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Does the Seed Bank Reflect Plant Distributions in a Coastal Dune?
David S. Messina and Tara K. Rajaniemi

Northeastern Naturalist, Volume 18, Issue 1 (2011): 107–114

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2011 NORTHEASTERN NATURALIST 18(1):107–114 Does the Seed Bank Reflect Plant Distributions in a Coastal Dune? David S. Messina1 and Tara K. Rajaniemi1,* Abstract - The spatial patterns in the seed bank were examined within a coastal sanddune system. Soil samples were collected from vegetated and bare plots in three zones, each dominated by different plant species. The total number of seeds and species diversity in the seed bank were low. There were no significant patterns in total seed number. However, there were significant differences in species composition. Seeds of Artemisia campestris (Tall Wormwood) were more abundant in vegetated plots and were found in all zones, although adults are more spatially restricted. Fruits of Cyperus grayii (Gray’s Sedge) were most common where adults are found. Dominant species from the site were absent from the seed bank. Overall, composition of the seed bank does not reflect composition of the adult community. Introduction A seed bank consists of all the potentially viable seeds contained in the soil or on the soil surface (Csontos 2007). These seeds provide a source for recruitment for future generations of plants (Harper 1977). Seed banks may make an important contribution to plant community structure, particularly in systems that experience severe, unpredictable disturbances, such as cropland, heathland, or wetlands that experience extreme flooding (Thompson 1992). While coastal sand dune communities experience disturbances such as wind, salt spray, and major storms, very little has been reported on their seed banks. Baptista and Shumway (1998) studied the seed banks of coastal dunes of Cape Cod, making comparisons among four sites. They found low abundance and low species diversity of seedlings emerging from soil samples from all sites. Our goals in this study were to examine the abundance and species composition in the seed bank of a coastal dune community at a single site, and to look for within-site patterns. Baptista and Shumway’s (1998) four sites were all located within Cape Cod National Seashore (the Lower Cape), whereas our study site is on the Upper Cape, closer to the mainland. Patterns in the seed bank may result from patterns in adult vegetation, especially if most seeds disperse only short distances, as appears to be typical for coastal dune species (Ehrenfeld 1990). Two reviews of studies comparing the seed bank to adult vegetation conclude that similarity between adults and seeds is greatest in frequently disturbed habitats (Hopfensperger 2007, Warr et al. 1993). The studies reviewed, however, consider the adult vegetation and the seed bank of an entire site. We examined this question at a smaller scale: given that there are patterns in distribution of adults within a coastal dune site, does the seed 1Biology Department, University of Massachusetts Dartmouth, North Dartmouth, MA 02747. *Corresponding author - trajaniemi@umassd.edu. 108 Northeastern Naturalist Vol. 18, No. 1 bank exhibit similar patterns? Previous studies of coastal sand dune seed banks (reviewed by Maun 2009) have not tested for such within-site patterns. Our study site consists of a barrier dune system with a single dune crest, 1–2 m in height. Adult vegetation at the site shows strong patterns of zonation with distance from the shore. Vegetation is patchy, and bare ground is common in all zones. The purpose of this study was to determine whether the spatial distribution of species in the seed bank follows the distribution of adult plants. We assessed the seed bank in the three vegetation zones, and in vegetated and bare plots. We predicted that more seeds would be found in vegetated than bare plots because seeds should physically get caught by existing vegetation (e.g., Li 2008). We also predicted that species composition in the seed bank of each zone would be similar to the composition of