Re-invasion of Spartina alterniflora in restored saltmarshes: Seed arrival, retention, germination, and establishment.

The invasive plant Spartina alterniflora presents a serious threat to the saltmarsh ecosystems in the Yangtze Estuary. Various measures have been implemented to control S. alterniflora and restore the natural saltmarshes in this area. However, many saltmarsh restoration activities often fail partly because of recursions of this invasive plant. In this study, we investigated the re-invasion of S. alterniflora in a restored saltmarsh in the Chongming Dongtan National Nature Reserve by analysing the aspects of seed arrival, retention, germination, and establishment, to better understand the potential factors that may influence the re-invasion of restored saltmarshes. The results showed that 1) tidal currents dispersed the seeds from the possible source area to the restored saltmarsh and adjacent mudflat. The spatio-temporal dynamics of arrived seeds were shown to vary greatly depending on the intertidal geomorphology, vegetation, and hydrodynamic processes. 2) Seed retention in the re-invaded area was shown to be greatly influenced by burial depth, and moderate sedimentation rates provided safe sites for the retention of arrived seeds. 3) Only when both the burial depth and inundation duration below certain thresholds, the retained seeds could germinate and establish in the recipient habitats successfully. The results from this study highlight that control efforts and the management of S. alterniflora should not only focus on the re-invaded areas of restored saltmarshes, but also on the possible source areas of re-invasive species.