| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Ecology |
2Marine-Estuarine-Environmental Sciences Program, University of Maryland, College Park, Maryland 20742 USA; 3Department of Biological Resources Engineering, 1423 Animal Science Building, University of Maryland, College Park, Maryland 20742 USA
Received for publication November 19, 2003. Accepted for publication April 22, 2004.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Key Words: landscape ecology • microtopography • rarefaction curves • spatial variability • species richness estimators • tidal freshwater wetlands
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
; Poiani and Dixon, 1995
), prairie pothole wetlands (van der Valk and Davis, 1976
), and playa wetlands (Haukos and Smith, 1993
). Because seeds of most wetland species are dispersed via water (based on Appendix 1 of Middleton, 1999
), wetlands connected by surface water may exhibit similar seed banks or, if environmental gradients exist, spatial trends in seed bank composition due to differential conditions for germination and establishment (Grubb, 1977
; Harper, 1977
; Schupp, 1995
). Additionally, most investigations of wetland seed bank composition have been conducted on relatively small spatial scales, on the order of 10– 15 ha (Titus, 1991
; Haukos and Smith, 1993
; Baldwin and DeRico, 1999
) or even 1–2 ha (Poiani and Johnson, 1989
; Schneider, 1994
). Other studies have been conducted in larger wetland ecosystems, but with sampling limited to a small area (Hopkins and Parker, 1984
; Schneider and Sharitz, 1986
, 1988
; Leck and Simpson, 1987
; Brock and Rogers, 1998
; Baldwin et al., 2001
). While it is important to describe fine-scale pattern in seed banks and their relationship to vegetation, it is also possible that small-scale studies do not adequately describe the diversity and composition of seed banks of a particular wetland type.
While little is known about landscape variation in the seed banks of wetland complexes, smaller scale studies have shown that seed bank composition can vary along elevation gradients, between elevations within sites due to microtopographic variation, and between plant associations or communities. For example, seed banks have been found to vary along elevation gradients within tidal freshwater marshes (Parker and Leck, 1985
) and nontidal swamps (Middleton, 2000
), and the density of woody seeds has been reported to be higher adjacent to logs (Schneider and Sharitz, 1988
) and on stumps (Titus, 1991
) as compared to open sites in floodplain forests. Dominant species composition was similar within but varied between vegetation associations across four herbaceous wetlands (Kirkman and Sharitz, 1994
), and herbaceous seed density and woody species composition differed between two vegetation associations in a floodplain swamp (Schneider and Sharitz, 1986
). Distinct seed bank species composition was seen within three vegetation associations of a tidal freshwater wetland, i.e., herbaceous high marsh, cattail-dominated marsh, and shrub forest, reflecting the vegetation in each association (Leck and Simpson, 1987
). These studies highlight the importance of elevation and vegetation structure on seed bank composition on relatively small spatial scales. However, they do not address seed bank variation within or between habitat types across a landscape of hydrologically connected wetlands.
We studied landscape variation in seed banks in tidal freshwater wetlands associated with the Nanticoke River and its tributaries Marshyhope Creek and Broad Creek, located on the Delmarva Peninsula in Maryland and Delaware, USA, in the area surrounding Sharptown, Maryland (38°32'33'' N, 75°43'11'' W). Tidal freshwater wetlands are ecosystems found worldwide along major coastal rivers in regions with low topographic relief. In the USA, they occur along the Atlantic and Gulf of Mexico coasts (Odum, 1988
; Mitsch and Gosselink, 2000
). These wetlands are located close enough to the coast to experience diurnal tides but receive sufficient freshwater flows to generally keep salinities less than 0.5 parts per thousand (ppt) (Cowardin et al., 1979
; Simpson et al., 1983
).
The Nanticoke River is a major tributary of the Chesapeake Bay, with a watershed encompassing approximately 86 200 ha in Maryland and 127 500 ha in Delaware, including approximately 2000 ha of freshwater tidal marsh, 2800 ha of freshwater tidal swamp forest, and 360 ha of freshwater tidal shrub swamp (McCormick and Somes, 1982
). The study was conducted in wetlands dominated by herbaceous and woody vegetation (i.e., marsh and swamp, respectively) across a landscape scale. A rectangle circumscribing our sample points encompasses an area of landscape of approximately 27 500 ha, with sampling points extending across approximately 48 km of river. The woody vegetation of the swamps promotes the formation of a hummock–hollow microtopography (areas of low and high elevation). Hummocks are mounds typically initiated as fallen trunks or branches covered with moss and rising as much as 20 cm above the swamp floor. The low-lying areas between the hummocks are referred to as hollows (Rheinhardt, 1992
; Ehrenfeld, 1995
).
