The effects of nutrient additions on mixtures of Typha latifolia L. and Typha domingensis pers. along a water-depth gradient

Aquatic Botany, 31 (1988) 83-92

83

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

THE E F F E C T S OF N U T R I E N T A D D I T I O N S ON M I X T U R E S OF T Y P H A LATIFOLIA L. A N D T Y P H A D O M I N G E N S I S P E R S . ALONG A WATER-DEPTH GRADIENT

JAMES B. GRACE

Department of Botany, Louisiana State University, Baton Rouge, LA 70803 (U.S.A.) (Accepted for publication 18 January 1988)

ABSTRACT Grace, J.B., 1988. The effects of nutrient additions on mixtures of Typha latifolia L. and Typha domingensis Pets. along a water-depth gradient. Aquat. Bot., 31: 83-92. Over a 2-year period, nutrients (N, P and K) were added to mixtures of Typha latifolia L. and T. domingensis Pets. growing across a water-depth gradient in order to determine: (1) the relationship between nutrient limitation and water depth; (2) the differential effect of nutrient addition on species performance. In addition, sediment characteristics were determined for untreated mixtures, monocultures and bare sediment. Additions of nutrients resulted in a rapid increase in total shoot density compared to controls. Increases in total shoot biomass, however, occurred gradually over time in response to nutrient additions. Nutrients enhanced T. latifolia in shallow to medium depths while T. domingensis was enhanced in medium to deep waters. Compared to bare sediment, pH was not affected by the presence of either species. Redox potential, however, was increased by T. domingensis in medium depths while unaffected by T. latifolia. Coupled with the observation that T. domingensis is capable of growing in deeper water than T. latifolia, the Eh data suggest that T. domingensis may have a greater capacity to aerate its roots and rhizosphere. Sediment ammonium concentrations were reduced by c. 90-95% by plant growth while potassium was reduced by c. 35-55%. Phosphate was unchanged compared to bare sediment. These results suggest that sediment nitrogen was the most limiting nutrient.

INTRODUCTION

Despite the rather large number of studies on competition between plant species that have been conducted in recent years, few cases exist in which the mechanism of interaction has been determined (exceptions include Christie and Detling, 1982; Caldwell et al., 1985; see also Tilman, 1982 for discussion). Because of this large gap in our knowledge, the discussions about niche differentiation and resource partitioning that often accompany studies of interspecific competition cannot be related to any features of the organisms involved. 0304-3770/88/$03.50

© 1988 Elsevier Science Publishers B.V.

84 Extensive studies of competition between the emergent aquatic plants Typha latifolia L. and Typha domingensis Pets. have revealed that T. latifolia (the shallow-water species) is a much better competitor than T. domingensis (the deep-water species ) under most circumstances (Grace, 1985 ). The reasons for the large differences in competitive ability observed are unclear considering the morphological characteristics of the two species. In general, T. domingensis has greater leaf height, a greater density of shoots, a more compact clone form, and a greater potential for biomass production than does T. latifolia (McNaughton, 1975; Grace, 1988). When the competitive abilities and growth potentials of the species were compared across a water-depth gradient, T. latifolia's competitive ability was found to increase with increasing water depth (Grace, 1985; Grace, 1987). In this paper the factors limiting the growth of T. latifolia and T. domingensis during competition are reported and how these factors relate to the characteristics of the species is discussed.

METHODS To examine competition between adults of T. lati/olia and T. domingensis, large replicate water-depth gradients were created by constructing an experimental pond (length 32 m, width 6 m, depth 1.4 m) at the agricultural farm site of the University of Arkansas. The pond was created as a long trough with a slope of approximately 30 ° tapering from the sides to the center (Fig. 1). The slope of the pond was chosen to resemble the slope found in earlier field studies of competition between Typha species (Grace and Wetzel, 1981a). Reinforced, non-reactive membrane (The Gagle Co., Bartlesville, Oklahoma) was used to line the entire pond. Individual gradients, 3.66-m long by 0.45-m wide with a relief along their long axes of 1.25 m were created by placing wooden dividers 0.2-m high on top of the pond membrane. The divider-membrane interface was sealed with reinforced polyethylene sheeting. Alluvial topsoil (pH 6.5 ) was added to a depth of 0.2 m. As soil was added it was packed in place, working from the bottom of the slope to the top, in order -25

