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Rapid Plant Community Response to a Water Level Peak in Northern Lake Huron Coastal Wetlands
J. Great Lakes Res. 31 (Supplement 1):160–170 Internat. Assoc. Great Lakes Res., 2005
Rapid Plant Community Response to a Water Level Peak in Northern Lake Huron Coastal Wetlands Joseph P. Gathman1, Dennis A. Albert2, and Thomas M. Burton3,* 1Michigan
State University Department of Zoology East Lansing, Michigan 48824 2Michigan
Natural Features Inventory Michigan State University Extension Mason Building, P.O. Box 30444 Lansing, Michigan 48909-7944 3Michigan
State University Departments of Zoology and Fisheries and Wildlife East Lansing, Michigan 48824 ABSTRACT. Aquatic plants were sampled in five coastal wetlands of northern Lake Huron during July 1996, 1997, and 1998. Mean annual water levels of Lake Huron changed during this period from 176.37 m (below the long-term average) in January 1996 to above average water levels of 176.83 m in July 1996 to 177.19 m in July 1997 and then declined to 176.88 m by July 1998. Boundaries of plant zones as indicated by distribution of the 1–3 dominant species along permanently established transect points across the wetland did not shift spatially over this 3-year period. Instead, relative abundance (percent of total stems per three 0.25 m2 quadrats per plot) and presence/absence of plant species responded individually to water level changes within major zones. In 1996, the first season sampled, the wet meadow had recently been inundated by rising water level. In 1997, after more than a year of above average and rising water levels, emergent stem densities were reduced in the Carex/Calamagrostis (sedge/blue-joint) dominated wet meadow and mixed transition sedge, narrow-leafed cattail, and hardstem bulrush (Carex, Typha angustifolia, and Schoenoplectus acutus) dominated zones compared to stem densities in 1996. Stem densities remained low in 1998, even though water levels dropped 31 cm from 1997 levels. The relative dominance (% of stems/3 quadrats/plot) and presence/absence of some plant species changed rapidly in the wet meadow zone in response to increases in water levels in 1997 and to decreases in water levels in 1998. In contrast, changes in emergent species were minimal in the deeper emergent zone dominated by hardstem bulrush. We conclude that temporary flooding and drying in response to water level changes are critical to maintaining a diverse arrray of plant species in the wet meadow zones of these marshes. Furthermore, short-term water level changes do not affect the relative spatial position of major plant zones within the marsh nor the relative abundance of emergent species in the deepest zone. INDEX WORDS:
Coastal wetlands, water level fluctuation, wetland plants, Great Lakes.
INTRODUCTION Plant zonation is a well-known phenomenon occurring in wetlands, but the causal mechanisms behind this familiar wetland characteristic are still incompletely understood. Past studies have shown that plant species respond to different, but often
*Corresponding
correlated, environmental variables. Factors that may directly affect distributions of certain wetland plant species include soil chemistry and redox potential (Lyon et al. 1986), seed and soil particle size (Keddy and Constabel 1986), water depth and subsoil water table height (Keddy and Constabel 1986), spatial differences in concentration of propagules in the soil seed bank (Zedler 1981, Keddy and Reznicek 1986), wave exposure on lakeshores
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FIG. 1.
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Location of the five Northern Lake Huron wetlands sampled.
(Keddy 1983, 1984), multi-year water-level cycles combined with muskrat activity in prairie potholes (Welling et al. 1988), interspecific competition (Keddy and Constabel 1986), and temporal variations in water levels (Hudon 1997). The fact that certain species tend to co-occur within zones is probably a coincidental outcome of each species consistently responding to its own set of environmental variables (Keddy and Ellis 1984). This Gleasonian assembly process was described by van der Valk (1981) as an “environmental sieve” model, in which each species must first “find” its potential habitat range with acceptable physico-chemical conditions for growth. Then, a plant can be further affected by biotic factors that determine its realized habitat. However, each of these factors is strongly affected by flooding regimes. This single, hydrological factor may be considered the ultimate cause of plant distribution patterns in wetlands. Such flooding regimes are particularly important in coastal wetlands of the Laurentian Great Lakes where water level changes occur on several overlapping time scales, including short-term changes over hours to days, seasonal variation from year to year from annual precipitation and evaporation patterns,
and multi-year changes due to less-predictable, long-term climate variation (Burton 1985; Minc 1997a, 1997b; U.S. Army Corp of Engineers 1987). Short-term changes that occur over hours to days are caused by seiches and storm surges. These changes, which can result in movement of plant propagules and erosion and redeposition of sediments, cause local alterations of wetland plants, such as breaking bulrush (Schoenoplectus) stems or uprooting cattails (Typha) or submergent plants. However, these changes do not result in large-scale alterations in the zonation within a wetland or within local wetland complexes such as those of the area studied (the Les Cheneaux Islands area of northern Lake Huron, Fig. 1). We did not focus on the effects of these short-term lake level changes on plants. Instead, our focus was on interannual differences resulting from seasonal variation over a 3year period of rapid water level change. Seasonal variation in water level resulting from annual precipitation and evaporation patterns is a more predictable form of water level fluctuation compared to storm surges and long term changes. Long-term water-level data for the Laurentian Great Lakes from the U.S. Army Corps of Engineers (US
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ACE) show a seasonal pattern of low water levels in autumn, winter, and spring, with highest water levels during late summer, typically in July and August in most years (Minc 1997a). This relatively predictable annual pattern is an important factor for wetland plants. Many wetland plants that cannot become established in permanently flooded areas are able to germinate in seasonally flooded zones that have an aerobic, non-flooded environment during the spring and early summer. Many wetland seedlings can then survive as water levels rise. Multi-annual fluctuations over periods greater than the 3 years of this study are caused by longterm climatic variation and represent the most important scale determining the lateral position of the boundaries between plant community zones in coastal wetlands. Over 100 years of water-level data demonstrate an irregular 7–10 yr pattern of high and low water levels in the Great Lakes, with the variation from lowest lake levels to highest lake levels exceeding 1.0 m (U.S. Army Corp of Engineers 1987; Minc 1997a, 1997b). When high or low levels last for a few years, they lead to changes in the lateral positions of wetland communities and their boundaries along the shore (Burton 1985), although the effects of short-duration highs and lows have not been studied. Extreme high and low conditions can occur over 2 or 3 years, creating vastly different hydrologic conditions and sediment characteristics over large parts of coastal wetlands. During relatively lowwater years, the upland portion of wet meadows is invaded by woody plant species. Long-term lows result in establishment of shrub swamp and occasionally swamp forest above the annual high water level (Minc 1997b), while marsh and wet meadow communities gradually shift downslope. When water levels rise again over several years, woody species are killed, and the marsh and meadow communities migrate upslope. Long-term water level changes therefore cause long-term spatial displacement of the entire zonation continuum, though the shifts are time-lagged behind the water level change (Burton 1985). This phenomenon has not been studied in detail in Great Lakes coastal wetlands, so the rate of plant community change associated with water level change is not known. This study took advantage of a natural experiment during which Lake Huron’s level rose 46 cm from January to July 1996, flooding the wet meadow zones of wetlands (Figs. 2, 3). This initial rise was followed by an additional 36 cm rise to a high of 177.19 m in July 1997. Water levels were
FIG. 2. Lake Huron monthly average water levels (m above International Great Lakes Datum (IGLD)), measured at DeTour Village, MI, 1980 through 1998 (Data source: U.S. Department of Commerce, NOAA/NOS, Silver Spring, MD). 31 cm lower in July 1998. This occurrence provided an opportunity to observe the initial plant community composition just after the initial water level rise with species composition a year later after additional water level increases and with community composition during the first year of water level decline. We hypothesized that significant changes would occur in the composition of the wet meadow and emergent marsh zones during this period of water level fluctuation. To test this hypothesis, we recorded changes in the plant communities of several wetland sites during this 3-year period.
FIG. 3. Schematic cross-section through representative sample transect for the five wetlands sampled.