The objectives of our study were to investigate (1) variation in composition of seed and spore banks (hereafter referred to as the seed bank) of tidal freshwater wetlands over a spatially broad and environmentally heterogeneous area and (2) variation in seed banks between marsh, swamp hollow, and swamp hummock habitats.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
). According to the Maryland Departments of Natural Resources and the Environment, agricultural land comprises approximately 38% of the watershed's total area (2002), and runoff from farm fields containing nutrients and sediment has been identified by The Nature Conservancy as the primary threat to the conservation targets in the watershed with freshwater intertidal wetlands judged to be the under the greatest stress (The Nature Conservancy, 1998
). Vegetation of the marshes consists primarily of annual and perennial emergent herbaceous vegetation typical of tidal freshwater marshes of the Atlantic Coast (Simpson et al., 1983
; Odum et al., 1984
; Tiner, 1993
). The swamps have floristically rich vegetation similar to the Fraxinus sp.-Nyssa biflora tidal freshwater swamps along the Pamunkey River in Virginia, USA, described by Rheinhardt (1992)
. Hummocks in many of the Nanticoke swamps are covered with a diverse assemblage of forbs, graminoids, and ferns. The hollows, in contrast, range from barren to having vegetation similar to that of the tidal marshes.
Mean temperatures ranged from –2°–9°C in January to 20°–32°C in July, with a mean annual precipitation of 1057 mm. (Climate data are from the National Climatic Data Center, website: http://lwf.ncdc.noaa.gov/oa/ ncdc.html, for Vienna, Maryland, approximately 5.15 km south of our southernmost sampling station; temperatures were averaged over 1991–2001 and precipitation over 1993–2001.) The mean tidal range was 0.73 m. Flooding for 0–4 h per tidal cycle, up to 30 cm deep, typically occurs in high marshes (Simpson et al., 1983
; Mitsch and Gosselink, 2000
). Our observations are consistent with these values. (Mean tidal range was determined from the difference in mean higher-high water [MHHW] and mean lower-low water [MLLW] at Sharptown, predicted by the National Oceanic and Atmospheric Administration [NOAA] Center for Operational Oceanographic Products and Services, website: http://www.co-ops.nos.noaa.gov/prodinfo.html. Precipitation and wind affect the water levels in these systems; therefore, actual tidal amplitudes and water levels often differ substantially from predicted values.) Southern areas of this watershed are susceptible to elevated salinity; we measured salinities of up to 7.0 ppt in the most southern areas of the typically tidal freshwater regions of the Nanticoke watershed during a major summer drought of 2002, while the upstream reaches of our study area continued to show typical salinities of 0.1–0.3 ppt (A. H. Baldwin, unpublished data).
Seed bank composition
This seed bank study is part of a larger project examining the influence of nutrients and sediment on plant species composition and diversity in the Nanticoke watershed. Surface soil samples were collected for seed bank analysis from 48 vegetation monitoring plots, 24 in marshes and 24 in swamps, set up as part of this larger project and located across the tidal freshwater reaches of the watershed. Two samples were collected at each swamp site, one from hollows within the vegetation plot and one from hummocks haphazardly chosen within a few meters of the plot. Thus, three habitats, marsh, swamp hollow, and swamp hummock, were sampled. Samples were collected during 17–20 March 2001. The temperature at this time ranged from –2.2°C at night to 12.8°C during the day (National Climatic Data Center, website: http://lwf.ncdc.noaa.gov/oa/ncdc.html).
Each sample was a composite of five 5 cm deep surface soil cores randomly taken within a 1 x 2 m herbaceous vegetation monitoring plot established at each site. An aluminum soil corer was used (4.8 cm diameter x 5 cm long), resulting in a sampling surface area of 90.4 cm2 (volume = 452.16 cm3).
Samples were stored at ambient temperature in coolers for transport back to the University of Maryland and stored in an environmental chamber at a temperature of 4–5°C until processing, which began on 5 April 2001. Despite an early sampling date, some seeds sprouted before the samples could be processed and transferred to the greenhouse. The majority of the seedlings were transient species of summer annuals, whose seeds only survive one germination season (Baskin and Baskin, 1998
), mostly Polygonum arifolium and Impatiens capensis. (At the time of sample collection, species of Polygonum and Impatiens were identified in plots of both the marshes and swamps, indicating that seed germination in the field had already begun.)
