Valk, 1978). In other cases, macrophyte detritus either retains its initial nutrient content or accumulates nutrients during 1-2 years of decomposition (Puriveth, 1980; Day, 1982; Davis and van der Valk, 1983), through the accumulation of microbial protein (Webster and Benfield, 1986 ) or humic N accumulation (Rice, 1982). Inconsistent findings concerning the role of macrophyte detritus in nutrient retention may result from variables such as nutrient supply (Howarth and Fisher, 1976; Saunders, 1976; Almazon and Boyd, 1978; Elwood et al., 1981 ), seasonality in temperature and flooding (Brinson, 1977; Puriveth, 1980; Day, 1982), and plant species (Day, 1982). There is little information concerning the capacity of macrophyte detritus to retain larger quantities of P and N with nutrient enrichment. This study presents a budget for nutrient retention through macrophyte growth, death and 2 years decomposition at eutrophic, transitional and oligotrophic sites along a gradient of surface water nutrient concentrations in the Florida Everglades. Net nutrient gains and losses are quantified in the process whereby Cladium jamaicense Crantz and Typha dorningensis Pets. leaf production leads to detritus accumulation and nutrient retention. Leaf production values of C. jamaicense and T. domingensis (Davis, 1990) are

CLADIUMJAMAICENSE AND TYPHA DOMINGENSIS IN FLORIDA EVERGLADES

205

combined with tissue P and N concentrations to estimate annual allocation to growing leaves and annual retention in standing dead leaves. The P and N content of C. jamaicense and T. domingensis leaf detritus is followed from the time the leaves die through 2 years of decomposition to estimate the detritus decomposition rate and the release or accumulation of P and N during decomposition. The effectiveness of Everglades C. jamaicense and T. domingensis in intercepting P and N is evaluated. Both above-ground and below-ground plant production contribute to nutrient immobilization in organic detritus in wetlands (Richardson and Marshall, 1986 ). In the Florida Everglades, Toth ( 1987, 1988 ) demonstrated that individual C. jamaicense and T. domingensis plants accumulate and hold P and N primarily through leaf production and above-ground detrital accumulation. Long-term nutrient retention by these species, through growth and decomposition of below-ground organs, amounts to less than 20% of the retention associated with leaf production and decomposition. These species thus appear to sequester nutrients primarily through above-ground production and accumulation of leaf litter in the Everglades. Soil water, rather than surface water, probably provides the major source of P and N nutrition for C. jamaicense and T. domingensis, but surface water is the source of anthropogenic nutrient enrichment to both soil and plants in the Everglades. Both surface water and soil concentrations are elevated in the Everglades near inflows of nutrient-enriched agricultural water (Davis, 1990). Anoxic conditions at nutrient-enriched Everglades sites (Reeder and Davis, 1983 ) may also influence nutrient solubility and availability in the substratum (Reddy, 1983; Reddy and Rao, 1983). Consequently, surface water P and N concentrations are used as correlates, rather than direct measures of nutrient enrichment in this study. STUDY AREA

The Everglades (Fig. 1 ) represent an oligotrophic wetland ecosystem which has received increased nutrient supplies for nearly 30 years as a result of water management practices. This subtropical, freshwater peatland historically occupied an approximately 1 000 000 ha basin which received water and nutrients mostly from direct rainfall (Davis, 1943; Parker, 1974). As a result, nutrients probably were in limited supply (Steward and Ornes, 1975 ). Most of the remaining Everglades are contained within approximately 500 000 ha of Water Conservation Areas and Everglades National Park. The nutrient supply into the Water Conservation Areas has increased due to pumped inflows of run-off water from drained lands in the Everglades agricultural area. Vegetation change has accompanied the increased nutrient inputs in the Everglades. Vast, nearly monospecific stands of C. jamaicense cover 65-70% of the Everglades marsh (Loveless, 1959 ). The dominance of this large sedge in

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208

S.M. DAVIS

the Everglades has been attributed to its low nutrient requirements (Steward and Ornes, 1975). Since nutrient supply has increased, T. domingensis has invaded where agricultural inflows enter the marsh (Davis, 1990). The middle of the Water Conservation Areas, designated Water Conservation Area 2A (Fig. 1 ), receives particularly large inflows of agricultural water and nutrients because of a convergence of canal systems on the inflow gates at the north end of the area. A nearly monospecific T. domingensis stand covers 2400 ha below the inflows in Water Conservation Area 2A. Scattered T. domingensis have permeated the sawgrass marsh to the south of this stand during the past decade. Surface water P and N concentrations decline as inflow water flows southward across the marsh, creating a gradient from high concentrations near the inflow structures to values approaching detection limits in the interior marsh (South Florida Water Management District, 1991 ) (Table 1 ). Declining soluble reactive phosphorus (SRP) and nitrate (NO3N) concentrations account for most of the falls in total P and total N along the gradient. The gradient is characterized by high temporal variability in surface water P and N concentrations at eutrophic sites in comparison to lower variability at transitional and oligotrophic sites (Table 1 ). The lower ends of the ranges of surface water concentrations are similar for all sites. Wide fluctuations in concentrations above lower levels, in comparison to consistency in concentrations near lower levels, differentiate eutrophic sites from oligotrophic sites along the gradient. The P and N gradient is further characterized by year-to-year fluctuations in surface water nutrient concentrations, depending on annual variations in water and nutrient inputs into the marsh. The marsh remained flooded year round throughout this study owing to a prescribed water regulation schedule for the area (Davis, 1990). METHODS

