Integrating UAV data to explore the relationship between microtopographic variation and Spartina alterniflora expansion during its early invasion

Abstract

As a major invasive species in coastal China, Spartina alterniflora (S. alterniflora) has seriously threatened the ecological functions of coastal wetlands. Based on multisource high-precision geospatial data obtained by unmanned aerial vehicles (UAVs), the shape and distribution of S. alterniflora patches were extracted and analysed by using object-oriented classification and the landscape pattern index in this study. Considering microtopographic variations, the trend-surface subregions were delimited. In this way, we examined the relationship between S. alterniflora distribution and expansion and the variation in microtopography and tidal hydrology in the early invasion stage. The results showed that the microtopography silted up acceleration from land to sea, and the inundation time was reduced significantly in the study area for 2020–2022. In 2020, the average area of S. alterniflora patches showed a decreasing trend from land to sea. The distribution of patches on both sides of the sea and land was relatively concentrated and dispersed in the middle. By 2022, S. alterniflora presented a rapid expansion, with the total area expanding from 8488.57 m2 to 18470.72 m2. With the variation in microtopography, the characteristics of S. alterniflora patch expansion presented regional differences: Due to the small microtopographic and tidal hydrological variations, the patches fluctuated outwards slightly (average variation distance was 0.93 m) near the dike. The patches in the seaside front area had an annular expansion, and the expansion speed positively correlated with the area of the original patch. In the central region, the small patches merged and filled to form an S. alterniflora belt, while a large number of new small patches appeared. In general, the heterogeneous environment formed by microtopographic variations was conducive to the colonization and growth of S. alterniflora, but the excessive slope could inhibit expansion. This study will provide a theoretical basis for coastal ecological management and rapid control of S. alterniflora expansion.