Buried Seeds Research Articles

Columnar and Subsurface Silicide Growth with Novel Molecular Beam Epitaxy Techniques

ABSTRACTMolecular beam epitaxy of silicides is conventionally carried out at temperatures ≤500°C, with stoichiometric Si:metal flux ratios or deposition of pure metal. We have found novel growth modes for epitaxial CoSi2 at high temperatures coupled with Si-rich flux ratios or low deposition rates. In the first of these modes, codeposition of metal and Si at 600-800°C with excess Si leads to the formation of epitaxial silicide columns surrounded by single-crystal Si. During the initial stages of the deposition, the excess Si grows homoepitaxially in between the silicide, which forms islands, so that the lateral growth of the islands is confined. Once a template layer is established by this process, columns of silicide form as a result of selective epitaxy of silicide on silicide and Si on Si. This growth process allows nanometer control over silicide particles in three dimensions. In the second of these modes, a columnar silicide seed layer is used as a template to nucleate subsurface growth of CoSi2. With a 100 nm Si layer covering CoSi2 seeds, Co deposited at 800°C and 0.01 nm/s diffuses down to grow on the buried seeds rather than nucleating surface silicide islands. For thicker Si caps or higher deposition rates, the surface concentration of Co exceeds the critical concentration for nucleation of islands, preventing this subsurface growth mode from occurring. Using this technique, single-crystal layers of CoSi2 buried under single-crystal Si caps have been grown.

The ecology of the seed in Trapa natans var. Japonica in a eutrophic lake

The production, density and environment of buried seeds and the temperature response of germinating seeds in Trapa natans var. japonica Nakai were ascertained. The mechanism of high reproductivity, tolerance of the plant for eutrophication and the stability of the stand in an eutrophic lake are discussed. The mean fresh weight of seeds was about 7.2 g, and the density of liver and dead seeds in April ranged 66.7 to 80.0 m-2 and 26.7 to 97.8 m-2, respectively. The threshold temperature that breaks dormancy and promotes germination in the lake was the low temperature, 8°C. The dormancy of buried seeds was almost broken under the field condition in January. The rate of germination obtained by the field experiment, and the estimation from the density of buried seed and the number of seeds germinated at the stand, were 87% and 60%, respectively. It was estimated that about 30% of the seedlings died before the floating leaf reached water surface. A rosette produced less than three seeds a year, which were fewer than for another Trapa, but the size of the former was larger. The number of seeds changed year by year, but the size-distribution pattern of seeds did not change with years, nor was it affected by the density of rosettes. The reason why T. natans var. japonica could succeed in eutrophic waters was attributed to reproductive strategy. The stand could recover by increasing the number of rosettes through the great branching ability of the main stem, even when the density of buried seeds was greatly decreased in the previous year.

Survival characteristics of buried seeds 10 years after the eruption of the Usu volcano in northen Japan

In the crater basin of the Usu volcano, where the vegetation has almost completely been destroyed by 1977–1978 eruptions, a study was conducted in 1987 to determine the survival characteristics of buried seed populations. A total of 17 618 seeds were extracted from six 25-L former topsoil blocks by a flotation test. Data showed that even after 10 years burial by 65–140 cm volcanic deposists, 16.6% of the seeds, representing 25 species, were viable. There were few predators in the former topsoil below the thick volcanic deposits. Seed volume of most species was less than 2.0 mm3 and smaller seeds had a greater rate of survival than larger seeds. Seed survival rates of Poa annua, Rumex obtusifolius, Alopecurus aequalis var. amurensis, and Viola grypoceras were positively correlated with the thickness of volcanic deposits, suggesting that the most important factor for seed survival is stable soil temperature with little diurnal fluctuations. Absence of light may be an important variable in seed survival, but this factor did not sufficiently explain survival under the thick volcanic deposits. Key words: buried seed, burial depth, seed size, seed survival, soil temperature, volcano Usu.

Colonization Dynamics of Herbs and Shrubs in a Disturbed Northern Hardwood Forest

Modes of recruitment of herbs and shrubs, and the extent to which availability of propagules affected distribution of the various species after disturbance were examined in a field experiment of canopy removal at the Hubbad Brook Experimental Forest, New Hampshire, U.S.A. The most common shrubs after canopy removal were Rubus idaeus and Viburnum alnifolium; the most common herbs were Aster acuminatus, Dennstaedtia punctilobula and Dryopteris spinolusa. All of the first-year recruitment or Rubus occurred through germination of buried seeds (...)