Processing of the seed bank samples consisted of pooling the five cores from each plot and removing roots and coarse organic matter from the soil after rinsing with water. The pooled sample was then split in half and one of the halves discarded. The second half was retained, yielding a sample collected over an area of 45.2 cm2. This sample was placed on a 3.5 cm thick bed of potting soil in aluminum pans (length x width x depth = 15.6 x 21.8 x 5.1 cm) with perforations of 2–3 mm diameter on the bottom (12 holes in three rows of four) and at the soil line for drainage. Pans were placed on a bench at the University of Maryland greenhouses and exposed to moist but nonflooded conditions under a misting sprayer at a temperature between 20° and 30°C from 24 April through 15 November 2001. Samples were checked once per week for seedling emergence. Seedlings and ferns were either pulled after identification or transplanted and grown to maturity for taxonomic identification. This technique is known as the emergence method and has been used elsewhere (e.g., van der Valk and Davis, 1978
; Kirkman and Sharitz, 1994
; Leck and Simpson, 1995
; Baldwin et al., 1996
) as an accurate measure of identifying viable seeds in wetland soil (Poiani and Johnson, 1988
; Gross, 1990
).
Data analysis
To graphically illustrate landscape-level variation in taxa density and seedling density in each habitat, maps were generated with a population-mapping program, POPulation MAPper (Molina, 2002
). The base map was created with data downloaded from the Fish and Wildlife Service National Wetlands Inventory website (http://www.nwi.fws.gov) and manipulated in AutoCAD 2000 (Autodesk, San Rafael, California, USA). Coefficients of variation of the density of seeds within each habitat were calculated to provide a basis for comparing variability in our study with that observed in other published studies. Additionally, a Pearson's correlation coefficient between taxa richness and log-transformed (log10(x + 1)) number of seedlings emerging was determined for each habitat, and regressions were conducted by habitat of taxa density and log-transformed (log10(x + 1)) number of seedlings emerging on the distance of sampling locations upstream from our southern-most sampling point in the watershed. Distance upstream was determined along transects of points along the approximate center of the Nanticoke River and its tributaries Marshyhope Creek and Broad Creek using MapSource version 3.02 software (Garmin, Olathe, Kansas, USA). A critical value of P = 0.05 was used in assigning statistical significance; P values between 0.05 and 0.1 were considered marginally significant. All analyses were conducted using the SAS system version 8.2 for Windows (SAS Institute, Cary, North Carolina, USA).
To summarize composition data, the arithmetic mean number of seedlings emerging for each species, standard error, and maximum values from the 24 samples collected from each habitat were determined (minimum was zero for all species). For all species combined, mean seedling emergence, mean taxa density, and the taxa richness of each habitat were also determined. The term "taxa density" is used to define the number of taxa emerging/surface area of sample (45.2 cm2) while "taxa richness" is used here to define the total number of taxa present in a study or community (see Gotelli and Colwell, 2001
).
The Chao 2 nonparametric estimator of species richness was used to estimate the total taxa richness for each habitat separately and for all habitats combined based on the number of uniques (i.e., taxa that occur in only one sample) and duplicates (i.e., taxa that occur in only two samples [SChao2 = Sobs + (Q12 ÷ 2Q2), where Sobs = total number of species observed in all samples pooled, Q1 = number of uniques, and Q2 = number of duplicates]). The computer program EstimateS version 5 for Windows (Colwell, 1997
) was used to calculate the estimators and their associated variances.
Species rarefaction curves were used to compare taxa richness across habitats, adjusting for differences in emerging seedling density. Rarefaction curves resample from a pooled group of individuals or samples to plot the increase in number of species against an increasing number of individuals. As the number of individuals included in the sample increases, the number of taxa recorded is also likely to increase. Rarefaction curves, unlike species area curves, allow comparisons of richness at a common number of individuals rather than at a common sample area, which would not account for differences in numbers of species resulting from variance in numbers of individuals (Gotelli and Colwell, 2001
). In the Nanticoke seed bank, the number of individual seedlings emerging varied roughly two-fold between the swamp hollows and other habitats, suggesting a need for analysis with rarefaction curves. Two rarefaction curves were produced: (1) a sample-based curve, with an increasing number of species in pooled samples plotted against an increasing number of individuals (Sobs vs. individuals); and (2) an individual-based sampling curve, with the increasing number of species of pooled individuals plotted against the increasing number of individuals (Coleman method, Cole vs. individuals). EstimateS version 5 for Windows (Colwell, 1997
) was used to analyze data for species rarefaction curves. The difference in the number of species between these curves at any constant level of individuals provides a rough measure of the patchiness, or the clumping of individuals of a species, in a habitat or location (Gotelli and Colwell, 2001
).