Leaf biomass turnover and nutrient flux Leaf production and nutrient flux were estimated in C. jamaicense and T. domingensis stands at four sites along the nutrient gradient in Water Conservation Area 2A (Fig. 1 ). These sites were located on a line extending approximately south-southwest into the marsh, at distances of 0.8, 1.6, 3.2 and 6.4 km from the north levee. Estimates were made for two sampling years. Sites B, C and D were sampled during April 1975-April 1976, while Sites A, B and D were sampled during April 1979-April 1980. This combination of three sites per year during two sampling years yielded six estimates of leaf production and nutrient accumulation for each plant species. Methods for estimating annual leaf production were detailed by Davis (1990). Two sampling techniques were employed. The first technique involved quadrat sampling to determine mean annual live leaf biomass. Five

CLAD1UM JAMAICENSE AND TYPHA DOMINGENSIS IN FLORIDA EVERGLADES

209

replicate 0.5 m 2 quadrats were collected monthly from C. jamaicense and T. domingensis stands during the first sampling year. Sampling frequency was reduced to bimonthly during the second year, except at Site A where samples were collected monthly. Living leaves within quadrats were weighed after oven-drying for 72 h at 90°C. The second sampling technique estimated annual leaf turnover rate. At least five newly emerged plants of each species were tagged at each site during each vegetation sampling year. Leaf lengths of each plant were measured monthly throughout the life of the plants. Cumulative life-time leaf growth of each tagged plant was divided by years longevity to estimate annual growth. Annual growth of each plant was divided by mean leaf biomass during its life span to calculate an annual turnover rate of leaf biomass. The mean leaf turnover rate for each stand was multiplied by mean annual leaf biomass, as determined by quadrat sampling, to estimate annual leaf production. Nutrient allocation to growing leaf biomass and release from dying leaf biomass were estimated by combining annual production estimates (Davis, 1990 ) with tissue P and N concentrations. Living leaves collected in biomass samples were analyzed for tissue P and N concentration. Intact standing dead leaves attached to living plants were also collected from quadrats for nutrient analysis, although biomass was not measured. Annual nutrient allocation to growing leaves was estimated by multiplying annual leaf production by mean annual tissue P and N concentrations in living leaves. Annual nutrient retention in dead leaves was estimated by multiplying annual leaf production by mean annual tissue P and N concentrations in freshly dead leaf material. Annual nutrient loss from dying leaves (amounts translocated or leached) was calculated as accumulation during growth minus retention in dead leaves.

Leaf decomposition and nutrient flux Litterbag experiments were initiated in July 1977, October 1977 and February 1979 at the locations of previous production studies. Experiments that began in July and October 1977 compared sites B, C and D, while 1979 experiments compared Sites A, B and D. This yielded nine combinations of sites and sampling periods. Intact standing dead leaves were collected from living C. jamaicense and T. domingensis plants at each site the m o n t h before litterbags were placed in the marsh. Leaves were cut into 10 cm lengths and oven-dried at 45 °C for 96 h. Litterbags were constructed from 30 X 30 cm squares of fiberglass window screening (six meshes per centimeter) which were loosely folded to contain the leaf material while allowing the entry of macroinvertebrates into the bags. Each bag contained 5 g dry mass of dead leaf material. Litterbags were placed in the water in the C. jamaicense and T. domingensis stands where the dead leaf material had been collected. The bags floated for the first few days until

210

S.M.DAVIS

the litter became waterlogged, after which they gradually sank to the bottom and became incorporated into the litter layer. Three to four replicate bags from each setout were retrieved after 1 month, 1 year, 2 years and varying periods in between. Litter remaining in retrieved bags was dried at 90°C for 48 h plus 45 °C for 24 h, weighed, and analyzed for tissue P and N concentration. Nutrient contents of retrieved litter were calculated by multiplying dry mass by P and N tissue concentrations.

Leaf tissue nutrient analyses Leaf material from quadrat samples and retrieved litterbags was ground in a Wiley mill after weighing. Analyses for P and N were made using a Technicon Autoanalyzer II, after solubilization of P by lithium metaborate fusion (Medlin et al., 1969) and Kjeldahl digestion of N using a block digester. For quality control, National Bureau of Standards NBS 1571 (National Bureau of Standards, 1979) was used as an external reference standard for each set of tissue and soil nutrient analyses. Analyses were accepted if values for the standard were _+ 10% of NBS values.

