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Spatial/temporal variations in shrub thicket soil seed banks on an Atlantic Coast barrier island¹

^a Department of Biology, Virginia Commonwealth University, Richmond, Virginia 23284

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Potential species replacement within low-diversity shrub thicket communities was investigated for a Virginia barrier island. Seed bank species composition was quantified in a glasshouse study using soil samples collected beneath closed Myrica cerifera thickets, as well as from thicket gaps. Samples were collected from productive and aging thickets, corresponding to differences in soil age. These data were compared to species presently occurring within the thickets and gaps. Seedbank species composition was not indicative of current community composition for either the intact thickets or the gaps. Seed banks resembled a more pioneer community. Thirteen families, 23 genera, and 25 species were identified from the seed bank beneath the M. cerifera thickets. Four species were woody. The within-gap seed bank included 19 families, 30 genera, and 34 species. Eight species were woody. The current community included 21 families, 33 genera, and 36 species beneath the intact thickets as well as within the thicket gaps. Eighteen species were woody. The species richness of gaps was more than three times that of intact thickets. For low-diversity shrub thickets, gaps enhance species richness.

Key Words: barrier islands • gaps • Myrica cerifera • seed bank • shrub thickets • species richness • succession

	INTRODUCTION

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Shrub thickets represent an intermediate seral stage in the development of maritime forests on many Atlantic coast barrier islands (Levy, 1983, 1990; Ehrenfeld, 1990). As a result of symbiotic nitrogen fixation, clonal growth, and high photosynthetic potential, Myrica cerifera L. (Myricaceae) rapidly forms dense thickets within protected mesic swales on many barrier islands (Young, 1992). Maritime forests may develop from thickets where island width and topography provide sufficient protection from storm exposure (Stalter and Odum, 1993). The establishment of late-successional species within these dense shrub thickets may be difficult due to above- and belowground competitive interactions.

Canopy gaps within thickets may facilitate establishment of woody competitors. Gap microclimate differs significantly from the surrounding thicket understory (Crawford and Young, 1998). Differences in microclimate influence germination and establishment; however, establishment is also a function of seed bank composition (Simpson, Leck, and Parker, 1989). Soil seed banks are essential to the regeneration of disturbed communities and ecosystems (Pickett and McDonnell, 1989) and may be similarly important to maritime successional processes stimulated by thicket gaps.

Generally, there has been less time available for soil development, species immigration, and evolutionary adjustments of local populations to barrier island environments as compared to mainland coastal dune systems (Ehrenfeld, 1990). Seed banks in dune soils are small to nonexistent (Fahrig, Hayden, and Dolan, 1993; Looney and Gibson, 1995), and few studies have focused inland of the strand and foredune (Ehrenfeld, 1990). Further, variations in soil age due to island accretion patterns may affect development of seed banks. The role of interdunal seed banks in the maintenance of species diversity and regeneration of woody species in low-diversity seral stages of barrier islands is poorly understood. The purpose of this study was to quantify soil seed bank species composition and density within thicket gaps as well as beneath intact thicket canopies of M. cerifera. Seed bank composition was examined for a young, productive thicket and for an aging thicket to identify potential differences related to landscape age.

	MATERIALS AND METHODS

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Field work was conducted during the summer and fall of 1994 on Hog Island (37°40' N, 75°40' W), located ~8 km east of the Eastern Shore of Virginia, USA. Hog Island is ~1177 ha, 10 km long, and 2 km across at the widest point. The accreting north end of the island has an established chronosequence with the youngest soils on the eastern side at the ocean beach followed by progressively older soils as one moves west towards the bayside marsh (Hayden et al., 1991; Young, Shao, and Porter, 1995). The soils range in age from 1 yr on the ocean side of the island to ~130 yr along the upland edge of the bay side of the island (Harris, 1992).

Shrub thickets of Myrica cerifera L. (Myricaceae) have developed in the protected swales along this chronosequence. The thickets were nearly monocultures of M. cerifera with <5% Baccharis halimifolia and a few vine species. The well-protected, ~20-yr-old, midisland thickets (~4 m tall) were established on young soils. In comparison, the ~40-yr-old bayside thickets (~5 m tall) were located on the oldest soils on the island. The latter were less productive than the younger midisland thickets (Young, Shao, and Porter, 1995).

Ground surveys identified a total of 21 thicket gaps, ten in the midisland and 11 in the bayside thickets. The midisland gaps were an order of magnitude smaller than the bayside gaps, 5.5 vs. 51.3 m, respectively. Midisland gaps contained live M. cerifera stems, a thick leaf litter layer, relatively little herbaceous vegetation, and almost no woody debris. The surface of bayside gaps was heterogeneous, with lianas and woody debris interspersed with bare soil, herbs and woody seedlings (Crawford and Young, 1998). Bayside gaps were apparently caused by senescence coupled with vine competition, while allogenic processes (e.g., ice loading or microbursts) may have formed the midisland thicket gaps (Crawford and Young, 1998).