An Experimental Study on the Effects of Earthworms on the Ecological Success of Fern Gametophytes

The successful establishment and maintenance of pteridophyte populations requires the co-occurrence of both the gametophyte and the sporophyte life history phases. Yet, most studies of pteridophyte ecology consider only the sporophyte stage (eg. Grime, 1985). While there are a few studies of gametophyte ecology (eg., Cousens, 1988; Duckett, 1985), there is little knowledge of the factors that limit the distribution and abundance of gametophytes. One factor relevant to the gametophyte phase is the earthworm. Modern families of earthworms likely radiated in soils characteristic of angiosperm forests (Satchell, 1983; Bouche, 1983; Piearce, 1989). Lovis (1977) suggests that the modern polypodiaceous ferns radiated in response to ecological changes brought about by the radiation of the angiosperms. Modern genera of earthworms and leptosporangiate ferns evolved during the Tertiary in communities dominated by angiosperms (Lovis, 1977; Bouche, 1983). Earthworms are known to be involved in dispersal of both the seeds of spermatophytes (McRill & Sagar, 1973) and the spores of pteridophytes (Hamilton, 1988). Earthworms may play a particularly important role in bringing buried seeds and spores to the surface and placing them in improved sites for germination (Grant, 1983). Earthworms naturally till the soil, providing increased aeration, increased porosity and a more even distribution of soil nutrients. They also positively influence soil nutrient content by acting as a first step in the decomposition of detritus, and cause an increase in the numbers and diversity among microbial populations in the soil, thereby increasing the rate of decomposition, and increasing nutrient turnover rates (Lee, 1985). Earthworms prey on small pieces of vegetation, fungi, algae, and even other earthworms (Darwin, 1881; Lavelle, 1983; Lee, 1985). The green, surface dwelling gametophytes characteristic of modern ferns would likely make excellent prey for earthworms. As earthworms have been shown to be influential in the gametophyte habitat, an experiment was undertaken to determine if the presence of earthworms influences the germination of spores, gametophyte establishment, and/or gametophyte reproductive success.

The effect of established plants on recruitment in the annual forb Sinapis arvensis.

The germination response of Sinapis arvensis to the presence of established plants was investigated in a greenhouse experiment. Established conspecific and heterospecific plants were found to inhibit germination and reduce the probability of recruitment of those seeds that germinate. Established plants have no effect on seed mortality in the soil. Using a simple recruitment model, it is demonstrated that the combination of variance in germination time coupled with the interaction between buried seeds and established plants can generate density dependence. The implications of these results for community processes, such as succession, are discussed.

Induction of extreme light sensitivity in buried weed seeds and its role in the perception of soil cultivations

Abstract. Light, probably acting through the photo‐receptor phytochrome, promotes germination of weed seeds when the soil is disturbed by tillage operations. A short period of burial is shown to induce an enormous ∼10000‐fold increase in light sensitivity in the seeds of the arable weed Datura ferox which is interpreted as a natural transition to the ‘very‐low‐fluence’ mode of phytochrome action. Field experiments indicated that germination of buried seeds may be triggered by millisecond‐exposures to sunlight and suggested a key role for the process of sensitization in the mechanisms whereby light requiring seeds detect the occurrence of soil cultivation events in arable lands.

Dormancy patterns in buried seeds of Datura ferox and D. stramonium

Seasonal changes in seed dormancy of Datura ferox L. and D. stramonium L. were studied during 2 years in seeds buried in the field and compared with changes in dormancy of dry-stored seeds. Dormancy levels were evaluated by measuring the germination of exhumed and dry-stored seeds under 32:27 °C (nondormant seeds) or 32:12 °C (relatively dormant seeds). Levels of relative or full dormancy did not change during 2 years of dry storage in either species. Seed burial at 20 cm accelerated after-ripening, and primary dormancy disappeared earlier than at 5 cm. However, burial depths greater than 10 cm prevented germination in situ. This inhibition was not due to smaller daily fluctuations of soil temperature with increasing depth, since seeds buried at 20 cm and kept at 32:12 °C also failed to germinate. Transferring seeds to shallower soil layers allowed their germination. Cyclic seasonal changes in dormancy were found in buried seeds of D. ferox but not in D. stramonium. In D. ferox, dormancy was alleviated during the winter and germinability was maximal in early spring; the increase in dormancy was preceded by high late spring – summer temperatures. In D. stramonium most seeds near the soil surface germinated in the field in a brief flush in early winter, at temperatures under which D. ferox did not germinate. Key words: Datura, germination, dormancy, buried seeds, temperature.