Additionally, the number of occurrences out of a possible 24 samples was determined for species that occurred in three or more samples collected in each habitat. A one-way analysis of variance (ANOVA) was conducted to examine variation in the number of seedlings emerging of all taxa combined and taxa density across habitats. Taxa density data required no transformation to meet assumptions of homogeneity of variance and normality, as tested by the Shapiro-Wilk statistic, box plots, and normal probability plots; the number of emerging seedlings was log-transformed (log10(x + 1)) to meet assumptions. Also, Sørenson quotients of similarity, qs (Sørenson, 1948
), were calculated to investigate the similarity of seed bank composition between habitats (qs = 2c ÷ (a + b); where c = number of common species, a = number of species in first group, and b = number of species in second group). Data on seedlings that germinated before transfer to the sample pans were included for calculation of the similarity index but no other calculations, due to the presence of unidentifiable seedlings.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
|
|
Taxa richness
A total of 57 taxa emerged from the 72 seed bank samples (Table 1). Many species were found infrequently; 17 species occurred in only one sample, and 14 species occurred in only two samples. The Chao 2 nonparametric estimator of taxa richness estimated the taxa richness of the seed bank, i.e., the total number of species present from all habitats combined, to be 67.3 ± 6.6 (mean ± SD). Twenty taxa found in the seed bank were not observed in the standing vegetation (Table 1).
|
Mean taxa density (i.e., species per sample) was about three in each habitat, while the mean number of seedlings emerging from each sample was about twice as high for the hollows as the hummocks and marshes, which were similar (Table 1). Taxa density and log number of seedlings emerging did not vary significantly between habitats (F2,69 = 0.04, P = 0.97 and F2,69 = 0.13, P = 0.88, respectively).
While taxa density varied little between habitats, differences emerge when we compare variability between habitats using rarefaction curves (Fig. 3). The swamp hollows had many more individuals (428) than the swamp hummocks (196) or marshes (246). The vertical difference between the sample-based and individual-based species rarefaction curves allow for comparison of the relative patchiness of the habitats (with patchiness defined as clumps of individuals of different species in different samples, as opposed to a homogeneous composition of similar species in all samples). In rarefaction graphs, patchier habitats show greater vertical distance between the sample and individual-based curves; that is, species accumulate more quickly by randomly sampling individuals than by sampling plots (Gotelli and Colwell, 2001
). Based on the rarefaction curves (Fig. 3), hummocks had a less patchy community than marshes or hollows. Swamp hollows and marshes had higher measures of heterogeneity (3.7 and 3.9, respectively) than swamp hummocks (1.1) (calculated as the difference in number of species between sample- and individual-based rarefaction curves at 100 individuals). At 156 individuals, swamp hollows were patchier than the marshes (3.0 vs. 2.0), and the swamp hummocks were still the most homogeneous (0.2). Swamp hollows had lower taxa richness at a common number of individuals than the other habitats, while the taxa richness of marshes and swamp hummocks were similar (Table 2). These results, in conjunction with the higher percentage of estimated total taxa richness sampled in the swamp hummocks, indicate that seed bank taxa composition within the swamp hummocks is more spatially uniform than that of either marshes or swamp hollows.
|
|
Taxa composition
Species composition varied across habitats, with three distinct seed bank communities emerging with some overlap in species (Table 1, Fig. 4). Of the most common species, Leersia oryzoides and Mikania scandens were the only two species to appear in all three habitats; however, Leersia oryzoides was more frequent in the marsh habitat than in either swamp habitat. Mikania scandens dominated the swamp hollows but appeared less frequently in the other habitats. Boehmeria cylindrica was more common in marshes and swamp hummocks and Typha sp. in swamp hollows and marshes. Half of the common species in the marshes occurred frequently only in that habitat (Amaranthus cannabinus, Peltandra virginica, Ludwigia palustris, and Zizania aquatica). The taxa composition of the two swamp habitats showed more overlap with each other than either did with the marsh: in addition to Mikania and Leersia, Cyperaceae sp., Glyceria striata, and Thelypteris palustris were common taxa in both swamp habitats. Some differences in composition did exist between the hummocks and hollows: Nyssa biflora and Viola cucullata, two species found primarily on hummocks in the vegetation, were also dominant in the seed bank of hummocks but not hollows. Bidens laevis, Juncus effusus, and Polygonum punctatum were most common in the swamp hollow seed bank. The greater overlap in taxa composition between the swamp hollows and hummocks is also reflected in higher Sørenson similarity quotient values (qs) for the hollow–hummock comparison (qs = 0.62), than for marsh–hollow (0.45) or marsh–hummock (0.42) comparisons.