Water sampling and nutrient analyses Water samples were collected and water depths were measured at C. jamaicense and T. domingensis sites monthly throughout the study. Samples were collected from all sites on the same date each month. Samples for the analysis of dissolved nutrient fractions were filtered through 0.45/tm Nucleopore filters. Samples for total P analysis were digested by autoclaving at 121 °C ( 15 PSI) using the persulfate procedure. Samples for total N analysis were digested by the Kjeldahl procedure. Analyses were made using a Technicon Auto Analyzer II according to procedures SM424G for total PO4 and SRP (American Public Health Association (A.P.H.A.), 1980), SM418F for NO3-N (A.P.H.A., 1980), SM417G for NH4-N (A.P.H.A., 1980) and EPA351.2 for total Kjeldahl nitrogen (Environmental Protection Agency, 1979 ). Total PO4 and SRP are reported as P. Analyses for SRP and NO3-N were conducted throughout the study, while analyses for total P, total N and NH4-N began in mid- 1976. RESULTS

Surface water SRP and NO3-N concentrations were used to examine correlations of plant nutrient retention to enrichment because these parameters were measured during both vegetation sampling years of 1975-76 and 1979-80. Mean annual SRP and NOa-N concentrations declined southward along the nutrient gradient and were higher during the second sampling year

CLADIUM JAMAICENSE AND TYPHA DOMINGENSIS IN FLORIDA EVERGLADES

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compared with the first (Davis, 1990). Nutrient gradients in combination with yearly variations yielded mean annual SRP concentrations of 0.0020.036 mg 1-1 and NO3-N concentrations of 0.005-0.050 mg 1-1 for the six combinations of vegetation sites and sampling years. The full range of measured surface water P and N parameters was used to examine correlations of litter nutrient retention to enrichment because all parameters were measured during the 1977-79 and 1979-81 litterbag experiments. Surface water nutrient concentrations during litterbag experiments showed differences between sites and sampling periods similar to those described for SRP and NO3N during vegetation sampling. Nutrient gradients, in combination with yearly variations, yielded mean surface water concentrations of 0.006-0.1 l 6 mg 1-l total P, 0.002-0.066 mg 1-1 SRP, 2.57-4.72 mg 1-l total N, 0.006-0.031 mg 1- l NO3-N and 0.02-0.58 mg 1- l NH4-N during the nine litterbag incubations.

Leaf biomass turnover and nutrient flux Leaf tissue concentrations of P, but not N, reflected surface water concentrations. Phosphorus concentrations in leaf tissue of both C. jamaicense and T. domingensis differed significantly ( P < 0.01 ) among the six site/sampling year treatments. Mean annual leaf tissue P concentrations increased logarithmically with mean annual SRP concentrations in surface water ( r = 0.84 and 0.93 for C. jamaicense living and dead leaves, respectively; r = 0.95 and 0.86 for T. domingensis living and dead leaves, respectively) (Fig. 2 ). Tissue N concentrations did not differ significantly along the gradient. A lack of significant differences for N may have resulted in part from the larger variability of tissue N concentrations compared with those of P (Fig. 2). Despite the lack of significant differences, a positive correlation of tissue N concentration to NO3-N was apparent for T. domingensis live leaves ( r = 0.95 ). Tissue nutrient concentrations were significantly higher ( P < 0.05 ) in live leaves compared with dead leaves. Concentrations in live C. jamaicense leaves exceeded those in dead leaves on average by factors of 2.4 for P and 1.6 for N (Fig. 2 ). Concentrations in live T. domingensis leaves exceeded those in dead leaves by factors 4.4 for P and 2.0 for N. The decline in tissue nutrient concentrations during leaf senescence was proportionately greater for P compared with N and for T. domingensis compared with C. jamaicense. Higher nutrient concentrations in live leaves, compared with dead leaves, indicated substantial translocation or leaching from leaves during mortality. Typha domingensis accumulated higher tissue nutrient concentrations than C. jamaicense during growth, although leaf concentrations after mortality were similar for both species. Concentrations of P and N in live leaf tissue were significantly higher ( P < 0 . 0 5 ) in T. domingensis compared with C. jamaicense. Phosphorus concentrations in live T. domingensis leaves averaged twice those in C. jamaicense, while T. domingensis N concentrations averaged 1.5

