Haplotype-specific interactions of Phragmites australis with Spartina alterniflora under salt stress

Biological invasions present a global environmental challenge, the competitive interactions between native and invasive species constitute a crucial factor in determining the success of invasions. Past research has often treated native species as a monolithic entity when conducting competition experiments with invasive species. In truth, different genotypes may possess substantial differences in functional traits and competitive abilities. Few studies have subdivided widespread native species into distinct subgroups to conduct competitive experiments with invasive species. The invasive Spartina alterniflora and the widespread native Phragmites australis coexist extensively along the coastal regions of China. Through field sample collection and greenhouse common garden experiments, we investigated the salinity tolerance of two common haplotypes of P. australis (haplotype O and haplotype P) in the Yellow River Delta, as well as their relative competitive ability against the invasive S. alterniflora across varying salinity levels. The results showed that under high salinity without competition, the biomass of haplotype O decreased by 46.89 % (haplotype P: 40.0 %), while under low salinity with competition, it decreased by 17.7 % (haplotype P: 25.8 %). The competitive dominance of both haplotypes against S. alterniflora gradually diminished as salinity increased, disappearing under high salinity conditions. Haplotype O exhibited a competitive dominance over S. alterniflora under control and low-salinity stress, whereas haplotype P only showed competitive dominance under control conditions. Our study demonstrates that different genotypes of native species possess varying resistance to invasive species, a consideration that is critical in the practice of using native species for biotic substitution to control invasions.