A new perspective on the impacts of Spartina alterniflora invasion on Chinese wetlands in the context of climate change: A case study of the Jiuduansha Shoals, Yangtze Estuary

Spartina alterniflora, an invasive plant, was introduced to the Chinese coastal zone in the early 90s. As an eco-engineering species, S. alterniflora not only alters saltmarsh species distributions, previously described as habitat degradation, but it also plays a vital role in coastal protection, especially for the development of recently emerged intertidal shoals. To provide a reference for coastal management under global change, we quantified the impact of the invasion process on provided ecological and coastal protection functions, exemplified at the emerging Jiuduansha Shoals (JDS) in the Yangtze Estuary. Results obtained by high-precision satellite monitoring and numerical modelling showed that the establishment and growth of S. alterniflora can exert considerable changes on local environment. The invasion of S. alterniflora to JDS wetland can be divided into three distinct phases, (1) establishment 1998-2003, (2) expansion 2003-2009, and (3) dominant 2009-2018 stages according to the changes in saltmarsh composition. Spatially, S. alterniflora continuously replaced Scirpus mariqueter, forcing S. mariqueter and Phragmites australis slowly to the lower and higher intertidal habitats, respectively. Notably, S. alterniflora expansion was the main driver that contributed to over 70 % of recent JDS wetland expansion even under sediment deficit conditions. Established S. alterniflora marsh (directly) dampens more waves because of aboveground stems, but it also causes more accretion and indirectly leads to higher "morphological" wave dampening. Thus, it increases coastal defense provided by the saltmarsh in the context of sea-level rise and strengthening storms. In conclusion, the role of S. alterniflora invasion to the local environment under global changes is controversial. For sustainable coastal management, we need context-dependent S. alterniflora management to maximize the benefit of coastal protection and minimize the impact on local ecology, especially in sediment-starving estuaries with expected coastline retreat.