|
). Atlantic white cedar trees occur sporadically in the tidal freshwater swamps of the Nanticoke watershed (A. H. Baldwin, unpublished data). With the exception of Murdannia keisak, there was an absence of non-native species in the seed bank. |
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
). Variation in the percent cover of understory vegetation within the swamp hollows may in turn be due to changes in forest canopy cover along a salinity gradient that fluctuates between seasons and wet and dry years and is sometimes not detectable. Periodically higher levels of salinity in the downstream reaches of our study area may stress and stunt woody vegetation, allowing for more light to reach understory vegetation and, through seed production, lead to development of a richer, denser seed bank. Levels of photosynthetically active radiation (400–700 nm) in July of 2000 below the forest canopy at the 24 swamp vegetation monitoring plots where seed bank samples were collected decreased significantly with increasing distance upstream (r2 = 0.35, P = 0.0023; A. H. Baldwin, unpublished data). Similarly, light levels measured in swamp hollows at ground level over the growing season in 2002 were significantly higher in a swamp in the lower vs. a swamp in the upper reaches of the watershed (Peterson, 2003
).
Typically, seed bank studies report large variances in seed density due to uneven horizontal distribution of seeds within a habitat (Baskin and Baskin, 1998
). Spatial variation in the density of seeds might be expected to increase as the scope of the study increases across the landscape; variation in seed density across the landscape in the Nanticoke watershed does appear to be substantially higher than variation seen in spatially smaller studies. However, variation in seed bank densities across the landscape depends in part on habitat type, as reflected in the lower coefficient of variation in the Nanticoke swamp hummocks (136%) as compared to the swamp hollows (175%) and marshes (180%). Other studies of seed banks of tidal freshwater marshes also have shown coefficients of variation of lower magnitude than those we found across the tidal freshwater portion of the Nanticoke watershed. In a 10-yr study in one vegetation association of a tidal freshwater marsh, Leck and Simpson (1995)
found lower coefficients of variation in seed banks, ranging from 36 to 89%, than we did. Similarly, a study of a tidal freshwater marsh on the Patuxent River in Maryland in one vegetation association showed a coefficient of variation of 61% in seed banks (Baldwin et al., 2001
). In another tidal freshwater wetland study, the seed banks of three differing vegetation associations had coefficients of variation of 55–117% (Leck and Simpson, 1987
), suggesting that greater variation can be expected as the number of vegetation associations sampled increases.
In studies in other types of wetlands, coefficients of variation ranged from less to greater than those we observed in the Nanticoke study. In eight small playa wetlands (averaging 13.1 ha) studied by Haukos and Smith (1993)
, coefficients ranged from 66 to 131%. Vegetation associations, excluding open water sites, in non-tidal marshes showed ranges of coefficients of variation from 85 to 121% in Ogden Bay Waterfowl Management Area, Utah, USA (Smith and Kadlec, 1983
) and 26–55% in Eagle Lake, Iowa, USA (van der Valk and Davis, 1978
). The woody seed bank community varied more (114 and 209%) than the herbaceous community (69 and 80%) in samples from two vegetation associations of a river floodplain swamp (Schneider and Sharitz, 1986
). These studies support our finding that seed banks are patchier in some plant associations than in others.
Taxa richness
The taxa richness we observed within each habitat across a landscape scale in the Nanticoke watershed appears higher than other single-habitat seed bank studies conducted over smaller spatial scales. Although comparisons in taxa richness should be made cautiously because of differences in sampling, in one type of vegetation association within a tidal freshwater marsh, Baldwin et al. (2001)
found 18 taxa. In another tidal freshwater marsh, within one vegetation association, Leck and Simpson (1995)
found between eight and 16 taxa annually over a 10-yr period. In contrast, we observed much higher taxa richness (34) in the marshes of the Nanticoke watershed.
