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Aboveground biomass and phosphorus concentrations of Lythrum salicaria (purple loosestrife) and Typha spp. (cattail) in 12 Minnesota wetlands.

INTRODUCTION

Lythrum salicaria (purple loosestrife) is an aquatic emergent plant native to Eurasia. It became established in North America during the early 1800s and has since spread throughout wetlands of the northern United States and southern Canada (Thompson et al., 1987). Lythrum salicaria distribution in North America ranges in the E from New Brunswick to South Carolina and from British Columbia to California in the W. Since 1940, it has spread at a mean rate of 645 [km .sup.2] per year (Thompson, 1991). The oldest herbarium specimen of L. salicaria from Minnesota was collected in 1994 in Ramsey County. Currently in Minnesota, L. salicaria covers 80 [km.sup.2] of wetlands (Anderson and Asher, 1993); there are over 1500 sites of loosestrife infestation in 68 of 87 counties (L. Skinner, 1994, pers. comm.). Lythrum salicaria displaces native vegetation in wetland communities, is a poor quality food and cover source for wildlife (Rawinski and Malecki, 1984) and its growth results in reduced wetland biodiversity.

Wetlands are also important sites for temporary nutrient storage. Water quality of flow-through waters can be improved by nutrient storage in short-term pools (plants) or long-term pools (soil). In wetlands, nutrients are transformed and assimilated by the processes of sorption, coprecititation, nitrification, denitrification and plant uptake (Sloey et al., 1978, Johnston, 1991). In northern temperate wetlands there are seasonal variations in community nutrient assimilation and release. During the growing season nutrients are absorbed by plants. After plant senescence nutrients are translocated within the plant or released during decomposition. Typically, a spring flush of nutrients from wetlands will occur due to increased water flow from snowmelt and precipitation (Lee et al., 1975).

Conversion of wetlands from native vegetation, primarily Typha spp. (cattails) to Lythrum salicaria, may result in significant alterations in wetland productivity and nutrient cycling. In this study, we compared aboveground standing crop biomass and phosphorus concentration of L. salicaria and Typha in 19 Minnesota wetlands. We hypothesized that L. salicaria biomass and phosphorus concentration would be significantly different from that of Typha, thus allowing wetland conversion to lead to significant alteration of productivity and phosphorus cycling. Phosphorus was chosen for this study because it often limits plant growth, it is frequently present in high concentrations in urban run-off (Payne et al., 1989; Brown, 1985) and phosphorus inputs can significantly increase lake eutrophication.

METHODS

Study sites, - Sites were selected using the Minnesota Department of Natural Resources loosestrife database and were subsequently field assessed. We selected sites which had all of the following criteria: (1) Typha and/or Lythrum salicaria as the dominant vegetation type within a designated wetland; (2) possessed a specific percentage of L. salicaria and Typha vegetation, such that the total study population had a vegetational gradient from 100% L. salicaria and 0% Typha to 0% L. salicaria and 100% Typha; (3) minimal human disturbance in the wetland; (4) wetland had no history of herbicide application, and (5) wetlands were located within the greater Minneapolis-St. Paul Metropolitan Area.

A total of 12 wetland sites were selected within Ramsey, Hennepin and Dakota counties, Minnesota. Wetland sites varied in size (3373 [m.sup.2] to 253,577 [m.sup.2]) and moisture (saturated soil to 1 m of standing water). We selected palustrine emergent wetlands (Cowardin et al., 1979) (i.e., type 2 and 3 of Shaw and Fredine, 1971) with the exception of two riverine lower perennial emergent sites (i.e., sites 2 and 11). Lake sites and wetlands with deep open water (i.e., wetland types 4 and 5 of Shaw and Fredine, 1971) were avoided to minimize variability in plant distribution and nutrient cycling due to wave action.

Peak standing crop biomass. - Before biomass harvest, each wetland was mapped to define its total area, as well as vegetation patches containing Lythrum salicaria and Typha. We defined a patch as a clumped spatial distribution of vegetation dominated by one species. Initial wetland mapping was carried out using 1:100 or 1:200 scale aerial photos (Balogh and Bookout, 1989). Initial maps for each wetland were field-checked in late July during peak L. salicaria bloom, and patch delineation was adjusted where necessary. Wetland and patch areas were determined by digitizing maps.

