Temperature affects the relative importance of phenotypic plasticity and natural selection contributing to the niche breadth of invasive plants

Plant species adapt to changing climates through phenotypic plasticity and natural selection, affecting invasive plants' niche breadth. However, our limited understanding of how temperature affects the relative importance of phenotypic plasticity and natural selection contributes to invasive plants' niche breadth across latitudes. Here, we used a model system, the salt marsh grass Spartina alterniflora , native to the United States but was introduced into China in 1979, spreading over 20° latitude. We collected seeds of S . alterniflora from nine locations across different latitudes in China, measured phenotypic plasticity, natural selection, and niche breadth of germination and post‐germination traits in three greenhouse common garden experiments (low‐latitude: 21°16'N, 110°20'E; mid‐latitude: 28°38'N, 121°24'E; and high‐latitude: 37°27'N, 121°23'E) spread across the latitude in China. We found that germination time, germination percentage, germination index, and seedling survival all increased with latitude within the common gardens. Germination time showed a lower slope at the high‐latitude common garden sites, but the other variables had higher slopes at the high‐latitude common garden sites. The phenotypic plasticity of germination and post‐germination traits across latitudes decreased with increasing latitude of origin, similar to germination niche breadth. Furthermore, temperature negatively affected the natural selection but positively affected the phenotypic plasticity. These results indicated that climate‐driven selection favors sexual reproduction at higher latitudes and higher phenotypic plasticity at lower latitudes. Our findings highlight that phenotypic plasticity and natural selection could drive the niche breadth across latitudes. This is critical for predicting the niche breadths of invasive species and the potential management of biological invasions in future climate conditions.