The Similarity between Species Composition of Vegetation and Soil Seed Bank of Grasslands in Inner Mongolia, China: Implications for the Asymmetric Response to Precipitation

The asymmetric response of productivity to precipitation was recently proposed as an early warning signal for the shifts in temperate grassland function in China. It was hypothesized that the asymmetry was influenced by the increased growth of the newly emerged seedlings from the soil seed bank. Therefore, the seed density in the soil seed bank and the similarity between species composition of the vegetation and the soil seed bank should be maximized where asymmetry was maximized. However, this knowledge was still limited and unconfirmed. In this study, the desert steppe, typical steppe and the transition zone between them (with the highest asymmetry) were selected for studying the similarity index in both 2018 (dry year) and 2019 (wet year). Plant species composition was monitored in situ using an unmanned aerial vehicle. Soil seed bank samples were collected, and the seed bank density and species composition were then examined and identified in the laboratory. Results showed that: (1) The variation in vegetation species richness between the two years was the highest (41%) in the transition zone (p < 0.05), while it was only 7% and 13% for the desert steppe and typical steppe, respectively. The presence of herbaceous species mainly caused the differences in variation among three grassland types. (2) Seed density was the highest in the transition zone (114 seeds/m2 and 68 seeds/m2 in the transient and persistent soil seed bank, respectively) (p < 0.05). Additionally, herbaceous species were the main components of the soil seed bank. (3) The similarity index was the highest in the transition zone (p < 0.05), with 38%/44% and 33%/44% for the transient/persistent soil seed bank in 2018 and 2019, respectively. Our study demonstrated that variation in vegetation species composition was very similar to the composition of the seeds accumulated in the soil seed bank. These results warrant further investigation for the mechanism of asymmetric response of productivity to precipitation.

Seeded growth of ultrathin gold nanoshells using polymer additives and microwave radiation

Nanoshells made of a silica core and a gold shell possess an optical response that is sensitive to nanometer-scale variations in shell thickness. The exponential red shift of the plasmon resonance with decreasing shell thickness makes ultrathin nanoshells (less than 10 nm) particularly interesting for broad and tuneable ranges of optical properties. Nanoshells are generally synthesised by coating gold onto seed-covered silica particles, producing continuous shells with a lower limit of 15 nm, due to an inhomogeneous droplet formation on the silica surface during the seed regrowth. In this paper, we investigate the effects of three variations of the synthesis protocol to favour ultrathin nanoshells: seed density, polymer additives and microwave treatment. We first maximised gold seed density around the silica core, but surprisingly its effect is limited. However, we found that the addition of polyvinylpyrrolidone during the shell synthesis leads to higher homogeneity and a thinner shell and that a post-synthetic thermal treatment using microwaves can further smooth the particle surface. This study brings new insights into the synthesis of metallic nanoshells, pushing the limits of ultrathin shell synthesis.

Interspecific competition in germination of bird-dispersed seeds in a habitat with sparse tree vegetation in South Africa

Background By transporting and scarifying the seeds during ingestion, avian frugivores reduce the competition with siblings, and may improve the germination which is critical for dispersal effectiveness and population recruitment. However, generally, there is limited knowledge on how deposited seeds interact/compete in the new microsite. We tested the hypothesis that the bird-dispersed seeds benefit from improved germination after their passage through the bird’s gut; and we investigated the potential impact of seed density on competition at the microsites by determining whether seed density and species diversity influence germination in the Free State Province, South Africa. Results Overall, the results partly supported the hypothesis. Germination trials with defecated seeds of five plant species compared with the manually depulped seeds showed that only Searsia lancea had significantly higher seed germination success and improved germination speed after passage through the bird gut while Ziziphus mucronata only benefited rapid germination. There was a significant correlation between seed size and the germination of bird-ingested seeds except in Olea subsp. africana possibly due to possession of extremely hard protective seed cover. Seed competition experiments pointed to Z. mucronata and O. subsp. africana having significant germination performance that was positively correlated to seed density and seed size while Ehretia rigida did not germinate at all. Seed species diversity in the germination trays did not have a significant impact since the seeds of two former plant species consistently displayed significantly higher germination across the competition levels. Conclusions We conclude that different plant species respond differently to seed ingestion by birds, and that further long-term tests for germination physiological responses of the seeds’ samples used in this study are required since poor germination observed in other tree/shrub species cannot be attributed to competition solely.