As in our study, seed bank studies sampling multiple habitats not surprisingly revealed higher numbers of taxa than studies focusing on one vegetation association; a study of a tidal freshwater marsh with six different vegetation associations showed an overall taxa richness of 52, comparable to the 57 we found in the Nanticoke; however, fewer taxa, 12–26, were found within each association (Leck and Graveline, 1979
). Poiani and Dixon (1995)
found 69 species over seven herbaceous, shrubby, or forested Carolina bays, with nine vegetation zones identified among herbaceous and shrubby bays; 9–26 species were recorded in each bay/vegetation zone combination.
Taxa composition
Variable environmental conditions between marshes and swamps, as well as heterogeneous microtopography, such as that found between swamp hollow and hummock communities of the Nanticoke watershed, are likely responsible for the variation in taxa composition we observed. Distinct communities emerged in each habitat. Only two of the commonly occurring species were seen in all three habitats, and nine of 16 common species occurred in only one habitat. Additionally, taxa density and density of emerging seedlings varied between plots, even between swamp hollow and swamp hummock samples taken from the same location.
It has long been known that microsite variation influences the distribution of species and the diversity of communities (Harper et al., 1965
), and field studies have shown differences between vegetation associations in swamps and marshes. For example, herbaceous seed density and woody species composition varied between two vegetation associations in a floodplain swamp (Schneider and Sharitz, 1986
). Across four isolated herbaceous Carolina bays, diversity, richness, and seedling density were found to be similar across vegetation associations, but the dominant species composition varied (Kirkman and Sharitz, 1994
). Light levels at the soil surface vary between the marsh and swamp communities, which may affect recruitment from the seed bank (Galinato and van der Valk, 1986
); seed bank composition was observed to vary between marsh and shrub forest vegetation associations within a tidal freshwater wetland (Leck and Simpson, 1987
). Similarly, seed bank composition of Carolina bays varied between associations, with seed banks of forested bays showing greater distinctions from herbaceous and shrubby bays, which were more similar to each other (Poiani and Dixon, 1995
).
Microtopographic variation has been shown to affect species composition and abundance in experimental mesocosms (Vivian-Smith, 1997
), in communities of adult plants (Beatty, 1984
; Paratley and Fahey, 1986
; Rheinhardt, 1992
), and in the distribution of woody seedlings (Huenneke and Sharitz, 1986
; Titus, 1990
). Additionally, significant variation in species composition was found along an elevation gradient in tidal freshwater marshes (Parker and Leck, 1985
) and in the woody seed bank of a forested wetland (Middleton, 2000
). In other forested wetlands, significantly higher seed densities of woody species were found in microsites that could trap seeds, such as stumps (Titus, 1991
) or logs (Schneider and Sharitz, 1988
), and lower densities were found in open sites. In contrast, we found that higher seedling densities emerged from samples collected from swamp hollows than from hummocks.
In summary, wetland seed banks exhibited a complex pattern across the tidal freshwater landscape of the Nanticoke watershed. Taxa composition varied with respect to the habitat sampled, and trends in taxa and emerging seedling densities were observed within habitats moving upstream along tributaries in the watershed, possibly due to the effects of a periodic salinity gradient on the structure of standing vegetation. The coefficients of variation of seed bank densities in this study are higher than studies of tidal freshwater marshes conducted on smaller spatial scales. We attribute this variation to sampling a heterogeneous landscape over a large spatial scale. However, because we did not investigate variability within smaller areas, no assumptions should be made on the variability of the seed bank within a site. Further research is needed at the Nanticoke and elsewhere to describe variation in the composition of the seed bank on both small and large spatial scales.