Dubbe et al. (1988) have shown that peak cattail leaf standing crop occurs at the beginning of September in Minnesota. We harvested standing crop aboveground biomass in each wetland between 29 August and 13 September 1990. We used vegetation maps to randomly select 1-7 patches for each wetland, Number of patches selected for each wetland depended on total number of patches within a wetland, plant diversity within a patch and wetland area. Within each selected patch, a 15-m transect was placed at random and three random 0.5-[m.sup.2] quadrats were harvested for Lythrum salicaria and Typha.

Plant material was processed at 60 C to obtain plant dry weight (DW) and ashed at 525 C to obtain ash-free dry weight (AFDW). Plant ash digested with 2N hydrochloric acid (Chapman and Pratt, 1961) was used to determine total phosphorus using the ascorbic acid-molybdenum blue colorimetric method (John, 1970).

Late summer harvest of aboveground Lythrum salicaria was assumed to represent peak standing crop. From these results, we calculated wetland biomass (g[multiplied by][m.sup.2]) and estimated annual productivity. Results for each wetland are presented at the patch and whole wetland level. Values for the whole wetland were calculated by multiplying patch values by their respective percent area for each wetland. Plant phosphorus concentration is presented as a function of plant weight (mg P[multiplied by]g D[W.sup.-1]). Phosphorus concentration per unit area (g P[multiplied by][m.sup.-2]) was calculated by multiplying the phosphorus value by the patch or wetland biomass.

Measurement of belowground plant biomass and phosphorus concentration were not part of this study. Belowground biomass and phosphorus levels for Typha are well-documented in previous research (Prentki et al., 1978; van der Valk and Davis, 1978; Davis and van der Valk, 1983; Garret et al., 1988). Shamsi and Whitehead (1974) measured Lythrum salicaria root biomass and found that it was approximately 20% of total plant biomass across a range of harvest and light intensities for a 16-h photoperiod. However, a photoperiod of 9 h resulted in an increase in root biomass and a decrease in leaf and stem percent biomass, as a proportion of total dry weight. Our other work on L. salicaria and Typha in artificial wetlands (Emery et al., 1991) indicated that senescent belowground phosphorus concentration was higher in L. salicaria (2.86 mg P[multiplied by]g D[W.sup.-1]) than Typha (2.18 mg P[multiplied by]g D[W.sup.-1]). However, levels of phosphorus in belowground plant tissue per square meter did not significantly differ between L. salicaria and Typha.

RESULTS AND DISCUSSION

Patch level biomass estimates ranged from 0 to 1100.3 g[multiplied by][m.sup.-2] for Lythrum salicaria and from 0 to 868.6 g[multiplied by][m.sup.-2] for Typha (Table 1). T-test comparisons of Lythrum salicaria and Typha biomass (excluding sites 1 and 12) indicated that Typha had significantly greater patch biomass (639.6 g[multiplied by][m.sup.-2], P [less than] 0.01) and wetland biomass (461.4 g[multiplied by][m.sup.-2], P [less than] 0.001) than did L. salicaria (patch = 444.8 g[multiplied by][m.sup.-2]; wetland = 213.9 g[multiplied by][m.sup.-2]). Wetland sites 1 and 12 were excluded from the t-test comparison because only one vegetation type was present at each site.

Highest patch values for Lythrum salicaria biomass (11000.3 g[multiplied by][m.sup.-2]) occurred at site 9, where plants were very robust and tall (2-3 m). Highest patch values for Typha (868.6 g[multiplied by][m.sup.-2]) occurred in site 2, which contained a dense healthy Typha stand. Patch biomass estimates were comparable to reported [TABULAR DATA FOR TABLE 1 OMITTED] literature values for Typha and L. salicaria. For example, McCormick (1970) reported that L. salicaria aboveground standing crop biomass in freshwater tidal wetlands was 560 to 1703 g[multiplied by][m.sup.-2]. Whigham et al. (1978) reported average aboveground standing crop biomass of 1616 g[multiplied by][m.sup.-2] for L. salicaria in freshwater tidal wetlands. Other reported biomass values for L. salicaria are not directly comparable to results from this study, because they primarily reported new seedling biomass and density (Shamsi and Whitehead, 1974, 1977a, b; Rawinski, 1982; Gabor and Murkin, 1990; Merendino et al., 1990) or mixed age stand density (Rawinski, 1982; Rawinski and Malecki, 1984; Balogh, 1986). Typha aboveground standing crop biomass in freshwater tidal wetlands has been reported as 642 to 1642 g[multiplied by][m.sup.-2] by McCormick (1970), and as 1215 g[multiplied by][m.sup.-2] by Whigham et al. (1978). In Iowa prairie glacial marshes, T. glauca aboveground peak biomass was estimated at 1156 to 2000 g[multiplied by][m.sup.-2] (Davis and van der Valk, 1983). Aboveground biomass for cattails in Minnesota was estimated at 430-1480 g[multiplied by][m.sup.-2] (T. latifolia), 1230 to 2110 g[multiplied by][m.sup.-2] (T. angustifolia) and 670 to 810 g[multiplied by][m.sup.-2] (T. glauca) (Dubbe et al., 1988).

