A process-based grid model for the simulation of range expansion of Spartina alterniflora on the coastal saltmarshes in the Yangtze Estuary

Spartina alterniflora has been widely introduced to many coastal and estuarine regions of the world as a species for ecological engineering and this species has been spreading rapidly and widely along the Chinese coastline for the past 30 years. Based on four years of field measurements at the Chongming Dongtan wetland in the Yangtze Estuary, a process-based grid model of spatio-temporal range expansion for S. alterniflora was developed. The model incorporated the seasonal patterns of seed bank dynamics, seedling establishment, clonal propagation and vegetative growth, while also considering the effects of hydrodynamic conditions on the range expansion of S. alterniflora in the coastal saltmarshes. Modelling of a survey strip over a single year on the Dongtan wetland showed that the simulated spread pattern agreed with the actual pattern recorded during the growing season (March-September) with an accuracy of 90-95%, based on the estimations of seed bank dynamics and seedling establishment. On a wider spatio-temporal scale, an 8 year simulation showed that the patterns of range expansion of S. alterniflora are amenable to spatially-explicit modelling that takes spatio-temporal processes into account, mainly due to the aggregation effects of clonal integration in patches. Hydrodynamic conditions and niche availability were the most important factors controlling the expansion rate of S. alterniflora on the seaward expansion front. However, the actual expansion rate of S. alterniflora has slowed down in recent years due to the reduction in the sediment load of the Yangtze River following the completion of the Three Gorges Dam Project. There remain several uncertain issues relating to the model setup and its predictive capacity in terms of environmental variability and the stochasticity that is inherent in the modelling of the reproduction, dispersal and survival of S. alterniflora. Furthermore, an update of the model is needed linked to the sediment dynamics seasonality of hydrodynamic conditions in the Yangtze Estuary. In conclusion, this modelling approach provided valuable insights into the life-cycle mechanisms and range expansion processes of S. alterniflora under the current conditions. We suggest the potential application of this model in comparing various control strategies.