Coastal Wetland Plants and High Water METHODS Study Sites Data were collected from five coastal wetlands in the vicinity of Cedarville and Hessel, Michigan, along the north shore of Lake Huron in the Les Cheneaux Islands region of Michigan’s upper peninsula: Mackinac Bay, Mismer Bay, Prentiss Bay, Duck Bay, and St. Martin’s Bay (Fig. 1). Wetlands in the Les Cheneaux Islands region occur within large, gently sloping embayments, with vegetation that displays relatively predictable zonation related to its position along the elevation gradient (Minc 1997a, 1997; Minc and Albert 1998). Along the upland edge, swamp forests flood only briefly during the growing season in the most extreme high-water years, although they often have saturated soils. Still closer to the lake is a shrub swamp zone, which is flooded more often, resulting in little or no successful establishment of tree species. Closer to the lake, flooding occurs more frequently and water depths can be greater, resulting in dominance by wet meadow plants such as grasses and sedges. Along the lake margin is a zone which is typically flooded continually and supports emergent marsh species, while further down the gradient, emergent species share the habitat with floating and submergent plants. Most of the five wetlands selected for study (Fig. 1) were typical of this region, with gently sloping shorelines and broad vegetation zones. Based on the dominant vegetation, we classified the zones we studied as: wet meadow (dominated by sedges, Carex stricta and/or Carex lasiocarpa and bluejoint, Calamagrostis canadensis), transition (dominated by narrow-leaved cattail, Typha angustifolia, sedges, Carex spp., and hardstem bulrush, Schoenoplectus acutus), and emergent marsh (dominated by hardstem bulrush). The study sites also had shrub and forested swamp zones above the wet meadow, as described by Minc (1997a), but these were not included in this study. Some coastal wetlands include a “strand” (Keddy and Reznicek 1986) or “shoreline” (Minc 1997a) zone between wet meadow and emergent marsh, but all study sites except St. Martin’s Bay were in protected bays lacking the wave exposure necessary to create this zone. Our wet meadow zone was similar to the Keddy and Reznicek (1986) description: a sedge meadow numerically dominated by the sedges, Carex aquatilis and C. lasiocarpa, at lower elevations, and the hummock forming sedge, C. stricta, and blue-joint, C. canadensis, and several shrub species
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(e.g., sweet gale, Myrica gale, and several species of willow, Salix spp.) at higher elevations. This was the most diverse zone, including many herbaceous annual species interspersed among the sedges. Below the wet meadow, our zonation differed somewhat from the other authors’ descriptions. The transition zone was visually dominated by cattails, predominantly T. angustifolia, but was fairly species-rich, with spike-rush (Eleocharis smallii) water smartweed (Polygonum amphibium) and species from both the adjacent wet meadow and emergent marsh zones commonly growing among the cattails. The emergent marsh contained both emergent and submergent macrophytes and was dominated by hardstem bulrush but also commonly included arrowhead (Sagittaria spp.), pickerel weed (Pontederia cordata), and spike-rush (E. smallii). Several genera of submergent plants (including coontail (Ceratophyllum demersum), several species of pondweed (Potamogeton), and bladderworts (Utricularia)) and floating-leaved plants (especially yellow pond-lily and pondweed (Nuphar variegata and Potamogeton natans)) grew within the emergent marsh. Each site contained unique characteristics. The St. Martin’s Bay wetland had open dune/swale topography, rather than the gradual, consistently sloping shore shared by the other four sites. This meant that sampling stations at St. Martin’s Bay alternated between wet meadow and emergent marsh along the entire transect. All other sites had gradual slopes and some vegetation patchiness. Duck Bay had the steepest slopes of 4.9 mm/m. A road separated the wet meadow and emergent marsh zones of Prentiss Bay, but hydrologic connection was maintained by a culvert through which Prentiss Creek flowed into the bay. This study focused on changes that occurred within the wet meadow and emergent marsh zones over a three year period (Figs. 2–4). From January to July 1996, water levels rose 46 cm and inundated parts of the wet meadow at the time of first sampling in July 1996. Water levels rose another 36 cm by the second sampling period in July 1997 but then declined 31 cm by July 1998. The regional slopes within these wetlands were 2.5 mm/m–4.9 mm/m, so a water level increase of 82 cm from January 1996 to July 1997 resulted in flooding of wetland areas 200 to 400 m up the gradient (Figs. 3, 4), potentially resulting in major changes in plant communities.
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FIG. 4. Water level changes at Mackinac Bay in relation to plant community zonation over the study period (Data source for water level: U.S. Department of Commerce, NOAA/NOS, Silver Spring, MD). Data Collection Water-level data over the 3 years were obtained from National Oceanic and Atmospheric Administration (NOAA) (Fig. 2). The data were monthly mean levels measured at the De Tour Village gauging station (Fig. 2) approximately 30 km east of the nearest (Prentiss Bay) study site and 60 km from the farthest site (St. Martin’s Bay). Since there were negligible differences between data from the Mackinaw City gauging station, located 30–60 km southwest of the study area across the Mackinac Straits, and the De Tour Village station, data from De Tour Village were used for the study. In 1996, a single permanent transect was established across each wetland from approximately the upslope margin of the wet meadow zone across the transition zone into the emergent zone to water approximately 75 cm deep at the time of sampling in 1996 (Fig. 3). Transects were perpendicular to the general shoreline contour, with the upland end marked by a tall wooden stake at the wet meadow/forest edge or at a point in the hummock/ sedge zone where herbaceous species were more common than shrubs (Fig. 3). Transects ran down slope with stakes placed every 20 or 25 m, depending on the width of the zone. At the Mackinac Bay site, the transect never reached 75 cm depth because the emergent marsh extended for a great distance over a barely sloping bottom before rising toward an island in the bay. Therefore, we estab-
lished the deep end of the transect at the point where there was no perceptible change in vegetation. Transect length varied among sites (160-320 m) because of different bottom slopes. Plant community data were collected from each wetland in late July 1996, 1997, and 1998. Sampling was initiated at the stake that marked the upland end of the transect. A point-centered circular plot with a 10 m radius centered at each stake along the transect was sampled. Essentially, this resulted in a series of circular plots extending along the transect that touched or were only 5 m apart at the outer edge of the circle. For each point centered plot, we measured water depth and depth of soil organic matter, and then selected three quadrat locations using random azimuths (0–360°) and distances (1–10 m) from the stake in the center of the plot. A 0.5 m × 0.5 m (0.25 m2) quadrat frame made of PVC conduit was dropped at the randomly selected location. This process was repeated each year so that new quadrat points were selected each year within each circular plot. Thus, the same plots were sampled from year to year but not the same quadrat locations within each plot. We identified all plant species present within the quadrats, counted stems of each emergent species (including shrubs), and visually estimated percent coverage of each floating and floating-leaved species. Stem counts included all individual stems of each species, including the individual stems that made up each hummock of the sedge, C. stricta, since it would have been too difficult to estimate how many actual individual plants made up each hummock. Stem counts were made at the base of the stem to avoid accidental counts of individual leaves. We estimated coverage of submergent and floating or floating leaved (SFF) species as percent of quadrat area covered by each species either by direct visual observation or by dragging a rake once across the bottom within the transect boundaries and estimating the proportion of collected plant material for each species. No grid was used for estimating percent cover. However, D. Albert and T. Burton participated in all surveys and trained crew members in how to make these estimates to reduce observer bias between plots and years. Percent cover was estimated for each species independent of other species. Thus, it was possible to have total coverage values greater than 100% if there were layers of submergent plants in the water column and/or coverage by floating or floating leaved plants as well as submergent ones.
Coastal Wetland Plants and High Water Because Mackinac Bay had the most gradual slope of the sites, it also had the longest transect and, hence, the greatest number of samples taken per unit of elevation change. Given this relatively high sampling intensity, Mackinac Bay provided the finest sampling resolution to detect subtle vegetation changes along the elevation gradient (Fig. 4). For this reason Mackinac Bay was the site chosen to delineate vegetation zones along the gradient. The cattail dominated zone was treated as a benchmark in the middle of the transect because cattails generally occur at the long-term mean water level in coastal wetlands (Keddy and Reznicek 1986). The Mackinac Bay maximum and minimum depths for cattails were defined as the upper and lower elevations for the cattail zone. This middle zone was designated as the transition zone, and all up slope elevations (shallower depths or no standing water) were designated as wet meadow and all down slope elevations (greater depths) were designated as emergent marsh. These zone labels were then applied to stations from all sites using water depth measurements taken during sampling in 1996 to estimate elevation and define the limits of the transition zone if cattails were not present. To verify that perceived zone divisions were based on real community differences, not just visual bias, hierarchical Ward’s matching cluster analysis was run on all Mackinac Bay 1996 samples. Clusters were defined based on Euclidean distance. According to this analysis, the three zone groupings represented natural community differences identifiable in the sampling data. Cluster analysis based on data from the other four wetlands also demonstrated similar zone designations. Analysis Data from the three quadrats of each point centered plot were either summed (stem counts) or averaged (coverage data) so that each three-quadrat composite from a single point centered plot comprised a sample. Relative abundances (percent of total stem counts), taxa richness (number of species), and Shannon diversity (H′) were calculated from these composite data for each sample. Note that Shannon diversity calculations were based on individual stem counts rather than counts of individual plants, and this would tend to overestimate the importance of hummock forming sedge species in these wetlands. Frequency distribution data could not be transformed to normality because of a high frequency of
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zero counts. Instead, all hypothesis tests were performed on each zone individually to reduce the number of zero values and nonparametric analyses were used. Each 2-year period (1996 to 1997 and 1997 to 1998) was treated as a separate experiment, and differences in response variables between years were analyzed using Wilcoxon rank-sum tests, the nonparametric equivalent of paired Student’s t tests. An a priori critical α level of 0.10 was used to determine statistical significance because of relatively low sample size (which varied among zones) and high data variability. RESULTS Community zones did not move in relation to the permanent points (stakes) along the transects during the study period, although we did observe several changes in composition within the wet meadow and emergent marsh zones. In general, the wet meadow experienced the greatest change in number of species present and in the stem counts of emergent species present, while the emergent marsh showed the least. Also, most changes observed from 1996 to 1997 were partially reversed from 1997 to 1998, although some of these reversals were not statistically significant. The most noteworthy changes in plant communities over the study period were: 1) increasing dominance by the sedges, Carex aquatilis and C. lasiocarpa, during the high-water year, with reversal of the trend for C. aquatilis as water levels declined (Table 1); 2) rapid establishment in the wet meadow by Utricularia (bladderwort) species, especially U. intermedia, during the wettest (second) year, with other submergent species appearing the next year, despite falling water levels (Table 1); and 3) a significant decrease in emergent plant stem density and Shannon diversity in the wet meadow and transition zones during the second year, followed by increases in these same measures during the third year (Table 2). Each change is explained in greater detail below. Density and Coverage Changes As water depth increased from 1996 to 1997, stem density of emergent plants in the wet meadow decreased from 376 stems/m 2 to 310 stems/m 2 (17% decrease, p = 0.038). As water depth decreased between 1997 and 1998, stem densities rose to 397 stems/m2 (28% increase, p = 0.019) (Table 2). Stem density changed more dramatically in the transition zone, falling from 226 stems/m2 to 138
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TABLE 1. Relative abundance (% of total stem counts for three 0.252 quadrats) for emergent plant species and percent coverage for submergent, floating, and floating-leaved (SFF) plant species that occurred in each plant zone during the 1996, 1997, and 1998 sampling seasons (n = total number of permanently established point centered plots per plant zone for all five wetlands sampled). Plant Zone Year of sampling Taxon Emergent Species Calamogrostis canadensis Campanula aparinoides Carex aquatilis Carex lasiocarpa Carex stricta Carex spp. Eleocharis smallii Equisetum fluviatilis Juncus spp. Lysimachia thyrsiflora Polygonum amphibium Pontedaria cordata Sagittaria spp. Schoenoplectus acutus Typha spp. Salix spp. All shrub spp. All others
Wet Meadow (n = 26) 1996 1997 1998
14.0% 6.2% 5.2% 17.1% 27.7% 62.8% 0.6% 0.9% 0.5% 0.7% 0.5% 0.0% 0.3% 3.4% 0.9% 0.2% 1.2% 8.0%
15.7% 8.8% 16.1% 18.3% 31.9% 66.6% 0.1% 1.0% 0.1% 0.9% 0.5% 0.0% 0.0% 1.5% 0.0% 0.3% 2.0% 2.7%
14.2% 5.2% 3.6% 26.3% 36.7% 67.6% 1.2% 0.6% 0.3% 1.6% 0.2% 0.0% 0.1% 4.5% 0.2% 0.4% 1.8% 2.3%
SFF* Species Chara spp. 0.0% 4.2% 0.0% Elodea canadensis 1.9% 0.0% 0.0% Myriophyllum spp. 0.3% 6.6% 2.6% Najas flexilis 0.0% 0.0% 0.0% Potamogeton spp. 2.6% 1.2% 0.0% Schoenoplectus subterminalis 0.0% 0.0% 0.0% Utricularia spp. 33.7% 60.0% 27.7% Vallisneria americana 0.0% 0.0% 0.5% Nuphar variegatum 0.0% 0.0% 0.0% * SFF = Submergent, Floating and Floating Leaved Plants
stems/m2 (39% decrease, p = 0.050) as water levels rose, then rising to 257 stems/m2 (86% increase, p = 0.008) with the subsequent drop in water levels. Emergent stem density in the emergent marsh began at 40 stems/m2 and did not change significantly (p > 0.1) during either year (Table 2). Submergent, floating, and floating-leaved species (SFF) coverage increased in the wet meadow when water rose from 1996 to 1997, from a very low mean level of 0.8% to 3.4% (p < 0.000), and continued to increase the following year to 8.4% despite the falling water levels. However, this increase in coverage was not statistically significant because of high variability among samples (Fig. 5).