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times those in C. jamaicense (Fig. 2 ). In contrast, dead leaf nutrient concentrations did not differ significantly between species. Although growing T. domingensis leaves were more effective than those of C. jamaicense in concentrating nutrients, dying T. domingensis leaves held a smaller proportion of assimilated nutrients. After growth and senescence, the two species were approximately equally effective in retaining nutrient concentrations in dead leaf tissue. Cladium jamaicense allocated 0.22-1.51 g m -2 year -1 P and 4.7-16.6 g m -2 year-l N to growing leaves, as shown by the upper sets of points in Fig. 3. These points represent estimates for the six combinations of sites and sampling years. Nutrient allocation to growing leaves corresponded to biomass and production estimates (Davis, 1990), and to tissue nutrient concentrations (Fig. 2). Dead leaves retained 0.07-0.74 g m -2 year -~ P and 2.9-10.8 g m -2 year-l N, as indicated by the lower sets of points. Differences between upper and lower points represent nutrient leaching or translocation from leaves during mortality. Because of falls in tissue P and N concentrations during leaf death, dying leaves lost 44-68% of the P and 31-46% of the N which they accumulated during growth. Nutrient allocation to growing C. jamaicense leaves and retention in dead leaves both decreased linearly with distance from the north levee where inflow structures were located ( r = - 0.97 and - 0.99 for P allocation and retention, respectively; r = - 0 . 9 8 and - 0 . 9 7 for N allocation and retention, re-

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spectively) (Fig. 3 ). Thus P and N allocation and retention were highest where concentrations were greatest in surface water and soil, toward the upper end of the nutrient gradient. Dying leaves also lost larger amounts of P and N where water and soil concentrations were higher, as indicated by the widening gap between uptake and retention toward the upper end of the nutrient gradient. Allocation of P and N to growing C. jamaicense leaves, and retention in dead leaves, did not reflect differences in surface water NOa-N and SRP concentrations between the two sampling years; correlations to mean SRP and NO3-N concentrations during specific sampling years were not found. Thus, nutrient allocation and retention resulting from C. jamaicense leaf turnover corresponded to general site characteristics of nutrient enrichment, as indicated by long-term surface water nutrient gradients and by distances from inflows. Typha domingensis allocated larger quantities of P and N to growing leaves, translocated or leached larger quantities from dying leaves, and retained larger quantities in dead leaves (Fig. 4) compared with C. jamaicense. Phosphorus allocation to T. dom ingensis leaves of 0.64-4.16 g m - e year-1 averaged 2.7 times that of C. jamaicense, while N allocation to T. domingensis leaves of

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9.6-33.4 g m - 2 year- ~averaged twice that of C. jamaicense. Greater nutrient allocation to T. domingensis leaves resulted from higher biomass and production rates (Davis, 1990), as well as from higher tissue P and N concentrations (Fig. 2 ). U p o n mortality, T. domingensis leaves translocated or leached proportionately larger amounts of nutrients than C. jamaicense. Dying T. domingensis leaves lost 71-83% of the P and 33-63% of the N which they accumulated during growth. After allocation during growth and translocation or leaching during mortality, P retention in dead T. domingensis leaves o f 0 . 1 1 1.00 g m - 2 year- ~ averaged 1.6 times that of C. jamaicense, while T. domingensis N retention of 3.6-15.9 g m -2 year -~ averaged 1.8 times that of C.

jamaicense. Typha domingensis differed from C. jamaicense in that nutrient allocation to leaves showed little correlation to the distance of sites from inflows, but increased logarithmically with mean annual nutrient concentrations in surface water during the particular sampling years ( r = 0.87 for both P allocation and retention in relation to SRP; r = 0.99 and 0.92 for N allocation and retention in relation to NO3-N) (Fig. 4 ). The gap between growing leaf allocation and dead leaf retention widened as SRP and NO3-N concentrations increased, indicating that dying T. domingensis leaves leached or translocated larger quantities of P and N when surface water concentrations were higher. Typha domingensis resembled C. jamaicense in that both species responded to higher surface nutrient concentrations by increasing P and N allocation to growing leaves, release from dying leaves and retention in dead leaves. However, only in T. domingensis were these processes correlated to yearly variations in surface water nutrient concentrations. C. jamaicense

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Leaf decomposition and nutrient flux Patterns of litter decomposition and nutrient flux, as illustrated for July 1977 litterbag experiments, were similar for all experiments. Approximately half of the litter mass remained intact in the bags after 2 years for sites and species combined (Fig. 5). Nutrient enrichment resulted in accelerated decomposition rates of both C. jamaicense and T. domingensis, as evidenced by the smaller litter mass remaining in bags at eutrophic sites relative to transitional and oligotrophic sites. Typha domingensis decomposed more rapidly than C. jamaicense at each site. Phosphorus and N concentrations in decomposing litter of both species increased over 2 years at eutrophic and transitional sites (Figs. 6 and 7). InC_.

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T

i

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i

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15-

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1978

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1979

nutrient e n r i c h e d site B ~ - - - - - -

I l i l l l l l l l l t l l l l l l l l l [ l l l l

1977

1978

transitional site C , - . . . .

