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Seed banks of Carolina bays: potential contributions from surrounding landscape vegetation.

INTRODUCTION

Carolina bays are one of the major wetland types in the south-eastern United States, consisting of shallow depressions located throughout the Atlantic Coastal Plain Province (Sheritz and Gibbons, 1982). In the upper Coastal Plain, bays are usually elliptical, with a sand rim and an impervious clay layer beneath the soil surface (Sharitz and Gibbons, 1982). Most bays are temporary wetlands, filling and drying in response to seasonal and annual changes in temperature and precipitation, although a few bays are permanent lakes. These wetlands are highly variable in size, substrate conditions, land use history, and hydroperiod, and do not have a single characteristic vegetation. Bays dominated by open water, herbaceous vegetation, shrubs or trees are all common. The gradual sloping contours that characterize Carolina bays result in hydrologic and soil moisture gradients across the basins which in turn cause discrete zones of vegetation (Sharitz and Gibbons, 1982). Vegetation zonation patterns in a bay are highly dynamic in response to climate, and a particular zone may shift position or change composition over time (Kirkman, 1992).

Carolina bays throughout the region are typically surrounded by upland areas used for agriculture or managed for timber production. On the Savannah River Site, bays are scattered patches in a mosaic of loblolly pine plantations and recent clearcuts, and upland weedy species and pines are common in bays during extended drawdown periods. Current management practice is to leave a 20-m-wide buffer of trees between a bay and clearcuts to minimize potential effects of clearcutting on the wetlands.

Little is known about the seed banks of Carolina bays and their interactions with wetland and surrounding nonwetland vegetation. Kirkman and Sharitz (1994) suggest that the seed banks in a particular bay are quite diverse and are important in vegetation dynamics. They did not assess, however, differences among bays with different types of dominant vegetation nor examine the influence of the surrounding landscape on seed bank composition.

A basic tenet of landscape ecology is that the features surrounding an ecosystem influence its properties and composition (Forman and Godron, 1986; Turner, 1989). Input to a bay seed bank could occur both from different patches of wetland vegetation within the bay and from the surrounding upland vegetation. Most studies of wetland seed banks have focused on within-wetland processes and have quantified the resemblance of the seed bank to the local zone or whole-wetland vegetation (Leck, 1989). Contributions from vegetation of the surrounding landscape are poorly known. Only one nonwetland study specifically examined the effects of adjacent vegetation on seed bank composition (Hume and Archibold, 1986). Transport of weedy seeds occurred between fallow fields and the edges of adjacent agricultural fields in that study, but there was little transport of weedy seeds farther than 10 m from the edge (Hume and Archibold, 1986).

The objectives of our study were to compare seed bank composition in Carolina bays with different types of dominant vegetation (herbaceous, shrubby, forested) and to assess the potential contribution from vegetation in the surrounding landscape. Direct measurement of seed dispersal by wind and water is difficult, particularly separating seed input from surrounding vegetation versus input from the local seed rain. A variety of other techniques have been used to explore the influence of dispersal on vegetation patterns, including simulation models (Hanson et al., 1990), mapping established seedling and vegetation patterns (Johnson, 1988; Matlack, 1994), dispersal manipulation experiments (Bergelson et al., 1993), and occurrences of nonlocal species in the seed bank (Hume and Archibold, 1986). Because of the difficulty in directly quantifying seed input in a large number of bays surrounded by pine plantation and clearcuts, we chose to measure seed input from the surrounding vegetation indirectly as the number of upland weedy species in a bay seed bank. Generally, pine plantations have few herbaceous species in the understory, but clearcuts include many upland weedy species (Golley and Gentry, 1966; Workman and McLeod, 1990). We expected that the number of upland weedy species in the seed bank would be greater in bays close to clearcuts than in bays relatively far from clearcuts. Statistically significant results from such indirect methods do not necessarily prove that an upland species dispersed from an adjacent clearcut but they suggest that contributions to wetland seed banks from surrounding landscape vegetation are important and highlight a need for further study.