Transition (n = 12) 1996 1997 1998
Emergent (n = 23) 1996 1997 1998
3.1% 0.7% 3.8% 19.7% 2.5% 30.4% 14.7% 1.4% 1.9% 0.4% 0.4% 0.0% 3.0% 40.6% 1.3% 0.0% 0.0% 2.1%
5.2% 1.5% 5.0% 38.7% 1.7% 46.3% 5.7% 2.7% 0.5% 1.3% 0.7% 0.0% 0.0% 22.5% 12.2% 0.2% 0.2% 1.2%
4.9% 1.4% 3.3% 37.8% 4.0% 45.9% 10.6% 0.4% 0.8% 3.6% 0.8% 0.0% 0.0% 18.8% 8.0% 0.0% 0.0% 4.8%
0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 15.0% 0.3% 0.0% 0.0% 0.0% 0.0% 8.6% 59.6% 1.1% 0.0% 0.0% 15.4%
0.0% 0.0% 0.0% 3.8% 0.0% 3.8% 8.8% 1.3% 0.0% 0.0% 0.0% 0.8% 13.9% 50.4% 0.0% 0.0% 0.0% 21.0%
0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 12.2% 0.0% 0.0% 0.0% 3.5% 1.8% 11.5% 54.2% 2.2% 0.0% 0.0% 14.6%
1.1% 0.0% 4.6% 4.6% 0.0% 30.4% 55.5% 3.7% 0.0%
6.9% 2.1% 5.7% 0.0% 2.1% 0.3% 74.3% 0.0% 0.0%
0.0% 0.0% 8.3% 0.0% 8.3% 8.5% 33.5% 0.0% 7.9%
8.1% 6.3% 14.4% 3.8% 6.9% 28.9% 9.5% 3.6% 3.0%
5.4% 2.9% 10.9% 0.6% 8.3% 30.8% 11.7% 0.3% 13.1%
13.0% 7.6% 0.6% 4.2% 4.5% 48.3% 5.7% 2.6% 0.5%
SFF coverage in the transition zone remained stable at 8.6, 6.7, and 6.4% respectively in 1996, 1997, and 1998. SFF coverage in the emergent marsh also remained stable at 9.9% in 1996 and 9.4% in 1997 as water levels rose, but coverage increased dramatically to 46.2% (p < 0.000) in 1998 as water levels dropped (Fig. 5). Species Diversity and Richness Changes For emergent plants, per-plot Shannon diversity and per-plot species richness decreased as water levels rose in the wet meadow from 1996 to 1997 and increased as water levels dropped again the fol-
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TABLE 2. Mean stem density per 0.25 m2 quadrat, species richness (species per quadrat), and Shannon diversity (H′) (based on stems per quadrat) for species for each zone and each year of sampling (n = total number of permanently established point centered plots per plant zone for all five wetlands sampled). Plant Zone Wet Meadow (n = 26) Transition (n = 12) Year 1996 1997 1998 1996 1997 1998 Emergent stem density 281.8 232.3 297.7 169.4 103.5 192.6 Emergent H’ 2.91 2.77 2.90 2.53 2.20 2.55 Emergent species richness 9.8 8.1 8.8 7.4 6.6 6.3 SFF* H’ 0.01 0.05 0.03 0.07 0.05 0.01 SFF* species richness 0.69 1.50 0.62 1.83 1.83 0.83 * SFF = submergent, floating, and floating leaved vegetation.
lowing year. However, the subsequent richness increase was not statistically significant (p > 0.1, Table 2). Shannon diversity of emergent plants responded similarly in the transition zone, but species richness showed a non-significant decrease (p > 0.1) during both years. All changes of emergent plant diversity and richness in the emergent marsh were small and non-significant (p > 0.1). In contrast, both SFF diversity and richness in the wet meadow increased as water level rose between 1996 and 1997 and then decreased as water levels dropped between 1997 and 1998 (p < 0.1 for all tests, Table 2). Transition zone SFF diversity and richness changes were small and non-significant (p > 0.1) from 1996 to 1997 as water levels rose,
FIG. 5. Changes in submergent plant coverage (with standard errors) in three wetland zones during 3 years of water level change in five Northern Lake Huron coastal wetlands (see Figs. 3 and 4 for water level data)(n = total number of permanently established point centered plots per plant zone for all five wetlands sampled, see Table 1 for details).
Emergent (n = 23) 1996 1997 1998 30.3 30.4 36.7 0.99 1.06 1.08 1.7 2.0 2.0 0.15 0.20 0.14 2.74 3.13 3.26
but both variables decreased in 1998 with dropping water levels (p < 0.05 for both tests). Emergent marsh SFF diversity increased as water level rose (p = 0.036), then decreased as water level dropped (p = 0.070), although species richness remained fairly stable. Thirty-five species recorded in 1996 were not recorded in 1997 after more than a year of inundation by rising water levels while 18 species were newly detected. Most of the “lost” species were wet meadow residents, including six sedge (Carex) species, four rush (Juncus) species, and four shrub species. Five submergent species were lost. Three wet meadow grasses, rattlesnake grass, cut grass, and reed (Glyceria canadensis, Leersia oryzoides, and Phragmites australis) were lost, while reed canary grass (Phalaris arundinacea) was dramatically reduced (87.5%). As water level dropped from 1997 to 1998, 25 species were lost, while 18 additional species were gained. Only two of those gained were species detected in 1996. Two sedge (Carex) and three rush (Juncus) species were among the gains. Abundances of most lost and gained species were very low, so failure to detect them may have been the result of random location of quadrats in each permanently established point centered plot along the transect. Community Composition Changes C. canadensis (blue-joint) and Carex (sedge) species (especially C. stricta, C. aquatilis, and C. lasiocarpa) accounted for over 90% of stems in the wet meadow (Table 1). Flooding of the wet meadow between 1996 and 1997 resulted in a threefold increase in relative abundance of C. aquatilis (p = 0.020), although the actual number of C. aquatilis stems decreased by 33%. Several species with low relative abundance values were lost. As water levels again dropped between 1997 and 1998,
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C. aquatilis relative abundance dropped to even lower levels than those seen in 1996 (p = 0.011), while representation of C. lasiocarpa in the community increased by over 40% (p = 0.017). The spike-rush, E. smallii, and narrow-leaved cattail, T. angustifolia, also increased in relative abundance in 1998, while the relative abundance of the marsh bellflower, Campanula aparinoides, decreased. Other relative abundance changes in 1998 included increased tufted loosestrife (Lysimachia thyrsiflora), a common herbaceous species in wet meadows that was likely responding to drier conditions, and decreased water horsetail (Equisetum fluviatile), whose weak surface roots may have resulted in it being eroded away during high water conditions in 1997 (Table 1). The actual number of stems of the four dominant sedge and grass species in the wet meadow, C. aquatilis, C. lasiocarpa, C. stricta, and C. canadensis, decreased 72.4% as a result of increased water levels in 1997. Only a moderate 16% increase in stems occurred between 1997 and 1998 as water levels dropped back almost to July 1996 levels (Fig. 3) when the wet meadow had just recently been inundated. In the transition zone, the sedge, C. lasiocarpa, and cattail, T. angustifolia, increased in relative abundance from 1996 to 1997 (p = 0.093 and p = 0.091 respectively), while the spike-rush, E. smallii, decreased (p = 0.028) in 1997, then increased (not statistically significant, p > 0.1) the following year. Emergent plant composition of the emergent marsh zone community was the most stable with the only statistically significant change (p < 0.1) detected being an increase in relative abundance of arrowhead, Sagittaria latifolia, from 8.6% to 13.9% from 1996 to 1997, the year of highest water level (Table 1). SFF plant composition changes were relatively few, but dramatic, in the wet meadow, where bladderworts (Utricularia), especially U. intermedia, expanded into the widely flooded wet meadow in 1997 and increased its dominance of the submergent plant assemblage from 34% in 1996 to 60% in 1997 (Table 1). This was followed by a similar decrease to 28% coverage of Utricularia spp. in 1998 when water levels were similar to those in 1996 (Fig. 3). Utricularia spp. exhibited a similar change in percent cover in the transition zone where coverage increased from 56% in 1996 to 74% in 1997 and then decreased to 34% in 1998. These changes were all significant at the p < 0.10 level. Utricularia spp. percent coverage in the emergent marsh remained low in all three years (6–12%, Table 1)