|
FOOTNOTES |
|---|
4 Present address: Penn State Institutes of the Environment, The Pennsylvania State University, University Park, Pennsylvania 16802 USA
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Baldwin A. H. M. S. Egnotovich E. Clarke 2001 Hydrologic change and vegetation of tidal freshwater marshes: field, greenhouse, and seed-bank experiments. Wetlands 21: 519-531
Baldwin A. H. K. L. McKee I. A. Mendlessohn 1996 The influence of vegetation, salinity, and inundation on seed banks of oligohaline coastal marshes. American Journal of Botany 83: 470-479[CrossRef][ISI]
Baskin C. C. J. M. Baskin 1998 Seeds: ecology, biogeography, and evolution of dormancy and germination. Academic Press, San Diego, California, USA
Beatty S. W. 1984 Influence of microtopography and canopy species on spatial patterns of forest understory plants. Ecology 65: 1406-1419[CrossRef][ISI]
Brock M. A. K. H. Rogers 1998 The regeneration potential of the seed bank of an ephemeral floodplain in South Africa. Aquatic Botany 61: 123-135[CrossRef][ISI]
Colwell R. K. 1997 EstimateS: statistical estimation of species richness and shared species from samples. Version 5. User's guide and application available at website: http://viceroy.eeb.uconn.edu/estimates
Cowardin L. M. V. Carter F. C. Golet E. T. LaRoe 1979 Classification of wetlands and deepwater habitats of the United States. FWS/ OBS-79/31. U.S. Fish and Wildlife Service, Washington, D.C., USA
Ehrenfeld J. G. 1995 Microsite differences in surface substrate characteristics in Chamaecyparis swamps of the New Jersey Pinelands. Wetlands 15: 183-189[ISI]
Galinato M. I. A. G. van der Valk 1986 Seed germination traits of annuals and emergents recruited during drawdowns in the Delta Marsh, Manitoba, Canada. Aquatic Botany 26: 89-102
Gotelli N. J. R. K. Colwell 2001 Quantifying biodiversity: procedures and pitfalls in the measurement and comparison of species richness. Ecology Letters 4: 379-391[CrossRef][ISI]
Gross K. L. 1990 A comparison of methods for estimating seed numbers in the soil. Journal of Ecology 78: 1079-1093[CrossRef]
Grubb P. J. 1977 The maintenance of species-richness in plant communities: the importance of the regeneration niche. Biological Reviews of the Cambridge Philosophical Society 52: 107-145[CrossRef]
Harper J. L. 1977 Population biology of plants. Academic Press, New York, New York, USA
Harper J. L. J. T. Williams G. R. Sagar 1965 The heterogeneity of soil surfaces and its role in determining the establishment of plants from seed. Journal of Ecology 53: 273-286[CrossRef]
Haukos D. A. L. M. Smith 1993 Seed-bank composition and predictive ability of field vegetation in playa lakes. Wetlands 13: 32-40[ISI]
Hopkins D. R. V. T. Parker 1984 A study of the seed bank of a salt marsh in northern San Francisco Bay. American Journal of Botany 71: 348-355[CrossRef][ISI]
Huenneke L. F. R. R. Sharitz 1986 Microsite abundance and distribution of woody seedlings in a South Carolina cypress–tupelo swamp. American Naturalist 15: 329-335
Kirkman L. K. R. R. Sharitz 1994 Vegetation disturbance and maintenance of diversity in intermittently flooded Carolina bays in South Carolina. Ecological Applications 4: 177-188
Leck M. A. K. J. Graveline 1979 The seed bank of a freshwater tidal marsh. American Journal of Botany 66: 1006-1015[CrossRef][ISI]
Leck M. A. R. L. Simpson 1987 Seed bank of a freshwater tidal wetland: turnover and relationship of vegetation change. American Journal of Botany 74: 360-370[CrossRef][ISI]
Leck M. A. R. L. Simpson 1995 Ten year seed bank and vegetation dynamics of a tidal freshwater marsh. American Journal of Botany 82: 1547-1557[CrossRef][ISI]
Maryland Department of Natural Resources and Maryland Department of the Environment. 2002 Maryland's Surf Your Watershed. http://www.dnr.state.md.us/watersheds/surf. Maryland Department of Natural Resources, Annapolis, Maryland, USA
Maryland Wildlife and Heritage Division. 2002 Rare, threatened, and endangered plants of Maryland. http://dnrweb.dnr.state.md.us/download/ rteplants.pdf. Maryland Department of Natural Resources, Annapolis, Maryland, USA
McCormick J. H. A. Somes Jr 1982 The coastal wetlands of Maryland. Maryland Department of Natural Resources, Coastal Zone Management, Jack McCormick & Associates, Inc., Chevy Chase, Maryland, USA
Middleton B. 1999 Wetland restoration, flood pulsing, and disturbance dynamics. John Wiley & Sons, New York, New York, USA
