1. Phenotypic variation plays an important role in successful plant invasions. The spread of invasive species over large geographic ranges may be facilitated if plants can match their phenotype to local abiotic conditions.Spartina alterniflora, native to the United States, was introduced into China in 1979 and has spread over 19ᵒ of latitude along the eastern coast of China.

2. We studied patterns in vegetative growth and sexual reproduction ofS. alternifloraat 22 sites at 11 geographic locations over a latitudinal gradient of ~ 2000 km from Tanggu (39.05ᵒ N, high latitude) to Leizhou (20.90ᵒ N, low latitude) in China. We further evaluated the basis of phenotypic differences by growing plants from across the range in a common garden for 2 growing seasons.

3. We found distinct latitudinal clines in plant height, shoot density and sexual reproduction across latitude. Some traits exhibited linear relationships with latitude; others exhibited hump-shaped relationships. We identified correlations between plant traits and abiotic conditions such as mean annual temperature, growing degree days, tidal range and soil nitrogen content. However, geographic variation in all but one trait disappeared in the common garden, indicating that variation largely due to phenotypic plasticity. Only a slight tendency for latitudinal variation in seed set persisted for 2

years in the common garden, suggesting that plants may be evolving genetic clines for this trait.

4.Synthesis. The rapid spread ofSpartina alterniflora(S. alterniflora) in China has probably been facilitated by phenotypic plasticity in growth and reproductive traits. We found little evidence for the evolution of genetic clines in China, even though these exist for some traits in the native range. The considerable variation among clones, within provenances, that persisted in the common garden suggests a potential for the evolution of geographic clines in the future. Low fecundity of low-latitudeS. alterniflorapopulations in China might result in a slower spread at low latitudes, butS. alterniflorais likely to continue to spread rapidly at high latitudes in China and into the Korean peninsula.

Fig 1.Map of China showing the geographical survey sites.