STUDY SITES

Seven bays on the Savannah River Site (Aiken and Barnwell counties, South Carolina) were chosen for evaluation ([ILLUSTRATION FOR FIGURE 1 OMITTED], Table 1). Three of the bays (Sarracenia, Dry and Woodward) were dominated by herbaceous vegetation, two by shrubs (Woods and Little Cypress) and two by trees (Buttress and Cypress). Forested bays had relatively homogeneous vegetation, but shrubby and herbaceous bays contained up to four distinct vegetation zones (Table 1). Zones were identified visually in the field and with the aid of unpublished vegetation maps for several of the bays. Different vegetation zones had distinctly different dominant species and were distinguished easily. Nine different vegetation zones occurred in the seven bays for a total of 19 distinct bay/zone combinations (Table 1). Existing vegetation in five bays (Cypress, Dry, Little Cypress, Sarracenia, and Woodward) was quantified in the summer of 1989 by Keough et al. (1990) just before seed bank sampling (March 1990). Seed bank samples were collected following a record ten-year drought (1980-1989), with precipitation during these years consistently below the preceding 30-year average (Kirkman, 1992).

The seven bays differed in their surrounding landscapes (Table 1). Three bays (Cypress, Woods and Sarracenia) were surrounded by pine plantation and were relatively distant from a clearcut. The nearest clearcut to these three bays was ca. 100 m. Two bays (Buttress and Little Cypress) were surrounded by pine plantation with the nearest clearcut between 50 and 75 m away. Two bays (Dry and Woodward) were adjacent to large clearcuts (i.e., 2025 m) with a small fringe of trees between the bay and the clearcut. Bays differed in the direction of the clearcut to the bay (Table 1), but there is no prevailing wind direction at the Savannah River Site (Zeigler et al., 1987).

METHODS

Seed bank. - In March 1990, six soil cores were extracted from each major vegetation zone in the seven Carolina bays for a total of 114 samples (6 replicates x 19 bay/zone combinations). [TABULAR DATA TABLE 1 OMITTED] Again, different vegetation zones were dominated by distinctly different species and were clearly distinguishable by eye. Core locations were arbitrarily chosen, but were dispersed throughout the bay. The total number of seed bank samples collected was set by greenhouse space limitations but appeared adequate for this study, particularly because we were interested in species presence/absence rather than seed densities. Cores were taken to a depth of five cm using a hand-made, 24-cm diameter, toothed, aluminum corer for optimal surface area. Each sample was thoroughly mixed and rhizomes, litter and other large debris were removed by hand.

Samples were stratified at 6 degrees C until July 1990 when they were placed in the greenhouse. The four month delay between collection of samples and placement in the greenhouse was unavoidable due to logistical considerations related to worker time and greenhouse space. Placement of samples in the greenhouse immediately following collection would have been optimal since field germination begins in March and April (although we did not notice significant germination in the field at the time of soil collection). Possible anaerobic conditions in plastic bags in storage and higher initial temperatures in July may have reduced seed viability and germination, respectively, but seemed unlikely given the large number of species and seeds that germinated.

Half of each sample was spread evenly across a sterilized vermiculite mixture in a 26 x 26 cm plastic flat. Flats were arranged in a randomized block design on greenhouse benches under natural light. Ten additional flats with only sterilized vermiculite mixture were distributed randomly among the experimental fiats to detect greenhouse contaminants. Temperature was maintained close to outdoor ambient conditions, although hotter temperatures could not be avoided. Flats were watered as needed to maintain moist conditions (not saturated). The remaining half of one of the six replicate samples collected in each bay/ zone combination (1 replicate x 19 bay/zone combinations) was inundated under about five cm of water to check for additional wetland species. Space limitations precluded inundating half of all six replicate samples.

