| Spartina alterniflora was introduced to the Yangtze Estuary for the purpose of land reclamation and has expanded rapidly thereafter. The rapid expansion of this exotic plant has threatened the native species richness and ecological security. Both the sexual reproduction by seeds and asexual propagation by tillering and rhizoming are the two main approaches by which Spartina aterniflora can keep fast rate of geographic spread. Therefore, studies on the spreading patterns and its role played on invasion of Spartina alterniflora will help understand their invasive ecological procession and mechanism, provide a scientific basis and guideline at a strategic control of this species to maintain the wetland ecological structure and function as well as the wetland biodiversity.In this study, the high, middle and low intertidal zones were divided from 1998 dyke to mudflat seaward according to their elevations, respectively. Using field investigation and laboratory test, the spreading patterns of exotic Spartina alterniflora at Chongming Dongtan Nature Reserve were studied, which included in seed production, viability and germination characteristic, seed floatation, spatial-temporal dynamic and type of soil seed bank, seed fate respecting to seed germination and survivor in soil after across intertidal dispersal, the range expansion patterns by sexual and asexual propagation at the advancing fronts and the reinvasion at controlled area. The ecological process and mechanism of Spartina alterniflora invasion by sexual and asexual reproduction were also discussed. The main results were as follows:1. Seed production, viability, germination characteristics and seed floatationSeed production and viability of Spartina aterniflora varied along an intertidal gradient, the middle intertidal zone (MIT) had the largest seed production (83638±11852 no./m2) and highest viability (59.7%±3.5%), the seed production and viability of high intertidal zones (HIT) were 54489±20433 no./m2 and 51.3%±2.9%,41955±8999 no./m2 and 28.0%±3.0% for low intertidal intertidal zones (LIT). The seed germination did not occurred at low temperature, the chilling treatment (at low temperature and in moist conditions) could significantly enhance the germinability of Spartina alterniflora seeds and shorten the time of onset seed germination. The seeds from the site MIT had much higher germinability than the sites of LIT and HIT. Spartina alterniflora seeds have capacity to remain afloat in water, the floating time differed among seeds from different intertidal zones, which were 14 d,12 d and 6 d for the site of HIT, MIT and LIT, respectively. There were no significant differences in floating time between the HIT and MIT, but both were significantly higher than that of LIT, which has very important role for good quality seed to survive in winter and increase the validity of seed dispersal on spring tides.2. Seasonal changes in seed germination and persistence in soil and spatial-temporal dynamics of soil seed bankSeed germination in soil started in February and ended in June whatever their burial depths (5cm,10cm and 20cm), over 80% germination occurred in March and April. The germinations significantly correlated with seed quality and burial depth, the MIT seeds buried at depth of 5 cm have the largest values, and the seed germination percentages decreased with increased burial depth, while the seed germination of LIT with 20cm burial depth was the lowest one. The germination was independent on the intertidal zones where they were buried, no significant differences in germination among the same burial depths across intertidal burial. Before the spring flush of germination, seed survival correlated with seed quality and was independent on the intertidal zones and burial depths. The seed survival decreased quickly at 5 cm burial with the flush germination in field, which were much lower than that of 10 cm and 20 cm burial. However, seeds of Spartina alterniflora did not survive 9 months even buried at 20 cm burial depth and no germinable seed were recorded after July.The size of soil seed bank of Spartina alterniflora depends on the seed production, viability and elevation of zones. The highest density and seed viability of soil seed bank was recorded at the site of MIT, where had the highest seed production, then the HIT, the smallest soil seed bank was at the site of LIT. By July, before there was any replenishment with fresh seeds from the current year, the soil seed bank was completely exhausted and the persistent time of soil seed bank for Spartina alterniflora was less than 9 months, which is in agreement with that of the transient seed bank.3. Range expansion patterns of Spartina alterniflora at advancing frontsTwo types of advancing fronts of Spartina alterniflora, i.e. Spartina alterniflora-mudflat (S-M front) and Spartina alterniflora-Scirpus mariqueter-mudflat (S-S-M front) could be found at the Chongming Dongtan nature reserve. Both sexual reproduction by seeds and asexual propagation by tillering and rhizoming were the two main means by which Spartina aterniflora maintained a fast rate of geographic spread at their advancing fronts, the roles which sexual reproduction and asexual propagation might play in the range expansion were probably dependent on their location and the type of habitat as well as local abiotic and biotic factors. Initial recruitment of seedlings did not occur until May, when seeds transported by the tidal water germinated in front of the continuous edge of dense Spartina alterniflora meadow. Comparing the seedling recruitment occurred at these two fronts, the mean number of seedling recruitment was much higher at the S-M (8.2±0.7 no./m2→0.2±0.1 no./m2) front than the S-S-M front (1.5±1.7 no./m2→0.1±0.3 no./m2). Once established, the seedling survivorship was high and there were no significant differences in seedling survivorship between the S-M (80.6%±86.7%) and S-S-M fronts (80.0%—84.0%). Once the seedlings established at the front of the continuous Spartina alterniflora meadows in May, they quickly formed tussocks by vegetative tillering and rhizoming and finally merged into dense meadows at S-M front. The mean distance of range expansion of Spartina alterniflora after one growing season at the S-M front was 25.4±3.1 m/year, while 2.7±0.5 m/year at the S-S-M front. The range expansion rate at the S-S-M front was much slower than the S-M front. These two patterns of range expansion of Spartina alterniflora on an expansion front scale revealed from this study confirmed the pattern on a large scale of range expansion of Spartina alterniflora at the salt marshes in the Yangtze Estuary. The colonization behaviors of Spartina alterniflora at the expansion fronts differed as a reaction to various external and internal factors.4. The reinvasion by Spartina alterniflora in the controlled areaSeeds and seedlings in the neighbouring Spartina alterniflora community were the basis for fast reinvasion of Spartina alterniflora in controlled area. Seeds and seedlings were transported by tidal water to occupy empty niches and quickly formed tussocks through vegetative propagation, and finally merge into dense continuous meadows. After two years'reinvasion by Spartina alterniflora, there were no significant differences in culm density, vegetation height, above biomass and sexual parameters between new formed continuous meadow and the former continuous meadow. However, the spreading distance by vegetative propagation from neighbouring continuous meadow edges were less than 1 m for two years, which contributed little to reinvasion. Since Spartina alterniflora mainly achieved reinvasion by seed and seedling dispersal, the most effective measures of controlling the reinvasion of Spartina alterniflora should be taken by preventing the dispersal of seeds and seedlings by tidal water.
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