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85 to minimize slumping. A slight amount of soil was added 6 months later to the top-most portions of the slope to compensate for soil compaction and slumping. "Landscape fabric" (burlap netting) and straw were used to cover the soil initially, in order to prevent soil erosion. The fabric and straw were removed when the pond was filled with water. Final soil depth was 0.2 + 0.03 m. On 22 and 23 July 1982, the pond was planted with young plants grown from seed in a heated glasshouse in 10-cm-diameter pots for 6 months prior to transplanting. Each individual gradient was initially planted with 11 equidistant rows each containing 2 transplants (thus, each gradient was a narrow rectangle, sloping along its long axis with 2 transplants at each of 11 equidistant positions along the slope). In order to create 50:50 mixtures in which both species were treated equally, the two species were alternated within each row and the sequence of species altered from row to row. Transplants were allowed to establish for 8 weeks as the water level was gradually raised to the level shown in Fig. 1. Water level was maintained by a fixed overflow and a continuous input of tapwater at a rate of 6.6 1 m i n - 1. The control treatment in this study was a 50:50 mixture of the 2 species planted as described above. Nutrient additions also used 50:50 mixtures in the same fashion. For the control treatment there were 6 replicate gradients arranged in a randomized block design with the pond divided into 2 blocks along its long axis. For the nutrient-addition treatment the arrangement of plants was similar to the controls except that the plants were confined to separate sections of the pond at either end. This treatment was isolated from the others to eliminate the possible contamination of the other treatments with the added nutrients. Each section contained 6 rows of plants and was, thus, equivalent in size to 3 regular gradients. To avoid an unequal contribution of edge effects, each section was divided into only two gradients instead of three. Thus, each of the 4 replicate gradients of nutrient additions was 50% larger than the gradients in the other treatments so that the total area was the same for all treatments even though the number of replicates was not. For fertilization, nutrients were added in the form of solid spikes (Jobe brand, 13N-4P-5K where N was 2% nitrate-N, 8% water-insoluble-N, and 3% urea-N). Spikes were mechanically injected into the sediment monthly from 4 April 1983 to 12 August 1984 using a specially-designed tube that placed each spike 5 cm beneath the sediment surface. Fifty-five spikes were positioned systematically within each replicate gradient each time nutrients were applied. For measurement purposes, each gradient was divided into 11 rows with Row 1 being above the water line and Row 11 at the deepest location (Fig. 1) by stringing crosswires at a height of 10 cm above the water surface. Each subsection thus created had horizontal dimensions of 0.45 m by 0.30 m. Plants were measured for height of tallest leaf and examined for the presence of inflorescences at monthly intervals from May through September in both 1983 and 1984. All measuring periods lasted no more than 4 days. In order to estimate

86 above-ground biomass, plants of each species were collected from local populations to determine the relationships between height of tallest leaf and aboveground biomass for both flowering and non-flowering plants. The results of these regressions are presented in Grace (1985). Above-ground biomass for each plant measured in this study was estimated from the regression relationships. Final harvests of above- and below-ground biomass for select treatments of T. latifolia and T. domingensis showed that the regression equations provided reasonably accurate estimates of shoot biomass (Grace, 1988). In order to compare the species in terms of their effects on the sediments, cores were collected and analyzed for pH, Eh, ammonium, phosphate and potassium (additional information on sediment characteristics are published in Grace, 1985). Sediment cores were collected on the following dates during 1984: 15, 18 May, 11, 14, 25, 26 June. On each date, one core was collected from each of 3 rows (1, 6, 11; Fig. 1 ) within one gradient of each of 4 treatments: control (a 50:50 mixture, described above), T. latifolia monocultures, T. domingensis monocultures and bare soil. Over the 6 sampling dates, 72 cores were analyzed. The monoculture treatments were exactly the same as the control treatment except that all transplants were of the same species. The bare soil treatment consisted of gradients that were constructed in the way same as all others except that no transplants were ever planted and these gradients were kept thoroughly weeded throughout the study. Sediment data for the nutrient addition treatment are not reported because initial analyses revealed that the addition of the nutrient spikes created an extremely patchy distribution of nutrient concentrations. On all sampling dates, cores were collected using 3.0-cm-diameter plastic cylinders between 08.00 and 10.00 h. Each sample was left intact inside its cylinder and sealed with rubber stoppers, both top and bottom. In the laboratory, sediment pH and Eh were measured for each core while still in its cylinder using electrodes inserted 5 cm into the sediment. After all samples were measured for pH and Eh, they were removed from their cylinders, thoroughly mixed, and subsamples were removed for extraction. All procedures, including extractions, were performed on wet sediment to avoid the impact of drying on extractable ion concentrations. Further, all analyses except potassium determinations were performed at once, always during the day of collection. Potassium extracts were frozen for later analysis. Ammonium was extracted using 6% NaC1 (Chapman, 1976) and measured using an Orion ammonium electrode (model 95-10). Initial analysis of nitrate found it to be undetectable at any water depth and further analyses were not performed. Phosphate-P was extracted using 2.5% acetic acid and analyzed by the molybdenum-blue method (Chapman, 1976). Potassium was analyzed using atomic-absorption techniques (Page et al., 1982). Statistical analyses of the data were conducted using the Statistical Analysis System (Helwig and Council, 1979). ANOVAs revealed no significant block