In order to assess the potential seed banks for future thicket gaps, ten random soil samples (0.25 m, 8 cm depth) were collected in March from beneath the intact M. cerifera thickets, both midisland and bayside. Samples were maintained at 5°C for 4 wk. The samples were spread thinly into trays over a base of sterilized sand, placed in the glasshouse under natural light and misted twice daily (Gross, 1990; Brown, 1991). Trays were monitored weekly for 6 mo. Emerging seedlings were identified using reference herbarium collections at Virginia Commonwealth University and floristic manuals (Radford, Ahles, and Bell, 1968; Godfrey and Wooten, 1981; Silberhorn, 1982; Duncan and Duncan, 1987). After identification, seedlings were removed from the trays. At the conclusion of the experiment, unidentified seedlings were transplanted into pots and grown to anthesis for proper identification. Dispersal mode for each species was determined from anatomical characteristics of seeds and fruits as described in the floristic manuals and as discussed in Stiles (1980), Estrada and Fleming (1986), and Moss (1993).

Similarly, to assess the actual seed banks for gaps, soil samples were collected from within each identified gap. Due to variation in gap size and spatial heterogeneity within the gaps, a 10 x 10 cm quadrat was randomly tossed ten times within each gap and the top 8 cm of soil was collected. The ten samples were combined to form a composite sample (0.10 m) for each gap. All samples were prepared and examined as previously described. For both seed bank experiments, seedling density and frequency were quantified for each species (Bonham, 1989).

The current community growing in the gaps and beneath the thickets was also quantified. Each gap was considered a sample unit; species present in each were identified. For the intact thickets adjacent to each sampled gap, all species within a 10-m radius were identified. Percent cover was estimated at <5%, therefore, the thicket understory community was not quantitatively sampled. Seed bank and species compositions were compared with Jaccard's index of similarity based on presence (Muller-Dombois and Ellenberg, 1974).

	RESULTS

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Seed banks of soils collected beneath intact M. cerifera thickets represent the potential community for the initial colonization of thicket gaps. From these samples, 13 families, 23 genera, or 25 species emerged (Table 1). Four species were woody. Myrica cerifera occurred most frequently in the soil samples (Table 1). Total density for all seedlings that emerged from the intact thicket soils was 30.6 individuals/m for the midisland and 73.6 individuals/m for the bayside. The only woody species to differ in emerging seedling density was B. halimifolia (P < 0.05, t test); nearly six times more seedlings emerged from the bayside thicket samples as compared to the midisland (Table 1). Cyperus esculentus was the most frequent herbaceous species, with a greater density (P < 0.05, t test) in the bayside samples. Midisland soil samples contained all four woody species and 14 herbaceous species; at least 56% of the species are bird dispersed (Table 2). Bayside samples also contained all four woody species and 17 herbaceous species; at least 62% are bird dispersed.

View this table: [in this window] [in a new window]   Table 1. Seedling density (no./m) and frequency (proportion of samples), based on a seed bank analysis of soil samples collected within Hog Island Myrica cerifera thickets. Values are means ± 1 SE; * denotes statistically significant differences (t test, = 0.05) between the thickets. Dispersal category (D/C): W = wind, B = bird, and ? = unknown.

View this table: [in this window] [in a new window]   Table 3. Seedling density (no./m) and frequency (proportion of samples), based on a seed bank analysis of soil samples collected within Hog Island Myrica cerifera thicket gaps. Values are means ± 1 SE; * denotes statistically significant differences (t test, = 0.05) between the thickets. Dispersal category (D/C): W = wind, B = bird, and ? = unknown.

For the current communities, there were 21 families, 33 genera, or 36 species identified growing beneath the intact thickets; 18 were woody (Table 4). In comparison, there were 35 different species growing within the thicket gaps (Table 4). Only four species were present in midisland thickets and ten in bayside thickets; three and six, respectively are bird dispersed (Table 2). Ten species occurred in the midisland gaps; six were woody. Eight are bird dispersed. The bayside gaps had three times greater species richness; 18 woody species were present. At least 21 species are bird dispersed (Table 2). Only two species from the current communities were exotics (Radford, Ahles, and Bell, 1968; Gleason and Cronquist, 1993). Both Chenopodium abrosiodes and Rumex acetosella occurred in bayside gaps (Table 4).

View larger version (15K): [in this window] [in a new window]   Fig. 1. Gap area vs. species richness for seed banks and species currently growing in midisland (filled circles) and bayside (open circles) gaps. The line in the lower panel represents the best-fit linear regression with area data log10 transformed.

	DISCUSSION

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Seed germination for pioneer species is typically cued to disturbance indicators including a high ratio of red to far-red light, soil temperature fluctuations, and elevated surfaces, such as tip-up mounds and large logs, where competition from other species is reduced (Brokaw, 1985; Veblen, 1989). Pioneer species become more abundant as gap size increases in many forests (Garwood, 1989); thus, regeneration of pioneers from the seed rain or from the seed bank predominates in larger gaps. Spatially heterogeneous zones of a given gap are colonized by different floristic assemblages that may determine a different floristic potential for gap regeneration (Núñez-Fárfan and Dirzo, 1988). The larger more spatially heterogeneous gaps of the bayside thicket allow for greater niche differentiation, which is evident in the high number of colonizing species.