1979

background

site D

Fig. 7. Tissue N concentration and content in sawgrass and cattail leaves during 2 years of decomposition. Values represent the mean_+standard error. N = 3.

creases in litter P and N concentrations were higher at enriched sites compared to transitional ones, and in T. domingensis compared to C. jamaicense. Phosphorus concentrations at the oligotrophic site changed little, while N concentrations at the oligotrophic site increased over 2 years. Increasing P and N concentrations in decomposing litter resulted in a net increase in content per bag and immobilization of these elements over 2 years, despite decreasing litter mass, at eutrophic and transitional sites. Litter at the oligotrophic site neither gained nor lost significant amounts of P, but accumulated N, during this period. Litter accumulated more P and N toward the upper end of the nutrient gradient. Typha domingensis litter accumulated more P

CLADIUMJAMAICENSEAND TYPHADOMINGENSISIN FLORIDAEVERGLADES C. jamaicense

217

T. domingensis

0.60.

B

0.50 •

0.40

g "~ 0.30

g 0 ,,,, 0.20

A

<3 A

010

z D ,~. P Content= -.074274-14.27565 (Total P) zD -104.66287 (Total p)2 r : 0.952

r

0.02

0.04

r

0.06

0.08

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zk p Content: =00683+17.58563 (Total P) -146.72790 (Total p)2 r = 0,800

i

010

i

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Fig. 8. Changes in the P content of sawgrass and cattail leaves during 2 years of decomposition in relation to mean surface water total P concentration. Values represent changes in mg P per g freshly dead leaf material_+ standard error. N = 3 for 1977 setouts and 4 for 1979 setouts.

TABLE 2 Net P and N immobilization ( g m -2 year- ~) by C. jamaicense and T. domingensis resulting from annual leaf turnover and 2 years decomposition. Values represent site means with standard errors in parentheses Site A ~ Phosphorus C. jamaicense T. domingensis Nitrogen C.jamaicense T. domingensis

1.11 1.42

17.4 18.3

Site B 1.34(0.03) 1.25 (0.12 )

15.2(0.5) 15.1 (2.1)

Site C 0.89(0.10) 1.68 (0.24)

10.7(1.6) 21.3(0.0)

Site D 0.07(0.01 ) 0.30 (0.13 )

4.2(0.3) 10.8(3.1 )

~N=I.

than C. jamaicense at each site. The amount of P that leaf litter accumulated during 2 years of decomposition was related to the mean surface water total P concentration during that period (Fig. 8 ). Litter of both species accumulated increasing amounts of P as surface water total P concentrations increased from background levels of less than 0.01 mg 1-~ to intermediate levels of enrichment of about 0.06 mg 1-~. However, once total P concentrations exceeded about 0.06 mg 1-1, litter P uptake leveled off, then declined with

218

S.M. DAVIS

higher levels of enrichment. Unlike P, litter N uptake was not clearly related to surface water N concentrations. Both C. jamaicense and T. domingensis sequestered P and N as the cumulative result of leaf growth, death and 2 years of decomposition (Table 2). Amounts of P and N that were sequestered were estimated by summing nutrient retention by dead leaves plus detritus nutrient immobilization. Nutrient immobilization by leaf detritus as it decomposed over 2 years was calculated by multiplying annual crops of dead leaf material (Davis, 1990) by changes in litter P and N content per unit mass of freshly dead leaf material (Fig. 8 ). Sequestration of P and N by C. jamaicense increased along the nutrient gradient from background Site D to enriched Sites A and B (Table 2 ). Typha domingensis P and N sequestration was also lowest at Site D, but peaked at transitional Site C, and then leveled off or declined further up the gradient at Sites A and B. The total amount of P that was sequestered as the result of annual leaf turnover and 2 years of decomposition ranged from 0.07 to 1.33 g m - 2 for C. jamaicense and from 0.30 to 1.42 g m - 2 for T. domingensis. Corresponding rates of N sequestration ranged from 4 to 17 g m -2 for C. jamaicenseand from 11 to 21 g m -2 for T. domingensis. DISCUSSION

Leaf biomass turnover and nutrient flux Nutrient allocation to growing leaves was greater for T. domingensis than for C. jamaicense. Leaves of T. domingensis accumulated more P and N than C. jamaicense under equivalent conditions, and T. domingensis was capable of allocating more nutrients to leaves during years of elevated nutrient enrichment. T. domingensis leaf turnover rates, being two to three times those of C. jamaicense (Davis, 1990), allowed nutrient allocation to leaves to better reflect temporal changes in surface water concentrations. The ability of T. domingensis to assimilate larger quantities of P and N in growing leaves corresponded to its observed spread into C. jamaicense stands in Everglades areas receiving high-nutrient inflows. A similar intrusion of Typha latifolia L. into macrophyte stands, with an eventual shift to a T. latifolia dominant community, was noted by Kadlec (1987) in the vicinity of effluent discharge in a Michigan wetland. These findings are compatible with models of Shaver and Melillo (1984) and Richardson and Marshall (1986), which predict a vegetation shift toward macrophyte species that are more efficient in nutrient uptake as nutrient supply increases. With nutrient enrichment in the Everglades, a plant species that was more effective in accumulating nutrients in leaf tissue invaded areas occupied by a species that was less effective. Contrasting responses of C. jamaicense and T. domingensis to the nutrient gradient are characteristic of low-nutrient-status plant species from infertile

