Effects of tidal creeks on Spartina Alterniflora expansion: A perspective from multi-scale remote sensing

Rapid expansion of Spartina Alterniflora ( S. alterniflora ) has led to severe ecological consequences in coastal wetlands in China. Better understanding of the mechanism of S. alterniflora expansion is of great importance for its control and management. Although previous research has reported that the expansion of S. alterniflora is closely related to tidal creeks via field investigations, few studies have quantitatively investigated their relationships using remote sensing at different scales. In this study, we utilized time series satellite remote sensing imagery with different spatial resolutions to investigate the effects of tidal creeks on S. alterniflora expansion at both landscape scale and patch scale. We took the Yellow River Delta (YRD), China as our study area, where S. alterniflora has spread across the intertidal wetlands during the past decade. At landscape scale, we first derived the yearly maps of S. alterniflora and tidal creeks during 2009 - 2020 based on Landsat-series satellite imagery, then analyzed the spatio-temporal relationship between S. alterniflora and tidal creeks. By using grid analysis method, we evaluated the change of S. alterniflora area and tidal creek length and compared their change time. The results showed that 30.46 % of the grids experienced no change in tidal creeks while expansion of S. alterniflora , and 54.02 % of the grids experienced S. alterniflora area increase after the lengthening of tidal creeks. The results indicated that relationship between tidal creeks and S. alterniflora in YRD was dominated by " tidal creeks promoting S. alterniflora expansion " . We then simulated the S. alterniflora expansion at patch scale using an improved spatially explicit individual-based model based on S. alterniflora maps derived from Landsat imagery (30 m) and GF-1 high spatial resolution imagery (2 m), respectively. The results showed that consideration of long -distance dispersal of seeds along the direction of tidal creeks considerably enhanced the simulation accuracy, regardless of the spatial resolution of the S. alterniflora maps. High-resolution S. alterniflora maps yielded higher simulation accuracies. Our study suggests that the long -distance dispersal of seeds led by tidal creeks is an important manner for S. alterniflora expansion. Satellite remote sensing imagery have great potential to discover the mechanism of S. alterniflora expansion, facilitating control of S. alterniflora at the early invasion stage.