Germination is the earliest life-history transition of a plant species. It determines the ecological breadth and geographic ranges of a species and has major effects on its invasion potential. The largest spread of the invasive salt-marsh cordgrass Spartina alterniflora in China, where it extends to latitudes lower than its native range in North America, provides an opportunity to examine germination trait variation across latitudes within and among its invasive and native ranges.

We studied seed germination traits of S. alterniflora using seeds collected from 10 locations across latitudes in its invasive range (China, 20°–40° N) and 16 locations across latitudes in its native range (USA, 27°–43° N) in growth chambers with 0 PSU sterilized distilled water. We further evaluated how climate and tide range in the original locations influenced germination traits.

Native populations showed higher (~10%) germination percentage and significantly higher (~20%) germination index than invasive populations did, but invasive populations germinated significantly earlier (~3 days) than native populations. Germination percentage and germination index increased with latitude in the invasive range but decreased with latitude in the native range. The mean germination time decreased with latitude in the invasive range and paralleled that in the native range. Germination percentage and germination index were negatively correlated with mean daily temperature (Tmean), mean daily maximum temperature (Tmax) and mean daily minimum temperature (Tmin), and inversely correlated with Tmean, Tmax and Tmin in the native range. However, the mean germination time was positively correlated with Tmean, Tmax and Tmean in both ranges. Our results demonstrate that invasive and native populations have evolved different latitudinal clines in germination percentage and index, but the mean germination time of the invasive population mirrored the latitudinal cline observed in the native range, suggesting that germination strategy across latitudes may change during invasion process.