and did not change significantly from year to year (p > 0.1). Schoenoplectus subterminalis, a submergent bulrush species characteristicly found growing in shallow water, decreased in percent coverage from 30.4% in 1996 to only 0.3% in 1997, then increased again to 8.5% in 1998 in the transition zone (Table 1). In the emergent marsh, there was little change in percent coverage of S. subterminalis from 1996 to 1997 (29 to 31%), but this was followed by an increase to 48% coverage in 1998 (Table 1). All increases or decreases for this species were significant at the p < 0.1 level. S. subterminalis did not occur in the wet meadow zone. DISCUSSION The Lake Huron water level change observed from 1996 to 1998 was atypical for high water events in the Great Lakes with water levels falling after only a year or two at the peak high water levels. These short-term peak events appear not to lead to the up slope zonation shifts described by Burton (1985). It is possible that the up- and downslope movements described by Burton (1985) only happen if high water events last for several years (e.g., similar to the event in Fig. 2 for the 1983–1988 time period). The short-term nature of the event we studied for 3 years does, however, provide information on which plant species respond quickly to water level changes in coastal Great Lakes wetlands. Our results suggest that the process of zone shifting does not involve entire plant assemblages moving up or down slope en masse. Instead, differential responses by individual species create a changing mosaic of plant community composition within each zone. The reduction in emergent stem density during the study period may have been caused, in part, by early-season animal activity in the transition and wet meadow zones, as has been observed in other studies (Burton 1985, Gathman et al. 1999). Carp (Cyprinus carpio) spawning occurred every year in the shallows during late May and early June. In 1997 and 1998, the high water in the early season allowed this activity to occur well up into the wet meadow zone. Also, muskrat activity was evident in the wetlands and high water may have encouraged these animals to extend stem harvesting into higher elevation zones as noted during the 1987 high water conditions by Albert et al. (1987). Whatever the cause, emergent stem density reduction may be a predictable and important component
Coastal Wetland Plants and High Water of shifts in community boundaries associated with water level rise. Other than reduced stem density, one of the greatest changes observed during the study period was a rapid increase in dominance (% of total stem counts) by two sedges, C. lasiocarpa and C. aquatilis, in the lower part of the wet meadow and transition zone as water levels rose. This change was short lived for C. aquatilis, whose actual stem numbers, as well as % of total stem counts, dropped rapidly in 1998, when water levels delcined from the 1997 high water level. At one site, Mackinac Bay, C. lasiocarpa appears to have expanded out into the shallow margins of the emergent marsh community in 1998. C. lasiocarpa commonly grows in shallow water, in shallow lakes and ponds, but it was undetected in plots sampled in the emergent zone in 1996 and 1997. Species characteristic of the transition zone, including narrow-leaved cattail and hardstem bulrush (T. angustifulia and S. acutus), did not show up slope shifts into the wet meadow under high water conditions. However, patches of hardstem bulrush (S. acutus) were observed in the wet meadow in 1997, suggesting that rhizomes of this species persisted in the wet meadow during low water periods and then produced a greater number of stems during wetter conditions, at the same time that the stems of competing grasses and sedges were declining in number. The pattern of plant distribution change in this 3-year study does not allow us to predict whether a true cattail-dominated transition zone would establish at higher elevations if high water levels were to persist, and if so, the length of time required for establishment. The changes observed in the wet meadow and transition zone, where flood timing and duration changed during the study years, indicated that the dominance and presence/absence of some species change rapidly following either an increase or decrease in water levels. However, the limited response of many persistent species provides stability in plant zones during short-term fluctuation events. Further, system resilience, as seen in the rapid “rebound” of some species in this study, is provided because many species lost from the wet meadow during water-level fluctuations remain in the soil as a seed bank, providing a rich abundance of new colonists when appropriate water-level conditions return (Keddy and Reznicek 1986). In contrast to the plant community changes noted above, patterns of plant response to water level changes in the emergent marsh were not evident.
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No changes in the emergent species were detected. Percent cover of several submergent and floating species, including stonewort (Chara spp.), common waterweed (Elodea canadensis), water-milfoil (Myriophyllum spp.), slender naiad (Najas flexilis), Schoenoplectus subterminalis, wild celery (Vallisneria americana), and yellow pond-lily (Nuphar variegatum), changed as water levels fluctuated, but there was no simple pattern evident in their responses. Annual and perennial emergent plant species, as well as submergent and floating plant species, responded rapidly to habitat changes in the transition zone, but there was no consistent pattern of plant responses to environmental change in this zone. Rapid change in species richness and emergent stem density, as described in this study, suggest that changes in the flood depth and duration is a key determinant of plant composition within the higher elevation portions of the lake-to-upland gradient, particularly in the wet meadow zone. These areas normally experience seasonal flooding followed by drying. However, year-round flooding without seasonal drying leads to reduction in species richness and diversity. This suggests that while temporary flooding is crucial to maintaining the diversity of this zone and likely contributes to the overall diversity of the wetland, long-term flooding would result in reduced diversity of plant species in the wet meadow. ACKNOWLEDGMENTS We thank The Nature Conservancy, the Michigan Department of Environmental Quality, and the United States Environmental Protection Agency for support of research conducted for this study. We also thank Mary Moffett and Peter Murphy for their many helpful comments provided during the review of this paper. REFERENCES Albert, D.A., Reese, G., Crispin, S.R., Wilsmann, M.R., and Ouwinga, S.J. 1987. A survey of Great Lakes marshes in Michigan’s Upper Peninsula. Michigan Natural Features Inventory Technical Report, Lansing, MI. Burton, T.M. 1985. The effects of water level fluctuations on Great Lakes coastal marshes. In Coastal Wetlands, eds. H.H. Prince and F.M.D’Itri, pp. 3–13. Chelsea, MI: Lewis Publishers. Gathman, J.P., Burton, T.M., and Armitage, B.J. 1999. Coastal wetlands of the upper Great Lakes: distribu-
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tion of invertebrate communities in response to environmental variation. In Invertebrates in Freshwater Wetlands of North America: Ecology and Management. Eds. D.P. Batzer, R.B. Rader, and S.A. Wissinger, pp. 949–994. New York, NY: John Wiley & Sons. Hudon, C. 1997. Impact of water level fluctuations on St. Lawrence River aquatic vegetation. Can. J. Fish. Aquat. Sci. 54:2853–2865. Keddy, P.A. 1983. Shoreline vegetation in Axe Lake, Ontario: effects of exposure on zonation patterns. Ecology 64:331–344. ——— . 1984. Quantifying a within-lake gradient of wave energy in Gillfillan Lake, Nova Scotia. Can. J. Bot. 62:301–309. ——— , and Constabel, P. 1986. Germination of ten shoreline plants in relation to seed size, soil particle size and water level: an experimental study. J. Ecol. 74:133–141. ——— , and Ellis, T.H. 1984. Seedling recruitment of 11 wetland plant species along a water level gradient: shared or distinct responses? Can. J. Bot. 63:1876–1879. ——— , and Reznicek, A.A. 1986. Great Lakes vegetation dynamics: the role of fluctuating water levels and buried seeds. J. Great Lakes Res. 12:25–36. Lyon, J.G., Drobney, R.D., and Olson, C.E. Jr. 1986. Effects of Lake Michigan water levels on wetland soil chemistry and distribution of plants in the Straits of Mackinac. J. Great Lakes Res. 12:175–183.