the adult vegetation in that zone. Field-site Description Field work was conducted at Waquoit Bay National Estuarine Research Reserve in Falmouth, Massachusetts (N41°33'5", W70°30'22"). The mean air temperature and precipitation are 9.8 °C and 1039.5 mm, respectively (averages from 1949–1970 and 1976–1982, respectively; WorldClimate.com). The study site is a barrier dune system separating the Atlantic Ocean from Waquoit Bay. The dune system consists of a single, low dune crest, 2 m or less in height and 5–20 m from the high tide line. A largely flat area behind the crest extends 100–200 m to saltmarsh or the bay. The soil throughout the dune system is a coarse sand with low organic matter content. The area shows a typical loose zonation of plant species. The windward side of the dune faces the Atlantic Ocean and is dominated by Ammophila breviligulata Fern. (American Beachgrass). The leeward side of the dune is dominated by Rosa rugosa Thunb. (Saltspray Rose). In the flat area behind the dune, Myrica pensylvanica Mirbel. (Northern Bayberry) is abundant. American Beachgrass is present at low density in all areas of the dune. Artemisia campestris L. ssp. caudata (Michx.) Hall & Clem. (Tall Wormwood) is common in open areas of the dune flats. Methods On 22 April 2009, substrate samples were collected in a sequence of paired plots within the study site’s three zones and along four transects. The sampling date was expected to be before the beginning of natural germination, as minimum daily temperatures were near or below freezing prior to this date (United States Historical Climatology Network, http://cdiac.ornl.gov/epubs/ndp/ushcn/ushcn.html). The three zones were dominated by American Beachgrass, Saltspray Rose, and Northern Bayberry. Transects were 120 m apart along the length of the dune. Each of the four transects contained two sets of plots within each zone, with beachgrass plots an average of 2 m in front of the dune crest and rose and bayberry plots an average of 11 and 48 m behind the dune crest, respectively. Each set of plots consisted of a bare area and a densely vegetated area. In the rose and bayberry zones, vegetated 2011 D.S. Messina and T.K. Rajaniemi 109 plots were located within shrubs, while bare plots were located at least 1 m from shrubs in areas with little to no vegetation (<5% plant cover in a 1-m2 plot). In the American Beachgrass zone, completely bare areas were rare; vegetated plots were located in areas of dense beachgrass growth (>40% plant cover) and “bare” plots had sparse beachgrass growth (<20% plant cover). A total of 48 plots were sampled. In each plot, soil was collected from an approximately circular area 15 cm in diameter to a depth of 10 cm. Each core was collected with a garden shovel, labeled, and bagged. We used the germination method (Gross 1990) to identify and count seeds in the seed bank. The 48 samples were each transferred into one half of a divided plastic tray measuring 25 cm x 25 cm x 6 cm. Each tray was filled with potting soil approximately 3 cm in depth and topped with the field samples (≈2 cm deep). The potting soil was used to retain moisture and improve conditions for seedling germination. Paired bare and vegetated plots were put in halves of the same tray. The trays were positioned randomly in the greenhouse, with positions rotated weekly, and watered three times a week. Every week, germination was recorded. Once a seedling was identified to species, it was counted and removed from the tray. The seed count data included many zero values and therefore did not meet the assumptions of ANOVA. Instead, we used unrestricted permutations of the data to calculate significance values for the effects of zone, plot type, and their interaction on total seed number, following the recommendations of Manly (2007). This analysis was conducted with R version 2.8.1. We used a χ2 test to assess whether seedling composition was similar among all zones and plot types. The seedling counts of all dune plant species, other than Tall Wormwood and Cyperus grayii Torr. (Gray’s Sedge), were grouped together for the χ2 test, because these species together