CLADIUM JA MAICENSE AND TYPHA DOMINGENSIS IN FLORIDA EVERGLADES

219

habitats (C. jamaicense) versus high-nutrient-status species from fertile habitats (T. domingensis) (Grime, 1977; Chapin, 1980). The relatively small leaf growth response of C. jamaicense to temporal variations in surface water nutrient inputs (Davis, 1990) is typical of low-nutrient-status plants. The relatively low nutrient allocation to C. jamaicense leaves, approximately half that of T. dorningensis, under high-nutrient conditions is another trait of lownutrient-status species. A lower rate of nutrient loss from senescing C. jamaicense leaves (via a combination oftranslocation and leaching) also indicates a low-nutrient-status species. The longer leaf longevity, lower leaf growth rate and slower leaf turnover rate of C. jamaicense (Davis, 1990), plus its welldeveloped leaf cuticle, are adaptations that reduce leaching loss in low-nutrient-status plants. In comparison, the larger growth response to changing nutrient availability, higher rates of nutrient allocation to growing leaves under high-nutrient conditions, higher rate of nutrient loss from senescing leaves, shorter leaf longevity, faster leaf growth rate and more rapid leaf turnover of T. domingensis are all characteristics of high-nutrient-status plants which tend to be competitive when the nutrient supply increases. In the fertile habitat near the upper end of the nutrient gradient in Water Conservation Area 2A, the high-nutrient-status traits of T. domingensis appear to have given that species competitive advantage over C. jarnaicense. The effectiveness of growing and dying leaves in retaining P and N remained constant as surface water concentrations of these elements increased. Nutrient retention in freshly dead leaves increased with enrichment at a slower rate than did uptake by growing leaves. Elevated quantities of nutrients which were allocated to growing leaves at enriched sites were mostly translocated or leached from the leaves by the time they died. As a result, dead leaves retained only slightly greater quantities of nutrients under enriched conditions in comparison to background conditions. The main effect of enrichment on leaf nutrient allocation was to increase translocation or leaching from dying tissue, rather than to increase retention in dead leaf material. Since leaf biomass turnover alone was an inefficient mechanism for trapping elevated surface water nutrient inputs, long-term nutrient retention appeared to depend largely on the nutrient dynamics of litter decomposition. The main role of leaf turnover in nutrient retention appeared to be the production of detritus as a substrate for subsequent physical-chemical and microbiological processes. Davis and van der Valk ( 1978 ) drew similar conclusions from temperate stands of Typha glauca Godr.

Leaf decomposition and nutrient flux The net effect of production and decomposition was an accumulation of leaf detritus. The finding that approximately half of the leaf litter remained intact after 2 years indicated that decomposition proceeded less rapidly than

220

S.M, DAVIS

production, as would be expected in a peatland. Litter decomposition rates in the Everglades were comparable with the 2 year litter half-life reported by Kadlec (1989) in a Michigan peatland. Nutrient enrichment appeared to influence both the annual rate of detritus accumulation and the physical structure of deposited sediments. Detritus accumulation was estimated by combining decomposition rates from this study with production estimates of Davis (1990). The interior C. jamaicense marsh at Site D produced about 894 g m - 2 of dead leaf material annually, of which about 45% was lost during 2 years decomposition and about 55% remained as relatively compact, fibrous organic sediment (S.M. Davis, personal observation, 1975-1980) on the marsh floor. In contrast, nutrient-enriched areas that had converted to T. domingensis (Sites A and B ) produced about 2417 g m - 2 of dead leaf material annually, of which about 52% was lost during 2 years decomposition and about 48% remained as relatively fine, flocculent sediment (S.M. Davis, personal observation, 1975-1980 ). As a net result of production and 2 years decomposition, background C. jamaicense deposited about 492 g m - 2 year-l of relatively intact and compact leaf detritus, while nutrient-enriched T. domingensis deposited about 1160 g m -2 year- Lof finer, more flocculent sediment. The results of this study indicate that Everglades C. jamaicense and T. domingensis communities deposited P and N in the detritus that resulted from leaf growth, death and 2 years decomposition. Cladium jamaicense and T. domingensis stands accumulated detritus after 2 years of decomposition, contributed to the accretion of organic sediment and deposited nutrients within these sediments. Rates of nutrient storage in organic matter found in this study represented values under conditions of continuous flooding; these values might have differed under more dynamic hydrologic conditions. Rates of nutrient storage reported here did not include contributions by below-ground biomass, which may have increased rates by about 12% for C. jamaicense and by about 16% for T. domingensis (Toth, 1987, 1988). The relevance of nutrient retention after leaf production and 2 years decomposition to longer term nutrient sequestration in accreting organic sediments was not evaluated in this study, but the two processes may have been comparable, as evidenced by the similarity of 2 year nutrient deposition estimates in the Everglades to nutrient accretion measurements in other wetlands. Nutrient deposition estimates for the oligotrophic Everglades site are approximately equivalent to accretion estimates for oligotrophic peatlands and backwater marshes. Mean retention rates of 0.18 g m - 2 year-~ P and 7.5 g m - 2 year-~ N at the Everglades oligotrophic site (Site D, species combined ) are comparable with calculated values of less than 0.1-0.2 g m -2 year- ~ P retention and 0.1-4.7 g m - 2 year -~ N retention through peat accretion in northern oligotrophic wetlands (Nichols, 1983). Nutrient retention estimates at the Everglades oligotrophic site are also similar to measurements of 0.5 g m -2 year-~ P and 9 g m -2 year- l N retention in accreting sediments in