Emerged seedlings were identified and removed periodically over the next 16 months to determine species presence/absence. No germination occurred in the control flats. Unknown specimens were transplanted to pots to encourage flowering. Nomenclature follows Radford et al. (1968).

The seedling emergence method has been shown to be reliable in assessing the readily germinable component of the seed bank in wetlands (Schneider and Sharitz, 1986; Poiani and Johnson, 1988). This method may underestimate composition, however, if environmental conditions are not suitable for germination of some species (Gross, 1990; Brown, 1992). In this study, presence of upland weedy species in the seed bank was particularly important. Weedy species were probably well estimated by the emergence method because their germination requirements are relatively general (Grime et al., 1981).

Data analysis. - Gradients in seed bank species composition among the 19 bay/zone combinations were described using DECORANA ordination (Hill and Gauch, 1980) for presence/absence data. Species lists from the six replicate samples from the moist-soil treatment and the one flooded replicate were pooled into a single species list for each bay/zone combination. Unidentified species were omitted since results were generally the same when all species were included (not presented). Analysis performed with unknowns added all unique unidentified species to the pooled species list for a bay/zone combination.

Upland weedy species (Table 2) were determined from habitat descriptions in Radford et al. (1968) and species lists in Workman and McLeod (1990). Species were checked for wetland indicator status in Reed (1988) if habitat characteristics were ambiguous. Upland weedy species were common on recent clearcuts but generally absent from mature pine plantation and Carolina bays (Workman and McLeod, 1990). The total number of weedy species in a seed bank was examined in this study because this measure best reflected seed immigration from adjacent habitats. Other measures such as percent weedy species (i.e., # weedy species/# total species) combine the effects of immigration with other ecological processes that influence total species richness, such as dominant vegetation type.

Distance from each bay to the nearest clearcut was determined from a 1989 aerial photograph of the Savannah River Site (1:24,000). Distance was measured from the closest edge of a clearcut to the closest edge of a bay. Thus, some samples in the bays actually were taken farther than the distance used in the analysis. Because seeds of most species were well distributed throughout a bay, results should not be confounded by these additional distances. Roadsides were a second type of disturbed habitat in the vicinity of the bays that could have contributed upland weedy species to wetland seed banks. Distance to the nearest road also was measured from the aerial photograph.

Differences in the number of upland weedy species among different vegetation zones (n = 9), among bays (n = 7), and the effects of distance to nearest clearcut and nearest road for each bay/zone combination (n = 19) were estimated using Poisson regression (Mead et al., 1993). Poisson regression models are similar to ANOVA/ANCOVA models except that the response variable (the number of upland weedy species) is assumed to have a Poisson distribution rather than a normal distribution. Poisson distributions usually are appropriate for count data like the number of upland weedy species (Mead et al., 1993). Preliminary analysis showed that the variance among replicate samples was approximately equal to the mean, which supports the use of a Poisson distribution. In these analyses, data from only the moist soil treatments were used and all replicates were maintained as individual observations to increase statistical power.

Statistical tests were based on the deviance, analogous to sums of squares in ANOVA. The computations can be illustrated using a test of the null hypothesis that the nine vegetation zones have the same number of weedy species. To test this, the deviances of two models are compared. One model allows the number of weedy species to differ among vegetation zones. The second model forces the number of weedy species to be the same in all vegetation zones. A large difference in deviance suggests that the null hypothesis, of no difference, is not appropriate. Differences in deviance were tested using Chi-square distributions (Mead et al., 1993). Calculations were performed with the S-plus statistical package (Chambers and Hastie, 1992). The analysis was repeated using the total number of species in the seed bank to examine landscape effects on species richness such as the position of a bay relative to other bays (i.e., distance to nearest bay).

RESULTS

A total of 69 species (16 unknown species) was found in the seed banks of the seven Carolina bays (Table 2). The number of species per bay ranged from 16 in Cypress Bay to 35 in Woods Bay (Table 1). The number of species per bay/zone combination ranged from 9 to 26 (Table 1). Bay size had no effect on the number of seed bank species. The Spearman rank correlation between bay size and number of species was -0.14 (n = 7, P = 0.70).

