A Study On The Expansion Patterns Of Spartina Alterniflora At Chongming Dongtan Wetlands And Its Underlying Factors

Since Spartina alterniflora was first found in the Chongming Dongtan nature reserve in 1995, this exotic plant has expanded rapidly and formed a large area of mono-dominant community at Chongming Dongtan by its fast sexual reproduction and asexual propagation. Studies on the expansion patterns of Spartina alterniflora and its mechanisms could provide a scientific basis and guideline for strategic control of this species and maintenance of wetland structure and function as well as wetland biodiversity. In this study, seed production patterns of Spartina alterniflora and its germination response to seasonal temperature variation were investigated at the Chongming Dongtan nature reserve. Two types of range expansion patterns of Spartina alterniflora at its advancing fronts and their interactions with hydrodynamics and accretion/erosion dynamics were also studied. The spatial-temporal patterns of sexual spreading and its formation mechanism of the range expansion patterns of Spartina alterniflora at different advancing fronts were discussed. The main results from this study were as follows:The results showed that along an intertidal gradient, the middle intertidal zone had the highest seed production of 44523/m2, followed by the higher intertidal zone of 31150/m2 and the lower intertidal zone of 17250/m2. The seeds of Spartina alterniflora from all intertidal zones could germinate at a temperature range of 10 to 30℃and showed a trend of increase followed by decrease in germination percentage and germination speed. Seed germination percentage reached the highest and germination speed was the fastest under the treatments of 25℃constant temperature and 20/30℃alternate temperature. Moreover, the germination percentage was higher under an alternate temperature than that under a constant temperature with a same cumulative temperature. The results indicated that the middle intertidal zone could provide an important source while the temperature in April and May could promote the seed germination for a fast sexual spreading. The results from this study could provide useful insights into the population ecology and the control of this invasive plant at the nature reserve.Two types of advancing fronts could be found, i.e. the Spartina alterniflora-mudflat (S-M front) and the Spartina alterniflora-Scirpus mariqueter-mudflat (S-S-M front) at Chongming Dongtan.The results showed that due to the higher flow velocity, the hydrodynamic force at the S-S-M front was stronger than that at the S-M front especially during summer. At the S-M front, the mudflat showed a pattern of autumn/winter erosion and spring/summer accretion while a pattern of autumn/winter stableness and spring/summer accretion occurred at the S-S-M front, along with a remarkable difference between their sedimentation patterns during spring which is the critical period for seedling recruitment. The mean number of seedlings of Spartina alterniflora recruited at the S-M front with a relatively slow accretion rate in May was much higher than that at the S-S-M front with a fast accretion rate at the same period. Through seedling recruitment, tussock development and coalesce, the range expansion rate after one growing season at the S-M front was much higher than that at the S-S-M front which was mainly achieved by vegetative growth. The different colonization behaviors at the advancing fronts could be related to the differences in hydrodynamics and accretion/erosion patterns. The knowledge on interactions of the range expansions of Spartina aterniflora with hydrodynamics and accretion/erosion dynamics at Chongming Dongtan wetlands could provide useful insights into the mechanisms underlying the formation of different range expansion patterns of Spartina alterniflora at its advancing fronts.