CLADIUM JAMAICENSE AND TYPHA DOMINGENSIS 1N FLORIDA EVERGLADES

221

a Louisiana backwater marsh (Hatton et al., 1982), although Louisiana sediments contained mineral matter in addition to organics. Litter breakdown has been reported to be largely complete after 2 years in some wetlands (Latter and Cragg, 1967; Chamie, 1976; Day, 1982). Nutrient release from detritus slowed before 2 years in the Everglades, as evidenced by little change in the litter nutrient content during the last 6-12 months of the 2 year decomposition period. Subsequent oxidation and mineralization of detritus may have been impeded by continuous flooding and reducing conditions in the detrital layer (Reeder and Davis, 1983 ). Perhaps rates of long-term nutrient sequestration in accreting organic sediment in the Everglades C. jamaicense community are comparable with rates in oligotrophic peatlands under natural lownutrient conditions. Similar litter half-lives in the Everglades and in a Michigan peatland (Kadlec, 1989) support this interpretation. Where surface water nutrient concentrations were higher in the Everglades, nutrient retention rates in 2-year-old detritus were higher, but apparently only up to a finite capacity. Rates of 1.1-1.4 g m - 2 year -I P and 17-18 g m -2 year- 1 N retention by C. jamaicense and T. domingensis at Site A may represent upper limits of nutrient retention resulting from leaf production and 2 years decomposition in eutrophic Everglades habitat; these rates were equaled or exceeded downstream along the surface water nutrient gradient at transitional sites between Sites A and D. Increased rates of nutrient retention at eutrophic Everglades sites might be expected, based on comparison with other studies. The finding of higher detrital nutrient retention at eutrophic Everglades sites, in combination with the decline in surface water P and N concentrations along the nutrient gradient below inflow structures, agree with the conclusion of Kadlec (1989) that the litter zone in a Michigan peatland receiving secondary effluent contained a large fraction of the nutrients added over a 10 year period. Greater detrital nutrient retention at enriched Everglades sites also agrees with the review of Richardson and Nichols ( 1985 ) of loading-retention relationships in northern wetlands receiving ~vastewater, where P removal increased up to 4.5 g m -2 year- 1as loading increased. However, a corresponding decrease in nutrient uptake efficiency (percentage of inputs) as loading increased (Richardson and Nichols, 1985 ) also suggests that the nutrient retention capacity of wetland may be limited, as appears to be the case for Everglades macrophyte stands. Thus, rates of long-term nutrient sequestration may increase with loading, to a limited extent, in a wide variety of wetland systems. The Everglades results suggest that the capacity of this system for long-term nutrient sequestration in accumulating detritus can increase, but only up to a finite level, as anthropogenic nutrient inputs increase. CONCLUSIONS

Everglades C. jamaicense and T. domingensis sequester P and N in the accumulated organic sediment that results from annual production, mortality

222

S.M. DAVIS

and 2 years decomposition. However, the capacity of vegetation for nutrient retention as a result of these processes is limited. As surface water concentrations of P and N increase along the gradient and during high-discharge years, retention of these elements reaches a maximum under moderate levels of enrichment, between Sites B and C in this study. The Everglades C. jamaicense community is adapted to low nutrient inputs primarily from direct rainfall, has limited capacity to retain higher nutrient inputs and is subject to displacement by a more competitive species ( T. domingensis) under higher-nutrient conditions. The observed spread of T. domingensis into C. jamaicense sites in nutrient-enriched areas of the Everglades, and contrasting competitive strategies of nutrient allocation by the two species, support this concept. Apparently, when the vegetation uptake capacity of stands already invaded by T. domingensis is exceeded during periods of high nutrient inputs, nutrients pass further downstream and the competitive strategies of T. domingensis allow it to invade C. jamaicense sites in the downstream area. Wetland ecosystems which developed under conditions of low nutrient supply, such as the Florida Everglades, may offer a finite potential for accelerated vegetation nutrient retention when the exogenous nutrient supply increases as a result of human activities. However, a plant species such as C. jamaicense, that is adapted to a low-nutrient Everglades environment, may have a low nutrient threshold before it loses its competitive capability and is replaced by a species such as T. domingensis that is better adapted to a highnutrient environment. ACKNOWLEDGEMENTS

I gratefully acknowledge the South Florida Water Management District for supporting this research. J.W. Dineen provided encouragement and support throughout the study. Water and soil nutrient analyses were performed by the South Florida Water Management District Chemistry Laboratory. I particularly wish to thank L. Haunert, M. Zaffke, M. Rosen, A.M. Superchi, N.H. Urban, F.E. Worth, D. Cook, S. Green and P.B. Reeder for major contributions to field data collection and laboratory analyses. J.B. Grace, A. Herndon, R.H. Kadlec, M.S. Koch, C.R. Richardson and L.A. Toth provided reviews of the manuscript.