Seed bank composition was dominated by wetland-dependent perennial or annual grasses, sedges and forbs (Table 2). Most (90%) of the 69 species were herbaceous plants. Woody species (10%) occurred only in forest- and shrub-dominated bays (Table 2). Forested bays had the fewest species in the seed bank (16 and 21 species), herbaceous bays had intermediate numbers (23, 25 and 30 species), and shrubby bays contained the most species (34 and 35 species) (Table 1). Although several species were ubiquitous, many species were restricted to one or a few bays (Table 2). Nine upland weedy species were found and of those nine, three occurred quite frequently: Eupatorium capillifolium in six bays, Cyperus globulosus in five bays and Andropogon scoparius in four bays (Table 2). Most of the weedy species that occurred in the bay seed banks had wind dispersed seeds with prominent wings, pappus or awns, and are common in clearcut habitats on the Savannah River Site (Golley and Gentry, 1966).

For the five bays surveyed by Keough et al. (1990), less than half of the species found in the vegetation occurred in the seed bank (Table 3). Some of the dominant species in the field were found in the seed bank (e.g., Panicum hemitomon, P. wrightianum, Rhynchospora sp., Leersia hexandra, Nymphaea sp.), but other species common in the vegetation did not germinate from the seed bank (e.g., Cephalanthus occidentalis, Taxodium distichum). Species [TABULAR DATA FOR TABLE 2 OMITTED] richness of the seed banks in each of the five bays was greater than that of the vegetation (Table 3).

The first axis (DCA 1) in the DECORANA ordination of seed bank species composition reflected a gradient from herbaceous, to shrubby, to forested bays [ILLUSTRATION FOR FIGURE 2 OMITTED]. The two forested bays (Buttress and Cypress) had seed banks that were well separated in ordination space from herbaceous and shrubby bays, but the shrubby bays (Little Cypress and Woods) were only slightly different from the herbaceous bays (Dry, Sarracenia and Woodward). Different vegetation zones within a bay formed compact clusters [ILLUSTRATION FOR FIGURE 2 OMITTED]. Alternate clustering methods, e.g., UPGMA clustering of the matrix of Jaccard similarities, produced similar results (not shown). In addition, pair-wise Jaccard similarity comparisons indicated that the seed banks of zones within a bay (e.g., open water vs. Panicum, open water vs. Andropogon, open water vs. Rhynchospora, etc, for Sarracenia Bay; open water vs. Panicum, open water vs. Cephalanthus, etc, for Dry bay; etc) were relatively homogenous (mean = 0.50, SD = 0.11, n = 21).
TABLE 3. - Species richness of the seed bank and the vegetation in
five Carolina Bays included in both this study and that by Keough
et al. (1990)


                                Number of species in:


Bay                   Seed bank       Vegetation       Both


Cypress                   21              13             3
Dry                       23              18             6
Little Cypress            34              15             6
Sarracenia                25              18             9
Woodward                  30              19             9


The number of upland weedy species in a single soil core varied between 0 and 4 (0 to 44% of the total number of species found in the soil core). The average number of upland weedy species in a bay/zone combination varied between 0 and 2.33 (0 to 30% of the average total number of species) [ILLUSTRATION FOR FIGURE 3 OMITTED]. There were significant differences in the number of weedy species among vegetation zones (Table 4, Test 1) and among bays (Table 4, Test 4), but there was no significant interaction between bays and vegetation zones (Table 4, Test 5). Distance to the nearest clearcut had a significant effect on the number of weedy species (Table 4, Test 2), but distance to the nearest road did not (Table 4, Test 3). Once distance to nearest clearcut was accounted for, there was little effect of bays on the number of weedy species (Table 4, Test 6). The number of upland weedy species in a bay seed bank was best estimated by [[Lambda].sub.ij] = [V.sub.i] exp(-0.0073[D.sub.j]) where [[Lambda].sub.ij] was the average number of weedy species found in bay j and vegetation zone i, [V.sub.i] was the intercept for vegetation zone i, and [D.sub.j] was the distance from bay j to the nearest clearcut. For example, any vegetation zone in a bay that was 20 m from the nearest clearcut had 79% more weedy species in its seed bank than a vegetation zone in a bay that was 100 m from the nearest clearcut.