Average plant phosphorus concentration for all wetlands ranged from 1.35 to 3.96 g P[multiplied by][m.sup.-2] for Lythrum salicaria and from 1.29 to 2.55 g P[multiplied by][m.sup.-2] for Typha (Table 2). Lythrum salicaria showed significantly greater phosphorus concentration than Typha (p [less than] 0.001). However, phosphorus concentration per area (g P[multiplied by][m.sup.-2]) at the patch level was not significantly different between the two plant types (p [greater than] 0.10). This implies that peak standing crop vegetative storage of phosphorus per unit area is similar for L. salicaria and Typha.

Prentki et al. (1978) compiled literature values for peak phosphorus standing stock of Typha. Values ranged from 3.2 to 4.6 g[multiplied by][m.sup.-2] for T. angustifolia and from 0.68 to 3.2 g[multiplied by][m.sup.-2] for T. latifolia. Davis and van der Valk (1983) reported peak phosphorus levels in an Iowa marsh during early July as 3.74 g[multiplied by][m.sup.-2] for T. glauca; they also reported an early September phosphorus concentration of approximately 2.2 g[multiplied by][m.sup.-2]. Garver et al. (1988) reported an early September phosphorus concentration of 1.6 g[multiplied by][m.sup.-2] for [TABULAR DATA FOR TABLE 2 OMITTED] Typha spp. in a Minnesota wetland. In this study, patch phosphorus concentration for Typha ranged from 0.55 to 1.75 g[multiplied by][m.sup.-2], which is similar to literature values. Variation in phosphorus content between this study and the literature could be due to timing of harvest, plant species and/or nutrient availability in individual wetlands. The lower phosphorus values reported in this study may be related to timing of peak harvest. Our plant material may have been harvested after phosphorus translocation to the rhizomes was initiated (usually occurs late July through August).

Emergent vegetation such as Typha adapts to the dynamic nature of wetlands, where water levels may vary substantially (Mitsch and Gosselink, 1986; Weller, 1987). Presently, a significant increase in wetland disturbance is occurring due to community development and agriculture throughout the Midwest. This disturbance, along with the natural dynamic nature of the wetland community, permits Lythrum salicaria to invade and become established in wetlands.

In the Minneapolis-St. Paul area, many wetlands receive stormwater runoff prior to flow into lakes and streams (Brown, 1985; Johnston et al., 1990). Wetlands also become passive wastewater treatment sites for nutrients and metals from non-point contaminant sources, such as roads and lawns. We found Typha had greater aboveground biomass, whereas Lythrum salicaria showed greater tissue phosphorus concentration. There was no significant difference between the two plant types in phosphorus concentration per trait area. Further studies would be necessary to quantify nutrient absorption and translocation within L. salicaria through the growing season. Data and findings presented here will be critical for designing and field-evaluating experimental work on alteration of wetland functions associated with invasion of Lythrum salicaria.

Acknowledgments. - This research was funded by the Legislative Commission on Minnesota Resources, with funds administered by the Minnesota Department of Natural Resources. We thank Chris Holm and Nels Troelstrup for research discussions and Leigh Vanderklein for editorial assistance. Also supported in part by the Minnesota Experiment Station under Project 42-25 of the McIntirre Stennis Cooperative Forestry Act. Paper No. 21,697 of the Scientific Journal Series, Minnesota Experiment Station.

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SARAH L. EMERY AND JAMES A. PERRY, Department of Forest Resources, University of Minnesota, St. Paul 55108. Submitted 16 May 1994; accepted 15 June 1995
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Author:Emery, Sarah L.; Perry, James A.
Publication:The American Midland Naturalist
Date:Oct 1, 1995
Words:3071
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