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Boyd, C.E., 1970. Vacular plants for mineral nutrient removal from polluted waters. Econ. Bot., 24: 95-103. Brinson, M.M., 1977. Decomposition and nutrient exchange of litter in an alluvial swamp forest. Ecology, 58: 601-609. Chamie, J.P.M., 1976. The effects of simulated sewage effluent upon decomposition, nutrient status and litter fall in a central Michigan peatland. Dissertation, University of Michigan, Ann Arbor, MI, 110 pp. Chapin, F.S., 1980. The mineral nutrition of wild plants. Annu. Rev. Ecol. System., 11: 233260. Davis, C.B. and van der VaIL A.G., 1978. The decomposition of standing and fallen litter of Typha glauca and Scirpusfluviatilis. Can. J. Bot., 56: 662-675. Davis, C.B. and van tier Valk, A.G., 1983. Uptake and release of nutrients by living and decomposing Typha glauca Oodr. tissues at Eagle Lake, Iowa. Aquat. Bot., 16: 75-89. Davis, J.H., 1943. The natural features of southern Florida. Fla. Geol. Soc. Geol. Bull. No. 25, 311 pp. Davis, S.M., 1990. Sawgrass and cattail production in relation to nutrient supply in the Everglades. In: R.R. Sharitz and J.W. Gibbons (Editors), Freshwater Wetlands and Wildlife. Office of Scientific and Technical Information, U.S, Department of Energy, Oak Ridge, TN, pp. 325-341. Day, F.P., Jr., 1982. Litter decomposition rates in the seasonal flooded Great Dismal Swamp. Ecology, 63: 670-678. Elwood, J.W., Newbold, J.D., Trimble, A.F. and Stark, R.W., 1981. The limiting role of phosphorus in a woodland stream ecosystem: effects of P enrichment on leaf decomposition and primary producers. Ecology, 62:146-158. Environmental Protection Agency, 1979. Methods for chemical analysis of water and wastes. EPA-600/4-79-020, 430 pp. Grime, J.P., 1977. Evidence for the existence of three primary strategies in plants and its relevance to ecological and evolutionary theory. Am. Nat., 111: 1169-1194. Hatton, R.S., Patrick, W.H., Jr. and DeLaune, R.D., 1982. Sedimentation, nutrient accumulation, and early diagenesis in Louisiana Barataria Basin coastal marshes. In: V.S. Kennedy (Editor), Estuarine Comparisons. Academic Press, New York, pp. 255-267. Hopkinson, C.S. and Schubauer, J.P., 1984. Static and dynamic aspects of nitrogen cycling in the salt marsh graminoid Spartina alterniflora. Ecology, 65:961-969. Howard-Williams, C. and Howard-Williams, W., 1978. Nutrient leaching from the swamp vegetation of Lake Chilwa, a shallow African lake. Aquat. Bot., 4: 257-267. Howarth, R.W. and Fisher, S.G., 1976. Carbon, nitrogen, and phosphorus dynamics during leaf decay in nutrient-enriched ecosystems. Freshwater Biol., 6:221-228. Kadlec, R.H,, 1987. Northern natural wetland water treatment systems. In: K.R. Reddy and W.H. Smith (Editors), Aquatic Plants for Water Treatment and Resource Recovery. Magnolia, Orlando, FL, pp. 83-98. Kadlec, R.H., 1989. Decomposition in wastewater wetlands. In: D.A. Hammer (Editor), Constructed Wetlands for Wastewater Treatment. Lewis Press, Chelsea, MI, pp. 459-468. Latter, P.M. and Cragg, J.B., 1967. The decomposition ofduncus squarrosus leaves and microbiological changes in the profile of Juncus moor. J. Ecol., 55: 465-482. Loveless, C.M., 1959. A study of the vegetation in the Florida Everglades. Ecology, 40: 1-9. Medlin, J.H., Suhr, N.H. and Bodkin, J.B., 1969. Atomic absorption analysis of silicates employing LiBO2 fusion. At. Absorpt. Newsl., 8: 25-29. Nichols, D.S., 1983. Capacity of natural wetlands to remove nutrients from wastewater. J. Water Pollut. Control Fed., 55: 495-505. Parker, G.G., 1974. Hydrology of the pre-drainage system of the Everglades in southern Florida.

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