Minc, L.D. 1997a. Great Lakes coastal wetlands: an overview of abiotic factors affecting their distribution, form, and species composition. Michigan Natural Features Inventory Technical Report, Lansing, MI. ——— . 1997b. Vegetative response in Michigan’s Great Lakes marshes to Great Lakes water-level fluctuations. Michigan Natural Features Inventory Technical Report, Lansing, MI. ——— , and Albert, D.A. 1998. Great Lakes coastal wetlands: abiotic and floristic characterization. Michigan Natural Features Inventory Technical Report, Lansing, MI. U.S. Army Corps of Engineers. 1987. Great Lakes Water Level Facts. U.S. Army Corps of Engineers, Detroit District. van der Valk, A.G. 1981. Succession in wetlands: a Gleasonian approach. Ecology 62:688–696. Welling, C.H., Pederson, R. L., and van der Valk, A.G. 1988. Recruitment from the seed bank and the development of zonation of emergent vegetation during a drawdown in a prairie wetland. J. Ecol. 76:483–496. Zedler, P.H. 1981. Microdistribution of vernal pool plants of Kearny Mesa, San Diego County. In Vernal Pools and Intermittent Streams, eds. S. Jain and P. Moyle, pp. 185–197. Davis, CA: Institute of Ecology. Submitted: 16 April 2004 Accepted: 10 September 2005 Editorial handling: John Janssen
Rapid Plant Community Response to a Water Level Peak in Northern Lake Huron Coastal Wetlands Joseph P. Gathman1, Dennis A. Albert2, and Thomas M. Burton3,* 1Michigan
State University Department of Zoology East Lansing, Michigan 48824 2Michigan
Natural Features Inventory Michigan State University Extension Mason Building, P.O. Box 30444 Lansing, Michigan 48909-7944 3Michigan
State University Departments of Zoology and Fisheries and Wildlife East Lansing, Michigan 48824 ABSTRACT. Aquatic plants were sampled in five coastal wetlands of northern Lake Huron during July 1996, 1997, and 1998. Mean annual water levels of Lake Huron changed during this period from 176.37 m (below the long-term average) in January 1996 to above average water levels of 176.83 m in July 1996 to 177.19 m in July 1997 and then declined to 176.88 m by July 1998. Boundaries of plant zones as indicated by distribution of the 1–3 dominant species along permanently established transect points across the wetland did not shift spatially over this 3-year period. Instead, relative abundance (percent of total stems per three 0.25 m2 quadrats per plot) and presence/absence of plant species responded individually to water level changes within major zones. In 1996, the first season sampled, the wet meadow had recently been inundated by rising water level. In 1997, after more than a year of above average and rising water levels, emergent stem densities were reduced in the Carex/Calamagrostis (sedge/blue-joint) dominated wet meadow and mixed transition sedge, narrow-leafed cattail, and hardstem bulrush (Carex, Typha angustifolia, and Schoenoplectus acutus) dominated zones compared to stem densities in 1996. Stem densities remained low in 1998, even though water levels dropped 31 cm from 1997 levels. The relative dominance (% of stems/3 quadrats/plot) and presence/absence of some plant species changed rapidly in the wet meadow zone in response to increases in water levels in 1997 and to decreases in water levels in 1998. In contrast, changes in emergent species were minimal in the deeper emergent zone dominated by hardstem bulrush. We conclude that temporary flooding and drying in response to water level changes are critical to maintaining a diverse arrray of plant species in the wet meadow zones of these marshes. Furthermore, short-term water level changes do not affect the relative spatial position of major plant zones within the marsh nor the relative abundance of emergent species in the deepest zone. INDEX WORDS:
Coastal wetlands, water level fluctuation, wetland plants, Great Lakes.
INTRODUCTION Plant zonation is a well-known phenomenon occurring in wetlands, but the causal mechanisms behind this familiar wetland characteristic are still incompletely understood. Past studies have shown that plant species respond to different, but often
*Corresponding
correlated, environmental variables. Factors that may directly affect distributions of certain wetland plant species include soil chemistry and redox potential (Lyon et al. 1986), seed and soil particle size (Keddy and Constabel 1986), water depth and subsoil water table height (Keddy and Constabel 1986), spatial differences in concentration of propagules in the soil seed bank (Zedler 1981, Keddy and Reznicek 1986), wave exposure on lakeshores
author. E-mail: [email protected]
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FIG. 1.
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Location of the five Northern Lake Huron wetlands sampled.
(Keddy 1983, 1984), multi-year water-level cycles combined with muskrat activity in prairie potholes (Welling et al. 1988), interspecific competition (Keddy and Constabel 1986), and temporal variations in water levels (Hudon 1997). The fact that certain species tend to co-occur within zones is probably a coincidental outcome of each species consistently responding to its own set of environmental variables (Keddy and Ellis 1984). This Gleasonian assembly process was described by van der Valk (1981) as an “environmental sieve” model, in which each species must first “find” its potential habitat range with acceptable physico-chemical conditions for growth. Then, a plant can be further affected by biotic factors that determine its realized habitat. However, each of these factors is strongly affected by flooding regimes. This single, hydrological factor may be considered the ultimate cause of plant distribution patterns in wetlands. Such flooding regimes are particularly important in coastal wetlands of the Laurentian Great Lakes where water level changes occur on several overlapping time scales, including short-term changes over hours to days, seasonal variation from year to year from annual precipitation and evaporation patterns,
and multi-year changes due to less-predictable, long-term climate variation (Burton 1985; Minc 1997a, 1997b; U.S. Army Corp of Engineers 1987). Short-term changes that occur over hours to days are caused by seiches and storm surges. These changes, which can result in movement of plant propagules and erosion and redeposition of sediments, cause local alterations of wetland plants, such as breaking bulrush (Schoenoplectus) stems or uprooting cattails (Typha) or submergent plants. However, these changes do not result in large-scale alterations in the zonation within a wetland or within local wetland complexes such as those of the area studied (the Les Cheneaux Islands area of northern Lake Huron, Fig. 1). We did not focus on the effects of these short-term lake level changes on plants. Instead, our focus was on interannual differences resulting from seasonal variation over a 3year period of rapid water level change. Seasonal variation in water level resulting from annual precipitation and evaporation patterns is a more predictable form of water level fluctuation compared to storm surges and long term changes. Long-term water-level data for the Laurentian Great Lakes from the U.S. Army Corps of Engineers (US
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ACE) show a seasonal pattern of low water levels in autumn, winter, and spring, with highest water levels during late summer, typically in July and August in most years (Minc 1997a). This relatively predictable annual pattern is an important factor for wetland plants. Many wetland plants that cannot become established in permanently flooded areas are able to germinate in seasonally flooded zones that have an aerobic, non-flooded environment during the spring and early summer. Many wetland seedlings can then survive as water levels rise. Multi-annual fluctuations over periods greater than the 3 years of this study are caused by longterm climatic variation and represent the most important scale determining the lateral position of the boundaries between plant community zones in coastal wetlands. Over 100 years of water-level data demonstrate an irregular 7–10 yr pattern of high and low water levels in the Great Lakes, with the variation from lowest lake levels to highest lake levels exceeding 1.0 m (U.S. Army Corp of Engineers 1987; Minc 1997a, 1997b). When high or low levels last for a few years, they lead to changes in the lateral positions of wetland communities and their boundaries along the shore (Burton 1985), although the effects of short-duration highs and lows have not been studied. Extreme high and low conditions can occur over 2 or 3 years, creating vastly different hydrologic conditions and sediment characteristics over large parts of coastal wetlands. During relatively lowwater years, the upland portion of wet meadows is invaded by woody plant species. Long-term lows result in establishment of shrub swamp and occasionally swamp forest above the annual high water level (Minc 1997b), while marsh and wet meadow communities gradually shift downslope. When water levels rise again over several years, woody species are killed, and the marsh and meadow communities migrate upslope. Long-term water level changes therefore cause long-term spatial displacement of the entire zonation continuum, though the shifts are time-lagged behind the water level change (Burton 1985). This phenomenon has not been studied in detail in Great Lakes coastal wetlands, so the rate of plant community change associated with water level change is not known. This study took advantage of a natural experiment during which Lake Huron’s level rose 46 cm from January to July 1996, flooding the wet meadow zones of wetlands (Figs. 2, 3). This initial rise was followed by an additional 36 cm rise to a high of 177.19 m in July 1997. Water levels were
FIG. 2. Lake Huron monthly average water levels (m above International Great Lakes Datum (IGLD)), measured at DeTour Village, MI, 1980 through 1998 (Data source: U.S. Department of Commerce, NOAA/NOS, Silver Spring, MD). 31 cm lower in July 1998. This occurrence provided an opportunity to observe the initial plant community composition just after the initial water level rise with species composition a year later after additional water level increases and with community composition during the first year of water level decline. We hypothesized that significant changes would occur in the composition of the wet meadow and emergent marsh zones during this period of water level fluctuation. To test this hypothesis, we recorded changes in the plant communities of several wetland sites during this 3-year period.
FIG. 3. Schematic cross-section through representative sample transect for the five wetlands sampled.