accounted for less than 4% of the total seedlings. Thus, the χ2 test compared the distribution of three species (or species groups) among six zone-plot type combinations. The initial χ2 test was then broken down into two separate contingency tests for the two most common species, Tall Wormwood and Gray’s Sedge. These 3 x 2 contingency tests tested whether zone and plot type had independent effects on seed distribution. Results Over the 15-week germination experiment, a total of 388 seedlings germinated and were identified. Seedlings of Tall Wormwood, Gray’s Sedge, Solidago sempervirens L. (Seaside Goldenrod), Oenothera parviflora L. (Small-flowered Evening Primrose), Suaeda maritima (L.) Dum. (Sea Blite), and Lepidium virginicum L. (Pepperweed) were found. Some additional germinants (67 total) were of common weed species (Chenopodium album L. [Lambsquarters], Medicago sativa L. [Alfalfa]) that had never been observed at the field site. Unfortunately, potting soil controls were not used in this experiment, but these species had been observed germinating at similar densities in other experiments using the same potting soil. Therefore, weed seedlings were assumed to have 110 Northeastern Naturalist Vol. 18, No. 1 emerged from the potting soil in the trays and were not included in analyses. There was little correspondence between the species composition of germinating seedlings and of adult vegetation in the three zones (Table 1). The three most abundant species in the adult vegetation for each zone were never observed in the seed bank. Tall Wormwood, Gray’s Sedge, and Seaside Goldenrod were the only species that emerged in samples from the windward (beach) side of the dune (Fig. 1, Table 1). Wormwood had the highest abundance; its seedling count in the vegetated plot samples was more than double that in the bare plot samples. In contrast, Gray’s Sedge had the highest seedling count in the bare plot samples—double that in the vegetated plot samples. Tall Wormwood, Table 1. Comparison of adult vegetation and seeds germinated in the three zones. Percent cover for adult vegetation was measured in 1-m2 plots from 10 transects in July 2009. For dune front, plots are at the dune crest or 1–11 m in front of the crest (n = 10). For dune back, plots are 1–11 m behind the dune crest (n = 13). For dune flat, plots are 20-80 m behind the dune crest (n = 19). Only species with >1% mean cover are shown. Mean and standard deviation of % cover are given, as well as total number of plots in which the species occurred. For seeds germinated, the total number of seeds (from n = 8 samples) is given. Bare = bare plot, Veg = vegetated plot. Adult vegetation % cover Seeds germinated Zone / Species mean ± s.d. [# plots] Bare Veg Dune front Ammophila breviligulata (American Beachgrass) 22.4 ± 20.5 [9] Lathyrus japonicus (Beach Pea) 13.9 ± 18.5 [6] Rosa rugosa (Saltspray Rose) 12.8 ± 30.9 [2] Toxicodendron radicans (Poison Ivy) 1.9 ± 5.0 [3] Solidago sempervirens (Seaside Goldenrod) 1.8 ± 2.8 [4] 2 2 Artemisia campestris ssp. caudata (Tall Wormwood) 11 32 Cyperus grayii (Gray’s Sedge) 7 2 Dune back Saltspray Rose 18.5 ± 29.5 [5] American Beachgrass 16.2 ± 18.7 [12] Poison Ivy 9.9 ± 15.9 [6] Wormwood 6.6 ± 7.0 [8] 30 13 Beach Pea 1.1 ± 2.9 [2] Lepidium virginicum (Peppergrass) 2 Gray’s Sedge 1 Oenothera parviflora (Small-flowered Evening Primrose) 1 Dune flat Myrica pensylvanica (Northern Bayberry) 24.0 ± 29.7 [10] Poison Ivy 11.4 ± 14.8 [11] Saltspray Rose 7.2 ± 16.4 [4] Tall Wormwood 5.3 ± 6.4 [11] 17 16 Poa sp. 3.7 ± 6.4 [7] American Beachgrass 2.5 ± 4.0 [10] Polygonella articulata (L.) Meisner (Jointweed) 1.4 ± 2.1 [7] Gray’s Sedge 1.2 ± 2.2 [5] 80 165 Seaside Goldenrod 3 1 Suaeda maritima (Sea Blite) 2 1 2011 D.S. Messina and T.K. Rajaniemi 111 Gray’s Sedge, Small-flowered Evening Primrose, and Peppergrass emerged in samples from the dune back. Wormwood again had the highest abundance, but in this zone it was more abundant in bare plots (Fig. 1). Tall Wormwood, Gray’s Sedge, Seaside Goldenrod, and Sea Blite emerged in samples from the Figure 1. Mean (± 1 standard deviation) of seed counts (n = 8) for each plot type and zone. “Other” includes Oenothera parviflora (Smallflowered Evening Primrose), Solidago sempervirens (Seaside Goldenrod), Suaeda maritima (Sea Blite), and Lepidium virginicum (Peppergrass). 