Spatio-temporal changes and hydrological forces of wetland landscape pattern in the Yellow River Delta during 1986-2022

Abstract

Context: Estuarine wetlands provide valuable ecosystem services but have undergone continuous change under the pressures of climate change and anthropogenic disturbance. The Yellow River Delta (YRD) wetland, which is one of the most vigorous delta areas in the world, has undergone dramatic changes. However, the long-term and high-resolution typical salt marsh vegetation evolution processes and hydrological drivers remain unclear. Objectives (1) Generate annual mapping of salt marsh vegetation in the YRD wetland from 1986 to 2022. (2) Analyze the trends of wetland patch area and landscape pattern, and explain the hydrological drivers of landscape pattern evolution. Methods Combining Landsat 5‒8 and Sentinel-2 images, vegetation phenology, remote sensing indices, and Random Forest supervised classification to map the typical salt marsh vegetation. We applied piecewise linear regression model to analyze YRD wetland changes, and stepwise multiple linear regression was used for assessing the impact of hydrological factors on landscape pattern. Results We identified three stages of landscape pattern evolution with 1997 and 2009 as turning points, including the rapid expansion stage, gradual decline stage, and bioinvasion stage. In the first phase, the wetland area was expanded by 70.45%, while the typical salt marsh vegetation, Phragmites australis area was reduced by 25%. In the second phase, the wetland was reduced by 21.33% and the Phragmites australis area was reduced by 15.96%. The third stage, Spartina alterniflora demonstrated an unstoppable trend of rapid expansion, with an area increase of 68 times relative to 2009, expanding at an average rate of 344 hm2 per year. Conclusions Areas of wetland, tidal flat, and Phragmites australis are significantly influenced by cumulative sediment and cumulative runoff, which total explain 61.5%, 75.7% and 63.8% of their variations, respectively. Wetland and tidal flat areas increased with the cumulative sediment, while cumulative runoff had a weak negative effect. As for Phragmites australis, cumulative runoff had a positive effect, whereas cumulative sediment had a negative effect. Water resources regulation measures should be taken to prevent the degradation of wetland ecosystems, and intervention measures can be implemented during the seedling stage to control the invasion of Spartina alterniflora.