Vegetation zones with deeper or more frequent inundation appeared to have fewer weedy species in the seed bank [ILLUSTRATION FOR FIGURE 3 OMITTED]. We tested this relationship by placing the vegetation zones into three groups: relatively deep inundation - open water, Rhynchospora and Taxodium zones; relatively moderate inundation - Panicum, Leersia and shortgrass zones; and relatively dry - Cephalanthus, Andropogon and Pinus zones. Zone inundation regime was based on field observations of water depth and hydroperiod in this study and long-term water level data for five of the seven bays (1989-1994; G. Guntenspergen, pers. comm.). Intercepts for each vegetation zone ([V.sub.i] in the model above) were estimated using Poisson regression and used in a 1-way ANOVA to test the hypothesis that the three vegetation groups had the same mean. Means of the three groups were significantly different ([F.sub.2,6] = 7.98, P = 0.02), with a significant linear trend ([F.sub.1,6] = 14.35, P = 0.0091). On average, relatively dry vegetation zones had 2.4 times more weedy species than did relatively deep water zones.

The Poisson regression analysis for the total number of species in a seed bank revealed a strong interaction between bays and vegetation zones (Chi-square = 13.42, df = 4, P = 0.009). Thus, it was not appropriate to test the effect of distance to nearest bay on total seed bank species numbers.

DISCUSSION

Species richness in the seed banks in the seven Carolina bays studied was intermediate (16-35 species/bay) compared to other types of wetlands, but lower than found in other Carolina bays. Richness reported for freshwater tidal marshes were generally higher (35-53 species; Leck and Graveline, 1979; Parker and Leck, 1985; Leck and Simpson, 1987), similar for prairie wetlands (25-45 species; van der Valk and Davis, 1976, 1978; Poiani and Johnson, 1989) and lakeshores (25-41 species; Keddy and Reznicek, 1982; Nicholson and Keddy, 1983), and generally lower in salt marshes (3-17 species; Ungar and Riehl, 1980; Hopkins and Parker, 1984; Kadlec and Smith, 1984). Higher species richness was reported for other herbaceous Carolina bays (79-108 species; Kirkman and Sharitz, 1994). Comparisons among these wetland types should be interpreted cautiously since different methods were used. Environmental conditions in the greenhouse, different methods, and the amount of effort used to differentiate taxa with similar vegetative appearance may substantially alter estimates of species richness.
TABLE 4. - Analysis of Deviance Tables for tests of the number of
upland weedy species in Carolina bay seed banks. 'Veg' and 'Bay'
are classifiation variables indicating the vegetation zone and bay.
'Clear-cut' and 'Road' are continuous variables equal to the
distances to the nearest clearcut and the nearest road. Under the
null hypothesis in all tests, the Change in Deviance has a
Chi-square distribution with the indicated number of degrees of
freedom


                              Change in
Model             df           deviance         P value


Test 1: No difference among vegetation zones


Intercept         113           113.54
Veg                 8            18.42           0.018


Test 2: No effect of distance to clearcut, after adjusting for
differences among vegetation zones


Veg               105            95.11
Clearcut            1             7.23           0.007


Test 3: No effect of distance to road, after adjusting for
differences among vegetation zones


Veg               105            95.11
Road                1             0.11           0.74


Test 4: No difference among bays, after adjusting for differences
among vegetation zones


Veg               105            95.11
Bay                 6            17.21           0.008


Test 5: No interaction between effects of bays and effects of
vegetation zones


Veg + Bay          99            77.91
Veg * Bay           4             7.97           0.092


