Spatiotemporal Dynamics of Salt Marsh Vegetation regulated by Plant Invasion and Abiotic Processes in the Yangtze Estuary: Observations with a Modeling Approach

The natural processes of mudflat accretion, vegetation succession, and plant invasion and associated anthropogenic impacts make the Yangtze Estuary an ideal area for the study of salt marsh dynamics. In order to understand the biotic and abiotic factors regulating the spatiotemporal dynamics of salt marsh vegetation in relation to the expansion of the exotic plant (Spartina alterniflora) and the consequent responses of native species (Phragmites australis and Scirpus mariqueter), a grid-based model was developed incorporating the key ecological processes at Chongming Dongtan and Jiuduansha wetlands. During the simulation period of 2000-2008, the area of S. mariqueter decreased significantly at both wetlands, due to the rapid expansion of S. alterniflora. The expansion rate of P. australis was slow. When compared with observations, the simulated spatiotemporal dynamics of salt marsh vegetation showed a percentage match of 73-91 %. The accuracy of the distribution area for each salt marsh species was even higher (89-97 %). Both simulation and observation revealed that the period of early salt marsh succession was crucial for colonization and establishment of S. alterniflora after its introduction. Projections for the period 2008-2015 indicated that the expansion rate of S. alterniflora would slow down. In contrast, the model predicted that the distribution of S. mariqueter and P. australis would increase at a steady pace. The probable reason behind this is that the accretion of habitat above the elevation threshold at the current rate of sedimentation is marginal compared to the previously rapid rate of habitat colonization by S. alterniflora over the past decade. This study indicates the model's potential for simulating and predicting the dynamics of salt marsh vegetation in the Yangtze Estuary and demonstrates that, when appropriately parameterized, the model could be successfully applied elsewhere. Finally, the limitations of the model and its potential for monitoring and controlling invasive species in coastal wetlands are briefly discussed.