87 effects and, therefore, this factor was ignored in the subsequent analyses. For graphical presentations of variability and for comparing means, Least Studentized Ranges (LSRs) were calculated (Steel and Torre, 1960). The LSR is the basis for the Duncan's Multiple Range Test and provides a rigorous, unbiased method for making unlimited a posteriori comparisons among means (Steel and Torre, 1960). Any two means in a comparison that differ by more than the appropriate LSR are declared significantly different with the probability of error less than 5%. For complete analysis of means using the Duncan's test, means were ranked from high to low and a different LSR was used depending upon how far apart in the ranking were the two means being compared. On many of the figures presented in this paper, the "minimum LSR" and the "maximum LSR" are presented for comparison between adjacent ranked means, and between the most dissimilar means (the range). RESULTS The density of Typha shoots (both species combined) averaged across the gradient was significantly enhanced by the addition of nutrients (Fig. 2 ). The enhancement of shoot density owing to nutrient addition was apparent by the first sampling date and persisted throughout the study. Changes over time in absolute shoot density were small but there was a significant decrease between May and June of 1983 for both the control and nutrient addition treatments. The total biomass of Typha shoots (both species combined) averaged across the gradient became increasingly enhanced by nutrient additions during the course of the study (Fig. 2 ). By September 1984, shoot biomass was enhanced approximately 45% by the addition of nutrients. The distribution of total shoot biomass across the gradient, however, was not equally affected by the addition of nutrients (Fig. 3). In Rows 1, 8 and 9, there was no significant effect but both medium and deep water showed increases in biomass ranging from 20 to 5O%. E 8o

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Fig. 3. Distribution of shoot biomassacross the gradient, September 1984. (A) Both speciescombined. (B) Typha latifolia. (C) Typhadomingensis. When the species are viewed separately, it can be seen t h a t they did not respond equally to the nutrient additions (Fig. 3). T. latifolia was significantly enhanced by nutrients in Rows 2-5 (5-58 cm) and reduced in Row 8 (95 cm) (Fig. 3). Where significant enhancements occurred, they ranged from 50 to 170%. In Row 8, however, T. latifolia was completely eliminated by nutrient additions. T. domingensis, in contrast, was enhanced in deeper waters by nutrient additions but was unaffected in Rows 1-5 (0-58 cm) and 9 (105 cm) (Fig. 3). E n h a n c e m e n t s for T. domingensis ranged from 60 to 150%. Sediment pH did not differ significantly between bare sediment and the other treatments at any depth (Table 1, Fig. 4 ). Significantly higher values did occur, however, at Row 1 (5 cm above water) compared to Rows 6 (70 cm) and 11 (115 cm) for all treatments except bare soil. Redox (Eh) showed significant row and t r e a t m e n t by row effects (Table 1, TABLE1 Analysisof variance statistics for sediment data (Figs. 4-6) Variable

Source of variation

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Treatment Row Treatment*row Treatment Row Treatment*row Treatment Row Treatment*row Treatment Row Treatment*row Treatment Row Treatment*row

0.10 13.07 1.16 1.83 266.88 2.26 122.60 75.08 12.62 1.15 1.02 1.26 14.95 3.47 1.02

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Fig. 4). For all treatments, Row 1 exceeded an Eh of + 100 mV and was much higher than Rows 6 or 11. Only T. domingensis monocultures showed a difference between Rows 6 and 11 (within treatment) with Row 6 having an almost 100 mV higher redox. Comparing across treatments, T. latifolia monocultures and control mixtures were not different from bare sediment at any row. T. domingensis monocultures, however, were 100 mV higher at Row 6 than any of the other treatments. Ammonium-ion concentration differed on two levels (Fig. 5). Comparing among all treatments (Fig. 5A), major differences occurred between treatments, with bare sediment significantly higher than other treatments at all depths (Table 1 ). For bare sediment, ammonium concentration increased with