Test 6: No differences among bays, after adjusting for distance to
clearcut and vegetation zones


Veg + Clearcut    104            87.89
Veg + Bay           5             9.99           0.076


Seed banks of Carolina bays, like those of many other wetland types (Leck, 1989), did not resemble the existing vegetation. Between 20% and 60% more species were found in the seed banks than in the vegetation and many of these species occurred infrequently. Kirkman and Sharitz (1994) found patterns similar to ours and attributed seed bank diversity to a high rate of vegetation change following disturbance. The absence in the seed bank of some of the dominant species in the field may be due to many factors including the reliance on vegetative reproduction in some species (Kirkman and Sharitz, 1994), our inability to identify a number of unknowns, shorter seed longevity in woody species (Schneider and Sharitz, 1986), unsuitable germination conditions in the greenhouse (Poiani and Johnson, 1988; Gross, 1990; Brown, 1992), lack of seed production during the dry years preceeding seed bank collection, seed predation and loss of viability during storage.

Seed bank composition of vegetation zones within a single bay was relatively homogeneous but seed bank composition among bays with different dominant vegetation types was distinctly different. Kirkman and Sharitz (1994) reported remarkable similarity in seed banks within and among vegetation zones in herbaceous bays. Within-bay similarity patterns may be attributed to two factors: (1) dynamic vegetation changes associated with fluctuating water levels, and (2) the dispersal and postdispersal ability of seeds of wetland plants. Sequences of wet and dry years result in a dynamic distribution of vegetation zones within a bay. A continuum of vegetation zones from open water to upland can occur at a single position in a bay over several years. Seed bank composition in Carolina bays, similar to other wetland and nonwetland systems, likely integrates these changes over time and reflects species that could have existed at that topographic position during a drier or wetter period (van der Valk and Davis, 1979; Rabinowitz, 1981; Poiani and Johnson, 1989).

Within-bay similarity also illustrates the far-ranging dispersal and postdispersal ability of seeds of wetland plants. Seed banks in different vegetation zones within other types of wetlands were often similar, due to movement of seeds throughout a basin (van der Valk and Davis, 1978; Parker and Leck, 1985; Smith and Kadlec, 1985; Poiani and Johnson, 1989). Although some of the dominant grasses such as Panicum hemitomon and Leersia hexandra primarily reproduce vegetatively (Kirkman and Sharitz, 1994), seed production in many wetland emergents and annuals typically is high (van der Valk and Davis, 1979) and movement of seeds to and from different zones in a wetland appears to be common, particularly during periods with standing water (Smith and Kadlec, 1985). Dispersal ability of submerged species was reported to be much more limited (Haag, 1983; Smith and Kadlec, 1985). In this study, Nymphaea sp. seeds, a floating-leaved aquatic, were restricted to the open water samples.

Differences in dominant vegetation forms among Carolina bays reflect a complex interaction among hydrology, land use history and geomorphology (Keough et al., 1990; Kirkman and Sharitz, 1994). Bay seed banks reflected these major differences in vegetation type in spite of their poor resemblance to the existing vegetation. The separation of bays along the second ordination axis [ILLUSTRATION FOR FIGURE 2 OMITTED] possibly reflected a flooding gradient among the herbaceous and shrubby bays with the more permanently flooded bays (Dry Bay and Sarracenia Bay) separated from the drier bays (Little Cypress, Wood and Woodward). Water depth measurements from this study as well as six years of water level data for several of the bays (G. Guntenspergen, pers. comm.) support this hypothesis. Dry Bay was the wettest bay and contained surface water throughout 1989-1994 (maximum depth = 75 cm in our study). Sarracenia was the second wettest bay and contained surface water throughout 1989-1994 except for several months in 1989 and 1990 (maximum depth = 60 cm in our study). Little Cypress, Wood and Woodward Bays all were completely dry at the time of our sampling, and long-term hydrologic data for two of the bays showed both Little Cypress and Woodward Bays were dry in 1989, most of 1990, 1993, and 1994 (G. Guntenspergen, pers. comm.).