Dormancy and viability of buried seeds of two deciduous broad-leaved species, Fraxinus mandshurica var. japonica and Acer mono var. mayrii

Dormancy and viability of buried seeds of two deciduous broad-leaved species, Fraxinus mandshurica var. japonica and Acer mono var. mayrii

Role of temperature and light in the germination ecology of buried seeds of Potentilla recta

SummaryFresh, mature seeds of the perennial weed, Potentilla recta L. (Rosaceae), were buried in early July of 1984 and 1985 and exposed to natural yearly temperature cycles. At monthly intervals for 28 (1984 seeds) and 25.5 (1985 seeds) months, seeds were exhumed and tested in light and darkness at 12/12 h alternating temperature regimes of 15/6, 20/10, 25/15, 30/15 and 35/20°C. At the time of burial, 2–38% of the seeds germinated in light at 15/6 and 20/10°C, but none germinated at any of the other test conditions. During summer, seeds after‐ripened, and by October they germinated to 41–87% in light at all thermoperiods. Seeds never gained the ability to germinate in darkness. Although buried seeds subsequently were exposed to low winter and high summer temperatures, they did not re‐enter dormancy or exhibit cyclic changes in germination responses. Thus, buried seeds can potentially germinate during any month of the growing season, if a disturbance brings the seeds to the soil surface and moisture is nonlimiting. However, only 23 to 50% of the exhumed seeds germinated at the July and August temperature (35/20°C) in July and August.

Germination Ecophysiology of Seeds of the Winter Annual Chaerophyllum Tainturieri: A New Type of Morphophysiological Dormancy

(1) Freshly matured seeds of the native North American winter annual Chaerophyllum tainturieri have morphophysiological dormancy (MPD), which is a combination of morphological (underdeveloped embryos) and physiological dormancy of the non-deep type. (2) Seeds after-ripen at high diurnally fluctuating temperatures (25/15 'C, 30/15 'C and 35/20 ?C) but not at low-constant or fluctuating temperatures (5 'C, 15/6 'C and 20/10 ?C); thus, the breaking of physiological dormancy occurs in the field during summer. Gibberellic acid promotes after-ripening. (3) Light is required for embryo growth, but it is ineffective while seeds are physiologically dormant. Thus, embryo growth and germination occur in autumn, after seeds are no longer physiologically dormant. However, if seeds are exposed to light for two or more days in summer, after the breakage of dormancy has started, they will germinate in darkness at normal autumn temperatures. (4) October habitat temperatures (c. 20/10 ?C) induce dormancy, and consequently no germination occurs in spring. Thus, the species behaves as a strict winter annual, germinating only in autumn. (5) Buried seeds exhibit an annual dormancy-non-dormancy cycle. Seeds become nondormant during summer and re-enter dormancy in autumn. This is the first species in which it clearly has been demonstrated that seeds with MPD can go into and out of dormancy annually. (6) Embryos did not grow while seeds were buried for thirty-two months. A light requirement for embryo growth ensures that buried seeds do not germinate in autumn while they are non-dormant. This is one reason why seeds can form persistent seed banks. (7) Seeds have a type of MPD that has not been described before, and we suggest that it be called 'non-deep simple MPD.' This is the eighth type of MPD so far described for seeds.

The role of light and alternating temperatures on germination of Polygonum aviculare seeds exhumed on various dates

Summary:The annual dormancy cycle was investigated in buried seeds of Polygonum aviculare L. exposed to natural temperature changes in Lexington, Kentucky, U.S.A. Seeds were exhumed monthly from December 1984 to February 1987 and tested in light (14‐h daily photoperiod) and continuous darkness at 12/12‐h daily alternating temperature regimes of 15/6, 20/10, 25/15, 30/15 and 35/20°C. During autumn and winter, seeds became non‐dormant, and in March 1985 they germinated to 95‐100% at all thermoperiods in light and to 7‐61% in darkness. Seeds remained non‐dormant during spring but became more specific in their germination requirements in early summer. During July and August 1985, seeds germinated to 17‐53% in light at 30/15 and 35/20°C but to 0‐10% at all other test conditions. By September, about 65% of the seeds were dormant, but the others were able to germinate under the higher alternating temperatures in light. A similar seasonal cycle was recorded in the following year through to the spring of 1987. The results confirm the seasonal pattern of dormancy in this species (Courtney, 1968) but indicate that alternating temperatures combined with light are important in determining germination potential in P. aviculare.