Coastal Wetland Plants and High Water METHODS Study Sites Data were collected from five coastal wetlands in the vicinity of Cedarville and Hessel, Michigan, along the north shore of Lake Huron in the Les Cheneaux Islands region of Michigan’s upper peninsula: Mackinac Bay, Mismer Bay, Prentiss Bay, Duck Bay, and St. Martin’s Bay (Fig. 1). Wetlands in the Les Cheneaux Islands region occur within large, gently sloping embayments, with vegetation that displays relatively predictable zonation related to its position along the elevation gradient (Minc 1997a, 1997; Minc and Albert 1998). Along the upland edge, swamp forests flood only briefly during the growing season in the most extreme high-water years, although they often have saturated soils. Still closer to the lake is a shrub swamp zone, which is flooded more often, resulting in little or no successful establishment of tree species. Closer to the lake, flooding occurs more frequently and water depths can be greater, resulting in dominance by wet meadow plants such as grasses and sedges. Along the lake margin is a zone which is typically flooded continually and supports emergent marsh species, while further down the gradient, emergent species share the habitat with floating and submergent plants. Most of the five wetlands selected for study (Fig. 1) were typical of this region, with gently sloping shorelines and broad vegetation zones. Based on the dominant vegetation, we classified the zones we studied as: wet meadow (dominated by sedges, Carex stricta and/or Carex lasiocarpa and bluejoint, Calamagrostis canadensis), transition (dominated by narrow-leaved cattail, Typha angustifolia, sedges, Carex spp., and hardstem bulrush, Schoenoplectus acutus), and emergent marsh (dominated by hardstem bulrush). The study sites also had shrub and forested swamp zones above the wet meadow, as described by Minc (1997a), but these were not included in this study. Some coastal wetlands include a “strand” (Keddy and Reznicek 1986) or “shoreline” (Minc 1997a) zone between wet meadow and emergent marsh, but all study sites except St. Martin’s Bay were in protected bays lacking the wave exposure necessary to create this zone. Our wet meadow zone was similar to the Keddy and Reznicek (1986) description: a sedge meadow numerically dominated by the sedges, Carex aquatilis and C. lasiocarpa, at lower elevations, and the hummock forming sedge, C. stricta, and blue-joint, C. canadensis, and several shrub species
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(e.g., sweet gale, Myrica gale, and several species of willow, Salix spp.) at higher elevations. This was the most diverse zone, including many herbaceous annual species interspersed among the sedges. Below the wet meadow, our zonation differed somewhat from the other authors’ descriptions. The transition zone was visually dominated by cattails, predominantly T. angustifolia, but was fairly species-rich, with spike-rush (Eleocharis smallii) water smartweed (Polygonum amphibium) and species from both the adjacent wet meadow and emergent marsh zones commonly growing among the cattails. The emergent marsh contained both emergent and submergent macrophytes and was dominated by hardstem bulrush but also commonly included arrowhead (Sagittaria spp.), pickerel weed (Pontederia cordata), and spike-rush (E. smallii). Several genera of submergent plants (including coontail (Ceratophyllum demersum), several species of pondweed (Potamogeton), and bladderworts (Utricularia)) and floating-leaved plants (especially yellow pond-lily and pondweed (Nuphar variegata and Potamogeton natans)) grew within the emergent marsh. Each site contained unique characteristics. The St. Martin’s Bay wetland had open dune/swale topography, rather than the gradual, consistently sloping shore shared by the other four sites. This meant that sampling stations at St. Martin’s Bay alternated between wet meadow and emergent marsh along the entire transect. All other sites had gradual slopes and some vegetation patchiness. Duck Bay had the steepest slopes of 4.9 mm/m. A road separated the wet meadow and emergent marsh zones of Prentiss Bay, but hydrologic connection was maintained by a culvert through which Prentiss Creek flowed into the bay. This study focused on changes that occurred within the wet meadow and emergent marsh zones over a three year period (Figs. 2–4). From January to July 1996, water levels rose 46 cm and inundated parts of the wet meadow at the time of first sampling in July 1996. Water levels rose another 36 cm by the second sampling period in July 1997 but then declined 31 cm by July 1998. The regional slopes within these wetlands were 2.5 mm/m–4.9 mm/m, so a water level increase of 82 cm from January 1996 to July 1997 resulted in flooding of wetland areas 200 to 400 m up the gradient (Figs. 3, 4), potentially resulting in major changes in plant communities.
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FIG. 4. Water level changes at Mackinac Bay in relation to plant community zonation over the study period (Data source for water level: U.S. Department of Commerce, NOAA/NOS, Silver Spring, MD). Data Collection Water-level data over the 3 years were obtained from National Oceanic and Atmospheric Administration (NOAA) (Fig. 2). The data were monthly mean levels measured at the De Tour Village gauging station (Fig. 2) approximately 30 km east of the nearest (Prentiss Bay) study site and 60 km from the farthest site (St. Martin’s Bay). Since there were negligible differences between data from the Mackinaw City gauging station, located 30–60 km southwest of the study area across the Mackinac Straits, and the De Tour Village station, data from De Tour Village were used for the study. In 1996, a single permanent transect was established across each wetland from approximately the upslope margin of the wet meadow zone across the transition zone into the emergent zone to water approximately 75 cm deep at the time of sampling in 1996 (Fig. 3). Transects were perpendicular to the general shoreline contour, with the upland end marked by a tall wooden stake at the wet meadow/forest edge or at a point in the hummock/ sedge zone where herbaceous species were more common than shrubs (Fig. 3). Transects ran down slope with stakes placed every 20 or 25 m, depending on the width of the zone. At the Mackinac Bay site, the transect never reached 75 cm depth because the emergent marsh extended for a great distance over a barely sloping bottom before rising toward an island in the bay. Therefore, we estab-
lished the deep end of the transect at the point where there was no perceptible change in vegetation. Transect length varied among sites (160-320 m) because of different bottom slopes. Plant community data were collected from each wetland in late July 1996, 1997, and 1998. Sampling was initiated at the stake that marked the upland end of the transect. A point-centered circular plot with a 10 m radius centered at each stake along the transect was sampled. Essentially, this resulted in a series of circular plots extending along the transect that touched or were only 5 m apart at the outer edge of the circle. For each point centered plot, we measured water depth and depth of soil organic matter, and then selected three quadrat locations using random azimuths (0–360°) and distances (1–10 m) from the stake in the center of the plot. A 0.5 m × 0.5 m (0.25 m2) quadrat frame made of PVC conduit was dropped at the randomly selected location. This process was repeated each year so that new quadrat points were selected each year within each circular plot. Thus, the same plots were sampled from year to year but not the same quadrat locations within each plot. We identified all plant species present within the quadrats, counted stems of each emergent species (including shrubs), and visually estimated percent coverage of each floating and floating-leaved species. Stem counts included all individual stems of each species, including the individual stems that made up each hummock of the sedge, C. stricta, since it would have been too difficult to estimate how many actual individual plants made up each hummock. Stem counts were made at the base of the stem to avoid accidental counts of individual leaves. We estimated coverage of submergent and floating or floating leaved (SFF) species as percent of quadrat area covered by each species either by direct visual observation or by dragging a rake once across the bottom within the transect boundaries and estimating the proportion of collected plant material for each species. No grid was used for estimating percent cover. However, D. Albert and T. Burton participated in all surveys and trained crew members in how to make these estimates to reduce observer bias between plots and years. Percent cover was estimated for each species independent of other species. Thus, it was possible to have total coverage values greater than 100% if there were layers of submergent plants in the water column and/or coverage by floating or floating leaved plants as well as submergent ones.
Coastal Wetland Plants and High Water Because Mackinac Bay had the most gradual slope of the sites, it also had the longest transect and, hence, the greatest number of samples taken per unit of elevation change. Given this relatively high sampling intensity, Mackinac Bay provided the finest sampling resolution to detect subtle vegetation changes along the elevation gradient (Fig. 4). For this reason Mackinac Bay was the site chosen to delineate vegetation zones along the gradient. The cattail dominated zone was treated as a benchmark in the middle of the transect because cattails generally occur at the long-term mean water level in coastal wetlands (Keddy and Reznicek 1986). The Mackinac Bay maximum and minimum depths for cattails were defined as the upper and lower elevations for the cattail zone. This middle zone was designated as the transition zone, and all up slope elevations (shallower depths or no standing water) were designated as wet meadow and all down slope elevations (greater depths) were designated as emergent marsh. These zone labels were then applied to stations from all sites using water depth measurements taken during sampling in 1996 to estimate elevation and define the limits of the transition zone if cattails were not present. To verify that perceived zone divisions were based on real community differences, not just visual bias, hierarchical Ward’s matching cluster analysis was run on all Mackinac Bay 1996 samples. Clusters were defined based on Euclidean distance. According to this analysis, the three zone groupings represented natural community differences identifiable in the sampling data. Cluster analysis based on data from the other four wetlands also demonstrated similar zone designations. Analysis Data from the three quadrats of each point centered plot were either summed (stem counts) or averaged (coverage data) so that each three-quadrat composite from a single point centered plot comprised a sample. Relative abundances (percent of total stem counts), taxa richness (number of species), and Shannon diversity (H′) were calculated from these composite data for each sample. Note that Shannon diversity calculations were based on individual stem counts rather than counts of individual plants, and this would tend to overestimate the importance of hummock forming sedge species in these wetlands. Frequency distribution data could not be transformed to normality because of a high frequency of
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zero counts. Instead, all hypothesis tests were performed on each zone individually to reduce the number of zero values and nonparametric analyses were used. Each 2-year period (1996 to 1997 and 1997 to 1998) was treated as a separate experiment, and differences in response variables between years were analyzed using Wilcoxon rank-sum tests, the nonparametric equivalent of paired Student’s t tests. An a priori critical α level of 0.10 was used to determine statistical significance because of relatively low sample size (which varied among zones) and high data variability. RESULTS Community zones did not move in relation to the permanent points (stakes) along the transects during the study period, although we did observe several changes in composition within the wet meadow and emergent marsh zones. In general, the wet meadow experienced the greatest change in number of species present and in the stem counts of emergent species present, while the emergent marsh showed the least. Also, most changes observed from 1996 to 1997 were partially reversed from 1997 to 1998, although some of these reversals were not statistically significant. The most noteworthy changes in plant communities over the study period were: 1) increasing dominance by the sedges, Carex aquatilis and C. lasiocarpa, during the high-water year, with reversal of the trend for C. aquatilis as water levels declined (Table 1); 2) rapid establishment in the wet meadow by Utricularia (bladderwort) species, especially U. intermedia, during the wettest (second) year, with other submergent species appearing the next year, despite falling water levels (Table 1); and 3) a significant decrease in emergent plant stem density and Shannon diversity in the wet meadow and transition zones during the second year, followed by increases in these same measures during the third year (Table 2). Each change is explained in greater detail below. Density and Coverage Changes As water depth increased from 1996 to 1997, stem density of emergent plants in the wet meadow decreased from 376 stems/m 2 to 310 stems/m 2 (17% decrease, p = 0.038). As water depth decreased between 1997 and 1998, stem densities rose to 397 stems/m2 (28% increase, p = 0.019) (Table 2). Stem density changed more dramatically in the transition zone, falling from 226 stems/m2 to 138
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TABLE 1. Relative abundance (% of total stem counts for three 0.252 quadrats) for emergent plant species and percent coverage for submergent, floating, and floating-leaved (SFF) plant species that occurred in each plant zone during the 1996, 1997, and 1998 sampling seasons (n = total number of permanently established point centered plots per plant zone for all five wetlands sampled). Plant Zone Year of sampling Taxon Emergent Species Calamogrostis canadensis Campanula aparinoides Carex aquatilis Carex lasiocarpa Carex stricta Carex spp. Eleocharis smallii Equisetum fluviatilis Juncus spp. Lysimachia thyrsiflora Polygonum amphibium Pontedaria cordata Sagittaria spp. Schoenoplectus acutus Typha spp. Salix spp. All shrub spp. All others
Wet Meadow (n = 26) 1996 1997 1998
14.0% 6.2% 5.2% 17.1% 27.7% 62.8% 0.6% 0.9% 0.5% 0.7% 0.5% 0.0% 0.3% 3.4% 0.9% 0.2% 1.2% 8.0%
15.7% 8.8% 16.1% 18.3% 31.9% 66.6% 0.1% 1.0% 0.1% 0.9% 0.5% 0.0% 0.0% 1.5% 0.0% 0.3% 2.0% 2.7%
14.2% 5.2% 3.6% 26.3% 36.7% 67.6% 1.2% 0.6% 0.3% 1.6% 0.2% 0.0% 0.1% 4.5% 0.2% 0.4% 1.8% 2.3%
SFF* Species Chara spp. 0.0% 4.2% 0.0% Elodea canadensis 1.9% 0.0% 0.0% Myriophyllum spp. 0.3% 6.6% 2.6% Najas flexilis 0.0% 0.0% 0.0% Potamogeton spp. 2.6% 1.2% 0.0% Schoenoplectus subterminalis 0.0% 0.0% 0.0% Utricularia spp. 33.7% 60.0% 27.7% Vallisneria americana 0.0% 0.0% 0.5% Nuphar variegatum 0.0% 0.0% 0.0% * SFF = Submergent, Floating and Floating Leaved Plants
stems/m2 (39% decrease, p = 0.050) as water levels rose, then rising to 257 stems/m2 (86% increase, p = 0.008) with the subsequent drop in water levels. Emergent stem density in the emergent marsh began at 40 stems/m2 and did not change significantly (p > 0.1) during either year (Table 2). Submergent, floating, and floating-leaved species (SFF) coverage increased in the wet meadow when water rose from 1996 to 1997, from a very low mean level of 0.8% to 3.4% (p < 0.000), and continued to increase the following year to 8.4% despite the falling water levels. However, this increase in coverage was not statistically significant because of high variability among samples (Fig. 5).