From the indirect assessment presented herein, vegetation patterns at the landscape scale at least partially influence bay seed bank composition. The average number of weedy species, most of which were wind dispersed, in a wetland seed bank increased with proximity to a clearcut. Several other wetland and nonwetland studies indicate that species from the surrounding landscape are input to local seed banks. Nonresident species found in salt marsh seed banks were more prominent in samples collected next to cultivated land (Milton, 1939), nonsaline meadows (Ungar and Riehl, 1980), and channels and bay fronts (Hopkins and Parker, 1984). Hume and Archibold (1986), in a study on the influence of a weedy habitat on seed banks in an adjacent cultivated field, found a large effect of the weedy habitat up to seven m into the field. Several species were observed infrequently even at distances of 45 m and 100 m.

In addition to clearcuts, it is possible that historical changes in the adjacent landscape and land use changes within a bay contributed to the occurrence of weedy species in a seed bank since bays farthest from clearcuts also contained some weedy species. Ditching, drainage and cultivation were common in most of the bays on the Savannah River Site before 1951 (Schalles et al., 1989), and clearcuts may have occurred in the past next to bays presently surrounded by mature forest. A temporally heterogenous landscape mosaic surrounding the bays offers a diverse input of seeds over time (Turner, 1989; Schiffman and Johnson, 1992). Seeds of weedy and ruderal species are known for their longevity, and they commonly occur in seed banks of mature forests, pastures and cultivated fields where they are absent from the vegetation (Cook, 1980).

The number of upland weedy species in a seed bank was influenced by flooding regime with fewer weedy species in more permanently flooded zones. These environmental conditions did not seem to influence the number of wetland species because wet zones had a diverse seed bank overall. Lower seed diversity or density was found in continually flooded zones of other wetland types including prairie marshes (Poiani and Johnson, 1989), flood-plain swamps (Schneider and Sharitz, 1986; Titus, 1991), inland brackish marshes (Smith and Kadlec, 1985), large lakes (Haag, 1983), and bays (Pederson, 1981), although upland weedy species were not a significant component of these seed banks. Anaerobic conditions associated with long hydroperiods likely reduce seed viability, particularly for upland species that are not adapted to flooded environments. In addition, upland weedy species can periodically replenish their seeds in the soil by establishing and reproducing in the drier zones during dry years (i.e., the ten-year drought previous to our sampling).

Although results from the indirect analysis in this study do not necessarily show that upland seed bank species are from adjacent clearcuts, they do indicate that contributions to Carolina bay seed banks from surrounding landscape vegetation may be significant and highlight a need for further study. Direct measurement of dispersed seeds into the bays from nearby clearcuts would further elucidate the importance of the various sources of weedy seeds. Because the composition of the seed bank influences patterns of regeneration following disturbance (van der Valk and Davis, 1978; Leck, 1989; Schiffman and Johnson, 1992; Kirkman and Sharitz, 1994), such information can be used to determine prudent management strategies for Carolina bays. For example, larger buffer zones between clear-cuts and bays may be needed to minimize the input of nonresident, weedy species.

Acknowledgments. - This research was supported by Contract DE-AC09-76-SR00-819 between the United States Department of Energy and University of Georgia's Savannah River Ecology Laboratory. We gratefully acknowledge the assistance of Kay Kirkman, Robert Wesley and Robert Kral in identifying plants and seedlings, Janet Dixon for help in harvesting plants, and Barbara Taylor for help in preparing Figure l. We thank Kay Kirkman, the Wetlands Ecology Reading Group at SREL and four anonymous reviewers for constructive comments on the manuscript.

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Author:Poiani, Karen A.; Dixon, Philip M.
Publication:The American Midland Naturalist
Date:Jul 1, 1995
Words:6047
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