Spatial pattern changes of Spartina alterniflora with different invasion ages in the Yancheng coastal wetland

Spatial pattern changes of Spartina alterniflora with different invasion ages affects the soil properties and ecological environment of the invaded area and changes the ecological process. Studies on the spatial pattern changes of S. alterniflora with different invasion ages will help to enhance our understanding of the effect of its invasion and reveal the relationship between spatial pattern changes and ecological processes in coastal wetlands. Therefore, based on remote sensing (RS) images and survey data collected during the period 1996-2015, spatial distribution pattern of S. alterniflora with different invasion ages was studied using space-for-time substitution, overlay analysis, and landscape pattern index analysis, together with RS and geographical information systems (GIS) technologies. Moreover, spatial pattern changes of S. alterniflora with different invasion ages and its influence on the ecological environment of the invaded area were determined by analyzing the characteristics of seaward and landward expansion, centroid shifts, and landscape index transformations. The results indicated that: (1) the spatial pattern changes of S. alterniflora with different invasion ages have stage characteristics: the patch area of S. alterniflora with invasion age of 15-20 years was very large with an average area of 425.40 hm2, formed in zones; the patch area of S. alterniflora with invasion age of 8-14 years became smaller with an average area of 153.64 hm2, and was formed in blocks with patchiness; the patch area of S. alterniflora with invasion age of 1-7 years was the smallest with an average area of 87.47 hm2; (2) in the early stage of invasion, the patch centroid of S. alterniflora shifted to the northeast, and later moved to the southwest. The seaward distance of S. alterniflora increased by 917.78 m at a rate of 48.30 m/year, whereas the landward distance of S. alterniflora increased by 697.70 m at a rate of 36.72 m/year. Therefore, seaward expansion was the predominant S. alterniflora invasion pattern; and (3) the landscape shape index of the S. alterniflora with middle invasion ages was higher than those of lower and higher invasion ages; therefore, with decreasing invasion age, the interspersion juxtaposition index and mean patch fractal dimension indexes showed an increasing trend, which was contrary to the trend indicated by the aggregation index. In conclusion, the patches of S. alterniflora distributed from the center to surrounding areas increased with invasion age. Higher S. alterniflora invasion ages showed more regular patch shapes and greater degrees of fragmentation.