Middleton B. 2000 Hydrochory, seed banks, and regeneration dynamics along the landscape boundaries of a forested wetland. Plant Ecology 146: 169-184[ISI]
Mitsch W. J. J. G. Gosselink 2000 Wetlands. John Wiley & Sons, New York, New York, USA
Molina J. 2002 POPulation MAPper. Documentation and utility available at website: http://popmap.sourceforge.net/
Odum W. E. 1988 Comparative ecology of tidal freshwater and salt marshes. Annual Reviews of Ecology and Systematics 19: 147-176
Odum W. E. T. J. Smith III J. K. Hoover C. C. McIvor 1984 The ecology of tidal freshwater marshes of the United States east coast: a community profile. FWS/OBS-83/17. U.S. Fish and Wildlife Service, Washington, D.C., USA
Paratley R. D. T. J. Fahey 1986 Vegetation–environment relations in a conifer swamp in central New York. Bulletin of the Torrey Botanical Club 113: 357-371[CrossRef][ISI]
Parker V. T. M. A. Leck 1985 Relationship of seed bank to plant distribution patterns in a freshwater tidal wetland. American Journal of Botany 72: 161-174[CrossRef][ISI]
Peterson J. P. 2003 Factors regulating expression of seed banks in vegetation of tidal freshwater wetlands. M.S. thesis, University of Maryland, College Park, Maryland, USA
Poiani K. A. P. M. Dixon 1995 Seed banks of Carolina bays: potential contributions from surrounding landscape vegetation. American Midland Naturalist 134: 140-154[CrossRef][ISI]
Poiani K. A. W. C. Johnson 1988 Evaluation of the emergence method in estimating seed bank composition of prairie wetlands. Aquatic Botany 32: 91-97
Poiani K. A. W. C. Johnson 1989 Effect of hydroperiod on seed-bank composition in semipermanent prairie wetlands. Canadian Journal of Botany 67: 856-864[ISI]
Rheinhardt R. 1992 A multivariate analysis of vegetation patterns in tidal freshwater swamps of lower Chesapeake Bay, U.S.A. Bulletin of the Torrey Botanical Club 119: 192-207[CrossRef][ISI]
Schneider R. 1994 The role of hydrologic regime in maintaining rare plant communities of New York's coastal plain pondshores. Biological Conservation 68: 253-260[CrossRef][ISI]
Schneider R. L. R. R. Sharitz 1986 Seed bank dynamics in a southeastern riverine swamp. American Journal of Botany 73: 1022-1030[CrossRef][ISI]
Schneider R. L. R. R. Sharitz 1988 Hydrochory and regeneration in a bald cypress–water tupelo swamp forest. Ecology 69: 1055-1063[CrossRef][ISI]
Schupp E. W. 1995 Seed-seedling conflicts, habitat choice, and patterns of plant recruitment. American Journal of Botany 82: 399-409[CrossRef][ISI]
Simpson R. L. R. E. Good M. A. Leck D. F. Whigham 1983 The ecology of freshwater tidal wetlands. BioScience 33: 255-259[CrossRef][ISI]
Smith L. M. J. A. Kadlec 1983 Seed banks and their role during drawdown of a North American marsh. Journal of Ecology 20: 673-684
Sørenson T. 1948 A method of establishing groups of equal amplitude in plant sociology based on similarity of species content and its application to analyses of the vegetation on Danish commons. Det Kongelige DanskeVidenskabernes Selskab, Biologiske Skrifter 5: 1-34
The Nature Conservancy. 1998 Nanticoke river bioreserve strategic plan. Maryland/District of Columbia and Delaware Field Offices, Bethesda, Maryland, USA
Tiner R. 1993 Field guide to coastal wetland plants of the southeastern United States. University of Massachusetts Press, Amherst, Massachusetts, USA
Titus J. H. 1990 Microtopography and woody plant regeneration in a hardwood floodplain swamp in Florida. Buletin of the Torrey Botanical Club 117: 429-437[CrossRef]
Titus J. H. 1991 Seed bank of a hardwood floodplain swamp in Florida. Castanea 56: 117-127
van der Valk A. G. C. B. Davis 1976 The seed banks of prairie glacial marshes. Canadian Journal of Botany 54: 1832-1838[ISI]
van der Valk A. G. C. B. Davis 1978 Role of the seed bank in the vegetative dynamics of prairie glacial marshes. Ecology 59: 322-335[CrossRef][ISI]
Vivian-Smith G. 1997 Microtopographic heterogeneity and floristic diversity in experimental wetland communities. Journal of Ecology 85: 71-82[CrossRef]
This article has been cited by other articles:
|
|
 |
|
  R. Shimamura, N. Kachi, H. Kudoh, and D. F. Whigham Hydrochory as a determinant of genetic distribution of seeds within Hibiscus moscheutos (Malvaceae) populations Am. J. Botany, July 1, 2007; 94(7): 1137 - 1145. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