112 Northeastern Naturalist Vol. 18, No. 1 leeward dune slope. Gray’s Sedge had the highest abundance, and was more abundant in vegetated plots than bare plots (Fig. 1). Wormwood was the second- most abundant species with almost equal counts in both plot types. The total seedling counts (combining all species) were not significantly affected by zone (P = 0.212 by permutation test), plot type (P = 0.710), or their interaction (P = 0.899). Results were similar when P-values were derived from the parametric two-way ANOVA (zone F2,42 = 1.674, P = 0.200; plot type F1,42 = 0.206, p = 0.652; zone*plot type F2,42 = 0.218, P = 0.805). Significant differences in species composition were found among the six zone-plot type combinations (χ2 = 221.234, P < 0.001). When examining individual species, we found interacting effects of zone and plot type on seedling abundance for both Wormwood (χ2 = 16.942, P < 0.001) and Gray’s Sedge (χ2= 8.387, P < 0.001). Discussion We found no statistically significant within-site patterns in numbers of seeds in the germinable seed bank, either among zones or plot types. The dune flat samples tended to have higher seed numbers, due to a few samples with high densities of Gray’s Sedge (note the high standard deviation in Fig. 1). Adults of this species occur in isolated patches (T.K. Rajaniemi, unpubl. data); if most achenes disperse only short distances, samples with high numbers of sedge fruits may have been collected near patches of adults. The windward and leeward slopes of the dune had similar seed densities to each other. The presence of dense grass or shrubs also did not affect seed density. We had hypothesized that, in the windy dune environment, vegetation would physically trap seeds (Li 2008). Several studies in desert systems have found that shrubs are associated with either higher or lower seed numbers in the seed bank than gap areas. These studies indicate the importance of local dispersal: when annuals are more abundant under shrubs, seeds are also more numerous under shrubs (Guo et al. 1998, Holzapfel et al. 2006), and when annuals are more abundant in gaps, seeds are more numerous in gaps (Holzapfel et al. 2006, Sternberg et al. 2004). In our system, the species represented in the seed bank are found primarily in gaps, but seeds are equally numerous under shrubs, suggesting that the wind may move seeds over distances of meters to tens of meters. We did find some patterns in species composition of the seed bank. These patterns can be attributed to the two most common species in the seed bank, Tall Wormwood and Gray’s Sedge. Tall Wormwood, a biennial herb, is a pioneer species that colonizes areas of major wind disturbance (C. Weideman, Waquoit Bay NERR, Falmouth, MA, pers. comm.). It is found on the leeward slope of the dune and particularly in the dune flat, but rarely under shrubs and never on the windward slope of the dune (Table 1; T.K. Rajaniemi, unpubl. data). Its seeds were found in all vegetation zones, and were associated more with dense vegetation on the windward slope and with bare areas on the leeward slope. Wormwood was also the most common species found in a previous study of Cape Cod sand dune seed banks, but its distribution was clumped both between and within sites 2011 D.S. Messina and T.K. Rajaniemi 113 in that study (Baptista and Shumway 1998). The wide spatial distribution of Tall Wormwood seeds may reflect a ruderal strategy. We have yet to determine why adult Tall Wormwood are not present on the dune front, where seeds are found. As noted above, Gray’s Sedge adults are found only in the dune flat, where its fruits are also most common. A few fruits of this species experience some wind dispersal, and are found in the dune front, 50–100 m from the adults. Adults are often found near Northern Bayberry shrubs, and these appear to capture windblown seeds, so that seed numbers are higher under shrubs in the dune flat. Species diversity in the