Geographical variation in germination traits of the salt-marsh cordgrass Spartina alterniflora in its invasive and native ranges

Aims 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. Methods We studied seed germination traits of S. alterniflora using seeds collected from 10 locations across latitudes in its invasive range (China, 20 degrees-40 degrees N) and 16 locations across latitudes in its native range (USA, 27 degrees-43 degrees 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. Important Findings Native populations showed higher (similar to 10%) germination percentage and significantly higher (similar to 20%) germination index than invasive populations did, but invasive populations germinated significantly earlier (similar to 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 (T-mean), mean daily maximum temperature (T-max) and mean daily minimum temperature (T-min), and inversely correlated with T-mean, T-max and T-min in the native range. However, the mean germination time was positively correlated with T-mean, T-max and T mean 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.