Transition (n = 12) 1996 1997 1998
Emergent (n = 23) 1996 1997 1998
3.1% 0.7% 3.8% 19.7% 2.5% 30.4% 14.7% 1.4% 1.9% 0.4% 0.4% 0.0% 3.0% 40.6% 1.3% 0.0% 0.0% 2.1%
5.2% 1.5% 5.0% 38.7% 1.7% 46.3% 5.7% 2.7% 0.5% 1.3% 0.7% 0.0% 0.0% 22.5% 12.2% 0.2% 0.2% 1.2%
4.9% 1.4% 3.3% 37.8% 4.0% 45.9% 10.6% 0.4% 0.8% 3.6% 0.8% 0.0% 0.0% 18.8% 8.0% 0.0% 0.0% 4.8%
0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 15.0% 0.3% 0.0% 0.0% 0.0% 0.0% 8.6% 59.6% 1.1% 0.0% 0.0% 15.4%
0.0% 0.0% 0.0% 3.8% 0.0% 3.8% 8.8% 1.3% 0.0% 0.0% 0.0% 0.8% 13.9% 50.4% 0.0% 0.0% 0.0% 21.0%
0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 12.2% 0.0% 0.0% 0.0% 3.5% 1.8% 11.5% 54.2% 2.2% 0.0% 0.0% 14.6%
1.1% 0.0% 4.6% 4.6% 0.0% 30.4% 55.5% 3.7% 0.0%
6.9% 2.1% 5.7% 0.0% 2.1% 0.3% 74.3% 0.0% 0.0%
0.0% 0.0% 8.3% 0.0% 8.3% 8.5% 33.5% 0.0% 7.9%
8.1% 6.3% 14.4% 3.8% 6.9% 28.9% 9.5% 3.6% 3.0%
5.4% 2.9% 10.9% 0.6% 8.3% 30.8% 11.7% 0.3% 13.1%
13.0% 7.6% 0.6% 4.2% 4.5% 48.3% 5.7% 2.6% 0.5%
SFF coverage in the transition zone remained stable at 8.6, 6.7, and 6.4% respectively in 1996, 1997, and 1998. SFF coverage in the emergent marsh also remained stable at 9.9% in 1996 and 9.4% in 1997 as water levels rose, but coverage increased dramatically to 46.2% (p < 0.000) in 1998 as water levels dropped (Fig. 5). Species Diversity and Richness Changes For emergent plants, per-plot Shannon diversity and per-plot species richness decreased as water levels rose in the wet meadow from 1996 to 1997 and increased as water levels dropped again the fol-
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TABLE 2. Mean stem density per 0.25 m2 quadrat, species richness (species per quadrat), and Shannon diversity (H′) (based on stems per quadrat) for species for each zone and each year of sampling (n = total number of permanently established point centered plots per plant zone for all five wetlands sampled). Plant Zone Wet Meadow (n = 26) Transition (n = 12) Year 1996 1997 1998 1996 1997 1998 Emergent stem density 281.8 232.3 297.7 169.4 103.5 192.6 Emergent H’ 2.91 2.77 2.90 2.53 2.20 2.55 Emergent species richness 9.8 8.1 8.8 7.4 6.6 6.3 SFF* H’ 0.01 0.05 0.03 0.07 0.05 0.01 SFF* species richness 0.69 1.50 0.62 1.83 1.83 0.83 * SFF = submergent, floating, and floating leaved vegetation.
lowing year. However, the subsequent richness increase was not statistically significant (p > 0.1, Table 2). Shannon diversity of emergent plants responded similarly in the transition zone, but species richness showed a non-significant decrease (p > 0.1) during both years. All changes of emergent plant diversity and richness in the emergent marsh were small and non-significant (p > 0.1). In contrast, both SFF diversity and richness in the wet meadow increased as water level rose between 1996 and 1997 and then decreased as water levels dropped between 1997 and 1998 (p < 0.1 for all tests, Table 2). Transition zone SFF diversity and richness changes were small and non-significant (p > 0.1) from 1996 to 1997 as water levels rose,
FIG. 5. Changes in submergent plant coverage (with standard errors) in three wetland zones during 3 years of water level change in five Northern Lake Huron coastal wetlands (see Figs. 3 and 4 for water level data)(n = total number of permanently established point centered plots per plant zone for all five wetlands sampled, see Table 1 for details).
Emergent (n = 23) 1996 1997 1998 30.3 30.4 36.7 0.99 1.06 1.08 1.7 2.0 2.0 0.15 0.20 0.14 2.74 3.13 3.26
but both variables decreased in 1998 with dropping water levels (p < 0.05 for both tests). Emergent marsh SFF diversity increased as water level rose (p = 0.036), then decreased as water level dropped (p = 0.070), although species richness remained fairly stable. Thirty-five species recorded in 1996 were not recorded in 1997 after more than a year of inundation by rising water levels while 18 species were newly detected. Most of the “lost” species were wet meadow residents, including six sedge (Carex) species, four rush (Juncus) species, and four shrub species. Five submergent species were lost. Three wet meadow grasses, rattlesnake grass, cut grass, and reed (Glyceria canadensis, Leersia oryzoides, and Phragmites australis) were lost, while reed canary grass (Phalaris arundinacea) was dramatically reduced (87.5%). As water level dropped from 1997 to 1998, 25 species were lost, while 18 additional species were gained. Only two of those gained were species detected in 1996. Two sedge (Carex) and three rush (Juncus) species were among the gains. Abundances of most lost and gained species were very low, so failure to detect them may have been the result of random location of quadrats in each permanently established point centered plot along the transect. Community Composition Changes C. canadensis (blue-joint) and Carex (sedge) species (especially C. stricta, C. aquatilis, and C. lasiocarpa) accounted for over 90% of stems in the wet meadow (Table 1). Flooding of the wet meadow between 1996 and 1997 resulted in a threefold increase in relative abundance of C. aquatilis (p = 0.020), although the actual number of C. aquatilis stems decreased by 33%. Several species with low relative abundance values were lost. As water levels again dropped between 1997 and 1998,
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C. aquatilis relative abundance dropped to even lower levels than those seen in 1996 (p = 0.011), while representation of C. lasiocarpa in the community increased by over 40% (p = 0.017). The spike-rush, E. smallii, and narrow-leaved cattail, T. angustifolia, also increased in relative abundance in 1998, while the relative abundance of the marsh bellflower, Campanula aparinoides, decreased. Other relative abundance changes in 1998 included increased tufted loosestrife (Lysimachia thyrsiflora), a common herbaceous species in wet meadows that was likely responding to drier conditions, and decreased water horsetail (Equisetum fluviatile), whose weak surface roots may have resulted in it being eroded away during high water conditions in 1997 (Table 1). The actual number of stems of the four dominant sedge and grass species in the wet meadow, C. aquatilis, C. lasiocarpa, C. stricta, and C. canadensis, decreased 72.4% as a result of increased water levels in 1997. Only a moderate 16% increase in stems occurred between 1997 and 1998 as water levels dropped back almost to July 1996 levels (Fig. 3) when the wet meadow had just recently been inundated. In the transition zone, the sedge, C. lasiocarpa, and cattail, T. angustifolia, increased in relative abundance from 1996 to 1997 (p = 0.093 and p = 0.091 respectively), while the spike-rush, E. smallii, decreased (p = 0.028) in 1997, then increased (not statistically significant, p > 0.1) the following year. Emergent plant composition of the emergent marsh zone community was the most stable with the only statistically significant change (p < 0.1) detected being an increase in relative abundance of arrowhead, Sagittaria latifolia, from 8.6% to 13.9% from 1996 to 1997, the year of highest water level (Table 1). SFF plant composition changes were relatively few, but dramatic, in the wet meadow, where bladderworts (Utricularia), especially U. intermedia, expanded into the widely flooded wet meadow in 1997 and increased its dominance of the submergent plant assemblage from 34% in 1996 to 60% in 1997 (Table 1). This was followed by a similar decrease to 28% coverage of Utricularia spp. in 1998 when water levels were similar to those in 1996 (Fig. 3). Utricularia spp. exhibited a similar change in percent cover in the transition zone where coverage increased from 56% in 1996 to 74% in 1997 and then decreased to 34% in 1998. These changes were all significant at the p < 0.10 level. Utricularia spp. percent coverage in the emergent marsh remained low in all three years (6–12%, Table 1)