Germination requirements and seed banks of some nymphaeid macrophytes: Nymphaea alba L., Nuphar lutea (L.) Sm. and Nymphoides peltata (Gmel.) O. Kuntze

SUMMARY. Germination experiments demonstrated that the innate dormancy of the seeds of Nymphaea alba L., Nuphar lutea (L.) Sm. and Nymphoides peltata (Gmel.) O. Kuntze could be overcome by a cold treatment. Light stimulated the germination of the three species. Hypoxic conditions stimulated the germination of Nymphaea alba and Nuphar lutea seeds but the seeds of Nymphoides peltata did not germinate under these conditions. Experimental seed banks of Nymphaea alba, Nuphar lutea and Nymphoides peltata were laid out in three water bodies, varying in pH and alkalinity. Germination patterns indicated that Nymphaea alba and Nuphar lutea produce transient seed banks, but that Nymphoides peliata produces a persistent seed bank. Sampling of natural seed banks and subsequent germination tests were in concordance with the results of the seed bank experiment. The experimental above‐ground seed banks of Nymphaea alba, Nuphar lutea and Nymphoides peltata showed similar germination patterns in the three selected water bodies, despite the differences in pH and alkalinity between them. However, the distribution of Nymphoides peliata is restricted to well‐buffered waters, so that its absence from soft and acid water bodies must be due to post‐germination mechanisms and/or processes. In aquatic systems where Nymphoides peltata co‐exists with the other nymphaeid species studied, it is largely restricted to a bell between the helophytes and the vegetation at deeper sites. The deeper sites were dominated by Nuphar lutea and Nymphaea alba. Germination requirements and seedling emergence from buried seeds of Nymphaea alba, Nuphar lutea and Nymphoides peliata play an important role in the establishment of the zonation pattern of these nymphaeid macrophytes.

Phenology, seed ecology, and age structure of Spanish heath (Erica lusitanica) in Canterbury, New Zealand

Abstract Seasonal growth patterns and phenology of Spanish heath (Erica lusitanica Rud. in Schrad.) were studied at several sites in Canterbury, South Island. Shoot growth begins very early in spring but is affected by frosts above 400 m a.s.l. and by drought at lower altitudes. Flower buds are formed in autumn, flowering is in spring, and mature seed is produced early in summer. A typical 4–6 year-old bush with c. 20 branches produces c. 9 million seeds annually. Seed germination is highest under fluctuating temperatures and in full light, but 42% to 75% of seeds are dormant. These become incorporated in the soil seed bank, mostly in the upper few centimetres where they can survive for at least 4 years and possibly much longer. The highest number of buried seeds, 480,000 m−2, was found beneath dense heath scrub and 1500 m−2 are present up to 30 m beyond isolated heath clumps. Fire stimulates a small proportion of buried seeds to germinate. The distribution and age structure of heath clumps shows that heath is still expanding in the Geraldine area. The ability of heath to resprout after fire or damage and to produce seeds with the capacity for dormancy help to ensure its persistence on the landscape after the native shrub, Leptospermum scoparium, with which it is frequently associated, has disappeared.

Seed bank dynamics of Mediterranean pastures subjected to mechanical disturbance

Abstract. Vegetation and buried germinable seeds were examined in Mediterranean pastures differing both in time elapsed since last ploughing and in intensity of disturbance. Densities of buried germinable seeds in the greenhouse were determined from 10 × 10 × 3 cm soil cores taken during the autumn prior to the time of germination. Abundance of pasture species was recorded on the same sites during the following spring.The species composition of the seed bank was very similar to that of the vegetation: 80.9 % similarity (Sorensen's index). The number of species and density of buried seeds ready to germinate increased with suc‐cessional age from 0 to 4 yr and decreased later to a constant level after 9 yr. Regarding disturbance intensity, the highest species numbers are found at intermediate intensities while there is a drastic decrease at high disturbance intensity. The floristic composition of the seedbankwas analyzed through Correspondence Analysis (CA) and Binary Discriminant Analysis (BDA). Seed bank composition reflected both successional stage and intensity of disturbance. Characteristic species of both gradients were identified.

Stimulation of Germination of Acacia pulchella: Laboratory Basis for Forest Management Options

(a) The leguminous shrub Acacia pulchella is a valued understorey species of jarrah (Eucalyptus marginata)forests in south-western Australia. It regenerates from buried seeds mainly after fire, which breaks seed dormancy.