seed bank was low, and the dominant species on the dune were not represented in the germinable seed bank. Previous studies have also observed low diversity in coastal dune seed banks (Baptista and Shumway 1998, Looney and Gibson 1995). Baptista and Shumway (1998) found mostly subordinate species in the seed bank, and Looney and Gibson (1995) noted that most woody species were absent from the seed bank. Dominant woody species including Saltspray Rose, Bayberry, and Toxicodendron radicans (L.) Kuntze. (Poison Ivy), as well as herbaceous species such as Lathyrus japonicus Willd. (Beach Pea), produce copious seeds at our site, and these seeds germinate well in the greenhouse (Poison Ivy germination has not been tested; T.K. Rajaniemi, unpubl. data). Some of these seeds may be lost in the field to predators; we have observed weevil damage in about half of the Beach Pea seeds collected and noted many chewed seeds of Saltspray Rose on the ground while collecting soil samples, suggesting fruit and/or seed consumption by small mammals. Short seed life span, attack by pathogens, deep burial of seeds, or transport out of the dune system might also account for some seed loss (Baptista and Shumway 1998). In general, seed banks of coastal sand dunes appear to be largely transient, although the fates of dispersed seeds have not been well studied (Maun 2009). In conclusion, the species composition of the germinable seed bank at this coastal dune site did not reflect the species composition of the adult vegetation, for the site as a whole or along gradients within the site. The seed bank may contribute to the ability of Tall Wormwood to colonize blowouts, but it does not appear to contribute to maintenance of the dominant vegetation. Acknowledgments We wish to thank Chris Weideman and the Waquoit Bay National Estuarine Research Reserve for advice and access to the study site. Literature Cited Baptista, T.L., and S.W. Shumway. 1998. A comparison of the seed banks of sand dunes with different disturbances histories on Cape Cod National Seashore. Rhodora 100:298–313. Csontos, P. 2007. Seed banks: Ecological definitions and sampling considerations. Community Ecology 8:75–85. Ehrenfeld, J.G. 1990. Dynamics and processes of barrier island vegetation. Reviews in Aquatic Sciences 2:437–480. 114 Northeastern Naturalist Vol. 18, No. 1 Gross, K.L. 1990. A comparison of methods for estimating seed numbers in the soil. Journal of Ecology 78:1079–1093. Guo, Q., P.W. Rundel, and D.W. Goodall. 1998. Horizontal and vertical distribution of desert seed banks: Patterns, causes, and implications. Journal of Arid Environments 38:465–478. Harper, J.L. 1977. Population Biology of Plants. Academic Press, London, UK. 892 pp. Holzapfel, C., K. Tielbörger, H.A. Parag, J. Kigel, and M. Sternberg. 2006. Annual plantshrub interactions along an aridity gradient. Basic and Applied Ecology 7:268–279. Hopfensperger, K.N. 2007. A review of similarity between seed bank and standing vegetation across ecosystems. Oikos 116:1438–1448. Li, F.-R. 2008. Presence of shrubs influences the spatial pattern of soil seed banks in desert herbaceous vegetation. Journal of Vegetation Science 19:537–548. Looney, P.B., and D.J. Gibson. 1995. The relationship between the soil seed bank and above-ground vegetation of a coastal barrier island. Journal of Vegetation Science 6:825–836. Manly, B.F.J. 2007. Randomization, Bootstrap, and Monte Carlo Methods in Biology, 3rd Edition. Chapman and Hall, London, UK. 480 pp. Maun, M.A. 2009. The Biology of Coastal Sand Dunes. Oxford University Press, New York, NY. Sternberg, M., S.L. Yu, and P. Bar. 2004. Soil seed banks, habitat heterogeneity, and regeneration strategies in a Mediterranean coastal sand dune. Israel Journal of Plant Sciences 52:213–221. Thompson, K. 1992. The functional ecology of seed banks. Pp. 231–258, In M. Fenner (Ed.). Seeds: The Ecology of Regeneration in Plant Communities. CAB International, Oxon, UK. Warr, S.J., K. Thompson, and M. Kent. 1993. Seed banks as a neglected area of biogeographic research: A review of literature and sampling techniques. Progress in Physical Geography 17:329–347.