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increasing depth while for T. latifolia monocultures and control mixtures, Row 11 had elevated concentrations. Statistical analysis of the subset of data that excluded the "bare sediment" treatment revealed that ammonium concentration at Row 6 was lower in the T. domingensis monoculture than in the T. latifolia monoculture (Fig. 5B). Examination of sediment phosphate concentrations revealed high variability from sample to sample (Fig. 6). None of the treatments containing plants deviated significantly from the bare sediments at any depth (Table 1). The only significant difference found was a higher concentration in T. latifolia monocultures than in T. domingensis monocultures at Row 6. For potassium concentrations, bare sediments had higher values than the other treatments at all depths (Table I, Fig. 6). Within treatments, only T. latifolia monocultures showed differences between rows, with Row 11 being higher than Rows 1 and 6. DISCUSSION

A key prerequisite to understanding competitive interactions is to understand what factor (s) act to limit plant growth. In this study, both shoot density and biomass were enhanced by the addition of nutrients (Fig. 2). Previous studies have shown~ for several different wetlands, that the addition of nutrients increases biomass production (Barko and Smart, 1979; Grace and Wetzel, 1981b; Krolikowska, 1981 ). In this study, individual nutrients were not tested separately to determine which of the added nutrients (N, P or K) was the most limiting. Most studies of wetland plants, however, have found ammonium and, less frequently, phos-

91 phorus to be the primary limiting factor (Pigott, 1969; Barko and Smart, 1979; Shaver and Chapin, 1980; Smart and Barko, 1980; Grace and Wetzel, 1981b). Additions of potassium have generally not been reported to stimulate plant growth unless added in combination with nitrogen and phosphorus (Shaver and Chapin, 1980; Grace and Wetzel, 1981b; Krolikowska, 1981 ). Examination of the sediment chemistry in unfertilized gradients (Figs. 5-6) revealed that ammonium concentrations were greatly reduced by plants (compared to bare sediment) while phosphate concentrations were not. Potassium was significantly reduced, but only by approximately 45 %. The association of plants with lowered concentrations of ammonium and potassium were also reflected in differences between rows within treatments. Typha latifolia monocultures did not contain surviving plants in Row 11 while T. domingensis monocultures had abundant growth at that depth (Grace, 1985). The control mixtures were intermediate in plant density to T. latifolia and T. domingensis monocultures at Row 11. Thus, the lower ammonium concentration at Row 11 within the T. domingensis monocultures further illustrates the strong impact of plants on this ion (Fig. 5 ). A similar pattern also occurred for potassium (Fig. 6). Taken together, the results presented here suggest that nitrogen, specifically ammonium, was probably the most limiting nutrient (though a lesser effect by potassium cannot be completely ruled out). The sediment data presented in this paper suggest that the Typha species differ in the capacity to aerate their rhizospheres. The distribution of plants along the water-depth gradient clearly showed that T. domingensis is capable of growing in deeper water than T. latifolia (Grace, 1985). Further, the available Eh data show that T. domingensis monocultures had elevated redox levels in Row 6 (compared to bare sediment) while T. latifolia monocultures did not (Fig. 4). In part, these results are consistent with the findings of Dean and Biesboer (1985) who also found T. latifolia to have no effect on sediment redox profiles compared to uncolonized sediment. While the available information is still incomplete, it appears that T. domingensis may supply more oxygen to its rhizosphere than does T. latifolia, though not necessarily at all depths. In summary, the available data indicate that nutrients, specifically nitrogen, were a primary limiting factor for both Typha species in this study. Thus, I hypothesize that the basis for the competitive superiority of T. latifolia over T. domingensis results from the greater capacity of T. latifolia for obtaining nitrogen. The possibility of a functional relationship between the apparently greater capacity of T. domingensis to aerate its rhizosphere and its apparently lower capacity to compete for nitrogen deserves further study. ACKNOWLEDGEMENTS I thank D. Smith for technical assistance. Also, J. Harrison, M. Carter, K. Hornberger, J. Dixon and many others assisted with the field work. The as-

92 s i s t a n c e o f t h e U n i v e r s i t y o f A r k a n s a s is also g r a t e f u l l y a c k n o w l e d g e d . I. M e n delssohn provided helpful comments on the manuscript. The work was supported by grants from the National Science Foundation.

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