Effects of sand burial on seed germination, seedling emergence, survival, and growth of Agropyron psammophilum

Studies were conducted to determine the effects of sand burial on seed germination, seedling emergence, survival, and growth of Agropyron psammophilum. In the field, seedlings of A. psammophilum emerged from burial depths ranging from 0 to 7 cm. The mean depth of seedling emergence was 2.96 ± 1.06 (mean ± SD) cm, with more than 80% individuals emerging from 0 to 4 cm depths of sand burial. In a greenhouse, seeds of A. psammophilum were artificially buried by sand to depths of 0, 2, 4, 8, 10, and 12 cm. Higher emergence percentages were obtained at 2 and 4 cm burial depths and no seedling emerged from depths greater than 8 cm. The percent germination of buried seeds, percent emergence, and emergence rte of seedlings showed negative correlation with burial depth. Burial of young seedlings up to a depth of 6 cm stimulated their growth in height, leaf and tiller production, and overall dry weight. When the burial depth exceeded 6 cm, some seedlings died and if seedlings were buried to 100% of their height (11 cm for 1-week-old and 22 cm for 2-week-old seedlings), all seedlings died.

Contribution of buried seeds to revegetation after eruptions of the volcano Usu, Northern Japan

Soon after the 1977–78 eruptions of the volcano Usu, there were created many gullies in which former topsoil, i.e., well-developed soil accumulated before the eruptions, was eroded in the crater basin, whereas the outside of the gully was covered with thick volcanic deposits. The short-dispersal-seed plants were the most abundant in the inside of gully where the former topsoil was exposed, however, they have not immigrated from external environments. The germination tests of the seeds extracted from the former topsoil demonstrated that viable seeds were buried at 1683.3/m2 for 9 years after the eruptions. At least, 12 herbaceous species, e.g.,Rumex obtusifolius, Geum macrophyllum var.sachalinense andPoa annua, were derived from the buried seeds in the former topsoil. Their contribution to revegetation was estimated to be 40.0% in the inside of gully where the former topsoil was exposed in 1983, suggesting that buried seeds are one of the most important seed source for revegetation even in the case of a volcano which produced pumice and ash by eruption.

Seed Banks, Fire Season, Safe Sites and Seedling Recruitment in Five Co-Occurring Banksia Species

Quantity of seed stored in the canopy of 5 co-occurring Banksia species varied by nearly 2 orders of magnitude. The 3 species which resprout vegetatively after fire, produced less seeds and retained smaller seed banks than 2 non-sprouting species. Contiguous patches of scrub-heath were burned in spring and autumn. The spring fire was cooler, and both sets of seed released subsequently did not germinate until the following (common) winter. Rate of seed release was higher after the autumn fire. The non-sprouting species released more seeds after each fire and yielded more seedlings per parent than the resprouting species. Percentage (field) germination of the non-sprouters was not consistently different from that of the resprouters. Seeds exposed on the soil surface during summer soon lost viability compared with buried seeds. The large number of seedlings established up to 8 months after the spring fire resulted from many seeds escaping exposure by dispersal into litter-covered safe' sites. Although litter microsites covered only 30% of the ground surface after the spring burn, they accounted for 80% of seedlings both before and after the summer drought. Litter microsites covered only 14% of the autumn-burned site but accounted for 60% of seedlings before summer and 40% after summer, suggesting density-dependent thinning of seedlings. The autumn-burned site supported more than twice the density of seedlings than the spring-burned site by the end of the 1st winter. Summer mortality of seedlings was 32% in the spring-burned site and 65% in the autumn-burned site, equalizing the seedling:parent ratio at both sites. Net recruitment of 1-yr-old seedlings varied from 0 for 2 resprouters to >100 per parent for a non-sprouter.

Mechanisms of seed harvest by heteromyid rodents: soil texture effects on harvest rate and seed size selection.

Several lines of evidence show that soil texture plays an important role in the distribution of desert-dwelling heteromyid rodents. This is not surprising, since texture influences the energetic cost of digging burrows and of scratching at the soil surface to harvest buried seeds. Texture also may influence the efficiency with which seeds can be separated from the soil particles with which they are mixed. To explore mechanisms of "particle separation" by foraging heteromyids we measured seed harvest rates and size selection in the laboratory for a variety of seed sizes and soil textures. Harvest rate declined with increasing soil coarseness, and the preference for seeds of intermediate size that was apparent in fine soil disappeared when seeds were mixed with soil slightly coarser than the preferred seed size. In addition, there was evidence that particle separation efficiency is sensitive to the relative sizes of seeds and soil. A discontinity in the function relating harvest rate to soil texture occurred at finer soil textures for small seeds than for large seeds, suggesting that harvest techniques change once soil particle diameter equals or exceeds that of seeds. These results are consistent with the hypothesis that heteromyids use a combination of gravity-and rake-sorting mechanisms for particle separation.