and did not change significantly from year to year (p > 0.1). Schoenoplectus subterminalis, a submergent bulrush species characteristicly found growing in shallow water, decreased in percent coverage from 30.4% in 1996 to only 0.3% in 1997, then increased again to 8.5% in 1998 in the transition zone (Table 1). In the emergent marsh, there was little change in percent coverage of S. subterminalis from 1996 to 1997 (29 to 31%), but this was followed by an increase to 48% coverage in 1998 (Table 1). All increases or decreases for this species were significant at the p < 0.1 level. S. subterminalis did not occur in the wet meadow zone. DISCUSSION The Lake Huron water level change observed from 1996 to 1998 was atypical for high water events in the Great Lakes with water levels falling after only a year or two at the peak high water levels. These short-term peak events appear not to lead to the up slope zonation shifts described by Burton (1985). It is possible that the up- and downslope movements described by Burton (1985) only happen if high water events last for several years (e.g., similar to the event in Fig. 2 for the 1983–1988 time period). The short-term nature of the event we studied for 3 years does, however, provide information on which plant species respond quickly to water level changes in coastal Great Lakes wetlands. Our results suggest that the process of zone shifting does not involve entire plant assemblages moving up or down slope en masse. Instead, differential responses by individual species create a changing mosaic of plant community composition within each zone. The reduction in emergent stem density during the study period may have been caused, in part, by early-season animal activity in the transition and wet meadow zones, as has been observed in other studies (Burton 1985, Gathman et al. 1999). Carp (Cyprinus carpio) spawning occurred every year in the shallows during late May and early June. In 1997 and 1998, the high water in the early season allowed this activity to occur well up into the wet meadow zone. Also, muskrat activity was evident in the wetlands and high water may have encouraged these animals to extend stem harvesting into higher elevation zones as noted during the 1987 high water conditions by Albert et al. (1987). Whatever the cause, emergent stem density reduction may be a predictable and important component
Coastal Wetland Plants and High Water of shifts in community boundaries associated with water level rise. Other than reduced stem density, one of the greatest changes observed during the study period was a rapid increase in dominance (% of total stem counts) by two sedges, C. lasiocarpa and C. aquatilis, in the lower part of the wet meadow and transition zone as water levels rose. This change was short lived for C. aquatilis, whose actual stem numbers, as well as % of total stem counts, dropped rapidly in 1998, when water levels delcined from the 1997 high water level. At one site, Mackinac Bay, C. lasiocarpa appears to have expanded out into the shallow margins of the emergent marsh community in 1998. C. lasiocarpa commonly grows in shallow water, in shallow lakes and ponds, but it was undetected in plots sampled in the emergent zone in 1996 and 1997. Species characteristic of the transition zone, including narrow-leaved cattail and hardstem bulrush (T. angustifulia and S. acutus), did not show up slope shifts into the wet meadow under high water conditions. However, patches of hardstem bulrush (S. acutus) were observed in the wet meadow in 1997, suggesting that rhizomes of this species persisted in the wet meadow during low water periods and then produced a greater number of stems during wetter conditions, at the same time that the stems of competing grasses and sedges were declining in number. The pattern of plant distribution change in this 3-year study does not allow us to predict whether a true cattail-dominated transition zone would establish at higher elevations if high water levels were to persist, and if so, the length of time required for establishment. The changes observed in the wet meadow and transition zone, where flood timing and duration changed during the study years, indicated that the dominance and presence/absence of some species change rapidly following either an increase or decrease in water levels. However, the limited response of many persistent species provides stability in plant zones during short-term fluctuation events. Further, system resilience, as seen in the rapid “rebound” of some species in this study, is provided because many species lost from the wet meadow during water-level fluctuations remain in the soil as a seed bank, providing a rich abundance of new colonists when appropriate water-level conditions return (Keddy and Reznicek 1986). In contrast to the plant community changes noted above, patterns of plant response to water level changes in the emergent marsh were not evident.
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No changes in the emergent species were detected. Percent cover of several submergent and floating species, including stonewort (Chara spp.), common waterweed (Elodea canadensis), water-milfoil (Myriophyllum spp.), slender naiad (Najas flexilis), Schoenoplectus subterminalis, wild celery (Vallisneria americana), and yellow pond-lily (Nuphar variegatum), changed as water levels fluctuated, but there was no simple pattern evident in their responses. Annual and perennial emergent plant species, as well as submergent and floating plant species, responded rapidly to habitat changes in the transition zone, but there was no consistent pattern of plant responses to environmental change in this zone. Rapid change in species richness and emergent stem density, as described in this study, suggest that changes in the flood depth and duration is a key determinant of plant composition within the higher elevation portions of the lake-to-upland gradient, particularly in the wet meadow zone. These areas normally experience seasonal flooding followed by drying. However, year-round flooding without seasonal drying leads to reduction in species richness and diversity. This suggests that while temporary flooding is crucial to maintaining the diversity of this zone and likely contributes to the overall diversity of the wetland, long-term flooding would result in reduced diversity of plant species in the wet meadow. ACKNOWLEDGMENTS We thank The Nature Conservancy, the Michigan Department of Environmental Quality, and the United States Environmental Protection Agency for support of research conducted for this study. We also thank Mary Moffett and Peter Murphy for their many helpful comments provided during the review of this paper. REFERENCES Albert, D.A., Reese, G., Crispin, S.R., Wilsmann, M.R., and Ouwinga, S.J. 1987. A survey of Great Lakes marshes in Michigan’s Upper Peninsula. Michigan Natural Features Inventory Technical Report, Lansing, MI. Burton, T.M. 1985. The effects of water level fluctuations on Great Lakes coastal marshes. In Coastal Wetlands, eds. H.H. Prince and F.M.D’Itri, pp. 3–13. Chelsea, MI: Lewis Publishers. Gathman, J.P., Burton, T.M., and Armitage, B.J. 1999. Coastal wetlands of the upper Great Lakes: distribu-
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tion of invertebrate communities in response to environmental variation. In Invertebrates in Freshwater Wetlands of North America: Ecology and Management. Eds. D.P. Batzer, R.B. Rader, and S.A. Wissinger, pp. 949–994. New York, NY: John Wiley & Sons. Hudon, C. 1997. Impact of water level fluctuations on St. Lawrence River aquatic vegetation. Can. J. Fish. Aquat. Sci. 54:2853–2865. Keddy, P.A. 1983. Shoreline vegetation in Axe Lake, Ontario: effects of exposure on zonation patterns. Ecology 64:331–344. ——— . 1984. Quantifying a within-lake gradient of wave energy in Gillfillan Lake, Nova Scotia. Can. J. Bot. 62:301–309. ——— , and Constabel, P. 1986. Germination of ten shoreline plants in relation to seed size, soil particle size and water level: an experimental study. J. Ecol. 74:133–141. ——— , and Ellis, T.H. 1984. Seedling recruitment of 11 wetland plant species along a water level gradient: shared or distinct responses? Can. J. Bot. 63:1876–1879. ——— , and Reznicek, A.A. 1986. Great Lakes vegetation dynamics: the role of fluctuating water levels and buried seeds. J. Great Lakes Res. 12:25–36. Lyon, J.G., Drobney, R.D., and Olson, C.E. Jr. 1986. Effects of Lake Michigan water levels on wetland soil chemistry and distribution of plants in the Straits of Mackinac. J. Great Lakes Res. 12:175–183.
Minc, L.D. 1997a. Great Lakes coastal wetlands: an overview of abiotic factors affecting their distribution, form, and species composition. Michigan Natural Features Inventory Technical Report, Lansing, MI. ——— . 1997b. Vegetative response in Michigan’s Great Lakes marshes to Great Lakes water-level fluctuations. Michigan Natural Features Inventory Technical Report, Lansing, MI. ——— , and Albert, D.A. 1998. Great Lakes coastal wetlands: abiotic and floristic characterization. Michigan Natural Features Inventory Technical Report, Lansing, MI. U.S. Army Corps of Engineers. 1987. Great Lakes Water Level Facts. U.S. Army Corps of Engineers, Detroit District. van der Valk, A.G. 1981. Succession in wetlands: a Gleasonian approach. Ecology 62:688–696. Welling, C.H., Pederson, R. L., and van der Valk, A.G. 1988. Recruitment from the seed bank and the development of zonation of emergent vegetation during a drawdown in a prairie wetland. J. Ecol. 76:483–496. Zedler, P.H. 1981. Microdistribution of vernal pool plants of Kearny Mesa, San Diego County. In Vernal Pools and Intermittent Streams, eds. S. Jain and P. Moyle, pp. 185–197. Davis, CA: Institute of Ecology. Submitted: 16 April 2004 Accepted: 10 September 2005 Editorial handling: John Janssen