In tropical forests long-term accumulation of dormant seeds makes the seed bank more important in gap regeneration than fresh seed rain (Lawton and Putz, 1989). In comparison, seed rain via wind or avian dispersal may play an important role in contributing to potential gap-colonizing species within the relatively young soils (<130 yr) of these barrier island thickets. The bayside gaps were an order of magnitude larger than the midisland gaps, and many gaps contained standing snags that could be used as perches. Altered wind speed, convectional currents above heated, exposed soils, and turbulence created by air flow across a broken canopy may increase the probability of seed deposition in gaps (Schupp, Howe, and Augspurger, 1989). Along the Atlantic and Gulf Coast barrier islands, avian transport of fleshy fruits contributes disproportionately to the total species diversity of the tree stratum (Ehrenfeld, 1990; Moss, 1993). Most of the woody species present in bayside gaps are bird dispersed (Silberhorn, 1982). The M. cerifera shrub thickets on the islands provide stopover habitat for migratory songbirds in the fall, with migrant abundance and species numbers greater on the barrier islands than on the adjacent mainland coasts (Maybe et al., 1993). Bird dispersal to disturbed sites can be enhanced by bird-attracting structures such as snags or perches, increasing the abundance and diversity of bird dispersed seeds (Chambers and MacMahon, 1994). Thus, seed rain may play an important role in determining gap colonizers of the large bayside gaps.

The results of the thicket and gap seed bank analyses suggest that the intermediate M. cerifera seral stage has the potential to be much more diverse in terms of species composition and density. However, the intact thicket canopies preclude the establishment of many species within the seed bank. Gap seed banks contained more species than the thicket seed banks. This difference may be attributable to an increase in seed rain from wind and bird dispersal. Also, the number of species present within gaps increased with increasing gap size. The larger gaps associated with the aging bayside thicket had three times the species richness than did the smaller gaps associated with the younger midisland thicket. Thus, the larger gaps of the bayside thicket may facilitate the establishment of a more diverse community within the dense M. cerifera thicket.

The proximity of seed sources and seed dispersal agents has an effect on the species composition of maritime forests on barrier islands (Stalter and Odum, 1993). Tree species and numerous vine and shrub species occurring within the bayside gaps are typical of maritime forest communities occurring along the Mid-Atlantic seaboard (Silberhorn, 1982; Stalter and Odum, 1993). Myrica cerifera is perceived to be an important successional species in the coastal regions of Virginia (Levy, 1983, 1990). Cyclic succession with Prunus serotina and directional successional towards a maritime forest have been observed (Tyndall and Levy, 1978; Levy, 1990). As M. cerifera thickets develop, the dense canopy produces an environment at the soil surface that may be unfavorable for recruitment of replacement species. Over time, the soils beneath these thickets are subjected to seed rain, nitrogenous inputs, and increasing organic matter. Senescence of individual shrubs (i.e., gap formation) within these thickets over time provides microsites for the establishment of a more diverse soil seed bank.

Despite the importance of the seed bank for future community structure, the thicket and gap seed banks did not reflect the existing community composition or a more mature successional community. Instead, the seed banks resembled a more pioneer community. This is reflected in the low Jaccard index of similarity values (<35%) when comparing the seed bank composition to the existing community. This was similar to a comparison of soil seed banks to vegetation growing on a barrier island in northwest Florida, USA (Looney and Gibson, 1995). For swales, similar to the present study, Jaccard values were 34–39%, and over a wider range of microtopography and communities, the overall index of similarity was 36%. Looney and Gibson (1995) concluded that the dissimilarity between the seed bank and the community was due to a general pattern of increasing seed bank development and a persistent rather than transient seed bank with decreasing frequency and increasing time since disturbance and successional maturity. In the present study, the relatively high Jaccard similarity (56–59%) when comparing seed bank compositions for differing soil ages, gaps, or thickets, may reflect persistence, but also that relative to other landscapes, barrier island soils are very young, subject to primary succession, and begin with a seed bank of zero (Looney and Gibson, 1995).

In conclusion, gaps enhance species richness in the nearly monoculture M. cerifera thicket community typical of Atlantic Coast barrier islands. There was considerable spatial variation in seed bank species composition when comparing gaps to adjacent thickets. In contrast, comparing the two sites differing in soil age indicated little temporal variation in seed bank composition. The relatively high richness for the current community within gaps relative to intact thickets is less related to the seed bank, but perhaps more a function of seed rain after a gap has formed.

	FOOTNOTES

 
¹ The authors thank the staff of the Nature Conservancy and the LTER laboratory, particularly Mr. James Spitler, for minimizing logistical problems associated with field work at the Virginia Coast Reserve, Mr. David Martin for capable field assistance, Dr. Miles F. Johnson for seedling identification, and Dr. Patricia P. Williamson for data analysis. This work was supported by NSF grant DEB-9211772 to the University of Virginia for LTER-related work at the Virginia Coast Reserve.

² Author for correspondence (dyoung{at}saturn.vcu.edu ).

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