Toward Ultrathin Gold Nanoshells: Exploring Seed Density, Polymer Additives and Microwave Radiation Effects

Nanoshells made of a silica core and a gold shell possess an optical response that is sensitive to nanometre-scale variations in shell thickness. The exponential red shift of the plasmon resonance with decreasing shell thickness makes ultrathin nanoshells (less than 10 nm) particularly interesting for broad and tuneable ranges of optical properties. Nanoshells are generally synthesised by coating gold onto seed-covered silica particles, producing continuous shells with a lower limit of 15 nm, as a result of an inhomogeneous droplet formation on the silica surface during the seed regrowth. In this paper, we investigate the effects of three variations of the synthesis protocol to favour ultrathin nanoshells: seed density, polymer additives and microwave treatment. We first maximised gold seed density around the silica core but surprisingly its effect is limited. However, we found that the addition of polyvinylpyrrolidone during the shell synthesis leads to higher homogeneity and a thinner shell and that a post-synthetic thermal treatment using microwaves can further smooth the particle surface. This study brings new insights into the synthesis of metallic nanoshells, pushing the limits of ultrathin shell synthesis.

Soil Seed Banks of Tree Species from Natural Forests, Restoration Sites, and Abandoned Areas in Chiang Mai, Thailand

Soil seed banks have been used for investigation of natural regeneration of forests. In this study, we compared seed density and species composition of soil seed banks of trees among natural forests, restored forests of different ages, and abandoned agricultural land. The soil seed banks were collected from a natural forest (NF), 12-year-old and 17-year-old restoration sites (RF12y and RF17y), and 17-year-old abandoned site (AA) at Ban Mae Sa Mai, Chiang Mai, Thailand. A seedling emergence technique was used to assess seed density and species of emerged seedlings was identified. We found 5-8 tree species at each site. Seed densities in the study areas ranged from 43 to 298 seeds/m2. The seed density of RF12y was significantly higher than that of both NF and AA but not significantly different than RF17y (p < 0.01). Although there was no significant relationship between the restoration ages and the seed densities of the soil seed banks, the species composition of standing vegetation was related to the seed bank species. Sorensen’s similarities between the species composition of the soil seed banks and the existing trees in each area were between 0 and 13.79%, suggesting seed dispersal of both within and across study sites. Eight out of fourteen species in the soil seed banks were dispersed into restoration sites without standing vegetation of those species. Seven of those were animal-dispersed species. The selected native trees, framework species, attracted small seed dispersers into the study areas, especially at the restoration sites. This finding suggests that active forest restoration improved natural regeneration in restoration sites as well as neighboring areas via seed dispersal.

Natural vegetation regeneration from soil seed banks in the cultivated edges of Sudd wetlands in Juba, Southern Sudan

Soil seed banks are important for regeneration of degraded wetlands ecosystems. The Sudd wetlands of Juba city have long been encroached for crop cultivation. Seedling germination was monitored in a greenhouse to establish possible natural regeneration in Mindiari, Rejaf and Roton wetlands in the Sudd. Sixty-four species germinated from the soil seed bank of which 12.5% were dominated by Cyperus difformis and Typha capensis. The findings showed that median wetland species richness in Mindiari was 1.5 (interquartile range = 0.75?3.5), Rejaf 2.5 (interquartile range = 1.0 ? 4.0), Roton 3 (interquartile range = 1.0 ? 5.0) while median Shannon-Wiener diversity was 1.5 (1.14 ?1.73), 1.43 (1.01?1.66), 1.15 (0.98?1.67) for Mindiari, Rejaf and Roton respectively. Both the median seed species richness and diversity were not significantly different among the study wetlands. The median of seed density (56.1) was significantly higher in Roton than in Mindiari (36.7) and Rejaf (29.4) wetlands. The NMDS results showed that species composition of Mindiari and Rejaf was different from Roton. It is concluded that growing crops in wetlands did not influence species richness and diversity but it reduced seed density and altered species composition. Although wetland species were not significantly different in the three-wetland categories, dominance of canopy species belonging to Typhaceae and Cyperaceae indicates that these species are resilient to cultivation and could facilitate natural regeneration of cultivated wetlands edges of the Sudd region in Juba. Further research should examine effect of cultivation duration and flooding regimes on soil seed bank species richness, diversity, and density and composition.